Mains paralleling application

Two diesel generating sets are being used by electricity suppliers in the UK for live paralleling operations. The units are equipped with Cummins' Power Command Control (PCC) system allowing work on high voltage mains equipment to be carried out in complete safety and without interrupting the supplies to customers.

The two Cummins Power Generation 250 kW gensets have been supplied to regional electricity companies (recs) in the south and south-west of England by Power Electronics (Bristol). The recs regularly hire the gensets whenever maintenance is required on overhead lines and transformers.

Each generating set is a self-contained portable unit comprising a Cummins NT855G6 diesel engine, a Newage alternator, a base fuel tank, and the Power Command Control system. The units are installed in a silenced acoustic housing.

Each genset has an output of 312 kVA, 415 V at 50 Hz. Connection to the mains is either via the busbar of the transformer or directly to the overhead cables, using special live line clamps.

An umbilical lead is temporarily connected to the mains, providing the PCC system with a reading of the sine wave from the mains supply. The PCC system can then gradually increase or reduce the engine speed until the alternator output matches the mains cycle.

The synchronization process is entirely automatic, and the system will only allow the two supplies to run in parallel when they are within specific parameters. This eliminates fluctuations in the supply.

When the cycles are identical, the genset takes over the load and the mains supply is disconnected. When the maintenance work is complete, the mains supply is restored and the PCC system re-synchronizes the genset output to that of the mains before the unit is shut down.

The PCC system allows much greater precision than with other methods of live paralleling. It also combines control of the fuel and the alternator excitation to achieve a soft ramp without excess fuel or overshoot. Black smoke emissions are therefore reduced.