Transition Between Paralleling and Stand-alone Operation:
Requirements for a Turbine-Driven Synchronous Generator

When the utility power fails, a turbine-driven synchronous generator can provide emergency back-up power. The plant as a whole, however, must be prepared to coordinate with the turbine generator in providing that power. For instance, provisions must be made for maintenance of sufficient steam supply. Unless the generator capacity is great enough for the full plant, non-critical loads must be shed, since the plant’s electrical load, (including starting KVA), cannot exceed the capacity of the generator(s) providing that power. The generator and critical loads must be isolated from the dead utility line, and the generator must be re-synchronized with the utility when it returns.

It is important to understand that when the turbine generator is the primary source of power, it is controlled by electrical load, rather than steam load, as is otherwise the case. This means that as the plant requires more or less electricity, the turbine will use more or less steam. If the steam used for electricity exceeds the plant’s process steam requirements, the excess steam must be vented or sent to a condenser. For the case when the process requires more steam than the turbine, a by-pass pressure-reducing valve is needed to send steam around the turbine to the process.

Perhaps the most intricate aspect of running stand-alone is the transition from paralleling to stand-alone and back again. A variety of schemes are possible, two of which are outlined below.

Black Transition Scheme

The simplest scheme involves allowing the turbine generator to go down when the utility power is lost. Any sheddable electrical loads are turned off, leaving only loads within the turbine generator’s capacity. A source of power, typically a diesel generator, is then used to power the boiler. The turbine generator is then synchronized with the diesel generator. The diesel can be shut down or left running.

When the utility comes back on line, the entire system may need to be brought down again. In this case utility power is used to power the boiler, and the turbine generator is restarted and then synchronized with the utility. Figure 1 shows a system that can make a black transition.

Figure 1

Ride-Through Capability

In order to “ride through” a utility power loss, some additional components are required. The plant entrance must have an automatic utility tie-breaker and associated protective relays, as shown schematically in figure 2. When utility power is lost the utility tie-breaker opens and isolates the generator and the emergency loads from the utility.

Figure 2

The system for detecting a loss of utility power must be carefully designed for each application. In some cases this must take the form of a telemetry signal from the utility. In some cases it is possible use the reverse power relay, the frequency relay, and/or the overcurrent relay at the plant entrance to detect the utility outage. The protective relays at the entrance and at the generator must be coordinated so that the utility tie breaker opens before the generator breaker.

In addition to opening the tie breaker, the controls must also simultaneously switch the turbine to follow the electrical load and maintain the proper electrical frequency. If required, non-critical loads must be shed instantaneously. In some plants, an automatic load control system is needed to shut down sheddable loads, to bring the electrical load to within the generator capacity.

Finally, when utility power returns, the generator must be synchronized with the utility across the utility tie breaker, whereas under normal start-up conditions, the generator is synchronized across the generator circuit breaker. The synchronizing equipment can be designed to accommodate both situations.

Custom Controls for Stand-Alone Operation

If a turbine generator system is intended to provide emergency power during utility outages, special attention must be paid to the needs and functions of the plant. Turbosteam engineers can provide your plant with a customized control system to meet your stand-alone needs. We will work with you to design a control system to optimize the utilization and reliability of your equipment.

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