www.distributionsystem.blogspot.com

Distribution Transformers

2010-04-11T09:24:00.000-07:00

The purpose of a distribution transformer is to reduce the primary voltage of the electric distribution system to the utilization voltage serving the customer.

A distribution transformer is a static device constructed with two or more windings used to transfer alternating current electric power by electromagnetic induction from one circuit to another at the same frequency but with different values of voltage and current.

Types according to use and application:

Pole-mounted Transformers
    10 to 63kVA, single phase and 25 to 500kVA, three phase

Ground-mounted Transformers
    300 to 1500kVA, 11 and 22kV, integrated and/or industrial styles complete with HV and/or LV swirtchgear.

3 Main parts in the distribution transformer:

    1. Coils/Winding - where incoming alternate current (through primary winding) generates magnetic flux, which in turn develop a magnetic field feeding back a secondary winding

    2. Magnetic core - allowing transfer of magnetic field generated by primary winding to secondary winding by principle of magnetic induction
These two parts are known as the active parts

    3. Tank - serving as a mechanical package to protect active parts, as s holding vessel for transformer oil used for cooling and insulation ans bushing (plus auxiliary equipment where available)

Earth Fault Indicator (EFI)

2009-10-13T10:16:00.001-07:00

The picture shown tells how this device works.

Earth Fault Indicator (EFI) is a part of a distribution network that is provided for each of the incoming cables in a ring circuit to indicate the faulty section during earth fault.

EFI is composed of an earth fault relay and indicator which is directly operated by a secondary current from the split-core type core-balance transformer. The indicator is automatically re-set by single phase 230V supply.

The split-core type core-balanced transformer is fixed to the cable termination box and has a ration of 60:1, accuracy of class 5 and burden of 1.5VA. Generally speaking, it should be sensitive to low magnitude earth fault current.

Commissioning of 150MVA, 132/11kV DLFF Substation

2009-10-13T10:05:00.000-07:00

I had the privilege to be part of the commissioning team for one of the substations of Dubai Electricity and Water Authority (DEWA), namely Dubai Logistics Freight Forwarders or DLFF 132/11kV, 3x50MVA Substation. This means that now, the electrical energy for the whole city of Dubai has again increased.

The team composed of the Commissioning Engineer, Protection Engineer, Transmission Operator, SIEMENS contractor, and yours truly from the Distribution System Operation.

The scope included commissioning of gas-insulated switchgear (GIS) by SIEMENS; 2×132kV line feeder; 3×50MVA inter-distribution transformer feeder; and a double busbar including bus-section disconnectors and earthing switches; automation system is used for control and monitoring. The man-machine interface is the BAILEY SCADA by ABB.

After much thorough pre-commissing, proper inspection, functional testing of protective relays and circuit breaker control circuits to confirm that protective equipment operates as intended, including current and potential circuits, which are critical sensing inputs, the substation was officially commissioned at 1347H on Jan. 21, 2009.

The Ring Main Unit (RMU)

2009-10-13T09:37:00.000-07:00

In my work as an Assistant Engineer, Distribution Operation here at Dubai Electricity and Water Authority, I always meet this robot look-a-like machine. But it's not a robot, it's called RMU.

The Ring Main Unit (RMU) is a type of switchgear used in the distribution networks that is normally formed by interconnecting a number of ring shaped grids. It is designed to operate at the rated nominal system voltage of 11kV and is normally consist of switches on both sides of the T-off, one to open the ring and another at the connection to the distribution transformer (usually rated 1000kVA) using one of the following:

    Swith and fuse separately
    Switch-fuse combination
    Circuit breaker

Basic Construction Aspects of RMU

    The equipment is metal enclosed, free standing or transformer mounted type (in DEWA, they call it Substation and Pocket types respectively)
    Free standing RMU shall be stable and rigid on its own support
    The unit is provided with lifting facilities of proven design for easy handling
    The equipment is complete with necessary cable termination boxes, earth fault indicating device with current transformers, fuse switch and switching equipment with necessary interlocks for safety and designed for minimum maintenance.
    The mechanical design and strength of the unit and components are able to bear the mechanical stresses on the switch terminals when installed and in operation
    The equipment can withstand the electro-dynamic forces without reduction of the switches reliability or current carrying capacity.

Cable Terminations

    The ring cable terminations are provided on the sides of the RMU. The T-off termination is provided on its rear in the case of free standing type. In the case of transformer mounted type, T-off connection is brought out in such a manner to connect direct to the transformer HT side.
    The cable terminations is suitable to receive three core underground cables of sizes up to 240mm. sq. of XLPE Copper cables.

Operational Switches

    RM Switches is composed of a bank of three positions (ON, OFF, EARTH) single break type suitable for the three phase current carrying capacity
    Tee-off switch is composed of a bank of three positions (ON, OFF, EARTH) double-break type suitable for the phase current carrying capacity
    Current limiting device is composed of the striker pin type and also fuse links capable to actuate a common tripping bar to protect the distribution transformer and to avoid chances of single phasing.

Quite a job eh!.. 'til next time.