I. The impact closing test of the transformer does not necessarily have to be conducted from the high voltage side, which is related to the application of the transformer. Generally, this test is combined with the operation of the transformer. Since most of the transformers we use are step-down transformers, the incoming call is naturally from the high-voltage side, so it can only hit 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 side. For the main transformer with reverse power transmission capability, it can be done from the high voltage side.

There will definitely be excitation inrush current when the transformer is charged at full voltage, but the magnitude is different each time. The size of the excitation inrush current, residual magnetism, and closing angle (non-periodic component) factors are all controlled! The result is: the maximum voltage doubles, the magnetic flux doubles, oversaturation, and the current suddenly increases.

II. Number of impact tests:

Before the main transformer is put into operation for the first time, it should be surge-closed five times at rated voltage. The duration after the first power supply should be no less than 10 minutes, and the interval between each time should be more than 5 minutes. After the overhaul, the main transformer should be impacted three times; the float under gas should be tripped before the main transformer is impacted and closed. The impact closing is normal. If conditions permit, it should be charged for 24 hours without load; when starting the transformer of 110 kV and above, it should be used if conditions permit. Voltage boost from zero; the on-load voltage regulating device of the transformer should be put into use after a switching test is normal when the transformer is put into operation.

III. The reasons why new transformers or transformers after overhaul need to be subjected to impact tests before they are officially put into 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 overvoltage occur when switching off a no-load transformer? What measures are generally taken to protect the transformer?

Theoretically, removing any inductive load will produce operating overvoltage;

Because the inductive load has an inductance L, and the energized inductive load has a magnetic field Φ, it also has electromagnetic energy W. This is a parameter that cannot change suddenly (W=1/2*L*I*I). When the current is cut off, the current It will not change to 0 instantly. There is a short time process dt. According to Faraday's law of electromagnetic induction E=-LdI/dt, because dt is very small, a very high voltage will be induced in the coil. This is the operation. Overvoltage; in addition to its value related to the performance of the switch, the structure of the transformer, etc., the grounding method of the neutral point of the transformer also affects the overvoltage of the no-load transformer. Generally, for ungrounded transformers or transformers grounded by arc suppression coils, the overvoltage amplitude can reach 4-4.5 times the phase voltage, while for transformers with the neutral point directly grounded, the operating overvoltage amplitude generally does not exceed 3 times the phase voltage. This is also the reason why the neutral point of the transformer required for impact testing is directly grounded.

In a system with a directly grounded neutral point, when the 110∽330 kV no-load transformer is disconnected, the overvoltage multiple generally does not exceed 3.0Uxg. In a 35 kV power grid with an indirectly grounded neutral point, the overvoltage multiple generally does not exceed 4.0Uxg. , at this time, a valve-type 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 arrester, this protection is reliable and can be used in non-lightning situations. Seasons should not quit either. )

2) Assess the mechanical strength of the transformer under the action of large excitation inrush current and assess whether the relay protection will malfunction under the action of large excitation inrush current.

IV. There are two purposes for conducting impact closing test on transformer:

1. When unloading the transformer, operating overvoltage may occur. When the neutral point of the power system is not grounded or is grounded through an arc suppression coil, the overvoltage amplitude can reach 4 to 4.5 times the phase voltage; when the neutral point is directly grounded, the overvoltage amplitude 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 the unloaded transformer is powered on, an excitation inrush current will occur, which can reach 6 to 8 times the rated current. The excitation inrush current begins to decay quickly, generally reducing to 0.25 to 0.5 times the rated current value in 0.5 to 1 second. However, the overall decay time is longer, and it can reach tens of seconds for large-capacity transformers. Since the excitation inrush current generates a large electric force, in order to assess the mechanical strength of the transformer and at the same time whether the attenuation of the excitation inrush current can cause malfunction of the relay protection device, an impact test is required.

(Reference: First of all, we must understand why the transformer needs to undergo a no-load closing test before it is officially put into operation. The reasons are:

1. This is to replace the lightning impulse test with the operating overvoltage test. There are lightning impulse generators in transformer manufacturing plants. It is impossible to have it at the installation site.

2. However, when the transformer is in operation, it will indeed experience lightning impact and operating overvoltage impact. This is the insulation performance index that the transformer must meet.

3. When doing lightning impact in the transformer manufacturing factory, there are strict indicators, such as full wave, intercepted wave and how much time. But it is impossible to have such strict and precise control on site. And in many cases, the multiple of the operating overvoltage often does not reach the multiple of the lightning impact.

4. Therefore, increase the number of closing times to compensate. Theoretically, it is known that when the switch is closed at the moment when the voltage crosses zero, the operating overvoltage multiple is the highest. We hope to get it once in five times.

5. Before the new transformer is put into operation, it must be closed without load 5 times, with an interval of not less than 5 minutes each time, so that the transformer can restore its insulation. For transformers after overhaul, the number of repairs can be 3 times. )

New transformer protects charging process

Step 1: Change the setting value to the charging setting value before charging. Put in differential protection (verify that differential protection can reliably avoid excitation inrush current.) Non-electrical protection. Other protections are put in according to the situation.

There are two general charging methods. The first one is to use the main transformer itself to switch the charging transformer, cancel the backup overcurrent protection locking conditions (directional element, re-voltage locking element), and become a pure overcurrent protection. The time is generally set to 0.2 seconds. Or 0.3 seconds. This time cannot avoid the need to increase the current or extend the action time according to the superior protection setting when charging the transformer. The second is to charge using sectional or bus tie switches. The charging setting is the same as above.

Step 2: After charging, the differential protection should be turned off before loading. After the phase measurement under load is correct, the differential protection should be turned on.

Step 3: The fixed value is restored to the official fixed value.

Only changes in the current circuit of the main transformer differential protection will change the backup protection settings.

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

HZYDP-80KV VLF Hipot Tester Integrated Machine