Electrical Interview Questions & Answers

Silicon Carbide (SIC) Lightning Arresters

2013-05-12T00:02:00.000-07:00

Silicon Carbide Arresters (SIC):

The Non linear lightning arrester basically consists of set of spark gaps in series with the silicon carbide non linear resistor elements. Lightning arresters are connected between the phase conductors and ground. During normal system operating voltage conditions, the spark gaps are non conducting and isolate the high tension (HT) conductors from the ground. However whenever an overvoltge of magnitude dangerous to the insulation of the apparatus protected occurs ( these over voltages or over surges may be caused due to lightning strikes on the conductors or due to Extra High Voltage (EHV) switching) the spark gap breaks down and allows the high voltage surge current to flow through the ground.

Working Principle of Silicon Carbide (SIC) Lightning Arresters:

The volt-ampere characteristics of the non linear resistor in the lignting arrester can be approximately described by expression V = KI^β. Where K and β are dependent on the composition and manufacturing process of the Non linear Resistor (NLR). The value of β lies generally in the range of 0.3 and 0.45 for modern silicon carbide (SIC) lightning arresters. If the voltage across the Non Linear Resistor (NLR) doubles, the current would increase approximately by 10 times.

Therefore, with multiple spark gaps arresters can withstand high Rate of Recovery Voltage (RRRV). The non-uniform voltage distribution between the gaps (which are in series in lightning arresters) presents a problem. To overcome this, capacitors and non-linear resistors are connected in parallel across each gap. In case of lightning arresters employed for high voltage applications, capacitors and nonlinear resistors are connected across the stock of gaps and NLRs. With the steep voltage wave surge the voltage is mainly controlled by the capacitor and at the power frequency by the non-linear resistors. It is obvious that when the over voltages cause the break down of the series gaps, the current would be very high so as to make the voltage to subside very fast. The highest voltage that appear across the lightning arrester would be either the spark over voltage of the arrester or the voltage developed across the non-linear resistor during the flow of surge current. The lowest spark over voltage of the arrester is called the hundred percent impulse spark over voltage of the arrester. The voltage developed across the non-linear resistor during the flow of surge current is called residual voltage. The lower the value of the voltage developed the better the protection of the lightning arrester.

Disadvantages of Silicon Carbide (SIC) Arresters:

Some of the disadvantages of silicon carbide arresters compared to gapless arresters are given below:

Silicon Carbide (SIC) arresters have inferior V-I Characteristics compared to Zno arresters (Metal oxide arresters).
Decrease in energy absorption (surge wave) capability compared to Zno arresters.
Probability of sparking between the gaps.

Advantages of Silicon Carbide (SIC) Arrester :

Due to the presence of gaps the normal power frequency voltage during normal operation is negligibly less compared to gap less arresters. Hence no leakage current flow between the line and earth in SIC arresters

Conductor Size Selection in Distribution Power System

2013-05-11T21:43:00.003-07:00

In power distribution system both aluminium and ACSR are commonly used. Mostly aluminium conductors are used in the distribution system because of cheaper in cost. Some of the factors which decides the size of the conductors designed for distribution system are given below:

Current carrying capacity of the conductor or distribution line
Allowable voltage drop or line regulation
Breakdown strength of the conductor

Current Carrying Capacity of Line:

The current carrying capacity of a conductor decided by the maximum conductor temperature rise or operating temperature. Operating temperature is limited by mechanical aspects such as allowable span, mid-span sag, joints, creep in conductors and long term mechanical efforts. Generally 85^oC (AAAC), 75^oC, 70^oC, 65^oC, or 60^oC (ACSR) maximum operating temperature is used. Lower temperature is used for long spot lines particularly in rural distribution system. where jumpers may give trouble at higher loading.
The permissible operating temperature of an overhead conductors depends on the maintenance of adequate clearances and limitations of the loss of strength through annealing. Generally, the maximum current which an over-head conductor size designed to carry must not cause it to be heated such that it may result in the annealing of the metal of the conductor or reduction in the clearances specified. Usually for normal day loading, a maximum operating temperature 75^oC is permitted which is allowed to reach up to 100^oC for emergency loading.

Voltage drop and Voltage Regulation:

The allowable voltage drop is considered as critical factors in determining the conductor size for 11kV and Low Tension (LT) distribution line with thermal loading (ampere loading) about 80 percent of the normal thermal rating based on the maximum operating temperature. Large conductor size (cross section) employed in distribution lines reduces the resistance of the line and hence the I²R losses and voltage drop in the line; and hence voltage regulation of the line improves. But using large cross section conductor size will increase the cost as the material required is more. Hence an optimum value must be chosen in between the cost and improving voltage regulation while designing the conductor size for distribution power system.

Mechanical Characteristics of Conductors:

The choice of conductor size from mechanical view point depends on the :

External Loading: Wind speed, ice loading and ambient temperature

Internal Characteristics: Stranding, modulus of electricity, thermal expansion of the creep. For example, considering the creep and economics AAC is used in LT distribution lines. The line characteristics includes voltage regulation is influenced by distribution line parameters and system frequency, current carrying capacity is assessed from the heat balance (amount of heat generated and heat dissipated).

Creepage Distance for Insulators in Substation

2013-04-14T06:50:00.003-07:00

Creepage Distance for Insulators:

The creepage distance for insulators is the shortest distance along the insulator surface between the metal parts at each end of the insulator. Creepage distance can also be refer as leakage distance for insulators.
Insulators in substation are provided to avoid any leakage current from live electrical conductors to flow to the earth through supports. The atmospheric dust sticks to the insulator surface forming a conducting layer. The leakage current flows from the live conductor to the earth through such surface layers. The leakage properties (creepage properties) of an insulator s in substation are characterized by the length of the leakage path. While designing the insulator sheds, the leakage distance for insulators requirement should be satisfied. The requirement of the leakage distance or creepage distance for insulators depends on the

Rated phase to ground voltage
Degree of atmospheric pollution

If the surface of the insulator is clean, smooth and well gazed the dust particles tend to fall down. When an a.c voltage is applied to the apparatus, the dust particles have a tendency to align to the direction of electrical field lines on the surface of the insulator. If the surface of the insulator is rough and moist this alignment will be faster. The continuous application of voltage causes slow alignment of these particles. To prevent a continuous conducting track, the insulator should have sufficient ceepage distance or leakage distance.

During power frequency withstand test of an unclean insulator, the flash-over occurs along the dirty surface of the insulator. In case of internal gas filled or oil filled apparatus, the internal surface should also be free from moisture and dust. Otherwise internal flash over can occur along the surface by tracking. Some of the typical creepage distance for insulators in substation provided based on the level of pollution are tabulated below

S No.	Degree of Pollution	Recommended Creepage Distance for Insulators
1	Clean areas	16 mm/kV
2	Moderately polluted areas	20 mm/kV
3	Industrial areas	22 mm/kV
4	Heavily polluted areas	25 mm/kV

Underground Cable Fault Identification Methods

2013-04-13T23:03:00.003-07:00

If a fault occurs in the underground cable, it is essential that the type of the fault and location of the fault should be determined as quickly and accurately as possible. Accuracy is important in order to avoid excessive trenching work. The type of fault which is most likely to occur is single conductor to ground fault. In multi-core cables, the fault current will likely give rise to excessive heating at the fault causing further damage to the insulation and extending the fault to remaining conductors. Open circuit faults may occur occasionally which be usually at cable joints.

Cable fault type identification:

Prior to the location of the fault on the power system it is important to determine the type of fault so as to make a better choice of the method to be used for fault location

Isolate the faulty cable and test each core of the cable for earth fault. One terminal of the insulation tester is earthed and each conductor of the cable is in turn touched with other terminals. If the insulation resistance tester indicates zero resistance during any measurement, conductor to earth fault for the particular conductor is confirmed
Then check the insulation resistance between the conductors. In the case it is a short circuit fault, the insulation resistance tester will indicate zero resistance
After the above step, short and earth the three conductors of the cable at one end. Check the resistance between the conductors and earth and between individual conductors (at the other end). This procedure is carried out to check the open circuit faults
In case in order to test any other faults. the insulation test of the individual cores with sheath or armour and between the cores is essential. The test should also be done by reversing the polarity of the insulation resistance tester. In the case of any difference in the readings, the presence of moisture in the cable insulation is confirmed. The moisture in the cable forms a voltage cell between the lead sheath and conductor because of the difference in the conductivity of these metals and the impregnating compound forms an organic acid when water enters

Cable fault location Identification:

After the fault type identification, suitable fault location method should be employed to pinpoint the location of the fault. Some of the fault identification methods generally employed are:

Murray loop test method
Fall of potential test
dc charge and discharge method
Induction test
Impulse wave echo test
Time domain reflectometry test

Factors Consider for Distribution Cable Selection

2013-04-13T20:56:00.002-07:00

The factors to be considered for evaluating the suitability of a cable for particular application are load, system voltage, cable insulation, short circuit rating, environmental conditions, sheath and protective coverings, heat dissipation losses, economic considerations etc. The following points are important

Maximum continuous current rating:

While choosing the conductor size for continuous rating, proper care should be taken of all the rating factors depending on actual conditions of installation. Like any other equipment selection, current carrying rating of the cable is governed by permissible rise in temperature of the cable which the insulation provided to the cable should have to withstand. This depends on the amount of heat produced and the surrounding temperature of the soil. The current rating of the cable apart from the above factors also depends on the large number of factors such as method of cable laying employed, spacing between the cables, number of cores of the cable and thermal conductivity of the soil.

System Earthing:

The type of system: Earthed or un-earthed?Is it solid grounding resistance/reactance grounding? The unearthed system will require full insulation from the core to the ground and the cable will be costly compared to the earthed system. For un-earthed cables, more insulation is provided in order to withstand the higher voltage stresses during the short circuit faults

Voltage drop:

The cable should be selected such a way that at full load, the voltage drop should be within the permissible limits.

Conditions of installation:

Methods of installation, estimated thermal resistivity of soil, type of covering, type of armouring, the need if any for additional corrosion protection.

Expected Short Circuit Level of System:

On the basis of the expected short circuit current and time of clearance, an appropriate conductor size for the cable may be selected. The cables should be selected such that it has to withstand stresses and temperature rise in the event of short circuit faults

Temperature rise:

During short circuit temperature of cable rises, cables should be able to withstand the rise in temperature for a prescribed duration without getting damage. It has to allow continuous current during normal operation without rise in the temperature and should be within desired limits

Economic evaluation:

One of the important factors to be considered while choosing the cable is cost evaluation

Voltage Control Methods at Distribution System

2013-04-13T09:55:00.001-07:00

All the equipments connected to the power utility system is designed to be used within the voltage range. Voltage drop exist in each part of the system. Consumers who are electrically connected on the primary distribution feeder near to the substation will have maximum voltage levels compared to consumers who are located at the tail end or far end of the distribution (higher voltage drops attributes to the poor voltage levels). But for satisfactory operation of the electrical equipment voltage levels to these equipments should be within the operational range. Therefore in order to provide supply voltage within prescribed limits some voltage correction or voltage control measures should be taken.

Some of the methods available to achieve voltage control at the distribution end of the power system are explained below:

Voltage Control Methods:

By operating grid or main sub-distribution on load/off load tap changing arrangement of the transformer.
By addition of extra feeders. This reduces the load carrying of the feeder and hence results in reduced voltage drop and improve regulation
By increasing the conductor sizes of the existing conductors. This reduces the losses in thee conductor resulting in improvement in the voltage profile
By rearrange the system and transfer loads
By balance load between different phases
Converting thee single phase system or single wire earth return system to three phase system
By adding distribution transformer capacity
By altering tap settings on the distribution transformers
Providing line drop compensator such as shunt capacitors for improving the power factor and series capacitors for improving the voltage profiles and stability of the system. Switching on and off theses devices whenever they are necessary
By installing voltage regulators at the distribution side
Increase distribution voltage levels.

Substation Interview Questions Answers

2013-04-12T20:07:00.001-07:00

What are different insulators employed in substations?

Answer: Some of the insulators employed in substations are:

Pin insulators
Post insulators
Strain insulators
Suspension insulators
Hallow apparatus insulators
Dead-end insulators
Solid core insulators

What is creepage distance?

Answer: Creepage distance is the shortest distance between two conducting points along the surface of the insulating material.

What is Basic Impulse Level (BIL)?

Answer: Basic Impulse Level (BIL) refer to the peak value of lightning impuse voltage withstand capability of the equipment

What are the disadvantages of corona?

Answer: Some of the disadvantages of corona are:

Corona causes power loss
Corona causes unacceptable noise
Corona cause radio interference

What is the significance of Corona rings in substations?

Answer: It is observed that corona discharge occurs mostly at sharp corners, edges near the conductor fixing points. Corona rings also called grading rings are hallow metallic rings having large diameter and will have a smooth surface. These corona rings are placed suitably to metal clamps, joints to encircle the conductors surface. This have the advantage of shifting of high stress point shifted from conductor surface to corona ring surface. As the diameter and surface area of the ring is higher than that of the conductor. This helps in:

Field distribution is made uniform
Surface stress is reduced to much lower value
Corona discharge is eliminated
Dust deposition is also minimized

What is the neutral point earthing and its advantages?

Answer: In substation all the neutrals of rotating machines, transformers, busbars and other equipment are connected to ground. Some of the advantages are:

Earth fault protection is based on the method of neutral grounding
The system voltage during earth faults depends on the neutral grounding. During line to ground faults, grounding of neutral helps to reduce the over-voltages which occur on the one healthy phases

Single Phase Earth Return System Advantages Disadvantages

2013-04-09T17:20:00.003-07:00

Generally in secondary distribution system, single phase two wire distribution system is generally followed. But for remote distribution systems in rural areas single phase ground return system has found economical. In this system earth is used as return conductor and only one phase wire or conductor is used. This system requires grounding rods deep into the soil at each consumer's site in order to establish a sufficiently low impedance to the ground. This single phase earth return system has certain limitations like increase in earth potential and interference with telecommunication lines. Some of the advantages and disadvantages of single phase earth return systems are:

Advantages:

Single Phase earth return system is economical to the extent of 30-40 percent conventional three phase system under favorable conditions. The galvanized steel line conductor system affords further economy
Better voltage regulation which is the main problem in long conventional feeders
Improved power factor as compared to the conventional system, nearly about 0.95 whereas conventional system usually have power factor between 0.7 to 0.8
The outage on the single wire earth return system are comparatively lesser than on the conventional system due to the less number of components per span

Disadvantages:

Maximum ground return current permitted is 7 to 8 amps. This limitation is to avoid communication interference and safety hazards due to voltage gradient. A 1:1 isolation transformer is used to minimize the possibility of telephone interference
Maximum power limit of distribution of single wire earth return system is limited up to 3 to 3.5kVA/km of the line
Particular configuration and the maintenance of the earth electrode is an important point while adopting this system
Single phase earth return system without the isolation transformer may create extraordinary interference with the telecommunication circuits

Shunt Vs Series Capacitors Advantages

2013-04-09T11:16:00.001-07:00

Capacitors aid in minimizing losses and helps in reducing operating expenses. Capacitors should be placed where the reactive power demand is more as capacitors generate reactive power and helps to maintain the voltage. Basically reactive power demand will be more at load side of the power system, hence capacitors should be placed as close as possible to load. In specific, capacitor banks should be placed where the low voltage problem occurs.Shunt capacitors are series capacitors are employed in power system for different purposes

Shunt capacitors and series capacitors in the power system generate the reactive power to improve the power factor and voltage, thereby enhancing the power system capacity and reducing the losses. In series capacitors reactive power generation is proportional to the square of the load current (I²X_c), whereas in shunt capacitors reactive power generation is proportional to the square of the voltage (V²/X_c).
The cost of installation of series capacitors is higher than that of the shunt capacitors. This is because the protective equipment for the series capacitors is often complicated. In addition to that, series capacitors are generally designed for higher power to cope up with the future increase in the load
For the same voltage improvement, the rating of the shunt capacitors will be higher than that of series capacitors. Series capacitors compensation may create certain disturbances: ferro-resonance in transformers, sub-synchronous resonance during motor starting, shunting of motors during normal operation and difficulty in protection of capacitors from system faults.
Series capacitors are more effective on distribution circuits with higher X/R ratio and for load variations involving a higher reactive content.
Series capacitors are generally employed to improve the stability of the system and shunt capacitors are generally employed to improve the power factor of the system

Some of the factors which influence the choice between the shunt and series capacitors are tabulated below:

Objective	Series Capacitors	Shunt Capacitors
Improving Power Factor	Secondary	Primary
Improving Voltage level in overhead line system with a normal and low Power Factor	Primary	Secondary
Improving Voltage level in overhead line system with a high Power Factor	Not Used	Primary
Reduce Line Losses	Secondary	Primary
Reduce Voltage Fluctuations	Primary	Not used

Air Blast Circuit Breakers Advantages Disadvantages

2013-04-08T10:29:00.002-07:00

In air blast circuit breakers, compressed air at a pressure of 20-30 kg/cm² is employed as an arc quenching medium. Air blast circuit breakers are suitable for operating voltage of 132kV and above. Switching resistors and equalizing capacitors are generally connected across the interrupters. The switching resistors reduce the transient over voltages and helps arc interruption. Capacitors are employed to equalize the voltage across the break. The number of breaks depends on the system voltage (eg: for 132kV 2 to 4 breaks and for 220kV 2 to 6 breaks)

Some of the advantages and disadvantages of air blast circuit breakers are discussed below

Advantages of Air Blast Circuit Breaker:

Cheapness and free availability of the interrupting medium, chemically stable and inertness of air
Air blast circuit breakers have advantage of high speed operation
In air blast circuit breaker fire hazard is eliminated unlike in oil circuit breakers
Short and consistent arcing time and therefore, less burning of contacts
Air blast circuit breakers require less maintenance
They are suitability for frequent operation
Facility for high speed re-closure

Disadvantages of Air Blast Circuit Breaker:

An air compressor plant has to be installed and maintained
In air blast circuit breaker upon arc interruption, the air blast circuit breaker produces a high level of noise when air is discharged to the open atmosphere
In air blast circuit breaker current chopping problem exists
There is a problem of re-striking voltage

Substation Safety Clearances

2013-04-07T12:16:00.003-07:00

Majority of electrical substations will be air insulated type. All the electrical equipment live parts will be exposed .All the electrical equipment in the substation shall be arranged in such a manner to ensure the clearance space between the live parts and other equipment of the substation (grounded or ungrounded). Adequate clearances should be provided for safety of the operating and maintenance personnel and also to prevent any conducting path between the phases. Safety clearances associated with different voltage levels are given in the table below.

Phase to phase clearance is the minimum clearance required to be provided between the live parts of different phases or is the minimum clearance between the same phases
Phase to earth clearance is the minimum clearance required to be provided between the live parts of the system and earth structures
Sectional clearance is the distance between the live parts of the phases and the terminals of the work section. This limits of work section or maintenance section may be a platform or ground on which operation personnel can carryout his task safely
Ground clearance is the vertical clearance between the live parts of the equipment and the earth surface

Safety Clearances in Substation:

Minimum Clearances (mm)	765 kV	400 kV	220 kV	132kV	33kV
Phase to Phase	9400	4200	2100	1300	320
Phase to earth	6400	3500	2100	1300	320
Sectional Clearance	10300	6500	5000	3800	3000
Ground Clearance	14000	8000	5500	4800	3700

Methods to Reduce Step Potential and Touch Potential in Substation

2013-04-07T08:28:00.002-07:00

Step Potential and Touch Potentials are very important in substations because during ground faults all the ground current returns to the substation transformer (as the substation transformer is grounded). The current that returns through the earth can create a significant voltage gradient along the ground and between ground and conducting objects.A step potential in substation creates a path through the legs from one foot to the other. A touch potential in substation is normally considered a hand-to-foot or hand-to-hand contact.

Step potential and touch potentials are of concern during normal conditions and during ground fault.Under normal conditions, unbalanced currents can rise the neutral to earth voltage. This is not normally dangerous but it can cause shocks.

Step potential and Touch potentials during faults are more dangerous. Therefore it is important to reduce the step potential and touch potential to be within limits during the substation design.

Methods to Limit Step Potential and Touch Potential:

Step potentials and touch potentials can be reduced by employing one of the following methods:

By proving low resistance path to ground
By providing insulation layer between operating personnel and earth
By proper placing of ground conductors

Some of the methods employed to reduce step potential and touch potential in substation are:

Multi-grounded neutral helps to reduce the dangerous step potential and touch potential during line-to-ground faults. By creating low impedance path back to the source, faults are quickly cleared by fault interrupters. Multiple grounding electrodes tied together helps to reduce the touch potentials at the fault point. With multiple neutrals, step potentials are usually not dangerous since fault currents spreads between several grounding electrodes
Using a reactor on the substation transformer neutral helps to limit the step and touch potentials. While utilities normally use the neutral reactors to limit the fault currents. the reduction of ground fault currents also reduce the step and touch potentials and reduces current in grounding and bonding connectors.
By wearing electric hazard shoes. These shoes when dry can have offer millions of ohms of resistance which can save the operating personnel against these dangerous potentials. By using insulating materials such as rubber gloves can protect the personnel.
By providing resistive surface layers in and around the substation. It is often provided with the surface of crushed rock or pebbles which acts as insulation medium between the operating personnel and earth.

Inrush Currents in Transformers - Causes

2013-04-06T22:32:00.000-07:00

When a transformer is energized after a short interruption, the transformer may draw high inrush currents from the system due to core magnetization being out of synch with the voltage. The inrush currents will be as high as short circuit currents in the transformer (almost 20 to 40 times the rated normal full load current of transformer). Inrush currents may cause fuse, relays or re-closers to falsely operate. It may also falsely operate the faulted circuit indicators or cause sectionalizers to mis-operate

When the transformer is switched in, if the system voltage and transformer core magnetization are not in synch, a magnetic transient may occurs. This transient may drive the core into saturation and drives a large amount current into the transformer causing transformer core to damage

Factors Significantly Impact Inrush Currents in Transformer:

A transformer that is designed to operate lower on the saturation curve draws less inrush currents as there is more margin between the saturation point and the normal operating. The extra flux during switching is less likely to push the core into saturation
Large transformers draw more inrush current. Large transformers will have smaller saturation impedance
Higher source impedance relative to the transformer size limits the currents that the transformer can pull from the system
The point where the circuit breaker close (position of flux wave in sine wave). The worst case will be when the flux is at maximum (peak) and voltage is minimum (in transformer the applied voltage lag behinds the flux by 90 deg).

Other factors have little significance. The load on the transformer does not significantly change the inrush currents. While switching transients cause high inrush, other voltage transients especially voltage transients with dc components can saturate the core of the transformer and cause inrush currents

When the nearby fault was cleared and transformer voltage is recovering from the voltage sag, the sudden rise in voltage can drive the transformer to saturation
Energizing a transformer can cause the nearby transformer to also draw inrush currents. The inrush currents into the switched transformer has a significant dc component that can cause the voltage drop. The dc component can push the other transformer into saturation and draws inrush
A lightning flash near the transformer can drive the transformer core to saturation

Three Winding Transformer Advantages

2013-04-05T10:08:00.000-07:00

Generally in power system mostly two winding transformers are employed. But three winding transformers are employed because of some advantages:

The most common reason for having a three winding transformer is to provide a delta connection tertiary winding
To limit the fault level on the low voltage system of the transformer by dividing the LV infeed (in order to provide double secondary windings)
Providing tertiary winding helps to interconnect different power system operating at different voltages (Three winding transformer helps provide power supply at two different secondary voltages, 220kV/11kV/6.6kV transformer can able to provide power at two different voltage levels (11kV and 6.6kV)
To regulate the voltage and reactive power of the system by providing synchronous capacitor connected to one of the terminals of the transformer

Why Delta winding prefer:

It is always desirable to have one delta connection winding in the three phase transformer as delta connected three phase winding will offer low impedance path for the three phase currents. Also the presence of delta connected three phase winding allows to circulate the current around the delta winding in the event of unbalance loading condition

Although power system designers aims to avoid use of star/star transformer in power system but cases will arise when the phase shift between the star/delta and delta/star is not applicable such as in the power station supplying power to auxiliary system. Therefore it is common practice to have a star/star with delta tertiary three winding transformer supplying power to the plant auxiliary system

B/H Curve of the magnetic material (core of the transformer) is not linear. Is a sinusoidal voltage (flux) is applied across the primary winding, the magnetizing current obtained will not be sinusoidal in nature and consists of fundamental component and several harmonics. Third harmonic components predominate with several other higher harmonic components. If there is no delta connected winding, or if the star connections of the transformers are not grounded, the line to earth capacitance currents supply system lines supply the harmonic components. If the harmonic components cannot flow in any one of these paths then, secondary voltage will be distorted

Factors affecting Soil Resistivity

2013-04-05T09:12:00.000-07:00

Soil resistivity is the resistance of certain volume of the soil between opposite faces of the cube of soil with a volume of 1m³. Soil resistivity vary widely. Soil resistivity varies significantly with increase in the depth. Some of the important significant factors that affect the soil resistivity are listed below:

Soil Resistivity is affected by:

Moisture content:

Moisture content in the soil is one of the major factors that determine the soil resistivity. The dryer the soil the more is the resistivity of the soil, wet soil will have low soil resistivity. Change in the moisture level of the soil through out the year during different seasons is the biggest reason for change in the ground electrode resistance. With increase in the moisture content in the soil, ground resistance or soil resistance decreases and about after 22% of moisture content there will be very little change in the soil resistivity

Temperature:

Above the freezing point of the water, temperature does not impact the soil resistivity significantly but the temperature below the freezing point of the water soil resistivity rises and will have significant impact

Salts in Soil:

The presence of soluble salts in the soil will significantly impact the resistivity of the soil. One of the option to reduce the electrode resistivity is by chemical treatment of soil near the electrode. Bentonite backfilles or salt treatment such as sodium chloride or calcium chloride can significantly reduce the soil resistivity. The only disadvantage is some chemicals enhance the metal corrosion rate which can damage the grounding rod to corrode.
If the grounding rod sustains significant current, the current flowing through the grounding rod may dry out the surrounding soil and that can increase the resistance of the electrode. When high currents flowing through the grounding rod or earthing electrode, this may be due to lightning or from faults the soil surrounding the electrode breaks resulting in the increase in the resistance

Magnetostriction:Transformer Noise

2013-04-04T06:23:00.002-07:00

Noise In Transformer:

The basic cause for the noise in the transformer is due to magnetostriction of the sheets in the magnetic circuit (core of the transformer). Variations in the magnetic induction subjected to the sheets to periodic variations in the length, the amplitude which is in the order of microns per meter length.
The fundamental frequency with which these vibrations occur is double that of the system frequency, (for 60Hz frequency vibration frequency will be of the order of 120Hz) and also constitute numerous harmonics. Also various parts of the transformer, starting with the magnetic circuit (core) are liable to vibrate due to magnetostricition effect.
The noise generated due to magnetostricition effect transmitted from the magnetic circuit to the tank of the transformer either through direct conduction to supporting points or through the oil and insulating material used in transformer. The transformer tank and the radiator radiate the acoustic noise or energy in to the ambient atmosphere
Another source is due to the vibration of magnetic sheets perpendicular to the surface either at the edge or at the core packets, or at the joints between the leg and the yoke
The current carrying windings is also a source of noise, however the amplitude of the noise is very less and is not detectable.
Cooling fans and pumps employed for cooling the transformer is also acts as source of noise

Methods to reduce Transformer noise:

The main source of noise in the transformer is due to magnetostriction effect of the magnetic circuit or core. In order to reduce the noise cold rolled grain-oriented plant, with low magnetostriction and improved flatness is employed
Ensuring uniform flux distribution and reduction in the cross flux also reduces the noise
Elimination of the clamp bolt holes, use of resin impregnated glass-fibre bands instead of core bolts, gluing of core packets can reduce the noise

These above specified remedial methods not only reduces the noise level by 5 to 10 dB, but also reduces the losses and no-load current

Different Transformer Internal Faults

2013-04-04T06:11:00.001-07:00

Faults in Transformers:

Some of the faults in the transformers are likely to be over-voltages which resulting from the atmospheric phenomenon (lightning) transmitted by overhead lines.
Switching in the power system (especially high voltage switching more than 400kV) can produce over-voltages of less steep but longer duration surges stressing both liquid and solid dielectrics (insulation). These over-voltages should be restricted in amplitude to a value below the transformer insulation breakdown withstanding level.
Short circuits in the power system subject the transformer to currents of 10 to 20 times the rated currents (short circuit current level will be severe when fault occur close to transformer). Power transformer is generally designed to withstand tens of short circuits, lasting not more than 2 sec duration in its life time. If there are more short circuits than the designed limit special construction is required. Short circuits should eliminated (by isolating faulty power system by opening circuit breakers) as quickly as possible to limit the short circuit intensity on transformer.
Overloads can arise in transformers from planned or fortuitous (unexpected) circumstances. In the first case, temperature increase in transformer insulating material should not exceed the standard value. In the second case, certain time limit can be tolerated but this will have certain cost in reduction in the life of the transformer.

Internal Faults in Transformer:

Electrodynamic faults: which occurs between insulation and current carrying conductors, HV and LV winding due to external and internal short circuits
Electromagnetic faults: Which occur due to eddy currents induced in the magnetic circuits or the clamping structure.
Electrical faults which occurs due to bad contacts in the leads or bad contacts in the tap changer Dielectric faults: Which occur due to shorting between windings or between live parts and earth, partial discharges
Thermal faults: Which occurs due to abnormal temperature rise, hot spot, thermal ageing or pollution in transformer oil
Mechanical faults: Which occur due to vibrations, leakages or defective operation of the tap changers

Different types of defects originated in transformers will have degree of gravity depending on the amount of damage it can do on transformers and their consequences. Some of the defects (vibrations, partial discharges) will not immediately endanger the equipment but care must be taken before causing major damage. On the other hand, defects such as (over-voltages and short circuits, and initial breakdown) requires immediate attention

Ferroresonance in Power Transformers

2013-04-03T11:54:00.001-07:00

Ferroresonance:

Ferroresonance phenomenon can be triggered off through interaction of the system capacitance with a non-linear inductance, when transformer is at no-load. This oscillatory phenomenon or resonance phenomenon result in over-voltages which can damage the cable and the power transformer. Series ferroresonance is dangerous to power transformer

Ferroresonance can occur when:

The power transformer is on no-load (even an active load which can be less than one tenth of the rated power can be sufficient to prevent the phenomenon of ferroresonance to occur by absorbing the energy).
A large capacitance exists in line or the cable connected to at least one terminal of the transformer while the other terminals remain at normal potential
An unbalanced situation is created by a single phase trip out, a break in the conductor or blowing of one or two fuses

Methods to Prevent Ferroresoance:

Ferroresonance can be reduced by reducing the length between the transformer terminals and the section circuit-breaker or fuses
Avoid non simultaneous switching of power transformer
Earth the neutral point directly or through a resistor limiting the single phase fault current

Auto-Transformers Advantages Disadvantages

2013-04-03T11:41:00.000-07:00

Auto-transformer Advantages:

Auto-transformers in comparison with the double winding transformers are generally advantageous of the voltage ratio is favorable from the point of view of the equivalent size
The reduction in the equivalent power in relation to the throughput, reduction in the weight and size, reduction in the no load (iron losses) and on load losses (cooper losses), the reduction in the no load current and the short circuit impedance
If P is the power of the transformer, linear dimensions of the auto transformer vary as P^0.25 and the weight and volume vary as P^0.75 thus reducing the losses and weight of the transformer
The short circuit impedance of the auto-transformer goes down with equivalent rating which is advantageous because of the lower voltage drop and better regulation.
For connections between the two systems at very high voltage, where the power is to be transmitted in the order of GVA (Giga Volt-amperes), the use of auto transformer make it possible for high power equipment and can be easily transportable. This will not be possible with normal transformers

Auto-transformer Disadvantages:

An electrical connection between the primary and secondary is not always desirable, particularly when the voltages levels on both the sides of transformer are quite different. If the neutral point is not solidly earthed, the lower voltage side can be subjected to high potential in the event of earth fault on high voltage side. In general practice, auto transformers are only used on distribution systems where the neutral is connected to the earth
Auto-transformers are particularly sensitive to the atmospheric over-voltages. Therefore the auto-transformer requires much highs standard of insulation compared to normal transformer. Surge Arrester protection is required
As the short circuit impendace values of the auto-transformer are low, short circuit currents can reach higher critical values.

Generating Station Grounding Principles

2013-03-21T10:17:00.000-07:00

Grounding systems in the generating stations should able to meet the following grounding principles:

Grounding Principles:

Metallic enclosure on electrical equipment and exposed non current carrying conductive materials in generating station which are capable of being energized due to insulation failure or accidental contact of the energized conductors shall be grounded
The grounding should limit the step and touch voltage to acceptable limits during a ground fault on the enclosed equipment.
The grounding in generating station should provide ground fault current return path so that ground faults can be detected by protective devices and can able to isolate the faults or can provide an alarm to the operator
The grounding conductors and connections should withstand the ground fault current for the duration of the fault, without being damaged by thermal, thermo-mechanical or electromechanical stresses.
The grounding conductors should be continuous; no switching device should be inserted in the grounding conductors (except where the operation of the switching device will also automatically disconnect all power sources to the equipment grounded by that conductor). Structural changes after installation should not interrupt the grounding conductor. In general, equipment enclosures should not be used as part of the grounding conductor.
Ground conductors should able to withstand any applied mechanical stresses and should be reliable.Ground connections which are exposed should be accessible for inspection
Grounding system in generating station should be designed in such a way to minimize the corrosion to adjacent structures and enclosures

Induction Motor Operating Characteristics

2013-03-20T12:18:00.000-07:00

All torques of an induction motor, at a given slip, vary approximately as the square of the voltage applied to its terminals
Low rotor resistance results in high full load speed (low slip), high efficiency (low rotor losses), and slightly higher starting current.
High rotor resistance results in high starting torque for line current drawn and slightly lower current during starting, but results in lower full load speed and lower efÞciency (high rotor losses).
The slip at which maximum torque occurs is proportional to rotor resistance
Rotor frequency and voltage are proportional to slip. Thus, both are zero at synchronous speed, but increase to a maximum at zero speed (for slip range of 0 to 100%).
Rotor I²R losses are proportional to slip and are in the rotor winding of a squirrel cage motor. On a wound rotor motor, the secondary losses divide in proportion to the inherent rotor winding resistance and any connected external resistance.
For a pure inertia load (no load torque), the heat energy added to the rotor winding during acceleration (starting) is equal to the kinetic energy of the rotating mass at full speed. The total kinetic energy added to the rotating mass during acceleration to full speed is always the same for a particular value of motor and load inertia regardless of load torque. The effect of load torque is to increase the heat energy added to the stator and rotor windings during acceleration to full speed due to longer accelerating time. The kinetic energy is added to the rotating mass at a rate determined by the accelerating torque (motor developed torque less load torque), and the heat added to the rotor winding is determined by the rotor current and the accelerating time. The accelerating time (and the time the high starting current exists) is inversely proportional to the accelerating torque. Under low-voltage starting conditions, the ratio of heat added to the rotor compared to the kinetic energy at full speed is even greater because the accelerating torque is reduced (developed torque varies approximately as the square of the voltage), the load torque is unchanged, and the ratio of accelerating torque to load torque is reduced

Harmonics Disadvantages in Power System

2013-03-06T12:07:00.001-08:00

Sources of Harmonics:

Several sources of harmonic currents that may be found on electrical power distribution networks are listed below:

Variable speed motor drives
Rectifiers
Arc furnaces
Wielding equipment
Uninterrupted power supplies
Switching mode power supplies
Compact fluorescent lamps
Electronic ballasts

Disadvantages of Harmonics:

Harmonics degrades the performance of power system. Some of the disadvantages of harmonics in the power distributed network are listed below:

The harmonics flowing in the distribution network downgrade the quality of the electrical power supply. There can have several negative effects on the operation of the power system
Increased losses on the distribution system due to increase in the effective rms current
Over-load in neutral conductors due to cumulative increase in the third harmonics created by the single phase loads
Overloads, vibration and premature ageing of the generators, transformers and motors as well as increase in the noise level
Overloads and premature ageing of the power factor correction capacitors
Distortion of the supply voltage that can disturb the operation of the sensitive loads
Disturbances in the communication networks and telephone lines
Resonance between the supply inductance and capacitance of the power factor correction capacitors

Dry Type Transformers Advantages

2013-03-06T04:48:00.003-08:00

Dry type transformers are most suitable in locations where conventional oil filled transformers present a safety hazard. Some of the locations where dry type transformers are best suited are: Oil refineries, Chemical plants, Marine applications, Metro railways where there is significant safety is required against fire.

Advantages of Dry Type Transformers:

Dry type transformers have no risk of fire
Dry type transformers are lighter in weight compared to oil filled transformers
It requires less floor area to accommodate
No toxicity unlike synthetic liquid filled transformers
Dry type transformers are have efficient noiseless operation
Reduced installation cost
They are Maintenance free

Type of dry type transformers:

Different type of dry type transformers are:

Cast resin type
Resin Impregnated type

Cast resin type transformers are Class F glass fibre reinforced epoxy resin. It can withstand 155^oC continuously. The disadvantages of cast resin type transformers is tat in the event of failure, the complete unit including HV and LV winding is to be replaced, whereas, with resin impregnated transformer, only the affected winding is required to be replaced.

DC Motors Application in Thermal Power Plant

2013-02-27T06:45:00.000-08:00

In thermal power plants dc motors are employed for certain control and critical emergency operations which are supplied by dedicated batteries. DC motors operate as backup drives for normal ac drive systems when ac power supply to the plant is lost.

In thermal power plant, the dc motors finds applications for performing control functions such as

Turbine governor motor
Governor limit setting
Motor operated rheostats
Emergency lubrication for the turbines (main, boiler feed pumps)
Generator (H2 oil seal).
DC motor operated valves

The emergency type services are of short duration. The emergency lube oil pump running time depends on the turbine-generator run-out time with vacuum or broken vacuum. The Hydrogen gas seal oil pump may be operated for 3 to 4 hours. The turbine-generator emergency bearing oil pumps provide lubrication, the emergency seal oil pumps maintain Hydrogen seal in case ac power is unavailable. The dc motor operated valves usually perform essential services, i.e., one ac and one dc operated valve in parallel or in series with each other will assure opening or closing, respectively.

DC motors employed in thermal plants are classified in to two types based on the type of application.

DC motors carrying out Control function
Dc motors carrying out Emergency function

Control functions:

This category consists of the turbine governor motor, governor limiting setting, motor operated rheostats, etc. These motors are small, about 1/8 hp or less. They are operated quite infrequently for short duration.

Emergency functions:

This category consists of turbine-generator emergency (lubrication) bearing oil pumps and emergency seal oil pumps. Such pumps may also be provided for steam turbine drives of feedwater pumps, fans, and other large loads. The lack of lubrication during a shutdown without ac power will ruin the linings of the bearings and damage the shaft.

Hydrogen seal oil pump is provided to prevent the escaping of hydrogen (for large turbine-generators hydrogen cooling is provided for efficient cooling) from the casing by providing a tight seal with high pressure oil.

Turbine Generator Hydrogen Seal System

2013-02-26T19:01:00.000-08:00

For large generators in power plants hydrogen cooling is provided for efficient heat transfer and electrical properties are superior to non explosive gases. High purity hydrogen will not support combustion. However when hydrogen mixed with air to the level of 95% purity, it is explosive in nature. Therefore a sealing system must be provided to prevent the hydrogen gas from escaping from the generator casing. This is done by maintaining pressure on an oil seal around the generator shaft. As long as the oil pressure is greater than the hydrogen gas pressure, a seal will be formed confining the gas inside the generator and preventing leakage to the areas outside the casing.

For a generating unit the seal oil motors would consists of the following:

AC main seal oil pump (MSOP)
DC emergency seal oil pump (ESOP)
AC motor driven vacuum pump

The main seal oil pump is used normally and is backed up by the dc emergency seal oil pump, which is fed from the station battery. These pumps circulate oil through the seals while the vacuum pump removes the hydrogen from the oil in the hydrogen drain tanks. Some vendors will furnish two seal oil pumps instead of one. These pumps, one for each side of the seal, are called the air side and hydrogen side seal oil pumps