As we all know, the American transformer, formally known as the American box transformer and abbreviated as the American box transformer or "Mei Bian," is an important set of electrical equipment composed of transformers, high and low voltage switchgear, GGD cabinets, and other electrical components. It serves the same function as European transformers (European box transformers) but has its own characteristics. There are differences between American and European transformers; please check for yourself for details. Many users want to understand the operation methods of the American transformer. This article provides a detailed explanation of the operation instructions for the American box transformer from the manufacturer's engineers.
Manufacturer's Explanation of American Transformer Operation Methods and Steps:
I. Working Principle of the American Transformer
Parallel Operation: The high-voltage bus of two American transformers operates in parallel, and the low-voltage bus jointly supplies power to the load.
Segmentation: The high-voltage bus connects two American transformers in parallel, while the low-voltage bus connects the circuit breaker in segments.
Segmentation: One main and one backup American transformer, with high and low voltage bus connections managed by the circuit breaker.
II. Protection Configuration of the American Transformer
Main Protection: Differential protection, gas protection
Backup Protection: Overcurrent protection and zero-sequence current protection initiated by composite voltage
Other Protection: Overload protection, over-excitation protection
III. Status of the American Transformer
Power System Equipment Status:
Running Status: Circuit breakers on both sides of the American transformer are in the closed position.
Hot Standby Status: Circuit breakers on either side of the American transformer are in the open position, and both sides have circuit breakers closed.
Cold Standby Status: Circuit breakers on both sides of the American transformer are in the open position.
Maintenance Status: Circuit breakers on all sides of the American transformer are in the open position, and safety measures are in place.
State of the American Transformer:
IV. Operating Conditions of the American Transformer
Operation: The act of changing the device from one state to another.
Dispatch Terms: "From XX state to XX state."
(Manufacturer's Instructions) *Main transformer from operation to hot standby: Open the circuit breakers on both sides of the American transformer.
(Manufacturer's Instructions) *Main American transformer switched from hot standby to cold standby: Open the circuit breakers on both sides of the American transformer.
(Manufacturer's Instructions) *Main transformer from operation to cold standby: After isolating the American transformer, open the circuit breakers on both sides (connect the American transformer using a 3/2 connection or specific switch but do not pull apart the switches from both sides).
(Manufacturer's Instructions) *Main transformer from operation to maintenance: Open the circuit breakers on both sides and install grounding lines on both sides (or turn off the grounding switch).
(Manufacturer's Instructions) *Main transformer from cold standby to operation: Close the circuit breakers on both sides unless it's not allowed due to maintenance needs or clearly unusable methods.
(Manufacturer's Instructions) *Main transformer from maintenance to cold standby: Disconnect the grounding on both sides of the American transformer (or pull a knife switch off).
(Manufacturer's Instructions) *Main transformer from maintenance to hot standby: Remove the grounding wires on both sides of the American transformer (or open the grounding switch), and close the switches unless not allowed due to maintenance needs.
() *Main transformer from inspection to operation: Remove the grounding wires from both sides of the American transformer (or pull out the grounding switch). Close the switches on both sides, except for switches that cannot be closed due to maintenance requirements or are clearly unusable.
V. Power Down and Power-Up Operation Instructions for the American Transformer
Sequence for Switch and Knife Operations:
Power Failure: Open the load side switch before the power source side switch.
Power Source: Open the power source side switch before the load side switch.
Operation Sequence for the American Transformer:
When powering down an American transformer, generally, the low-voltage side is stopped first, then the medium-voltage side, and finally the high-voltage side. The order of power transmission is the opposite.
During power-down operations, each side's circuit breakers should first be operated to the open position, followed by the isolating switches operated in order from low to high (when operating isolating switches, the isolating switch on the American transformer side must be pulled first, then the busbar side switch in sequence).
For American transformers rated above 3.110KV, before charging or powering down, the neutral grounding switch of the American transformer should be closed first, and removed only after operation is completed.
For American transformers operating in parallel, first connect the high-voltage side, and if needed, connect the medium and low voltage sides. The power-down operation sequence is the reverse. The tap changer of the American transformers in parallel must be adjusted to the same position.
Tap Changer Operation Under Load:
The main American transformer tap must not be adjusted above the load and voltage levels specified by the manufacturer.
For American transformers operating in parallel, voltage adjustment operations should be performed stepwise or synchronously, and should not involve two or more consecutive tap switches for a single transformer.
For multiple American transformers operating in parallel, during power-up, operate the transformer with relatively low load current first, then operate the transformer with higher load current to avoid excessive circulating currents; during power-down, reverse the order.
Operation of Arc Suppression Coils:
Arc suppression coil compensation methods: Over-compensation, under-compensation, full compensation.
Operating procedures for arc suppression coils:
a. When there is no ground fault in the network or the neutral point displacement voltage does not exceed 50% of the normal phase voltage and current does not exceed 10 amperes, the arc suppression coil should be operated.
b. To change the working tap of the arc suppression coil, it must first be stopped; after replacing the tap, check for good contact before restarting.
c. In networks operating under over-compensation mode, if it is necessary to de-energize the circuit to reduce the setting current value of the arc suppression coil, the line should be stopped first, and then the tap should be adjusted. If an increase is needed in the setting value of the arc suppression coil due to line input, adjust the tap first, then switch on the line.
d. The arc suppression coil should not be connected to the neutral points of two or more American transformers operating in parallel simultaneously; follow the principle of closing the switches sequentially after breaking.
e. It is strictly prohibited to pull or close the arc suppression coil switch during single-phase grounding in the network.
Configuration Principle of the Neutral Point and Grounding Point of the American Transformer:
In 110kV, 220kV, and 500kV systems, the neutral point is directly grounded. The number and network location of the American transformer's grounded neutral point are determined by the comprehensive insulation safety, short-circuit current reduction, and reliable operations of relay protection requirements.
When there is power on the low voltage side of the American transformer, the neutral point must be directly grounded to prevent the high voltage side circuit breaker from tripping, turning the American transformer into a neutral point insulated system.
If multiple American transformers are running in parallel, only one or two of the transformers' neutral points will be directly grounded during normal operation. While running, at least maintain the original number of directly grounded neutral points.
The neutral point of auto-transformer configurations should be grounded directly during operation.
For American transformers running with neutral points, enable zero-sequence overcurrent protection, and disable zero-sequence overvoltage and gap zero-sequence overcurrent protection; for those operating with neutral points, enable zero-sequence overvoltage and gap-zero sequence overcurrent protection (if no gap, and gap zero-sequence overcurrent protection is not enabled, disable zero-sequence overcurrent protection).
VI. Precautions for Operating the American Transformer
Before charging or stopping an unloaded American transformer in a system directly grounded at the neutral point, the neutral grounding switch must be turned off. Once normal, adjust the grounding point in accordance with the relay protection regulations.
For operation at the end of long high voltage lines with unloaded American transformers, due to capacitive effects, the terminal voltage of ultra-high voltage long lines will increase. Therefore, before operation, lower the starting voltage of the line and include it in the terminal station's reactor to avoid excessive American transformer voltage and prolonged running time with high short-circuit voltages.
During switching on or off of the American transformer, consider the changes in the network grounding point and arc suppression coil compensation to ensure correct operation of the neutral point.
When operating American transformers in parallel, ensure that the grounding switch of the non-grounded neutral point is closed before opening another American transformer's neutral grounding switch.
During the charging of the American transformer, complete relay protection should be in place.
When the 3/2 connections in the substation are disrupted, first cut off the main American transformer, then restore the bus or switch in series operation, and switch on the American transformer protection tripping board.
When switching American transformers, check that the merged American transformer is not loaded before stopping the other parallel transformers. Avoid overloading the operating American transformers.
When a new American transformer is put into operation, it should be closed five times at rated voltage. Transformers that have had their coils replaced should be closed three times at rated voltage. Generally, the impact closure should be performed with voltage applied to the high-voltage side of the American transformer, and the voltage applied must not exceed 105% of the rated voltage of the transformer tap switch.
Parallel American transformers must meet the conditions for parallel operation.
This concludes the detailed explanation of the operation methods for the American transformer provided by the manufacturer. If you have any further questions, feel free to contact our manufacturer.
