Methods for Checking Phase Sequence of Hydro-Generators

Phase sequence refers to the order in which the three-phase voltages of a generator reach their maximum values. Parallel operation of a hydro-generator with a different phase sequence from the power system is the most severe case of asynchronous paralleling, which will cause serious damage to the generator. It is necessary to check the phase sequence of newly installed hydro-generators and those that have undergone primary circuit maintenance before paralleling to prevent asynchronous paralleling. There are many methods for checking the phase sequence between a generator and the power grid, and the common ones are as follows.

1. Motor Method

Connect an ordinary three-phase induction motor to the station service power supply. First, power the station service system with the grid power supply, and record the rotation direction of the motor. Then, power the station service system with the generator, and observe the rotation direction of the motor. If the rotation direction is the same as that when powered by the grid, it indicates that the phase sequence of the generator is the same as that of the system; if the rotation direction is different, it means that the phase sequences of the two are different. If the phase sequences are different, stop the generator, swap any two outgoing lines, and then check the phase sequence again.

2. Phase Sequence Meter Method

The phase sequence meter can only be used when the voltage is below 500V. For a newly installed generator, after the first start-up without excitation, the residual voltage and phase sequence of the generator can be measured at the stator outlet of the generator. After the generator is boosted, for a high-voltage generator, the phase sequence meter can be connected to the secondary side of the voltage transformer of the busbar. Then, connect the system power supply and the generator to the busbar respectively, and observe the rotation direction indicated by the phase sequence meter. If the directions are consistent, the phase sequences are the same; if the directions are opposite, the phase sequences are different. For low-voltage units, the phase sequence meter can be directly connected to the busbar, and the method is the same as above.

3. Homemade Stationary Phase Sequence Indicator

When it is inconvenient to check the phase sequence on the busbar and station service power supply and there is no phase sequence meter, a homemade stationary phase sequence indicator can be used to check the phase sequence. According to experience, it is best to select a capacitor C with a capacitance of 8μF or more and a voltage resistance of not less than 450V; the light bulbs can be ordinary incandescent bulbs, preferably 15W bulbs produced by the same manufacturer. Conventionally, the terminal connected to the capacitor is connected to the middle phase, which is assumed to be phase B. Since the voltage vector on the capacitor lags behind its current vector, the light bulb in the next phase connected to it bears a slightly higher voltage. The brighter light bulb corresponds to phase C, and the dimmer one corresponds to phase A. For a 400V power supply, two identical light bulbs should be connected in series for each phase.

4. Voltage Transformer Method

Open the generator circuit breaker and isolating switch. Use 3 or 2 single-phase voltage transformers, connect both ends of the primary side of each transformer to the bushings on both sides of one phase of the circuit breaker, and connect incandescent bulbs with a voltage twice the secondary side voltage to the secondary side. Then, close the isolating switch, raise the generator speed and voltage to the rated values. If all the light bulbs light up and go out at the same time, it indicates that the phase sequence of the generator is the same as that of the power grid; otherwise, it is necessary to swap the positions of two outgoing phases of the generator and recheck. For power stations using high-voltage switchgear as the main generator switch, it is difficult to ensure the safe distance between the leads and the ground when using this method to check the phase sequence.

Guangxi Dingbo Generator Set Manufacturing Co.,Ltd. was founded in 1974 and is one of the earliest manufacturers of generators and diesel generator sets in China. If you want get more information, please feel free to send email to [email protected] we will pay highly attention on your question.