Paralleling means to conect a generator (parallel connection) to the grid.
Usually, there are more generators conected to a grid. This is one of the reasons why a hydro power plant operates at least with two hydro generators conected in parallel either direct at the terminals or after the step-up transformers.
Paralleling a generator with a voltage grid without a sudden raise of current is accomplished using the synchronization method. With this method both during and after the established connection, the circulating currents between the hydro generator and network should be negligible.
This is the reason why we should respect a few conditions:
- both hydro generator and network voltages should be:
- equal in value,
- have the same frequency and
- have the same phase;
- the hydro generator's phase should have the same name with the network's phase (4).
First of all we bring the hydro generator at the rated speed using the turbine's governor.
By regulating the speed with the hydraulic turbine governor, we bring the frequencies at the same value.
Having the frequency at the terminals of the same value with the one of the grid, we excite it using the excitation system until we reach the rated voltage at the terminals of the stator.
We bring the voltages at the same value using the Automatic Voltage Regulator (AVR) by regulating the field current. This condition is verified using two voltmeters, one of them conected to the terminals of the generator, and the other one to the grid.
To check out the conditions mentioned above, we usually use devices called synchronoscopes like the one on the right side picture.