Synchronous machines are frequently operated in parallel to meet the varying load demands of power systems. This allows for greater reliability, efficiency, and flexibility compared to operating a single large machine. Understanding the conditions for successful parallel operation and how load is shared is crucial for power system engineers.

For two or more synchronous generators to operate successfully in parallel, several conditions must be met. These ensure that the machines synchronize properly and share the load without causing undesirable circulating currents or mechanical stress.

The incoming machine's frequency must match the busbar frequency before it can be connected. If frequencies differ, a large circulating current will flow, causing oscillations and potential damage. The prime mover speed controls this.

The voltage of the incoming machine must be equal to the busbar voltage. If there's a voltage difference, a reactive current will flow, leading to reactive power exchange and potential voltage instability. The excitation voltage of the incoming machine is adjusted to match.

The phase sequence of the incoming machine must be identical to that of the system. If the phase sequences are different, connecting the machines will result in a phase reversal, causing a large current to flow, equivalent to a short circuit, which can damage the machines and protective relays.

Ideally, the phase angle difference between the incoming machine and the busbar should be zero at the moment of connection. If there's a phase angle difference, a power surge will occur, causing transient oscillations. This is typically achieved by adjusting the prime mover speed of the incoming machine.

Once connected in parallel, synchronous generators share the total load of the system. The distribution of this load depends on the characteristics of the prime movers and the excitation of the machines.

The real power (kW) supplied by each synchronous generator is determined by the power output of its prime mover. If the prime movers have drooping speed-load characteristics (common in diesel engines and turbines), the load will be shared proportionally to their rated capacities. A machine with a steeper droop characteristic will take a larger share of the load at a given frequency.

The reactive power (kVAR) supplied by each synchronous generator is controlled by its excitation voltage. Machines with higher excitation voltages will supply more reactive power and tend to control the busbar voltage. If the excitations are not matched, circulating currents will flow, leading to inefficient operation and potential overheating.

Synchronization is the process of bringing an incoming synchronous generator into phase with the running system before closing the circuit breaker. This involves matching frequency, voltage, and phase angle.

In some systems, one generator (the master) dictates the system frequency and voltage, while other generators (slaves) follow. The master's prime mover governor is typically set to a fixed speed, and its excitation is adjusted to control voltage. Slave generators adjust their prime mover speed and excitation to match the master's frequency and voltage, thereby sharing the load.