Can transformers be operated in parallel?

Table of Contents

    In technical terms, the so-called parallel operation of transformers is called transformer paralleling.
There are conditions for parallel operation of transformers, which are: 1) the primary circuit of the transformer must be the same grid; 2) the wiring of the transformer must be the same, and the impedance voltage must be the same; and 3) there must not be too much deviation in the voltage of the low-voltage side windings of the transformer.

    Of these 3 conditions, the first one is the most important. If the primary circuit of the transformer comes from different grids, a major accident will occur once the transformer is paralleled. In addition, the relay protectionon the medium voltage sidewill operate and the transformer will not be able to be put into operation at all. The second is also necessary, if the wiring ofthe power transformeris not the same, for example, one is a 12-point connection, the other is an 11-point connection, and there is aphase difference on the low-voltage side of the transformer, the transformer is strictly prohibited from being paralleled.
Let’s look at the following diagram:

transformers be operated

    Figure 1: What will happen if two transformers run in parallel and a short circuit occurs at the load?

    We can see from Figure 1 that there are two power transformers T1 and T2 in the system. If we close the two incoming circuit breakers QF1 and QF2 and the bus tie circuit breaker QF3, the transformers T1 and T2 will be operating in parallel.

    When the transformers were running in parallel, we saw a short circuit accident occurred at the load of the I busbar. Note that transformer T1 will inject short-circuit current into the short circuit, and transformer T2 will also inject short-circuit current into the short circuit. It can be seen that if the power transformers must be operated in parallel during the project, the ultimate short-circuit breaking capacity Icu of the feeder circuit breaker of the power supply and distribution equipment must be twice as large as the Icu of the incoming circuit breaker! This will increase the supporting costs of power distribution equipment.

    Let’s take the 630kVA transformer as an example to illustrate. As for the provided transformer capacity of 630VA, is the k here deliberately omitted, or what does it mean? If the transformer capacity is only 630VA, it should be an isolation transformer used for load driving, right? !

    Rated current In and short-circuit current Ik of 630kVA power transformer:

Rated current: In=Sn3Un=6301.732×0.4≈909AI_n=\frac{S_n}{\sqrt{3}U_n}=\frac{630}{1.732\times 0.4}\approx 909A

Short circuit current: Ik=InUk%=9090.06×10−3≈15.2kAI_k=\frac{I_n}{U_k\%}=\frac{909}{0.06}\times 10^{-3}\approx 15.2kA

    Therefore, for the incoming circuit breaker of a 630kVA power transformer, its rated current must be greater than 909A, and 1250A is optional; its ultimate short-circuit breaking capacity Icu must be greater than 15.2kA, and 25kA is optional. As for the feed circuit breaker in the power distribution equipment, because the system bus will limit the short-circuit current, its ultimate short-circuit breaking capacity Icu can be selected to be equal to or slightly less than 15.2kA.
    Now, we let two 630kVA transformers run in parallel. The ultimate short-circuit breaking capacity Icu of the incoming circuit breaker is still 25kA, while the ultimate short-circuit breaking capacity of the feeder circuit breaker must be equal to or slightly less than 2×15.2=30.4kA. Optional 30kA. It should be noted that the molded case circuit breaker with the ultimate short-circuit breaking capacity Icu=30kA is much more expensive than the one with Icu=15kA! Since there are many feeder circuits, the manufacturing cost of power distribution equipment will increase.
    Now, if you want to run three 630kVA transformers in parallel, you should think about how to select the feeder circuit breakers.
    The following two pictures are for reference and reflection:

transformers be operated

Figure 2: Three transformers in parallel operation

Figure 3: Three transformers operating in parallel2
    What is the difference between the short-circuit currents occurring on the bus and in the feeder circuits in Figures 2 and 3? What is the maximum value? How should the feeder circuit breakers be selected for their ultimate short-circuit breaking capacity?
    As to whether the neutral N can be connected together, it depends on the form of grounding. If TN-C grounding system is adopted, the zero line PEN can be connected together. Note: There is no ground PE in the TN-C grounding system. if TN-S grounding system is adopted, it is recommended to adopt TN-C-S to connect the neutral wires together under the incoming circuit breaker of the distribution equipment, i.e., inside the distribution equipment, to construct a uniform neutral copper row. At the same time the neutral wire is directly grounded and leads to a uniform ground copper row, which is a little better.

    Introducing the similarities and differences between self-transfer and reversing operations
We call the mutual throw operation of two transformers and single busbar sectionalized system as self-transmission, and the operation of transformer parallel operation is called reverse gate. Obviously, the transformer parallel process has both self-provisioning process and reverse gate process.
    The design must ensure that the two parallel operation of the power transformer to meet the parallel conditions. If a violation of the reversal requirements occurs during the operation, the reversal is immediately terminated and the transformer is returned to self-provisioning.
The following are the closing conditions for each circuit breaker under self-provisioning:

Closing QF1. closing = QF2¯ + QF3¯ QF_{1. closing} = \bar{QF_2} + \bar{QF_3}

Close QF2.close=QF1¯+QF3¯QF_{2.close}=\bar{QF_1}+\bar{QF_3}

Close QF3.close=QF1¯+QF2¯QF_{3.close}=\bar{QF_1}+\bar{QF_2}

Note that to the right of the equal sign, there is a horizontal line at the top of each term, indicating that each term here is a normally closed contact.

Figure 4: Self-provisioning control circuit for 1-section incoming circuit breaker

In Figure 4, the sky blue area on the right is the self-provisioning interlocking logic.

Figure 5: Self-provisioning control circuit for a busbar circuit breaker


    In Figure 5, the second green area on the right is the self-provisioning interlocking logic.
For reverse gate operation, it is clear that the above three self-provisioning formulas will fail, and the interlocking logic in Figure 4 and Figure 5 must be shorted to undo. Therefore, when designing the circuit control logic or PLC measurement and control logic, there must be a choice to determine whether to reverse the operation or self-provisioning, and self-provisioning is the priority.
When the reverse gate is in effect and put into operation, closely monitor the incoming current and closely monitor the short circuit at the load side. Since the short-circuit current at the load side is larger than the short-circuit current at the feeder side, the faster the short-circuit current is disconnected, the faster the better, and the feeder circuit adopts a current-limiting circuit breaker if necessary.

What are the conditions for transformer parallel operation?

First, let’s see what the demand for transformer paralleling is, and then we will discuss the conditions for paralleling.

(1) General conditions

Look at the following figure:

    In this diagram we see two transformers, labeled T1 and T2.In actual operation, the two bus sections are fed by their respective transformers. So, both incoming circuit breakers QF1 and QF2 are closed, while the bus breaker QF3 of the single bus section is open; if there is a problem on the transformer or MV side of one of the incoming sections, such as a serious voltage depression (under-voltage or loss of voltage) or a fault, the incoming circuit breaker for that section is opened, and then the bus breaker QF3 is closed; when the system is restored, there are two methods of restoration:
    Recovery method 1: Open the busbar circuit breaker QF3 and then close the corresponding incoming circuit breaker. This method is simple, but the loads on the busbar such as motors need to go through a power failure restart again after experiencing a power failure restart.
Recovery method 2: First close the corresponding incoming circuit breaker, at which time the transformer is running in parallel, and then open the busbar circuit breaker. This method is slightly more complicated, but the load does not have to go through a second power failure restart.

Let’s look at the conditions for paralleling the transformer:
First: the conditions of the transformer itself
Including: transformer wiring method and ratio consistent, transformer impedance voltage consistent, transformer secondary voltageconsistent.
Second: line conditions
Including: the medium voltage side must come from the samedistribution network, their phase,initial phase angleand frequency are consistent, and the voltage amplitude is also consistent. At the same time,the medium-voltage sidemust be able to withstand the up-start shockof the low-voltage side.
(2) System equipped with a generator
Let’s look at the following diagram again:

This diagram is a little more complex than the previous one, in that it has a captive generator and there is an interlocking and mutual throwback relationship between the generator’s circuit breaker and the utility’s incoming circuit breaker. Because of the complexity of the throwback relationship, at ABB, PLCs are often used to build the throwback logic. Let’s describe it briefly:


1) During normal operation the busbar is open and the incoming lines of each section are closed.
(2) If a section of the utility power supply failure, then open the section of the feeder, followed by the closure of the busbar.
3) When the fault is lifted and restored, the system is restored in both transformer-parallel and non-parallel ways. Transformer paralleling conditions are the same as above.
(4) If a section of the utility power supply failure is not restored, and another section of the utility power supply fails, or both sections of the utility power supply fails at the same time, then the system starts the generator. Depending on the generator starting operation, it is decided whether the busbar is put into operation or not, and the situation is similar to the utility power supply.
(5) When the utility power is restored, there are two ways to deal with it: the first method is shown in Figure 2, the utility inlet line and the generator inlet line are interlocked, allowing only one side to be closed. At this time, the generator inlet line will be opened, and then close the utility inlet line can be; the second method of utility inlet line and generator inlet line has no interlocking relationship. After the utility power is restored, under the guidance of the system, the generator will do quasi-simultaneous processing to the utility power, and then close the utility power inlet line, and then evacuate the generator.
The second method prevents the load from restarting with a second power failure.
We see that the conditions for transformer paralleling are the same as in the general case.
(3) singletransformerload capacity is insufficient when the parallel operation of transform ers
Transformer parallel conditions as before.
In this condition, once the load side of the short-circuit, the short-circuit current value should be multiplied by the number of transformers in parallel. We look at the following figure:

    In the figure both feeders and busbars are closed and transformers T1 and T2 are in parallel operation.
When a short-circuit occurs in the load of a section of bus, both transformers contribute short-circuit current to the short-circuit point, so that the short-circuit current at the load is equal to twice the short-circuit current of a single transformer.
    It is therefore a condition for parallel operation of the transformers that thebreaking capacityof the feeder circuit breakerson each bus sectionmust be twice that of the incoming circuit breakers. If this is not done, the transformers are not allowed to operate in parallel.
    The code provides that for parallel operation of transformers during a short period of inverting operation, the breaking capacity of the load-side circuit breakers may be selected under general conditions without doubling.

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