Synchronization of Turbo Alternator or an AC generator is the process of connecting the generator with grid power supply which is an interconnection of large pool of generators and power consumption loads. Simply the grid is parallel operation of some number generators with same frequency. So to connect the Generator in power plant in this pool of parallel running generators, The incoming generator parameters like frequency, phase angle and voltage should be matching with the existing grid frequency.
Before going detail description first let us understand what is the need of synchronization of generator. Generator is connected with the prime mover which provides the rotating magnetic field and hence this rotating magnetic field will induces the voltage in the stationary part. The frequency and phase angle of the voltage signal is controlled by the prime mover speed and magnitude of the voltage signal is controlled by the generator excitation.
To understand the phenomenon let us correlate the entire operation with the person wants to catch a running travel bus. Consider the travel bus is grid power supply and the person is incoming generator. Now if the person wants to get in to the bus then he should equally or little faster than the bus same the generator tries connect to the grid should run equally or little faster than the grid. Here the speed is measured with the frequency because speed is proportional to the frequency( 50 Hz, 60 Hz). The person is now running with the same speed of the bus but the bus door is one end of the bus and he is at another end of the bus so he needs to match with the door to get in to the bus. Like the same if the generator is running at the same frequency of grid it cannot be synchronized until unless the phase of the two voltages matches.
What is the need of synchronizing two different power sources?
"Suppose you have a trolley that can only drawn by either pushing or pulling it ,two workers are there to drive it if one of them is pushing in one direction but the other one is in another direction ..What will happen?
they can't move it with different speed or different direction."
Similar is gonna happen with power source.
Phase angle indicates the direction, If there is a phase difference between two power sources it can't operate any load.
In order to synchronize a generator to the grid, four conditions must be met:
1. Phase Sequence
The phase sequence (or phase rotation) of the three phases of the generator must be the same as the phase sequence of the three phases of the electrical system (Grid).
The generator or transformer power leads could actually be interchanged during maintenance orthe potential transformer leads could be interchanged during maintenance..
2. Voltage Magnitude
The magnitude of the sinusoidal voltage produced by the generator must be equal to the magnitude of the sinusoidal voltage of the grid.
If all other conditions are met but the two voltages are not the same, that is there is a voltage differential, closing of the AC generator output breaker will cause a potentially large MVAR flow.
3. Frequency
The frequency of the sinusoidal voltage produced by the generator must be equal to the frequency of the sinusoidal voltage produced by the grid.
The synchroscope would be rotating rapidly counter clockwise. If the generator breaker were to be accidentally closed, the generator would be out of step with the external electrical system. It would behave like motor and the grid would try to bring it up to speed.
In doing so, the rotor and stator would be slipping poles and damage (possibly destroy) the generator as described previously. The same problem would occur if the generator were faster than the grid.
4. Phase Angle
As previously mentioned, the phase angle between the voltage produced by the generator and the voltage produced by the grid must be zero.
The phase angle (0 to 360°) can be readily observed by comparing the simultaneous occurrence of the peaks or zero crossings of the sinusoidal waveforms..
Effects of poor synchronization:
Prime mover damages if the speed and rotor angle is not matches with grid voltage frequency and phase angle due to rapid acceleration or deceleration. Let us suppose generator has to connected to the grid frequency of 60 Hz. But the breaker has closed with poor synchronization at the generator frequency of 58Hz (i.e for two pole generator speed is 3480 out of 3600 rated), now once the breaker closes the generator is connected in the pool of parallel generators which forces the incoming generator to rotate at the same grid frequency. Due to this sudden acceleration of the rotor from 3480 to 3600 rpm and a sudden break at 3600 rpm damages the rotor mass. Same way in the reverse when the generator is running higher frequency than the grid frequency.
A large currents may suddenly flow through the Generator windings and Generator transformer windings due to poor synchronizations which damages the windings.
There will be power and voltage oscillations because of this sudden acceleration and deceleration of the rotor.
It may leads to activation of the generator protective relays which causes the major interruption so the process should be started once again after clearing the protection.
