Testing of ACDB, DCDB Panels and Auxiliary Transformer

Auxiliary Testing of ACDB, DCDB Panels

AC Distribution Panel Board (DPB) shall control the AC power from PCU, and should have necessary surge arrestors. Requirement/specifications of DCDB and ACDB may be changed as per site conditions. An ACDB to be provided at the cable terminating point emanating from 5 KVA PCU for interconnection control of dedicated electrical loads.

All switches at the, circuit breakers, connectors should confirm to IEC 60947, part I, II and III. 

A distribution board (also known as panel board or breaker panel) is a component of an electricity supply system which divides an electrical power feed into subsidiary circuits, while providing a protective fuse or circuit breaker for each circuit. The ACDB receives the AC power from the solar inverter and directs it to AC loads through the distribution board


  • Check tightness of all connections
  • Inspect for physical damage / defects
  • Panel condition, cleanliness, organization, labeling and readiness for service, panel doors, handles…etc
  • CT shorting checked
  • Indications checked
  • Contact resistance of tripping and alarm checked
  • Check the ferrules as per specification
  • Confirm that each panel has been properly secured to the floor in its final service location.
  • Panel Earthing checked
  • Confirm that panels are constructed and wired as per SEC relevant specification
  • Check case cover and gasket for proper seal against dust.
  • Check all installed equipment nameplate information for compliance to approved drawings and equipment /material lists.
  • Check that panel equipment is mounted securely and protected against mal operation due to vibration, shock, etc
  • For all internal and external panel wiring, confirm that all screw terminations are tight and that crimp connectors are firmly secured to the wire and to the termination point.
  • Ensure that no part of the wire is bent at the termination point. Check Ferrules.
  • Use of ring type terminals for wire termination for current circuit wires.
  • Check that panel equipment is mounted securely and protected against mal operation due to vibration, shock, etc


  • Check Output Of  Ac Outlet
  • Check Illumination Lamp
  • Check Door Switch
  • Check Heater / Thermostat
  • Identification label for each compartment is fixed properly
  • Manual spring charging operation
  • Circuit breaker manual closing by Push button
  • Circuit breaker manual opening by Push button
  • Check under voltage relay operation
  • Check over current relay operations


  • Electrical spring charging motor operation
  • Circuit breaker closing by local & remote electrical operation
  • Circuit breaker opening by local & remote electrical operation
  • Circuit breaker emergency trip checked
  • Circuit breaker close interlock verified as per drawing
  • Circuit breaker “ON “ indication checked
  • Circuit breaker “OFF “ indication checked
  • Circuit breaker “TRIP “ indication checked
  • Cubicle illumination lamp circuit checked
  • Cubicle heater circuit checked
  • All contactor & relay operation is checked

Testing of Auxiliary Transformer

Auxiliary transformers are used for lighting purposes, heating the train wagons or for producing single-phase auxiliary for the safety systems supply or the substation’s own supply.

Product scope

  • Voltage level: up to 36 kV
  • Power rating: up to 2 MVA

Auxiliary Transformer Testing

  • Insulation Resistance Test
  • Ratio Test
  • Vector Group Test
  • Winding Resistance Test
  • Magnetizing Current Test
  •  Magnetic Balance Test

1.Insulation Resistance Test:

To check the insulation level of transformer winding & oil this test is conducted 
Apply the 5 KV on the primary to earth for 15 seconds & 60 seconds values to find polarization index value
Apply 2.5KV between primary to secondary and note down the 15 seconds & 60 second value.
Apply 1 KV between secondary to earth and note down the 15 seconds & 60 seconds value.

2. Ratio Test:

To find the transformation error this test is carried out. Apply 3-phase Voltage on the primary side and measure the secondary side voltage in TAP 3 & TAP 5.Calculate the ratio error by actual and measured value.

3. Vector Group Test:

To verify the vector group as specified by the manufacturer this test is carried out.
For example: Dyn11 Transformer
to verify, superimpose the 1U and 2U terminals and measured the voltages as per below equation and justify

1.1U 1V = 1U 2n + 1V 2n
2.1V 2W=1V 2V
3.1W 2W < 1W 2V.

4. Magnetic Balance Test:

To verify the correct identification of cores and phases and to verify mutual flux inductance, this test is carried out. 
Apply the voltage on one winding and measure the voltage sharing on the other windings. By theory the adjacent placed winding should share more flux than the other. That is if we apply on 1U –1V winding, 1V-1W should share more voltage than 1W-1U.

 5. Magnetizing Current Test:

To Asses the no load losses of the transformer this test is carried out. Measure the magnetizing current of the each winding / phase by tong tester or multi meter and verify with factory report.

6. Winding Resistance Test:

To verify the resistance value as specified by the manufacturer and to asses any damage on winding this test is carried out. Apply the DC current of 10Amps through the winding with the feedback of voltage drop the kit will calculate winding resistance value. Compare and verify with factory test report.


A Ring Main Unit (RMU) is a totally sealed, gas-insulated compact switchgear unit. The primary Switching devices can be either switch disconnectors or fused switch disconnectors or circuit breakers.

scada engineer

In case a circuit breaker is the switching device, it is also equipped with Protective relaying, either with a very basic self-powered type or a more advanced one with communication capabilities.

The rated voltage and current ranges for RMUs typically reach up to 24 kV and 630 A respectively.

 With many of the manufacturers of RMUs, the basic construction of the unit remains the same for the whole of the voltage range.

The increase in rated voltage is handled by an increase in the insulating gas pressure.

 RMU configuration

The figure above shows a typical RMU configuration where load disconnectors are the switching devices for the incoming cable feeders and circuit breaker works as the switching device for distribution transformer feeder.

Three-position design Closing, Opening and Earthing

All of the switching devices are of three-position design, having the possibility to close or open or earth the feeder in question.


Closing the moving contact assembly is manipulated by means of a fast-acting operating mechanism. Outside these manipulations, no energy is stored. For the circuit breaker and the fuse-switch combination, the opening mechanism is charged in the same movement as the closing of the contacts.


Opening of the switch is carried out using the same fast-acting mechanism, manipulated in the opposite direction. For the circuit breaker and fuse-switch combination, opening is actuated by:

     A pushbutton

A fault.

A specific operating shaft closes and opens the ear thing contacts. The hole providing access to the shaft is blocked by a cover which can be opened if the switch or circuit breaker is open, and remains locked when it is closed.


The figure below shows typical outlook of a three-feeder RMU. In the figure, the combination consists of load disconnectors for the incoming two feeders and a fused load disconnectors for the distribution transformer feeder. The incoming and outgoing medium-voltage cables are attached using elbow-type plug-in cable ends.

Whereas the RMU type of units represents the very compact gas-insulated design for a Auxiliary dedicated purpose, the secondary medium-voltage switchgears represent an air-insulated, quite freely extendable and configurable solution.


Neutral Earth Resistors (NER) protects equipment from damage in the event of earth faults. They do this by limiting the current that flows through the neutral point of a transformer to a safe level so as prevent Auxiliary damage to equipment but still operate protection devices. Neutral Earth Resistors can be used to protect critical equipment including switchgear, generators and transformers.

  1. Neutral Earthing Resistors (NERs) are used in an AC distribution networks to limit transient over voltages that flow through the neutral point of a transformer or generator to a safe value during a fault event such as a phase to phase, phase to neutral or phase to earth fault.
  2. Generally connected between ground and neutral of a transformer or generator, NERs reduce the fault current to a pre-determined maximum level such that damage to equipment or network shutdown is avoided while allowing a sufficient amount of fault current to activate protection devices.
  • A fast response time allows protection relays and current transformers to Auxiliary operate and quickly identify, isolate and clear the fault.
  • Subsequent faults are also avoided. Damage to equipment is therefore minimized and the risk of hazardous arc flash is reduced or eliminated.
  • Given that an NER must absorb and dissipate a huge amount of energy during a fault without exceeding temperature limitations, the design and selection of an NER is critical to ensure equipment and personnel safety as well as continuity of supply.

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