The parallel operation of transformers means that the primary windings of two or more transformers are connected in parallel on the busbar of the same voltage, and the secondary windings are connected in parallel on the busbar of another voltage.
Its meaning is: when one transformer fails, other transformers running in parallel can still continue to operate to ensure the power consumption of important users; or when the transformer needs to be repaired, the standby transformer can be connected in parallel, and then the transformer to be repaired is powered off and repaired. , which can not only ensure the planned maintenance of the transformer, but also ensure that the power supply is not interrupted and improve the reliability of the power supply.
In addition, due to the strong seasonality of the electricity load, some transformers can be withdrawn from operation in the light load season, which can not only reduce the no-load loss of the transformer, improve the efficiency, but also reduce the reactive excitation current, improve the power factor of the grid, and improve the efficiency of the power grid. system economy.
So what are the requirements for parallel operation of transformers? This is a very common question. First, let's take a look at what the requirements for juxtaposition of transformers are, and then discuss the conditions for juxtaposition.
(1) In general, see the figure below:
In this image, we see two transformers, labeled T1 and T2.
System recovery method:
In actual operation, the two busbars are powered by their own transformers.
As a result, both incoming circuit breakers QF1 and QF2 are closed, while the bus tie breaker QF3 of a single busbar segment is open; if there is a problem with the transformer or the medium voltage side of a certain incoming line, such as severe voltage sag (undervoltage or voltage loss) or failure, the incoming line breaker of this section is opened, and then the bus tie breaker QF3 is closed; when the system is restored, there are two recovery methods:
Recovery method 1: Open the bus tie breaker QF3, and then close the corresponding incoming circuit breaker. This method is simple, but after a load on the bus, such as a motor, needs to be restarted after a power failure.
Recovery method 2: first close the corresponding incoming circuit breaker, then the transformers run in parallel, and then open the bus tie breaker. This method is slightly more complicated, but the load does not need to go through a second power failure to restart.
Let's look at the conditions for the transformers to be juxtaposed:
First: the conditions of the transformer itself
Including: the wiring method of the transformer is the same as the transformation ratio, the impedance voltage of the transformer is the same, and the secondary voltage of the transformer is the same.
Second: line conditions
Including: the medium voltage side must come from the same distribution network, their phase, initial phase angle and frequency are the same, and the voltage amplitude is also the same. At the same time, the medium-voltage side must be able to withstand the power-on start-up shock of the low-voltage side.
(2) When the system is equipped with a generator, let's look at the following figure:
This figure is a little more complicated than Figure 1. There is a self-contained generator in the figure, and the circuit breaker of the generator and the incoming circuit breaker of the mains have an interlocking and mutual investment relationship.
Due to the complexity of the throw-out relationship, in ABB, PLC is often used to construct the throw-out logic. Let's briefly describe:
1) During normal operation, the bus tie is opened, and the incoming lines of each section are closed.
2) If the mains power supply of a certain section fails, open the incoming line of this section, and then close the bus tie.
3) When the fault is removed and restored, the system will be restored in two ways: parallel and non-parallel transformers. Transformer parallel conditions are the same as above.
4) If the failure of a certain section of the mains power supply fails to recover, and the other section of the mains power supply fails again, or two sections of the mains power fail at the same time, the system will start the generator. Depending on the starting operation of the generator, it is determined whether the bus tie is put into operation.
5) When the mains is restored, there are two ways to deal with it: the first method is as shown in Figure 2, the mains incoming line and the generator incoming line are interlocked, and only one side is allowed to be closed. At this time, open the generator incoming line, and then close the mains incoming line; the second method has no interlocking relationship between the mains incoming line and the generator incoming line. After the mains is restored, under the guidance of the system, the generator is quasi-synchronized with the mains, and then the mains incoming line is closed, and then the generator is evacuated.
The second method can prevent the load from restarting after the second power failure. We can see that the condition of parallel transformers is the same as the general situation.
(3) Parallel operation of transformers when the load capacity of a single transformer is insufficient
Transformer parallel conditions are the same as before. Under this condition, once a short-circuit occurs on the load side, the short-circuit current value should be multiplied by the number of transformers in parallel. Let's look at the picture below:
In the figure, the two incoming lines and the bus tie are closed, and the transformers T1 and T2 are in parallel operation.
When the load of a section of bus is short-circuited, both transformers contribute short-circuit current to the short-circuit point, so the short-circuit current at the load is equal to twice the short-circuit current of a single transformer.
Therefore, the conditions for parallel operation of the transformers are: the breaking capacity of the feeder circuit breaker on each section of the bus must be twice that of the incoming circuit breaker. If this is not done, the transformers are not allowed to operate in parallel.
It is stipulated in the specification that for parallel operation of transformers in a short period of switching operation, the breaking capacity of the load-side circuit breaker can be selected under normal conditions without doubling.
(4) Advantages and purposes of parallel operation of transformers
Improve the economy of transformer operation. When the load increases to the point where the capacity of one transformer is not enough, the second transformer can be put in parallel, and when the load is reduced to the point where the two transformers are not required to supply power at the same time, one transformer can be taken out of operation.
Especially in rural areas, the seasonal electricity consumption characteristics are obvious, and the parallel operation of transformers can be switched according to the size of the electricity load. In this way, the loss of the transformer itself can be minimized and the purpose of economic operation can be achieved.
Improve power supply reliability. When one of the transformers running in parallel is damaged, as long as it is quickly removed from the grid, the other or two transformers can still supply power normally; when a transformer is repaired, it will not affect the normal operation of other transformers, thereby reducing faults and damage. The scope and frequency of power outages during maintenance can improve the reliability of power supply.
Save electricity, realize electricity saving and increase efficiency. For example, a substation is equipped with two transformers of 4000kVA and 3150kVA. After calculating the operating conditions of the two transformers, after one year of parallel operation, the power saving is 102,000 Kwh, the power saving effect is very obvious, and the capital investment is reduced.