15 Fundamental Questions about Power Transformers

1.The Importance of Transformer Core Grounding The core must have and can only have one point of grounding to eliminate the floating potential and avoid discharge faults. Grounding at two or more points will generate circulating currents, causing the core to heat up. In severe cases, the silicon steel sheets need to be replaced for repair. Proper grounding ensures the safe operation of the transformer!

2.The Protection Scope of the Gas Protection
The gas protection is mainly used to protect the transformer from the following faults:
Multiphase short circuits inside the transformer;
Turn-to-turn short circuits, short circuits between the winding and the core or the casing;
Core faults;
Drop in the oil level or oil leakage;
Poor contact of the tap changer or insecure soldering of the wires.

3.Handling of Main Transformer Cooler Failures
When the main transformer cooler fails, the following measures should be taken:
If the working power supplies of Section I and Section II are lost, report to the dispatcher immediately and deactivate the relevant protection;
When the switching of the working power supply fails, report to the dispatcher and switch manually in the same way;
If any circuit of the cooler fails, isolate the faulty circuit.

4.The Conditions for Parallel Operation of Transformers
When transformers are operated in parallel, the following conditions need to be met; otherwise, faults such as circulating currents or short circuits may occur:
The same voltage ratio;
The percentage impedance is consistent;
The same connection group.

5.The Reasons for Abnormal Noise of Transformers
Abnormal noise from the transformer may be caused by the following reasons:
Overloading;
Poor internal contact or sparking due to discharge;
Loosening of individual parts;
System grounding or short circuit;
Large load changes caused by the starting of large motors.

6.Adjustment Limitations of the Tap Changer of the On-load Tap Changer Device
Under the following circumstances, it is not allowed to adjust the tap changer of the on-load tap changer device of the transformer:
The transformer is operating under overload (except in special cases);
The light gas protection frequently gives signals;
There is no oil in the oil gauge;
The number of voltage adjustments exceeds the specified value;
The voltage adjustment device malfunctions.

7.The Meanings of the Rated Values on the Transformer Nameplate
The rated values on the transformer nameplate include:
Rated capacity: The guaranteed value of the output capacity of the transformer under the rated condition;
Rated voltage: The guaranteed value of the terminal voltage under no-load conditions;
Rated current: The line current calculated according to the rated capacity and rated voltage;
No-load current: The percentage of the exciting current to the rated current during no-load operation;
Short-circuit loss: The active power loss when one side of the winding is short-circuited and the other side of the winding reaches the rated current;
No-load loss: The active power loss during no-load operation;
Short-circuit voltage: The percentage of the applied voltage to the rated voltage when one side of the winding is short-circuited and the other side of the winding reaches the rated current;
Connection group: The connection mode of the primary and secondary windings and the phase difference between the line voltages.

8.The Relationship between the Current Source Inverter and the Transformer Capacity
Why does the current source inverter require a larger transformer capacity?
Because the power factor of the input-side transformer is equal to that of the asynchronous motor. Compared with the voltage source inverter, it requires a larger rated capacity.

9.Methods to Improve the Efficiency of Transformers
Select transformers with low losses, high efficiency and energy conservation;
Select a transformer with a reasonable capacity according to the load situation;
Keep the average load factor of the transformer greater than 70%;
When the average load factor is less than 30%, consider replacing it with a transformer of a smaller capacity as appropriate;
Improve the load power factor;
Allocate the load reasonably and reduce the number of operating transformers.

10.Influencing Factors of Transformer Capacity
The capacity of a transformer is mainly related to the heat generation. The selection of the core is related to the voltage, and the selection of the wires is related to the current.
In addition, the nominal capacity of the transformer is also related to the allowable temperature rise.

11.Technical Transformation of High-energy Consumption Distribution Transformers
The iron losses and copper losses of high-energy consumption distribution transformers (such as SJ, SJL, SL7, S7 series) are higher than those of the S9 series transformers.
Replacing high-energy consumption transformers with new-type transformers (such as 510, S11 series or amorphous alloy transformers) can significantly improve the energy conversion efficiency and save electrical energy.
12.The Generation and Hazards of Eddy Currents
Eddy currents are the induced currents generated inside the whole conductor in the alternating magnetic field generated around the wire when the alternating current passes through the wire.
Eddy currents not only consume electrical energy and reduce the efficiency of electrical equipment but also cause the electrical appliances to heat up. In severe cases, it will affect the normal operation of the equipment.
13.The Selectivity of the Instantaneous Protection of Transformers
The instantaneous protection of transformers needs to avoid the low-voltage short-circuit current to ensure the selectivity of the operation of the relay protection.
If the maximum short-circuit current on the low-voltage side is not avoided, the protection scope will be expanded to the low-voltage outgoing lines, and the selectivity will be lost.
14.Considerations for the Neutral Point Grounding of Transformers in Parallel Operation
It is not allowed for the neutral points of two transformers in parallel operation to be grounded simultaneously, mainly considering the coordination of the zero-sequence current and zero-sequence voltage protection.
The method of grounding the neutral points of some transformers for operation can limit the level of the grounding fault current and improve the sensitivity of the system zero-sequence current protection.
15.The Purpose of the Impulse Closing Test of Transformers after New Installation or Overhaul
Transformers after new installation or overhaul need to undergo an impulse closing test before being put into operation to check whether their insulation can withstand the rated voltage and the switching overvoltage during operation.
At the same time, consider the influence of the electrodynamic force generated by the inrush current on the mechanical strength of the transformer and the relay protection.