Electrical engineering

Electrical Transformer: Working Principle, Construction, and Types Explained

2021-08-21T10:39:00.029-07:00

Electrical Transformer: Working Principle, Construction, and Types

Understanding Electrical Transformers: Definition, Working Principle and Construction

An electrical transformer is a static electromagnetic device designed to transfer alternating current (A.C.) energy from a primary circuit to a secondary circuit while maintaining the same frequency. This transformation modifies the voltage and current to meet specific requirements in power transmission and distribution.
A transformer's core, typically made from cold-rolled grain-oriented (C.R.G.O.) steel sheets, is engineered to minimize energy losses and optimize magnetic flux. The device consists of two or more windings:
Primary winding: Receives the input A.C. energy.
Secondary winding: Delivers the transformed energy.
In an auto-transformer, the windings are both electromagnetically and electrically coupled.
Transformers are classified based on their windings and applications:
A transformer with two windings is called a double-wound transformer.
A transformer with three or more windings is known as a multi-winding transformer.
By design, transformers are further categorized into:
Single-phase transformers: Suitable for residential or small-scale applications.
Three-phase transformers: Common in industrial and commercial power systems.
Polyphase transformers: Feature interconnected windings to deliver consistent voltage in multi-phase systems.
The windings are also classified by voltage:
High-voltage (H.V.) winding: Connected to the circuit with higher voltage.
Low-voltage (L.V.) winding: Connected to the circuit with lower voltage.
Depending on the voltage transformation:
A step-down transformer decreases voltage.
A step-up transformer increases the voltage.
Some transformers are equipped with taps on their windings to adjust the voltage or current ratio, making them versatile for various applications.
For insulation and cooling, transformers are often immersed in transformer oil within a sealed tank. These are referred to as oil-immersed transformers. In contrast, dry transformers use air or other materials for insulation and are ideal for environments where liquid insulation isn’t practical.

Working Principle of a Transformer

A transformer is a static electrical device designed to transfer power from one circuit to another without altering the frequency. It operates by increasing or decreasing voltage levels while inversely adjusting current levels. This functionality is essential for efficient power transmission across electrical systems.
The principle of operation is based on mutual inductance: when an alternating current (A.C.) flows through the primary winding, it creates a magnetic field that induces voltage in the secondary winding. The primary winding receives the electrical input, and the secondary winding delivers the output.
Both windings are made from insulated copper or aluminum and are wound on a laminated core made of magnetic material. This core reduces energy losses and enhances the magnetic flux necessary for efficient energy transfer.

Fig.2.1 Magnetic circuit and windings of Transformer

The basis of the evolution of a transformer is the formation of mutual inductance between two coils sharing the same magnetic flux through a path of low reluctance as shown in Fig. 2.1.

The mutual inductance is extremely high between these two coils inside a transformer. If one coil is connected to a source of alternating voltage, an alternating flux gets formed within the laminated core, most of which is linked up with the secondary side, inside which the formation of mutually induced emf ( Electromotive force) takes place. The equation for Faraday's laws of electromagnetic induction, i.e.

Where, e = Induced EMF

M = Mutual Inductance

If the second circuit is closed, a current flows in it, so the electric energy is transferred from the primary one to the secondary one.

Understanding Transformer Ratings: Key Specifications and Measurements

A transformer has several important rated quantities, such as power, voltage, and frequency, which are typically specified on the manufacturer’s nameplate. These ratings provide essential information for understanding the transformer's capabilities and proper use. In addition to these, other rated quantities—such as efficiency and temperature conditions of the cooling medium—are also important but might not be explicitly listed on the nameplate.
Here are the key transformer ratings:
Rated Duty: The rated duty of a transformer is determined by the quantities provided on the nameplate, indicating its design limits for operation.
Rated Power: The rated power of a transformer is the output power at the secondary terminals, usually expressed in KVA (kilovolt-amperes). This is the maximum power the transformer can supply without overloading.
Rated Primary Voltage: This is the voltage indicated on the nameplate for the primary winding. If the transformer has taps on the primary, the rated tapped voltage will be specifically noted.
Rated Secondary Voltage: The secondary voltage is the voltage across the transformer’s secondary terminals when there is no load, and the primary side is operating at its rated voltage. If the secondary winding has taps, the rated voltages for each tap will be clearly specified.
Rated Current: The rated primary and secondary currents are calculated based on the rated power and voltage values provided on the nameplate. These indicate the maximum safe current levels for the transformer.

Kinds of transformers

The transformers mentioned in the list are the most important ones:

1. Power Transformers are used for the transmission and distribution of electric power over long distances.

2. Autotransformers are used for converting voltages within relatively small limits to connect power systems of different voltages to start A.C. Motors etc.

3. Transformers for feed installations with static converters for example Mercury arc rectifiers, ignitrons, semiconductor valves, etc are used in converting A.C. into D.C.(rectifying) and converting D.C. into A.C.(inverting).

4. Testing transformers are used for conducting tests at high and ultra voltages.

5. Power transformers can also be used for special applications like furnace and welding etc.

6. Radio transformers are used in radio engineering.

Transformer construction

All the transformers have the following essential elements

1. All the transformers except auto-transformers can have more than two windings insulated from each other and the core.

2. A core which in the case of a single-phase distribution transformers usually comprises a cold-rolled Silicon Steel strip instead of an assembly of punched Silicon Steel laminations such as are used in the larger power transformer cores. The flux flows parallelly to the direction of steel's grain or 'orientation' inside the core. This results in a reduction in core losses for a given flux density and frequency, or it allows the use of higher core densities by keeping the size of transformers small against different core losses.

Other necessary parts are:

1. A suitable container for the assembled core and windings.

2. A suitable medium for insulating the core and its windings from each other and from the container.

3. suitable bushings for insulating and bringing the terminals of the windings out of the case.

A transformer has two basic types of construction :

1. core type transformer.

2 shell-type transformers.

Both the type of transformer has a different arrangement of copper conductors and iron core. Core type transformers have an iron core visually surrounded by copper but in the shell-type transformers, the iron surrounds the copper winding.

Core type transformer

The complete magnetic circuit of the core-type transformer is in the shape of the hollow rectangle exactly as shown in fig.2. in which Io is the no-load current and fi is the flux produced by it. N1 and N2 are the numbers of turns on the primary and secondary sides respectively.

Fig.2. The magnetic circuit of a core-type transformer

The core is made up of silicon-Steel laminations which are either rectangular or

L-shaped with the coils wound on two legs, the appearance is that of fig.3. if the two coils shown here, are the respective high and low side coils as shown in fig.3, the leakage reactance would be much too great. In order to provide maximum linkage between windings, the group on each leg is made up of both high-tension and low-tension coils. This may be seen in fig.4, where a cross-sectional cut is taken across the legs of the core. By placing the high-voltage winding around the low voltage winding, only one layer of High-Voltage insulation is required, that between the two coils. If the high-voltage coils were adjacent to the core, an additional high-Voltage insulation layer would be necessary between the coils and the iron core.

Fig.3&4 cross-section of a core-type transformer

Fig.5 shows the coils and laminations of a core-type transformer with a cruciform Core and circular coils.

Fig.5.coils and laminations of a core-type transformer

Rectangular cores as shown in Fig.6a with rectangular cylindrical coils can be used for small-size core-type transformers. For large-size transformers, it becomes wasteful to use for rectangular cylindrical coils and so circular cylindrical coils are preferred. For such purposes 'square cores may be used as shown in fig.6b where circles represent the tubular former carrying the coils. Evidently, a considerable amount of useful space is still wasted. A common improvement on the square core is to employ a 'cruciform core', which is shown in fig.6c which demands, at least, two sizes of core strips. For very large transformers, further core stepping is done as shown in fig.6 where at least three sizes of core plates are necessary. Core stepping not only gives a high space factor but also results in reduced length of the mean turn and the consequent I2R loss. The three-stepped core is the most commonly used although more steps may be used for very large transformers as shown in fig.6e.

Shell type transformer

one can see in fig.7 that the shell-type transformer has the Iron fully surrounding the copper. E-shaped and F-shaped laminations are stacked together to form a core that looks like numeric eight.

Fig.6. various types of transformer cores

Fig.6 shows the different types of cores used in core type transformers.

Fig.7. shell-type transformer

The Centre leg has all the windings placed on it. A high-side coil arrangement is made adjacent to the low-side coil to reduce the leakage flux. Both the windows are fully occupied by the coils, which are flat or pancake in shape, and most of the time it is made up of copper strips. A low voltage coil arrangement is made adjacent to the iron core to keep the usage of high voltage insulation low.

Choice of core or shell-type construction

Core type transformers consists of a core with a higher mean length and lower coil turn. Core type transformers have a small cross-section of iron, and so to increase the flux, a higher no of turns of wire will be required across the core. Core type transformers can be better utilized at places where high voltage is required. Shell type core can manage the coil's high mechanical stress or forces exerted radially, better than core type at the time of dangerous short-circuits.

Fig.8.coils and laminations of a shell-type transformer

Core type and shell-type cores have similar characteristics but vary in cost.

Voltage rating, KVA rating, weight, insulation stress, mechanical stress, and heat distribution are the factors that can differentiate the choice for the two types of core.

Spiral core Transformer

One can see the spiral core in fig.9. The core is formed with a long thin strip of CRGO(cold rolled grain oriented) steel sheet wound in the shape of a circular or elliptical cylinder. The core can also be constructed with a group of short strips laminated with each other to form an elliptical type core.

Fig.9.spiral-core transformer

Such kind of core construction is always useful to achieve the desired flow of flux along with the 'orientation' or the grain of the iron used in assembling the core. It becomes possible for a transformer to operate at higher flux density with a reduced loss per kg and less weight per KVA with the help of cold-rolled steel of high silicon content.

Transformer windings, Terminals, Tappings, and bushings

Transformer windings

The most important requirements of transformer windings are:

1. The construction cost of winding should be cost-effective in both the cases such as the availability of copper in the market and the maintenance-free working of the transformer for a longer period.

2. The heating of the winding should not be in access to the minimum standard requirements. Neglecting overheating against the standard requirement will bring down the service life of the transformer.

3. The winding should have proper mechanical support against the forces exerted on it during a sudden short circuit.

4. The winding should have the necessary electrical strength with respect to overvoltages.

Different types of windings are classified and briefly discussed below:

1. Concentric winding types are cross-over, helical, and disc.

1. Concentric windings are used in core-type transformers where each limb is wound with many other coils including both primary and secondary turns that can also be concentric cylinders.

1. Concentric windings as shown in fig.10. These winding are used for core-type transformers. Each limb is wound with many coils in a group containing both primary and secondary turns which may also look like concentric cylinders. The placement of L.V. winding is done just next to the core and that of H.V. winding outside the core. In order to reduce leakage reactance, the two windings get subdivided and are tightly fixed with high tension and low tension section differently. These windings can be further divided as follows:

Fig.10.Concentric coils

1. Cross-over windings:-Cross-over windings can sustain currents up to 20A. Hence these are used in the H.V. winding of small transformers. The round wire type conductor comes with the covering of cotton and strip-type conductors come with the covering of paper. Initially, Cross-over coils are fixed and wound over the formers. Each coil is constructed with many no of layers and each layer comes with a different number of turns. Each winding is constructed with many layers connected in series. Both ends of the coil are brought out from inside and outside of the coil. The inside end of a coil is connected with the outside end of the other coil placed next to it.

2. Helical winding:-The construction of helical winding is done with the help of rectangular strips wound in the shape of a helix. The arrangement of copper strips is such that they are wound in parallel radially. Here the total no turns of the coil occupies the total radial section of winding. Helical coils are mostly used in the construction of L.V. windings of large transformers. With additional insulation between layers apart from the insulation of conductors, these can also be used in the construction of H.V windings for large transformers.

3. Continuous disc winding:- The construction of this kind of winding takes place with the help of a number of flat copper strip wound spirally from inside radially outwards. The thin strip of a copper conductor is used in sufficient length, needed for complete winding or section of winding between tappings. The copper conductor with a single strip or a number of strips put in parallel is also used in the construction of this winding. This kind of winding formation takes place with the help of the robust construction of each disc. The discs are wound on insulating cylinders. These cylinders are separated from the discs with the help of thin strips sticked along the length of the cylinder. The discs are insulated from each other with the help of pressboard sectors attached to the vertical strips. The vertical and horizontal space provide ducts for free circulation of oil which is in contact with every turn.

2. Sandwich coils:- Sandwich coils as shown in fig.11 are employed in transformers of shell type. Both high and low-voltage windings are split into a number of sections. The formation of this winding is in such a way that each High Voltage section lies in between the low-voltage sections. It is possible to achieve the desired leakage reactance by the way of division of windings inside the sandwich coils.

Fig.11.sandwich coils

Terminals and leads:- The windings are connected with insulated copper rods or bars. The Leads should be free from bends and corners to avoid corona and dielectric loss in High Voltage transformers. Winding Connections in air-cooled transformers are taken to the busbar while in the case of oil-cooled Transformers, winding connections are taken to the insulated bushings.

Tappings:- The voltage in the supply network is controlled by changing the transformation ratio. It is achieved by tapping the winding and changing the number of turns. The change in the number of turns may be affected when the transformer is out of the circuit known as off load tap changing or when onload known as on load tap changing. The tappings are provided on the high voltage winding because a fine voltage variation is obtained to a large number of turns. It is difficult to obtain voltage variation within closed percentage limits in low voltage winding as there are few turns and voltage per turn is a large percentage of the total voltage.

In transformers, the tappings can be provided at:

1. Phase ends

2. Neutral point or in the middle of the windings.

The advantage of providing tappings at phase ends is that the number of bushing insulators is reduced, this is important where the cover space is limited. Some transformers have reinforced insulation at the phase ends. It is essential that in such cases either the taping should not be provided at the end turns or the reinforcement should be carried beyond the lower tap.

At the time of construction of tappings at the neutral point, the insulation between various parts is kept small. This kind of arrangement is very cost effective in the case of High Voltage transformers.

Bushings:-The terminals of the winding are brought out of the tank to connect with the bushings and then the bushings are connected to the external circuit. The porcelain bushings are connected with the conductor at the point of entry at the top of the tank in low voltage transformers. Large size bushings are used in high voltage transformers. Large porcelain or composition bushings are used for 33kv type transformers, above that Oil Filled or condenser type bushings are used.

Transformer cooling methods

For dry-type transformers up to 25KVA, cooling methods are:-

1. Air natural:- The natural surrounding air is used in this method to cool the heat generated by losses.The winding enclosed by a sheet metal protects the winding from mechanical injury.

Fig.12. Air blast transformer

2. Air blast:-In this method, the transformer is cooled by a continuous blast of cool air circulated through the core and winding shown in fig.12. The blast is created by a fan attached outside to the transformer. The air supply must be dust-free to prevent the accumulation of dust in ventilating ducts.

Oil Immersed Transformers uses mineral oil for cooling and these are classified as:

1. Oil-immersed self-cooled transformers

The inside part of the transformer is immersed in oil and heat is generated in cores and windings is passed to the oil by conduction. The hot oil in contact with the core and winding rises and is displaced by cool oil from the bottom. The hot natural oil slowly transfers its heat to the tank walls from where heat is taken away by the ambient air. The oil gets cooler and falls to the bottom from where it is dissipated into the surroundings.

Fig.13 Transformer with cooling tubes

For large voltage rating transformers, the tank surface should be plain and large enough for proper heat dissipation with the help of auxiliary means like fans, etc. As both space and oil are costly, these auxiliary means should not increase the cubic capacity of the tank. The heat-dissipating capacity can be increased by providing corrugations, fins, tubes shown in Fig.13. and radiator tanks. The advantage of 'oil natural cooling is that it does not clog the ducts and the windings are free from the effects of moisture.

2. Oil-immersed forced air-cooled transformers:- This type of cooling is achieved, with the help of cool air directed over the outer surfaces of the tank of the transformer immersed in oil.

3. Oil-immersed water-cooled transformers:- Here the oil is cooled by pumping the cool water through the metallic coil immersed in the oil just below the top of the tank. Thereafter, the hot water is cooled in a spray pond or a cooling tower.

4. Oil-immersed forced oil-cooled transformers:- Here the oil is cooled by pumping it upward and downward to the winding with the help of radiators and in turn, the radiators are cooled by the way of external fans. The cooling of transformers having capacities from 10000 KVA and higher are shown in fig14. In such cases, airblast cooling of the radiator is used.

Fig.14.Air blast cooling of radiator

Transformer oil

It is a mineral oil obtained by refining crude petroleum. It serves the following purposes:

1. Provides additional insulation

2. carries away the heat generated in the core and coils

3. Protects the paper from dirt and moisture.

The transformer oil should possess the following properties:

1. High dielectric strength

2. low viscosity to provide good heat transfer.

3. Good resistance to the emulsion.

4. Free from inorganic acid, alkali, and corrosive sulphur.

5. Free from sledging under normal operating conditions.

6. High flash fire point.

Conservator:- The Transformer oil looses its insulating properties when it comes in contact with the surrounding natural air and attracts moisture hence it should be protected from coming in direct contact with the atmospheric air.

Also, air may cause acidity and sledging of oil. This is prevented by fixing the conservator over the top of the transformer tank. The function of a conservator as shown in fig.14 is to take up contraction and expansion of oil without allowing it to come in contact with outside air. The conservator has an airtight metal drum set above the level of the top of the tank and connected with it by a pipe. The main tank gets completely filled when the oil gets cold. The conservator is partially filled with oil. So the possibility of the oil surface coming in contact with air is greatly reduced. Here the sludge formed, sets in the conservator, and does not go up to the main tank.

Breather:- when the temperature changes the oil expands or contract and there is a displacement of air. During the cooling of the transformer, the air is drawn in and the oil level goes down. This is called breathing. When the air comes inside the transformer tank, it is passed through an external device called a breather for the purpose of extracting moisture. The breather has a small vessel that contains a drying agent like Silica Gel crystal impregnated with Cobalt crystal. This dry agent inside the breather helps its to filter the moisture from the air.

Sledging takes place when there is a slow formation of solid hydrocarbons due to the heating and oxidation process of the transformer oil. Slowly the sludge starts depositing itself on the windings and cooling ducts due to which the transformer gets overheated. The sledging formation is a continuous process, till the time, transformer remains hot.

Oxidation takes place when the oil comes in direct contact with air.

what is current transformer? | different types of CT connections

2020-01-05T03:21:00.039-08:00

Let's learn,  What is current transformer? and how does the current transformer works? with different types of CT connections.

POLE TYPE CURRENT TRANSFORMER

what is current transformer?
The term current transformer is used to describe a piece of equipment consisting of pairs of mutually coupled windings mounted around a core, usually of magnetic materials. such transformers are normally used to step down high current flowing in their primary windings to lower levels suitable for feeding to measuring and protective equipment.

Another good definition of what is a current transformer? by "James H Harlow"
The current transformer is often treated as a "BlackBox". It is a transformer that is governed by the laws of electromagnetic induction:

ε =k β Ac Nf (7.12)

ε = induced voltage

k = constant of proportionality

β = flux density

Ac = cross-sectional area

N = Turns

F = frequency

How does the current transformer works?
As previously stated, the CT is employed in series with the circuit to be monitored and it's this difference that results in its ambiguous description. the primary winding is to bring a constant current source of supply through a low impedance loop. Due to this low impedance, current passes through it with very little regulation. The CT operates on the ampere-turn principle:
primary ampere-turns = secondary ampere-turns, or

(7.13)
Since there is energy loss during transformation, this loss can be explained in ampere-turns: primary ampere-turns, magnetizing ampere- turns= secondary ampere-turns, or

(7.14)
The CT is not voltage-dependent but it is voltage limited. As current passes through and impedance, a voltage is developed, V=IZ. As this occurs, energy is depleted from the primary supply, thus acting as a shunt. This depletion of energy results in CT errors.
As the secondary impedance increases, the voltage proportionally increases. Thus the limit of the CT is magnetic saturation, a condition where the CT core flux can no longer support the increased voltage demand.
At this point, nearly all of the available energy is going into the CT core, leaving none to support the secondary circuit.

Let's learn how does the current transformer work with the help of the equivalent circuit.
To understand how does the current transformer works, one has to first understand the basic equivalent circuits. These transformers behave similarly to all two winding transformers and may be represented by the well-known circuit shown in figure 2.3(a) which is based on the winding directions and current and voltage polarities shown in figure 2.3(b).
Because the connected CT burdens are normally of low impedance, the secondary output VA and voltage are relatively low, typically of a maximum of 20VA and 20V for a current transformer with a secondary winding rated at 1 ampere.
The voltage across the primary winding, because of the turns ratio, is not therefore likely to exceed a fraction of 1 volt, a negligible value relative to the rated voltage of the primary circuit.
For this reason, the equivalent circuit of a current transformer can be simplified to that shown in figure 2.4. This circuit can be made to represent any current transformer operating with any CT burden and primary current under either steady-state or transient conditions.

Fig 2.3 Equivalent circuit of a current transformer

Fig 2.4 Equivalent circuit of a current transformer referred to the secondary

It will be seen that the exciting current Ie is dependent on the exciting impedance, presented by Rl in parallel with lm and the secondary E.M.F (Es) is needed to drive the secondary current Is through the total secondary circuit impedance.

Because the secondary current of a current transformer may vary over a wide range, that is from zero under no-load conditions to very large values but when there is a fault on the primary circuit the secondary E.M.F and excitation current may also vary greatly and in this respect, the behavior is very different from that of voltage transformers.

Because of the non-linearity of the excitation characteristics of the magnetic materials used for current transformer (CT)cores, the exciting impedance of a given current transformer is not constant, both the magnetizing inductance Lm and loss resistance R1 varying with the CT core flux needed to provide the secondary E.M.F es.

Allowance may be made for this non-linearity, if necessary, when determining the behavior of a particular Transformer under specified conditions, calculation than being done using step-by-step or other methods. If great accuracy is not required, however, simplifications can be affected by assigning constant values, the averages over a cycle, to Rl and Lm.

An alternative method of representing a current transformer is to employ the concept of mutual inductance Mps. By definition and based on the conventions used above:

when there is zero current in the secondary winding. It has been shown that the circuit shown in figure 2.5 is equally as satisfactory a model as that shown in figure 2.4.
The self-inductance, Lss of a secondary winding is given by:

in which Ls is the leakage inductance used in the circuit of figure 2.4. It is clear therefore that the value of Ls for a given Transformer may be determined experimentally by measuring itself Lss and mutual inductances Mps.

Saturation curve in Current transformer

The saturation curve, often called the secondary excitation curve, is a plot of secondary exciting voltage versus secondary exciting current drawn on log-log paper. The units are in RMS with the understanding that the applied voltage is sinusoidal.

This characteristic defines the CT core properties after the stress relief annealing process. It can be demonstrated by a test that CT cores processed in the same manner will always follow this characteristic within the specified tolerances. Figure 7.14 shows a typical characteristic of a 600:5 multi ratio CT. The knee point is indicated by the dashed line. Since the voltage is proportional to the turns, the volts per turn at the knee are constant.

The tolerances are 95% of saturation voltage for any exciting current above the knee point and 125% of exciting current for any voltage below the knee point.

These tolerances, however, can create a discontinuity about the knee of the curve, which is illustrated in figure 7.14. since the tolerance is referenced at the knee point, It is possible to have a characteristic that is shifted to the right of the nominal, within tolerance below the knee point.

But careful inspection shows that a portion of the characteristic will exceed the tolerance above the knee point. for this reason manufacturers, typical curves may be somewhat conservative to avoid this situation in regards to field testing. some manufacturers will provide actual test data that may provide the relay Engineer with more useful information.

knowing the secondary winding resistance and the excitation characteristic, the user can calculate the expected RCF under various conditions. using this type of curve is only valid for non-metering applications. the required voltage needed from the CT must be calculated using the total circuit impedance and the anticipated secondary current level. The corresponding exciting current is read from the curve and used to approximate the anticipated errors.



(7.15)

(7.16)

(7.17)




where,

Vex = secondary excitation voltage required at fault level

Isf = secondary fault current

Iex = secondary exciting current at Vex, obtained from curve

ZT = is the total circuit impedence

Rs = secondary winding resistance

RB = secondary burden resistance

XB = secondary burden reactance

RCF = ratio correction factor

RE     = ratio error

In the world of protection, the best situation is to avoid saturation entirely. this can be achieved by sizing the CT knee point voltage to be greater than Vex, but this may not be the most practical approach. This could force the CT physical size to substantially increase as well as cause dielectric issues. There must be some reasonable trade-off to reach a desirable condition. equation 7.15 provides the voltage necessary to avoid AC saturation. if there is an offset that will introduce a DC component, then the system X/R ratio must be factored in:

(7.18)

And if the secondary CT burden is inductive, equation 7.18 is rewritten as:

(7.19)

The saturation factor Ks is the ratio of the knee point voltage to the required secondary voltage Vex. it is an index of how close to saturation a CT will be in a given application. Ks is used to calculate the time, a CT will saturate under certain conditions:

(7.20)

Where,

Ts = Time to saturate

ω = 2πf, where f is the system frequency

Ks = saturation factor,

R = primary system resistance at the point of fault

X = primary system reactance at point of fault

Ln = natural log function

CT thermal rating factor in current transformer

The continuous current rating factor is given at a reference ambient temperature, usually 30-degree Celcius. The standard convention is that the average temperature rise will not exceed 55 degree Celsius for general purpose use, but it can be any rise shown in table 7.8. from this CT rating factor, a given CT can be derated for use in higher ambient temperature from the following relationship:

(7.21)

which can be simplified and written as:

(7.22)

where,

RFnew = Desired rating factor at some other ambient temperature,

RF std    = Reference rating factor at 30 degree Celcius.

AMB new = desired ambient temperature at < 85 degree celcius.

Equation 7.22 is valid only for 55 degrees Celsius rise ratings and maximum ambient less than 85 degrees Celsius.

The rating factor ensures that the CT will not exceed its insulation class rating. this expression follows the loading curves of figure 1 in IEEE C57.13.It assumes that the average winding temperature rise is proportional to the current squared and those CT core watt losses are insignificant under continuous operating conditions.

Conversely, this expression will work for ambient temperature lower than 30 degrees Celsius, but in this situation, the rating factor will be higher. the concern is not with exceeding the insulation system but, rather with the chance of increasing the errors of the CT accuracy limits. A number of factors can affect this, such as CT core material and compensation methods used. it is best to consult the manufacturer for overrating performances.

Another consideration is the stated rating factor versus the actual temperature rise. in most cases, the stated rating factor may be quite less than the permitted 55 degrees Celsius rise allowance. for example, the actual rating factor may be 2.67, but the stated rating factor is 2.0. if the actual rise data were known, then equation 7.22 could be rewritten as:

(7.23)

Figure 7.15 CT derating chart

RFactual = rating factor at an actual temperature

∆Trise    = actual measured temperature rise.

This may be more useful when operating a CT at a Higher ambient, where there is a need for the maximum rating factor. For example, if the CT has an actual rating of 2.67 based on temperature rise data but is only rated 2.0 and if it is desired to use the CT at 50 degree Celsius ambient with the stated rating factor 2.0, then this unit should work within its insulation rating and within its stated accuracy class.

If a higher temperature class insulation system is provided, then the rise must be in compliance with that class per table 7.8. In some cases, the temperature class is selected for the environment rather than the actual temperature rise of the CT.

Open circuit conditions in current transformer

The CT functions best with the minimum CT burden possible, which would be its internal impedance. This can only be accomplished by applying a short circuit across the secondary terminals. since the CT core MMF acts like a shunt, with no load connected to it's secondary, the MMF becomes the primary current, thus dividing the CT into hard saturation. with no load on the secondary to control the voltage, the winding develops an extremely high peak voltage.

This voltage can be in the thousands or even tens of thousands of volts. This situation puts the winding under incredible stress, ultimately leading to failure. This could result in damage to other equipment or present a hazard to personnel.it is for this reason that the secondary circuit should never be open. it must always have a load connected.

If it is installed to the primary but not in use, then the terminals should be shorted until it is to be used. most manufacturers ship CTs with a shorting strap for wire across the secondary terminals. the CT winding must be able to withstand 3500 Vpeak for 1 minute under open-circuit conditions. if the voltage can exceed this level, then it is recommended that overvoltage protection be used.

Overvoltage protection in current transformer
Under load, the CT voltage is limited. The level of this voltage depends on the turns and CT core cross-sectional area. the user must evaluate the limits of the CT burdens connected to ensure equipment safety. sometimes protective devices are used on the secondary side to maintain safe levels of voltage. these devices are also incorporated to protect the CT during an open circuit condition.

In metering applications, it is possible for such a device to introduce a direct current across the winding that could saturate the CT core or leave it in some state of residual flux. in High Voltage equipment, arrestors maybe used to protect the primary winding from high voltage spikes produced by switching transients or lighting.

Residual magnetism in current transformer
Residual magnetism, residual flux, or remanence refers to the amount of stored, or trapped, flux in the CT core. this can be introduced during heavy saturation or with the presence of some DC components. figure 7.16 shows a typical B-H curve for Silicon iron driven into hard saturation.
The point at which the curve crosses zero force, identified by +Bres, represents residual flux. If at some point the CT is disconnected from the source, this flux will remain in the magnetic CT core until another source becomes present. If a fault current drove the CT into saturation, when the supply current resumes normal levels, the CT core would contain some residual components.

Residual flux does not gradually decay but remains constant once steady-state equilibrium has been reached. Under normal conditions, the minor B-H loop must be high enough to remove the residual component. if it is not, then it will remain present until another fault occurrence takes place. the effective result could be a reduction of the saturation flux.

Fig 7.16 Typical B-H curve for Si - Fe steel

However, if a transient of opposite Polarity occurs, saturation is reduced with the assistance of the residual. conversely, the magnitude of the residual is also based on the polarity of the transient and the phase relationship of the flux and current. whatever the outcome, the result would cause a delayed response to the connected relay.

It has been observed that in a tape-owned toroidal CT core as much as 85% of saturation flux could be left in the CT core as a residual component. the best way to remove residual flux is to demagnetize the CT core.

This is not always practical. the user could select a CT with a class that is twice that required. this may not eliminate residual flux, but it will certainly reduce the magnitude. the use of hot-rolled Steel may inherently reduce the residual component to 40 to 50% of saturation flux.

Another way of reducing residual magnetism is to use and Air gapped CT core. normally the introduction of a gap that is, say 0.01% of CT core circumference could limit the residual flux to about 10% of the saturation flux. referring to figures 7.16, a typical B-H loop for and Air gapped CT core is shown.

The drawback is significantly higher exciting current and lower saturation levels, as can be seen in figure 7.3. to overcome the high exciting current, the CT core would be made larger. that coupled with the gap would increase its overall cost. This type of CT core construction is often referred to as a linearized CT core.

Let's understand current transformer( CT) connections

As previously mentioned, some devices are sensitive to the direction of current flow. It is often critical in three-phase schemes to maintain proper phase shifting. Residually CT connections in a three-phase ground fault scheme sum to zero when the phases are balanced.

The reversed polarity of a CT could cause a ground fault relay to trip under a normal balance condition. Another scheme to detect zero-sequence faults use one CT to simultaneously monitor all leads and neutral in differential protection schemes. current source phase and magnitude are compared.

The reverse polarity of a CT could effectively double the phase current flowing into the relay, thus causing a nuisance tripping of a relay. when two CTs are driving a three-phase ammeter through a switch, a reversed CT could show 1.73 times the monitored current flowing in the unmonitored circuit.

In CT connections, the wye-connected secondary circuit is the most commonly used. the CT will reproduce positive negative and zero sequence elements as they occur in the primary circuit. in the delta-connected secondary, the zero sequence components are filtered and left to circulate in the delta.

Figure 7.17 (a) over current and ground fault protection scheme (b)differential protection scheme (c)zero-sequence scheme

This is a common scheme for differential protection of delta-wye transformers. A general rule of thumb is to construct the current transformer (CT) connections for secondaries in wye when they are on the Delta winding of a transformer and, conversely, connect the secondaries in Delta when they are on the wye winding of the transformer.

Table of contents

Transformer - Working principle

Transformer-Types

Transformer-Elementary theory of an idle transformer, Emf Equation, voltage transformation ratio, transformer on no load

Transformer-ON Load

Transformer-Transformer with winding resistance but no Leakage reactance

Transformer-Equivalent resistance and MagneticLeakage

Transformer- resistance and Leakage Reactance

Transformer-Simplified diagram

Transformer-Total approximate voltage drop in a transformer,Exact voltage drop and its equivalent circuit

Transformer-Open-circuit test(no load test) and short circuit test(impedence test),seperation of core losses

Transformer-Why transformer rating is in KVA?

Transformer-Regulation

Transformer-percentage,resistance,reactance,impedence

Transformer-Kapp regulation diagram

Transformer-Sumpner back to back test

Transformer-Losses in a transformer

Transformer-Efficiency and condition for max efficiency

Transformer-All day Efficiency

Transformer-Auto Transformer

Transformer-Conversion of 2-windinding transformer into Auto-Transformer

Transformer-Parallel operation of single-phase Transformers

Transformer-Three Phase Transformer

Transformer-Star-Star Connection

Transformer-Delta-Delta Connection

Transformer- Y-DELTA connection, DELTA-Y connection, OPEN - DELTA Connection

Transformer-Scott connection or T-T connection

Transformer-Three phase to Two phase conversion

Transformer-Parallel Operation of Three phase Transformers

Questions on current transformer

1.what is the purpose of a current transformer?
Current transformers are made to measure, or monitor, the flow of current within the ac power circuit. They're very useful in high power circuits, where the flow of current is large, i.e more current than the ratings of so-called self-contained current meters. Other applications relate to overcurrent and undercurrent relaying for power circuit protection, such as, within the power lead of an inverter or converter.

2.what do you mean by current transformer?
A current transformer is an electrical machine, that works to measure high AC which is not supposed to be sensed or passed through normal ammeters and current coils of wattmeter, energy meters, etc.
A current transformer basically includes a primary coil of single or more turns of heavy cross-sectional area. In some, the bar carrying a high current can act as a primary. This is joined in series with the circuit carrying a high current. The secondary of the AC transformer is constructed up from a large number of turns of fine wire having a small cross-sectional area. This is usually rated for Five ampere. This is coupled with the coil of the normal range ammeter.

4. What are the different types of current Transformers?

There are 5 types of current Transformers
1.Portable type
2.Outdoor type
3.Indoor type
4.Bushing type
5.Multi-ratio type

Indoor type current transformers are of two types
1.Wound type
2.Bar and window type

Many outdoor type current transformers are actually used for higher voltage circuits. In the same way, Indoor types of current transformers are employed for low voltage circuits.

In a wound core type, the primary coil consisting of several turns is wound over the core. The core is mostly made up of a Ni-Fe alloy or specially treated steel and is used in such a way that the flux flowing always in the direction of grains as produced by Rolling the sheet metal. This improves the magnetic properties and helps in decreasing the ratio and phase angle errors.

Current Transformers using hot rolled Steel are made up of stacks of ring stampings which are insulated by means of the end-collars and circumferential wrap of pressboard protect the secondary winding from mechanical damage and also act as an insulating medium.

In a bar-type current transformer, the primary coil consists of a bar of suitable size that forms an integral part of the transformer as shown in figure 5.6. In bar type, the primary ampere-turns are numerically the same as the primary current. However, at a low current rating, the accuracy decreases because the magnetization of law demands a larger fraction of the total ampere-turns.

In the case of wound core type construction, higher accuracy can be achieved because of the higher value of primary ampere-turns.

The bushing type transformer is similar to the bar type. The core and secondary are mounted around a single primary conductor.

The secondary coil is placed over a circular core which is installed in the high voltage busing of a circuit breaker or power transformer.

The primary coil is the conductor in the bushing. In Precision type portable current Transformers, the choice of ratios can be obtained by changing the links on the top plate.

In laboratory-type current Transformers, the variable-ratio can be obtained by using the following methods:

1. The variable-ratio can be obtained by using the series-parallel connection of the primary coil. It should be ensured that the various coil sections used must have the same characteristics.

2. It can be obtained by using tappings on the primary or the secondary coil. This method provides a higher degree of flexibility. but when applied to the primary side, much of the copper space is left unused except on the lowest range. when used on the secondary-coil side, working ampere-turns are changed.

Fig 5.6 CURRENT TRANSFORMER WITH BAR TYPE PRIMARY

3.Using a variable primary formed by threading any desired number of turns of cable through the core. For this purpose, a ring core and a toroidal Core secondary is generally used.

Split core current Transformers
In some applications, it is desirable that the core of the current transformer is split into two halves. The conductor which is already fixed in location may be encircled thus making a bar primary type of current transformer.

It is also called a clamp-type ammeter in which the instrument and split-core transformer are combined to form a single unit. They have a self-aligning Hing Limb of split-core which is operated by a spring-loaded trigger to spread the core into two halves and clip around the current-carrying conductor as shown in figure 5.7.

Fig 5.7 SPLIT-CORE CURRENT TRANSFORMER

This arrangement avoids the necessity of breaking the circuit for the purpose of measurement. For increasing the range shunts are used and by varying the shunt-resistance of the milli-ammeter circuit, current ranges from 0 to 5 ampere to 0 to 600 ampere can be obtained.

Switchgear-i2t characteristics of fuse wire

2018-11-25T11:24:00.000-08:00

i²t characteristics of fuse wire

i²t characteristics of fuse wire. Fuses operate very rapidly under short circuit conditions when the current levels are extremely high clearance times being typically only of few milliseconds. Under these conditions the current waves shapes depend on the parameters of the protected circuit the instant in the voltage cycle at which the fault occurs and the current limiting effect of the fuse link itself. only a small portion of a cycle of power frequency current flows before interruption occurs and transient components are present. Such currents cannot be assigned a single value which may be used to determine the corresponding operating time from the time-current characteristics. Because of this use is made of a quantity termed i²t which is the time integral of the square of the instantaneous current which passes through a fuselink between the incidence of a circuit fault and the instant at which the fuse Arc is extinguished.

if it could be assumed that the resistance of the components of the protected circuit was constant throughout the operating Period of the fuse link, then the value of i²t would be proportional to the energy fed to the circuit and also the heating produced by it. In practice, however, the resistances must rise some significantly. nevertheless the i²t value is often described as the let-through energy, a term which is not strictly correct because it does not contain a resistance value and is not there for in energy units J but in a2s.

To assist in the application of fuses many manufacturers of components determine the i²t withstand limits of their products and publish them. The i²t values needed to operate fuse links at very high currents tend to be independent of the waveshapes and manufacturers publish these values as well as providing time-current characteristics to enable appropriate fuselinks to be selected by users.

In practice, fuse link manufacturers provide two sides of i²t values one set associated with the total clearance time and another set with lower values for the i²t let through during the pre-arcing period. Both sides are needed to enable coordination to be achieved between fuse links which are effectively in series in networks.

switchgear-Time-current characteristics of fuse wire

2018-11-24T09:46:00.001-08:00

Time-current characteristics of fuse wire

To achieve the above criteria fuse links should have time/current characteristics which lie close to the withstand curves of their associated circuit shown in figure 1.18. Obviously, the operating time of fuse should always be less at any current level then the period for which its associated circuit can withstand the condition of a very important factor which must be taken into account is that fuse operating times include the arcing period and change of state occurs before clearance is affected. Once arcing has commenced it is clearly impossible

for an element to return to its original form and even when melting begins distortion main result and an element may not return to its original shape on cooling. Fuselinks with low melting point materials on their elements which will be subjected to acceptable and Irreversible changes if overcurrents pass through them long enough to initiate the M effect diffusion process.

There is called a band below its time-current characteristics in which a fuse should if possible not be called upon to perform. while in practice it is not likely that a high current will flow through a fuse link for a period just less than its operating time the possibility nevertheless exists. if such a condition should arise then the probable result which must be accepted is that the fuse May operate more quickly than expected on a future occasion.

Difficulties would certainly arise if a fuse link Carried a current just below its minimum fusing value for prolonged periods. To ensure that the situation is avoided fuse links are assigned rated current somewhat below their minimum fusing levels. The ratio of the minimum fusing current to the rated value which is defined as the fusing factor usually has a value in the range of 1.2 to 2. The significance of this factor is that circuits must be able to operate continuously at levels significantly above the current of the fuse protecting them to satisfy Criterion one above namely that the continuous rating of the circuits should exceed the minimum fusing current. This situation arises with other protective equipment in which current settings about the full load level are used. clearly, circuits and equipment are not normally designed without some overload capacity, in providing factors of safety are usual and difficulties are not encountered in providing adequate fuse protection. clearly, relatively low fusing factors are nevertheless desirable on economic grounds.

A further factor which must be recognized is that not all fuselinks are capable of operating satisfactorily at all current levels between the maximum breaking capacities and minimum fusing values. Satisfactory arc extinction may not take place at certain current levels usually those just above the minimum fusing values. care must be taken to ensure that users are only used in circuits where currents of these levels will not be encountered or if this cannot be guaranteed then some other protective scheme must be provided to open the circuit before the fuse attempts to operate. In the USA the term 'full range' has been introduced to designate those fuses which can clear all levels of current satisfactorily.

Switchgear-Working criteria of fuses

2018-11-23T09:21:00.000-08:00

Working criteria of fuses

A fuselink which is to protect a piece of equipment and or a circuit should ideally satisfy several criteria. This may be illustrated by considering an example based on the simple circuit shown in figure 1.17.

The criteria are:

1. The minimum fusing current of the fuse should be slightly less than the current which can be carried continuously by the source, the supply, and connecting cables and the piece of equipment.

2. The item of equipment should be able to carry currents in excess of its rated current for a Limited period and the fuel should operate when carrying this overload current in times slightly shorter than those which the equipment can withstand.

3. The cables should also be able to cope with the above over currents without being damaged.

4. The high current may flow due to faults within the item of equipment and in these circumstances, consequential damage should not occur to the remainder of the circuit. The extreme case would occur if the input terminals of the piece of equipment become short-circuited. In this situation, clearance should be affected quickly enough to prevent damage to the cables.

5. A further possibility is a short circuit between the conductors of the connecting cables. The most serious situation would arise if the fault was at the input end that is between points A and B. For such faults the fuse should operate rapidly enough to prevent damage to the source and the supply cables.

Switchgear-Construction of cartridge fuses

2018-11-19T10:41:00.001-08:00

Construction of cartridge fuses

Fuse elements

from theoretical and other studies it is evident that fuse elements should generally be made from materials with low resistivities. If possible the materials should also possess the following properties:

(a) low specific heat

(b) low melting and vaporization temperatures

(c) low latent heats

(d) low density

(e) high thermal conductivity

Construction of cartridge fuses.properties (a) to (d) are desirable when rapid operation at high currents is necessary. At The rated current level, steady-state equilibrium conditions must be established in which the electrical power input to an element is conducted away from it and dissipated by the connecting cables and the outer surfaces of the fuse link. This is the reason for property (e). Of course, not only must the element material be of high thermal conductivity but so must the surroundings filling material and the body. In addition, because the power which can be dissipated from a particular fuse link is limited, it is necessary to limit the power input to the element by using elements of the adequate cross-sectional area.

After extensive studies of the available materials and taking the above factors into account, it has been found that silver and copper are the most suitable materials for elements and they are used in the majority of fuse links. Notched elements are produced in various forms a few being shown in figure 1.11.

The strip thickness is usually in the range of 0.05 to 0.5 mm and the widths seldom exceed 10 mm.

Occasions do arise when the necessary performance cannot be made using notched elements of a single material with a relatively high melting point. In these circumstances, advantage may be taken of the so-called' effect in which a low melting point material is deposited adjacent to one or more of

the restrictions on the notched strip, an example shown in figure 1.12. This method resulted from work done by Metcalf who published an article entitled 'A new fuse phenomenon' in 1939.

Fuselink bodies

bodies must possess good electrical insulating properties and should not allow the Ingress of moisture. They should be reasonably good thermal conductors and have an adequate emissivity constant so that they can transmit and radiate heat energy emanating from the elements within them. They should also be physically robust.

Filling material

The filling material invariably quartz of high chemical purity and grain sizes in the region of 300 microns, conducts some of the heat energy away from the fuse element. To ensure consistency of performance it is necessary that the packing density of the filling material be maintained at a high constant level during production. It has also been found that this factor may have a very significant effect on the fuse performance at high current levels. A low packing density is undesirable as it may allow arcs to expand more rapidly and thus adversely affect the extinction process.

Mountings and ratings

Low voltage cartridge fuse links of standardized performance and dimensions are available with rated currents ranging from 2 to 1250 ampere to provide overcurrent protection on three phase 415 volt a.c systems and up to 250-volt dc systems. Designs are also available at certain ratings for a.c circuits up to 660 volt and d.c circuits up to 500 volts.

Fuse links with rated currents up to 200 amperes are usually fitted into fully shrouded fuse holders comprising a carrier and base. such fuses are produced in large numbers and many are fitted in distribution fuse boards.

Fuse links with the current rating above about 200 amperes are large and produced in small quantities. They are normally installed directly in convenient positions without their own special or standardized housings. They are often incorporated in fuse switch units and example being shown in figure 1.13.

High Voltage cartridge fuse links are produced for use in a.c systems operating at frequencies of 50 hertz and 60 hertz with rated voltages exceeding 1000 volt. They are of the same basic design as low voltage fuses. Those with low rated currents have silver wire elements whilst those for higher current applications have elements of the silver strip with restricted sections along their lengths. All elements have low melting point metals applied to them to enable the 'M' effect referred to in section 1.5.1 to be obtained. The elements are long because to achieve satisfactory current interruption, the total voltage across the fuse link must be high and this is best achieved by having many restrictions and therefore many arcs in series. To accommodate such elements in a body of acceptable length, the elements are accommodated in a helical form on insulating formers with a star-shaped cross-section.

Whilst some high voltage fuse links are mounted in air, it is nevertheless common practice for them to be immersed in oil in the pieces of equipment they are protecting. This is advantageous because the cooling effect of the oil allows a given current rating to be achieved in a smaller fuse link then that required for use in the air. The smaller fuse link also operates more rapidly at very high current levels.

Fuse links suitable for use in systems operating at voltages up to 33kv at a range of rated current values are produced. As an example, voltage ratings of 3.6 to 7.2 KV with current ratings up to 500 A are available.

switchgear-Determination of fuselink performance

2018-11-14T11:52:00.000-08:00

Determination of fuselink performance

In the determination of fuselink performance, the performances of fuse links may be determined by testing in circuits such as that is shown in figure 1.10, in which the voltage, current level, and waveform may be controlled. The ability to interrupt very high currents may be ascertained as well as the operating times at lower currents. The minimum fusing current may also be determined. Because of the possible variability of fuses, each individual test must be performed on several fuse links.

When fuselinks are being developed for new applications for which the required characteristics are known, designers rely on their experience. Prototypes are produced in sufficient quantities to enable all the necessary tests to be done. should the performance not meet the requirements, design modifications must be made, further prototypes produced and then the tests must be repeated. This process can be both lengthy and expensive.

There has long been a desire to calculate the performance of fuselinks so that acceptable and optimum designs can be produced without recourse to trial and error methods and lengthy testing procedures. In 1941 Gibson published the results of investigations, done by professor R.Rudenberg, in which the high current pre-arcing behavior of fuselinks with wire elements were determined. Because the times to reach the melting point of the elements was very short, heat transfer from the elements was neglected. The current flow was axial and the current density constant.

With notched elements, the current flow is not axial or of constant density but nevertheless with various assumptions applicable over certain current ranges and by employing finite-difference or other methods, various workers have modeled Pre-arcing performance with high accuracy in recent years. It is now possible to determine the effects of changing the materials in fuselink as well as changing their dimensions and geometry.

The arcing process is more Complex than that which takes place during the pre-arcing period and accurate modeling is, therefore, more difficult. some workers have sought from experimental studies to establish the empirical relationship between the current through a fuse link and the voltage across it when arcing occurs. Gananalingam and Wilkins were able to predict the arcing proper performances of certain fuses with reasonable accuracy using the relationship:

They claimed that the simulation technique is useful for screening preliminary designs and investing the effects of system parameters such as frequency.

Such methods are not based on the phenomena taking place and do not assist in understanding the behavior. They may not be applicable if the use of other element or filling materials are being investigated. For this reason, Wright and Beaumont developed a model based on a number of simplifying assumptions. They considered it to be superior to earlier models because it was based on the process occurring during arcing and took account of energy changes. Work of this type is still proceeding to enable the amount of testing currently required to be reduced greatly in the future. Details of work done recently in this field were given in papers presented at the fourth International Conference of electrical fuses and their applications. It must be accepted, however, that it will be necessary to determine fuselink characteristics experimentally for many years.

Switchgear- arcing period in cartridge fuselinks

2018-11-12T06:43:00.001-08:00

Arcing periods in cartridge fuselinks

When an Arc or arcs have been established they must persist until the current reaches zero, at which time extinction will occur and it is clearly desirable that the arc or arcs should not then restrike. This process must always occur and it is desirable that satisfactory clearance should be obtained at all current levels. At relative zero currents, the duration of arcing is very short relative to the pre-arcing period and the effect of arcing on the energy let through to the protective equipment is not very significant. This is not the case at very high current levels, however, when the arcing durations may be comparable to or even greater than the pre-arcing periods.

During the pre-arcing period associated with short-circuit conditions that are high currents, the current varies with time in a manner determined by the circuit itself and the resistance of the fuse. It will be marginally lower than it would have been had the fuse not been present. The current which would have flowed in these latter circumstances is known as the prospective current and typical variations which would be obtained in a circuit containing significant inductance and some resistance are illustrated in figure1.7.

At the instant when arcs are initiated in a fuselink there is a rapid increase in the voltage drop across it. This voltage then rises as the arcs lengthen due to more material being vaporized from the element.

consideration of the circuit shown in figure 1.8, assumed to apply for a fault condition, shows that the following relationship applies:

e = iR+d/dt(L.i) +Vf

in which is the source of e.m.f R and L are the resistance and inductance of the circuit, Vf is the voltage across the fuselink and i is the current.

When the current is positive as arcing commences, it is necessary that the rate of change of current di/dt should become negative so that the current falls to zero.

Clearly, this situation will obtain more quickly and more rapid extinction will be obtained, the greater the voltage across the fuse arcs. These conditions are illustrated in figure 1.9 from which it can be seen that the current which flows during fault conditions is lower than that which would have flowed had the fuse not been present.

Fuses, therefore, have the beneficial effect of providing current limiting, which assists in reducing damage to the circuits they protect.

Fuselinks containing notched strip elements may be made to reduce the working period by increasing the number of restrictions and thus the number of arcs in series. Care must be taken however to ensure that the rate of current reduction di/dt is not so great that excessive voltages will be induced in inductive components. permissible upper limits for fuselink voltages are included in specifications.

The behavior of fuselinks with cylindrical wire elements is less well controlled because such elements do not have well-defined restrictions at which arcs occur during operation. In practice, however, conditions are not uniform throughout a wire element and distortions occur during the pre-arcing period. As a result, the cross-sectional area does vary along the element length, producing thinner and fatter sections, known as unduloids.

Gaps ultimately form at the centers of thin sections, their number not being fixed however as in notched elements. In some cases, sufficient arcs may be formed to cause excessive voltages to be produced in the protected circuits.

Switchgear-The electrical behaviour of cartridge fuselinks

2018-11-11T11:57:00.000-08:00

The electrical behaviour of cartridge fuselinks

These cartridge fuselinks incorporate one or more elements which melt and then vaporize when currents above a certain level flow through them for a certain time. Thereafter the arc or arcs which result have to be extinguished to complete the interruption process. The operating time is therefore made up of two periods, designated the pre-arcing and arcing periods. The behavior during this period is considered in the following sections.

The pre-arcing period

The element or elements and the other conducting parts of a cartridge fuselink using posses resistance and as a result a fuselink must absorb electrical power when it carries current. While the current is constant and below a particular level, the temperatures of the various parts of the fuse must be at levels above the ambient value so that Power, equal to the input power is dissipated to the surroundings and a state of equilibrium exists.

Should the current be increased and maintained above a certain level, a new state of equilibrium will not be achieved because, although the temperatures of the fuselink parts will rise, the power dissipated from the fuselink will not become equal to the power input by the time that parts of the element or elements melt. Disruption of the element or elements will result and circuit interruption will take place after a period of arcing. The time from the instant when the current exceeds the critical value until the melting and initial vaporization of the elements occurs is known as the pre-arcing period. The time taken thereafter to achieve interruption is known as the arcing period.

The more the current through of using exceeds the maximum value at which equilibrium can be achieved, the shorter is the time taken before melting of the element or elements occurs. This is because the power available to cause the temperatures to rise is equal to the difference between the input power, which is proportional to the square of the current, and the power dissipated from the cartridge fuselink. The latter quantity is limited because the temperatures of the fuselink parts cannot exceed the melting point of the element material.

As a result, all cartridge fuses have inverse processing time / current characteristics in the range of currents above that at which thermal equilibrium can be established. A current marginally above this latter level would theoretically likely cause operation after an infinite time.

Clearly, this current could not be determined experimentally and therefore in practice tests are done on a number of similar cartridge fuselinks to determine a current level at which each of them will operate in a particular time, typically 1-4 hours depending on the fuse rating. This current is termed the 'conventional fusing current'. Further, the tests are done on the same number of fuselinks to determine the current level, termed the 'conventional non-fusing current at which none of them will operate in the same time as that used in the earlier tests. A current between the above two current values is done designated as the minimum fusing current of the fuselink and for simplicity, only this value is referred to in the remainder of this chapter.

It will be appreciated that a cartridge fuselink cannot be operated near the minimum fusing current continuously and that the rated current of the circuit and the fuse must be at a lower level. The ratio of the minimum fusing current to the rated current of a fuse link is called the fusing factor.

After a part or parts of a fuse element have melted, the current must continue to flow in the liquid metal. The situation which then arises is not fully understood but there are a number of conditions which may exist during the initial stages of vaporization. In cartridge fuselinks during the transition period when current levels are very high, it is possible that the material in the sections of reduced cross-sectional area boils. There would then be bubbles present in the liquid metal which would cause the resistance to rise, thus increasing the power input. This would lead to rapid vaporization of the notched sections. Alternatively, it could be postulated that the temperatures must be highest at the centers of restricted sections, causing vaporization to commence at these points. The vapour escaping through the surrounding liquid could produce hairline cracks or a gap across the notch.

Whatever process takes place, the vapour in the gaps will not be ionized initially and capacitance will be present across them.

Because of the small cross-sectional areas of fuselink elements, these capacitances must be small. They will charge rapidly because of the current flow which will be maintained through them by the circuit inductance. The resulting voltage will cause the gaps to break down anthers initiate arcs. This transition period between melting and arcing must always be of very short duration in a cartridge fuselink.

switchgear-cartridge type fuse and cartridge fuse diagram

2018-11-10T08:25:00.001-08:00

Cartridge type fuse and cartridge fuse diagram

Cartridge fuse

Cartridge fuse is designed for high voltage, low voltage and miniature applications.Cartridge fuse link, which is replaceable is often fitted into a fuse holder that consists of a fuse carrier and fuse base, an example being shown in figure 1.4.

Cartridge fuse links for low current ratings contains a single element while those for higher ratings have a number of parallel connected elements. The elements are usually of silver or silver plated copper, it being necessary to ensure that oxidation will not occur as stated earlier. Wire elements are used in cartridge fuses with ratings below 10 amperes but for higher rated strips with one or more sections of reduced cross-sectional area are used. The elements are attached to plated copper or brass end caps which together with the body form an enclosure or cartridge. In cartridge fuse link, the bodies which must be good insulators, must also be robust and able to withstand the conditions which occur during the interruption. They were almost exclusively of ceramic or glass in the past but glass reinforced plastics have been introduced in recent years. For high breaking capacity cartridge fuse link, the space within the body is usually filled with quartz of controlled grain size and chemical purity. A cross-sectional view through cartridge fuse link is shown in figure 1.5.

switchgear-What is liquid type fuse?

2018-11-07T11:40:00.000-08:00

What is Liquid type fuse?

Liquid fuse

In the earliest liquid fuses, the arcs were quenched in a liquid and this principle has been used for many years to produce fuse links. Many liquid fuses are in use in the UK. Liquid fuse has a body consisting of a glass top with metal ferrules at each and within it is the element. This is normally of silver strip or wire, with a strain wire across it except for low current ratings, say10Amp and below, in which a wire is used as the element and this is made strong enough to make a separate strain wire unnecessary. In all liquid fuse designs, the element is positioned near the top of the tube so that it is shielded from Corona discharge by the upper ferrule. In liquid fuse the element is held in tention by a spring anchor to the lower ferrule and the tube is filled with an Arc extinguishing liquid usually a hydrocarbon. when the element melts during operation, the spring collapse and arc are extinguished in the liquid. In liquid fuse, to relieve the tube of excessive pressure, a diaphragm on the upper ferrule is ejected except under every moderate condition. Provision is usually made to allow the user to recharge by inserting a new fuse element assembly and new liquid. figure 5.4 shows a cross-sectional view of a typical liquid fuse-links. These fuses are only in use outdoors and provision is made for removing from and replacing the fuse links into their mountings by poll operation from the ground, a bayonet fixing arrangement being usual. Liquid fuses are intended for the protection of 11 and 33KV pole or pad mounted transformers on rural systems and also for Spurs, feeding a number of transformers.

The breaking capacity of these liquid fuses is limited to values below those of expulsion fuses and although, they have given and indeed still are giving good service. Liquid fuses are no longer recommended for new installations.

Switchgear- what is expulsion type fuse? and how does it works?

2018-11-06T03:49:00.002-08:00

Expulsion fuse

what is expulsion type fuse and how does it works?Expulsion fuse links contain a short element of 3 obtained copper wire in series with a flexible braid. These items are mounted in a fuse carrier incorporating a tube of organic material usually closed at the top with a frangible diaphragm and containing a liner of gases evolving material such as fiber. The fuse element carries a closely fitting sleeve of the gas generating fabric. The flexible bread is brought out of the open lower end of the tube and held in tension by a string attached to the lower end of the fuse base.

A cross-section through a typical Expulsion fuse link is shown in figure 5.1 and three popular physical connection arrangements designated button head double tail and universal are illustrated in figure 5.2. It will be seen that all these have a braid at the lower and it is being the upper terminations which differ.

The Expulsion fuse carrier has pins at the lower and which act as a hinge when it is mounted in the lower contacts of the fuse Mount.

In the service position, the fuse carrier is tilted from the vertical as can be seen from figure 5.3 which shows a complete assembly.

when the expulsion fuse element melts during operation, the release of the spring tension disengages a large that allows the fuse carrier to swing down by Gravity.

This provides isolation and prevents discharges along the tube which could lead to tracking it also indicates that operation has occurred.

Expulsion fuses are able to break a wide range of fault currents. when interrupting a small fault, the arc is extinguished within the fired film around the element, but at high current, this sleeve bursts and the arc is extinguished within the liner of gas evolving material. The diaphragm at the upper end of the fuse carrier ruptures when the current being interrupted is sufficiently high and Thus double end venting is provided, so relieving the fuse carrier tube of excessive pressure.

These Expulsion fuses are available for use in three phase circuits with current and line voltage ratings of 200 amperes and 72KV, respectively. Their maximum breaking capacity is typically limited to 150 MVA.

Expulsion fuses are for outdoor use only and they may be replaced using a pole from the ground. The pole can also be used to swing down the carrier of a healthy fuse for the purpose of isolation. It is necessary to be performed unless the equipment is arranged so that load current May be interrupted by the contacts. In some designs, this is done by including circuit breaker type arc chutes around the contacts which separate. An alternative, which is sometimes adopted is to use fuse links which are so designed that the element may be snapped mechanically by an operator inserting a pole in into it from the ground. The expulsion fuse link then functions in the normal way to clear the circuit after which the carrier may be swung down safely.

Switchgear-what is fuse?

2018-11-05T10:25:00.001-08:00

What is Fuse?

The fuse is a device which consists of small piece of metal connected in series circuit.When large amount of current passes through that piece of metal strip beyond a predetermined limit,it melts and saves the circuit from overcurrent.

TYPES OF FUSES

1.Expulsion fuse
2.Catridge fuse
3.Drop-out fuse
4.liquid fuse
5.Open fuse
6.Striker fuse
7.Switch fuse
8.HRC fuse
9.Rewirable fuse or semi-enclosed fuse

Switchgear-circuit breaker operating mechanisms

2017-04-11T08:04:00.001-07:00

Switchgear-circuit breaker operating requirements

2017-04-11T07:53:00.001-07:00

Switchgear- Vaccum Circuit breakers (VCBs)

2017-04-07T08:01:00.001-07:00

Switchgear-SF6 Circuit breakers

2017-04-07T07:53:00.001-07:00

Switchgear-Air blast circuit breakers (ABCBs)

2017-04-07T07:32:00.003-07:00

Switchgear-Air Circuit breakers (ACBs)

2017-04-06T07:43:00.002-07:00

Switchgear-Minimum oil circuit breakers(MOCBs)

2017-04-06T07:29:00.000-07:00

Switchgear-Bulk oil circuit breakers(BOCBs)

2017-04-06T07:20:00.003-07:00

Switchgear-Vaccum Arc

2017-04-05T07:44:00.004-07:00

Switchgear-Arcing properties

2017-04-04T08:00:00.000-07:00

Switchgear-Arc phenomenon process

2017-03-27T07:40:00.000-07:00

Single phase induction motor-Single phase Repulsion motors

2016-12-08T11:34:00.002-08:00

Single phase Repulsion motors with their types and working principle is explained in detail below.