GENERATOR SYNCHRONIZATION (PARALLEL) OPERATION

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Need of Paralleling of Generators

In most commercial power plants, several small units supply the power rather than single large unit. This is called as Parallel Operation of Generators. The reasons for preferring this practice are enumerated below.

Reliability

Several small units are more reliable than single large unit. This is because, if one alternator is failed, other alternators are still active and hence the whole system will not be shutdown.

Continuity of Service

In case of periodic maintenance, break-down or repairs of one alternator, it must be shutdown and removed from service. Since the other machines are operating in parallel, the interruption to supply the load is prevented.

Load Requirements

The load requirements in the central station changes continuously. During light-load periods, only one or two generators are operated to supply the load demands. During peak-load demands, additional alternators are connected in parallel to meet the demand.

High Efficiency

Generators run most efficiently when they are loaded at their rated values. Due to the operation of few generators at light-loads and more generators at high peak loads efficiently loads the generators.

Expanded Capacity

As the demand for electric power is increasing continuously, utility companies have been increasing the physical size of the generating plants by adding more alternators. So, these alternators have to be connected in parallel with the existing generator equipment.

Conditions for Synchronization or Paralleling of Generators

There are certain requirements that must be met for successful paralleling of alternators. The following conditions must be met in order to synchronize a generator to the grid or with other generators.

Phase Sequence

The phase sequence of the three phases of the alternator which is being connected to the power system bus must have the same phase sequence as that of the three phases of the bus bar (or electric grid). This problem comes mainly in the event of initial installation or after maintenance.

Voltage Magnitude

The RMS voltage of the incoming alternator should be same as the RMS voltage of the bus bar or electric grid. If the incoming alternator voltage is more than the bus bar voltage, there will be a high reactive power that flows from the generator into the grid.

If the incoming alternator voltage is lower than the bus bar voltage, generator absorbs the high reactive power from the bus bar.

Frequency

The frequency of the incoming generator must be equal to the frequency of the bus bar. Improper matching of frequency results in high acceleration and deceleration in the prime mover that increases the transient torque.

Phase Angle

The phase angle between the incoming generator voltage and voltage of the bus bar should be zero. This can be observed by comparing the occurrence of zero crossing or peaks of the voltage waveforms.

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