What is the operating sequence of the voltage transformer secondary switch and the high-voltage side power supply when the voltage transformer is shut down and powered on?

The operating sequence of the secondary small switch of the bus voltage transformer when the voltage transformer is shut down or powered on is generally as follows: when there is a power outage, first disconnect the secondary small switch, and then shut down the high-voltage power supply of the voltage transformer.

When transmitting power, the high-voltage side of the voltage transformer is supplied first, followed by the secondary breaker. This prevents the low-voltage side of the voltage transformer from feeding power back through the secondary breaker. This operating sequence is applicable to wiring arrangements such as dual busbars and single busbar segments, where the secondary sides of the voltage transformers may be parallel. To prevent the less likely occurrence of reverse power feed from the secondary side of the voltage transformers due to miswiring and to standardize operating procedures within a unit, voltage transformers in other wiring arrangements should also follow this operating sequence.

During the power outage operation of the bus voltage transformer with double bus or single bus segmented connection, if the secondary sides of the voltage transformers of the two busbars are connected together, first shut down the high-voltage side power supply of the voltage transformer (disconnect the bus tie switch and the segment switch); or open the high-voltage side knife switch of the voltage transformer (the secondary auxiliary node of the voltage transformer knife switch fails), and the secondary power supply of the other operating voltage transformer will be boosted and fed back to the high-voltage side of the power-off voltage transformer. The capacitive current to ground on the high-voltage side of the voltage transformer may cause the secondary small switch of the operating voltage transformer to trip (if there is a busbar with a capacitive current, it will be even greater), which will cause the line, transformer maintenance or active equipment operating on the bus to lose the secondary AC voltage. The relay protection or active equipment that is sensitive to the secondary AC voltage may malfunction and trip the output, causing equipment or power grid accidents.

Such accidents have occurred in the system. There have also been cases where the secondary small switch of the voltage transformer was not turned off first. The secondary voltage of the voltage transformer passed through the medium-voltage side switch voltage switching relay node of the transformer protection (the voltage switching relay should be disconnected after operation but was not disconnected), causing the operating secondary of the voltage transformer to be fed back to the power-off bus, causing the voltage switching relay of the transformer protection equipment to burn out and the transformer to be forced to shut down.

There are generally two types of operations for stopping and supplying power to the bus voltage transformer: one is that the voltage transformer stops and supplies power alone, and the other is that the voltage transformer and the bus stop and supply power together.

When operating a voltage transformer independently during a power outage, first open the transformer's secondary breaker, then open the transformer's high-voltage side switch. This procedure is reversed for power transmission. When operating a voltage transformer along with a busbar, first open the transformer's secondary breaker if the busbar is empty, then disconnect the busbar's bus tie or sectionalizer to shut down the busbar, and finally open the transformer's high-voltage side switch. This procedure is reversed for power transmission.

A voltage transformer (VT) is installed on the line side of a 500 kV line. This VT is directly connected to the line, and its secondary side is not connected to any other secondary power source. During power outages and maintenance on 500 kV lines, this VT is used as a key indicator for detecting the absence of voltage on the line (direct voltage testing is generally used for 500 kV equipment).

The operating sequence for power outage maintenance is as follows: shut down the power by operating the switches and circuit breakers on both sides of the line, then verify the line is voltage-free by checking the secondary voltage of the line voltage transformer to ensure there is no indication, then ground the line-side grounding switch, and finally, open the secondary circuit breaker of the line voltage transformer. The order of power restoration after line maintenance is reversed.

When powering new equipment, the issue of reverse power transmission from the voltage transformer secondary side is generally not considered. This is because the primary sides of two busbars (or sections) are not necessarily in parallel operation during charging, and the secondary sides of the busbar voltage transformers are also not suitable for parallel operation. Therefore, the charging operation of the busbar voltage transformer does not follow the "high voltage first, low voltage later" principle. Instead, the voltage transformer secondary side switch should be closed first, and then the high voltage side of the voltage transformer should be powered.

When charging a new bus voltage transformer, it is generally charged together with the bus. The operating sequence is: when the bus is not energized, first push up the high-voltage side knife switch of the bus voltage transformer, close the secondary small switch of the bus voltage transformer, and then charge the bus and the bus voltage transformer together through the switch (line switch, bus tie switch or section switch).

According to the above power transmission operation method, after charging the busbar and voltage transformer, the voltage indication can be directly detected on the secondary side of the voltage transformer as a basis for judging whether the operation is normal. If the secondary small switch of the voltage transformer is closed after charging the busbar and voltage transformer, and then the voltage indication is detected again for judgment, the correctness of the operation cannot be directly judged in time, and there is a certain risk of the operator going to the site to operate (such as the newly charged equipment malfunctioning).

With advances in technology, voltage transformers that transmit signals optically are now used in substations, eliminating the problem of the voltage transformer's secondary power being fed back to the high-voltage side. In smart substations, the voltage transformer's voltage signal is transmitted via network technology, eliminating the need for direct connection of the voltage transformer's secondary conductors.

In this case, there is no technically mandatory rule for the operating sequence of the high and low voltage sides of the voltage transformer during power-off and power-on operations. A corresponding operating sequence rule can be formulated based on operating practices.

The voltage transformer can be operated by first transmitting power to the low-voltage side and then to the high-voltage side; and by first shutting down the high-voltage side and then the low-voltage side during power outages. This operation sequence allows the voltage on the secondary side of the voltage transformer to be directly detected to determine whether the operation is normal. This makes it more intuitive and convenient to check the operation results.

During the switching operation, we should follow the principle of "weighing two benefits and choosing the better one, weighing two harms and choosing the lesser one" and arrange the operation sequence safely and reasonably according to the actual conditions of the on-site equipment to achieve a safe and smooth effect.