In Short
Switchgear decides where generator power goes. A generator breaker protects the set, an automatic transfer switch moves the load between the grid and the generator, and paralleling switchgear lets several sets work together and share the load. A clear specification of each part avoids problems at commissioning.
The alternator makes the electricity; the switchgear decides where it goes. Between a generator set and the loads sit a generator breaker that protects the set, a transfer switch that chooses between the grid and the generator, and - when several sets work together - paralleling switchgear that synchronises them and shares the load, plus a motor control centre that feeds the pumps, fans, heaters and chargers the plant itself needs. This guide explains each part and what a specification should say about it.
The Generator Breaker and the Control Panel
Every set has a circuit breaker at its terminals that protects the alternator and the cable against overload and short circuit and is opened and closed by the generator controller. The controller starts and stops the engine, watches its temperatures, pressures and speed, regulates the voltage and the frequency, and reports to the plant. On a single standby set the breaker and the controller sit in a panel on the set or in the enclosure; on larger plants they go into a switchgear container with the rest.
The Automatic Transfer Switch
The ATS stands between the grid, the generator and the load. It senses the grid failure, signals the generator to start, and transfers the load to the generator once its voltage and frequency are within limits, all within seconds. When the grid returns and has been stable for a set time, it transfers back and lets the generator cool down and stop. An open-transition switch breaks the connection for a fraction of a second during transfer, which the UPS bridges; a closed-transition switch synchronises the generator with the grid and transfers without a break, which the utility must allow. The ATS is rated for the full load current and, in critical plants, duplicated.
Paralleling Several Sets
Where several sets supply one load - for capacity, for redundancy or both - they are connected to a common busbar through paralleling switchgear. Each set is synchronised before its breaker closes: voltage, frequency and phase angle are matched, automatically, by the controller. On the bus the controllers share the active and reactive load between the sets, start additional sets as the load rises and stop them as it falls, and take a set off the bus for service without a flicker at the load. The same switchgear can synchronise the plant with the grid for grid-parallel operation - peak shaving, demand response, export - under the utility's protection requirements.
The switchgear is specified by voltage level (low or medium voltage with transformers), short-circuit rating, the number of sets and feeders, the protection scheme and the control system; it is usually housed in its own container or room with the MCC.
The MCC and the Auxiliaries
A generator plant has loads of its own: fuel transfer pumps, polishing systems, radiator and ventilation fans, jacket-water heaters, battery chargers, lighting, control power. The motor control centre - the auxiliary cabinet - feeds and protects them, with starters, breakers and the controls, and is itself supplied from the grid or from the generators through a small transfer so that the plant can serve itself in an outage. AXON builds circuit breaker panels, ATS and paralleling switchgear and the MCC for its containerized plants and as stand-alone panels, around generator sets of any make and to the plant's single-line diagram.
In short
- Generator breaker and controller: protect the set, regulate voltage and frequency
- ATS: senses the failure, starts the set, transfers the load and back
- Open transition breaks briefly; closed transition synchronises and does not
- Paralleling switchgear synchronises several sets and shares the load
- Grid-parallel operation needs the utility's protection scheme
- MCC feeds the plant's own pumps, fans, heaters and chargers
What to send with an enquiry
- Single-line diagram or the loads and the number of sets
- Voltage, frequency and the short-circuit level
- Transfer scheme wanted: open or closed transition, grid-parallel operation
- Redundancy and the duplication of switches
- Auxiliary loads for the MCC
- Control system interface and the protection requirements of the utility
Questions we are often asked
What does an ATS do?
It watches the supply, starts the generator when the supply fails, moves the load to the generator when it is ready, and moves it back when the supply returns - automatically and within the times the specification sets.
When is paralleling switchgear needed?
Whenever more than one set feeds the same load, for capacity or redundancy, and whenever a set is to run in parallel with the grid. A single set with a simple transfer needs only its breaker and an ATS.
What is synchronising?
Matching a generator's voltage, frequency and phase to the busbar or the grid before closing its breaker. Done automatically by the controller; closing unsynchronised damages the alternator and the switchgear.
Low voltage or medium voltage?
Smaller plants are usually low voltage; large plants and long distances use medium voltage through step-up transformers. The plant size and the site decide.
Can the plant run in parallel with the grid?
With paralleling switchgear and the protection the utility requires, yes - for peak shaving, demand response or export. The utility's connection rules set the protection and the tests.
Products
- Circuit Breaker Panel
- Auxiliary Cabinet (MCC)
- Containers for Power Generators
- Load Bank for Generator Testing