Transformer Impact Closing Test

The impact closing test of a transformer does not necessarily have to be conducted from the high voltage side, which is related to the application scenario of the transformer. Generally, this test is conducted in conjunction with the operation of the transformer. Due to the fact that most of the transformers we use are step-down transformers, the incoming power is naturally from the high-voltage side, so it can only be impacted from the high-voltage side. If the power plant's step-up transformer is on the low voltage side, it will be impacted from the low voltage. For main transformers with reverse power transmission capability, they can be installed from the high-voltage side.

There will definitely be excitation inrush current during full voltage charging of transformers, but the magnitude varies each time. The magnitude of excitation inrush current, residual magnetism, and closing angle (non periodic component) are all factors that affect the circuit! The generation is: the maximum voltage reaches twice, the magnetic flux reaches twice, supersaturation occurs, and the current suddenly increases.

Number of impact tests:

Before the first operation of the main transformer, it should be impulse closed five times at rated voltage, and the duration after the first power on should not be less than 10 minutes, with an interval of more than 5 minutes between each time. After major repairs, the main transformer will experience three impacts; The float under gas should be tripped before the main transformer impacts and closes, and the impact and closing are normal. If conditions permit, the air load can be charged for 24 hours; When starting transformers of 110 kV and above, if conditions permit, zero voltage boosting should be used; The on load voltage regulating device of the transformer should undergo a switching test before it can be put into use.

The reasons why new or overhauled transformers need to undergo impact testing before formal operation are as follows:

1) Check whether the insulation strength of the transformer can withstand the impact of full voltage or operating overvoltage.

Why does switching to an unloaded transformer generate overvoltage? What measures are generally taken to protect the transformer?

In theory, cutting off any inductive load will generate operational overvoltage;

Because inductive loads have inductance L, and energized inductive loads have a magnetic field Φ, there is electromagnetic energy W. This is a parameter that cannot jump (W=1/2 * L * I * I). When the current is cut off, the current will not instantly become zero. There is a brief time process dt in this process. According to Faraday's law of electromagnetic induction E=- LdI/dt, because dt is very small, a high voltage will be induced in the coil, which is the operating overvoltage; Its value is not only related to the performance of the switch and the structure of the transformer, but also to the grounding method of the neutral point of the transformer, which affects the overvoltage of the no-load transformer. Generally, transformers that are not grounded or grounded through arc suppression coils can have an overvoltage amplitude of 4-4.5 times the phase voltage, while transformers with a neutral point directly grounded generally have an operating overvoltage amplitude of no more than 3 times the phase voltage. This is also the reason why the neutral point of the transformer that requires impact testing is directly grounded.

In a neutral point directly grounded system, when disconnecting a 110-330 kV no-load transformer, the overvoltage multiplier generally does not exceed 3.0 Uxg. In a 35 kV power grid where the neutral point is not directly grounded, the overvoltage multiplier generally does not exceed 4.0 Uxg. At this time, a valve type lightning arrester should be installed between the high voltage side of the transformer and the circuit breaker. Since the magnetic energy of the no-load transformer winding is much smaller than the energy allowed to pass through the valve type lightning arrester, this protection is reliable and should not be withdrawn during non lightning seasons

2) Assess the mechanical strength of transformers under large excitation inrush currents and assess whether relay protection will malfunction under large excitation inrush currents.

The purpose of conducting impact closing tests on transformers is twofold:

1. When pulling open an unloaded transformer, there is a possibility of operating overvoltage. When the neutral point of the power system is not grounded or grounded through the arc suppression coil, the amplitude of overvoltage can reach 4-4.5 times the phase voltage; When directly grounded at the neutral point, it can reach 3 times the phase voltage. In order to check whether the insulation strength of the transformer can withstand full voltage or operating overvoltage, an impact test is required.

2. When an unloaded transformer is energized, it will generate excitation inrush current, which can reach 6-8 times the rated current. The excitation inrush current begins to decay rapidly, usually reducing to 0.25-0.5 times the rated current value within 0.5-1 second. However, the entire decay time is relatively long, and large capacity transformers can reach several tens of seconds. Due to the significant electric force generated by the excitation inrush current, in order to assess the mechanical strength of the transformer and whether the initial attenuation of the excitation inrush current can cause misoperation of the relay protection device, an impact test is required.

(Reference: First of all, it is necessary to clarify why the transformer needs to undergo no-load closing test before formal operation. The reason is:

1. This is using overvoltage testing to replace lightning impulse testing. There is a lightning impulse generator in the transformer manufacturing plant. And it is impossible to have it on the installation site.

2. However, transformers do experience lightning strikes and overvoltage impacts during operation. This is the insulation performance index that transformers must be able to meet.

3. When conducting lightning strikes in transformer manufacturing plants, there are strict indicators such as full wave, cutoff wave, and duration. But there cannot be such strict and precise control on site. Moreover, in many cases, the multiple of operating overvoltage often cannot reach the multiple of lightning impulse.

4. Therefore, the method of increasing the number of closing cycles is used to make up for it. In theory, it is known that the operating overvoltage multiplier is highest at the moment when the closing occurs at zero voltage crossing. We hope to have one out of five.

5. Before the new transformer is put into operation, it is closed under no-load for 5 times, with an interval of no less than 5 minutes between each time, in order to restore the insulation of the transformer. The transformer after major repairs can be used three times

New transformer protection charging process

Step 1: Before charging, change the set value to the charging set value. Implement differential protection (verify that differential protection can reliably avoid excitation inrush current) and non electric quantity protection. Other protections will be implemented according to the situation.

There are generally two types of charging methods. The first is to use the main transformer itself to switch the charging transformer, cancel the backup overcurrent protection blocking conditions (directional elements, compound voltage blocking elements), and switch to pure overcurrent protection. The time is usually set to 0.2 seconds or 0.3 seconds, which cannot avoid the transformer charging. The current should be appropriately increased or the action time should be extended according to the higher-level protection setting. The second method is to charge using segmented or bus tie switches. The charging setting is the same as above.

Step 2: After the charging is completed and before carrying the load, the differential protection should be turned off. After the phase measurement with load is correct, the differential protection should be put into operation.

Step 3: Restore the fixed value to the official fixed value.

Only changes in the current circuit of the main transformer differential protection will alter the backup protection setting.

Because after the current circuit of the main transformer differential protection is changed, the differential protection exits and the main transformer loses the main protection (differential protection) for electrical quantities. Therefore, the purpose of protecting the main transformer is achieved by shortening the backup protection time!

HZDL-7000Q 7000A Three Phase Primary Current Injection Tester