Method for checking generator and grid phase sequence

The phase sequence refers to the sequence in which the three-phase voltage of the generator reaches the maximum value. The phase sequence of the generator and the system is not simultaneous and parallel, which is the most serious case in which the generator will be seriously damaged. The phase sequence must be checked before juxtaposition between the newly installed generator and the generator that has overhauled the primary loop to prevent non-simultaneous juxtaposition. There are many ways to check the phase sequence of generators and grids, and the common ones are as follows.

1.Motor method

An ordinary three-phase induction motor is connected to the factory power supply. First, the system supplies electricity to the factory and records the rotation direction of the motor. Then, the generator supplies electricity to the factory and observes the rotation direction of the motor. If the steering is the same as the grid, the generator is in the same phase sequence as the system. If the steering is different, the phase sequence is different. When the phase sequence is different, stop the generator and switch any two outlet lines to check the phase sequence again.

2. Phase sequence table method

The phase sequence meter can only be used when the voltage is less than 500V. For the newly installed generator, the residual voltage and phase sequence of the generator can be measured at the exit of the generator stator under the condition that the excitation is not added after the first start. After the generator voltage boost, for the high voltage generator, the phase sequence table can be connected to the secondary side of the busbar voltage transformer, and then send the system power supply and the generator to the busbar respectively. Observe the rotation direction indicated by the phase sequence table. If the direction is consistent, the phase sequence is the same; if the direction is opposite, the phase sequence is different. For low voltage unit, the phase sequence table can be directly connected to the busbar, the method is the same as above. For a power station with several generators connected to a busbar, the phase sequence meter can be connected to the secondary side of the generator voltage transformer, disassemble the generator cable lead connector (keep a safe distance), and then close the isolation switch and circuit breaker of the generator. The system provides power and records the rotation direction of the phase sequence meter. Then open the generator circuit breaker and isolation switch, restore the cable connection, start the generator to raise speed and boost voltage to the rated value, then the voltage transformer connects to the generator power supply, observe the turn of the phase sequence table and compare with the last record, can judge whether the generator and the grid phase sequence is the same.

3.Self-made stationary phase sequence indicator

When it is inconvenient to check the phase sequence on the busbar and the factory power supply and there is no phase sequence table, the wiring is made up of stationary phase sequence indicator to check the phase sequence. According to experience, it is best to choose capacitor C above 8μF, withstand voltage not less than 450V; Light bulbs can be ordinary incandescent bulbs, preferably 15W bulbs from the same manufacturer. It is customary to connect the capacitor terminal to the middle phase, assumed to be phase B. Because the voltage vector on the capacitor lags behind its current vector, the voltage of the bulb behind it is slightly higher. The brighter bulb is phase C and the darker phase A. For each 400V power supply, use two identical bulbs in series. After the first measurement, it is best to measure the bulb position again after exchanging each other. After the bulb exchange, the original bright strain will be dark, and the dark strain will be bright, so as to prevent the error of judgment due to the different characteristics of the bulb. If the luminance of the two phase bulb is the same, it indicates that the phase-break line of the capacitor is connected.

If there is no suitable capacitor on site, the electromagnetic winding of the contactor can be used instead, the iron core armature is closed and tied with cotton thread. If there is A central phase of the electromagnetic winding, let's say phase B, because the voltage vector on the inductance is ahead of the current vector, then the light bulb is phase A, and the dark is phase C.

4. Voltage transformer

Pull the generator circuit breaker and isolation switch apart, use 3 or 2 single-phase voltage transformers, connect the two ends of the primary side of each to the bushings on both sides of the first phase of the circuit breaker, and connect the secondary side to an incandescent bulb with a voltage twice that of the secondary side. Then close the isolation switch and raise the generator speed and voltage to the rated value. If all the bulbs are on and off at the same time, the generator and the grid phase sequence is the same. Otherwise, the two-phase position of the generator outlet line must be switched and rechecked. It is difficult to guarantee the safe distance between the lead wire and the ground when the phase sequence is checked by this method for the power station using the high voltage switchgear as the main switch of the generator.

For low-voltage units, you can not use the voltage transformer, but each phase should have two 220V bulbs in series, straddle on both sides of the circuit breaker, as long as the generator and the grid phase sequence is consistent, and the frequency and voltage are very close, three groups of indicating bulbs will continuously light and dark at the same time, otherwise the three-phase light is not the same dark change, but rotating light.