Photo from IEC/IEEE 62271-37-13 standard just for show
Generator circuit breakers are fundamentally applicable for all kinds of power generation plants such as fossil-fired, nuclear, gas turbine, combined-cycle, hydro, and pumped storage power plants as well as for retrofit in existing power stations without generator circuit breakers.
General circuit breakers(GCB) benefits in the grid:
Generator Circuit Breakers were used in multi-unit stations where a number of relatively small generators were connected to a common bus. The rapid increase in generator size and system fault current levels soon exceeded the interrupting capabilities of this type of switchgear. The unit concept was then adopted where each generator had a separate steam supply auxiliary system directly connected to a step-up transformer and high side breaker(s).
The use of generator circuit breakers for the switching of generators at their terminal voltage offers many advantages when compared to the unit connection, e.g:
– simplified operational procedures.
– improved protection of the generator and the main and unit transformers.
– increased security and higher power plant availability.
– economic benefit.
Advances in circuit breaker design(s) have made the generator circuit breaker concept a viable alternative even at the 250 kilo-ampere level required for some applications.
A major advantage of the generator circuit breaker is that fault current contributions from the generator can be interrupted in 5 to 7cycles (now it’s 3 – 5 cycles) for faults in the isolated phase bus or on the high side of the
generator step-up transformer.
The major demands on the electrical layout of power plants can be summarised as follows:
– transfer the generated electric energy from the generator to the HV-transmission system considering operation requirements as well as availability, reliability, and economical aspects
– supply of electric power for auxiliary and station service systems to ensure a safe and reliable power plant operation
Examples of power station layouts that employ a generator circuit-breaker to connect the generator to the main transformer are shown in Figure 1
Photo 1: Different power plant layouts which employ generator circuit breakers (source from Generator Circuit-Breakers – Application Guide pdf ABB company)
MT: Main transformer, UT Unit transformer, ST Station transformer, GCB Generator circuit breaker, EHV Transmission system, HV Sub-transmission system, UX Unit auxiliaries, SS Starting switch, BS Braking switch
SFC Static frequency converter, PRD Phase-reversal disconnector
The generator circuit breaker operation duties:
- Synchronize the generator with the system voltage at the HV
- Separate the generators from the HV system (switching off the unloaded or lightly loaded generators).
- Interrupt load currents (up to the full-load current of the generators).
- Interrupt system-fed short-circuit
- Interrupt generator-fed short-circuit
- Interrupt current under out-of-phase conditions (up to an out-of-phase angle of 180°).
- Synchronize the generator-motor with the HV system when the machine (generator unit) is started in the motor mode (in case of pumped storage power plants, there are different synchronization methods such as a static frequency converter (SFC) starting or back-to-back starting)
- Close on and Interrupt the starting current of the generator-motor when the machine is started in the motor mode (in pumped storage power plants, with asynchronous starting)
- Interrupt generator-fed short-circuit currents at frequencies below 50/60 Hz (in a gas turbine, combined-cycle, and pumped storage power plants, depending on the start-up supply)
There are several synchronization methods in the case of pumped storage power plants.
The static frequency converter (SFC) starting scheme mainly consists of a thyristor converter connected to a unit transformer at the HV side and an inverter connected to a generator. The inverter starts the generator from a low power frequency and gradually increases it up to the rated power frequency. Then the generator is excited to produce the power which may have a different phase angle from that in the network. The generator is synchronized with the HV network by either a generator circuit breaker or an HV circuit breaker, at the instant when the phase difference between the generator and the HV network is minimized.
For another example, a back-to-back starting scheme can be used in a power plant with several generators. The power source produced by the generator operating at nominal condition is used to start the halted generator up to the rated power frequency. The generator is synchronized with the HV network with either a generator circuit breaker or an HV circuit breaker.
Generator circuit breaker operation duties in IEC/IEEE 62271-37-13:
According to IEC/IEEE 62271-37-13: the rated short circuit duty cycle of a generator circuit breaker shall be
made of two units of operations with a 30 minutes interval between operations
(duty cycle: CO – 30 minutes – CO)
This means two full short circuit interruptions separated by 30 minutes between each short circuit closing. This is designed to protect power plants and generators, in particular, because two close-opens at full short the circuit might damage the generator and the step-up transformers. These types of short circuit are very unlikely and after a full short circuit, it is very unlikely that the plant manager will try to close again after 30 minutes.
This time between two operations is necessary to restore the initial conditions and/or to prevent undue heating of parts of the generator circuit-breaker (this time can be different according to the type of
operation);
