Electrical Auxiliaries

The Electrical Auxiliary Equipment comprises all the electrical systems needed to operate and maintain the powerhouse. (It does not include the Ancillary Equipment, which are needed for maintenance and operation of the turbine-generator units.)

The mechanical auxiliaries and the electrical auxiliaries are often provided together under a separate “Balance of Plant” contract.

Power Systems

The Power Systems convey the electrical power from the generator terminals to the transmission line and include the following comonents.

Bus System

The bus system operates at 11 to 13.8 kV and carries high currents from the generator to the Main Generator Transformer (see below).

There are several types of bus in use:

Exposed bus

The conductors are mounted on insulators and surrounded by safety cages, but are open to the air. These are potentially dangerous, because accidental contact with the conductor would be fatal. Also, dropping a crowbar or large spanner on the bus can cause a short circuit between phases, resulting in massive arcing and large loads on the generator that can tear the stator from its mountings. Where exposed bus passes close to reinforced concrete, it can induce eddy currents in the steel rebar (reinforcement), causing them to heat and burst the concrete. For this reason, each piece of rebar needs to be insulated from all adjacent bars.

Isolated Phase Bus

ISB houses each conductor mounted on insulating spacers within a separate aluminum tube, and the tubes are from one another to provide air insulation. IPB is safe. The expansion and contraction of the conductors due to current flow is accommodated by flexible links at intervals in the bus.

Cable Bus

Cable bus comprises a set of high voltage cables housed in a square, sheet metal duct. The size and number of conductors is chosen to satisfy the current rating of the HV cables. The conductors or each phase are arranged in a specific pattern to limit the magnetic fields around the duct. 

Insulating Gas Bus

The conductors can be brought close together without arcing if they are surrounded by a gas with high insulating properties, such as sulfur hexafluoride (SF6). This allows the sizes of the bus and associated switchgear to be reduced, leading to a more compact bus arrangement. Gas insulated switchgear is usually called GIS.

Bus Components

There are several components built into the bus system. Depending on the main Single Line Diagram, these might include potential transformers (PTs), current transformers (CTs), circuit breakers (which can open or close the circuit even when current is flowing, disconnect switches (which can only be opened or closed when there is no current flowing, but which provide safe isolation of equipment during maintenance), power reactors (to limit short circuit currents), and taps to take off power for the excitation transformer (see the Generating Equipment page) and the Station Service Transformer (see below in this page).

Main Generator Transformer

The main transformer is responsible for stepping up the voltage from the bus voltage to the transmission line voltage (called the “Line Voltage”).  Main transformers on large generating units are contained within steel tanks that are filled with mineral-based oil for insulation and heat dissipation. Heat exchangers and fans are typically used to limit heat build-up inside the transformer. The insulating oil can break down if it gets too hot or if it absorbs moisture. Oxyidation of the oil produces acid and sludge. The sludge deposits on the transformer coils and can reduce heat dissipation.

Transformer oil is tested regularly for dielectric breakdown, acidity, interfacial tension, Colour, water content and sometimes dissolved gasses. When any of these parameters is outside its predetermined range, the oil is either reconditioned or replaced.

Failure to do so can lead to a transformer explosion and oil fire.

[Transformer bays have walls to contain the blast and fire and chilling sumps with coarse rock to hold the spilled oil and extinguish the fire. A deluge system sprays water on the transformer in the event of a fire, and this water must also be accommodated in the chilling sump.]

The power bus connects to the low voltage windings of the transformer. High voltage insulated porcelain bushings are connected to the high voltage windings, and these are connected to the transmission line via PTs, CTs, disconnects, surge arresters and transmission line communication equipment.

Grounding (Earthing) System

The station grounding system is one of the most important system in a powerhouse, because it provides essential life protection for personnel in the station. Any metal object in the powerhouse or switchyard may acquire an electrical charge through the presence of an electromagnetic field or a lightning strike on the facility or the connected transmission line.

The general principle of the grounding system is to connect all metallic components to the earth’s conductive surface. The connection to the earth may be provided by ground rods, remote ground grids (comprising bare copper conductors buries in damp earth) or by engaging the vast surface area of the concrete reinforcement bars in the facility foundations.

Grounding studies, often including field measurement of ground resistance, are carried out to design the grounding system. Because of the high localized voltage rises that can occur in outdoor switchyards, these studies pay special attention to “step and touch potential”. The step potential is the voltage difference that might occur between one foot and the other as a person walks across the ground, and is caused by a strong voltage gradient in the ground. Touch potential is the voltage difference between the ground where a person is standing a metal object, such as a fence post. In either case, the voltage differential can cause an electrical current to flow through the person’s body. Buried ground mats are installed to limit step and touch potential.

The station ground system is connected to every piece of generating, ancillary and auxiliary equipment in the facility, as well as every steel column, stair, handrail, gate, fence, etc..

SCADA System

SCADA stands for supervisory control and data acquisition. It is a computer system for monitoring and controlling the entire generating facility and is installed in the powerhouse control room and usually accessible from a remote operations centre.

The system continually gathers real time data from the generating equipment and auxiliaries and stores it in a secure database with an accurate time-stamp.  It also runs analyses on the data, checking the measured parameters and raising alerts and alarms when they deviate from pre-set limits. The user interface of the SCADA system displays a suite of screens, displaying the continually-updated status of each system in digital and analogue form. For example, the main screen might show the reservoir level, which intake gates are open or closed, the flow in the penstocks and the electrical output from each generator. A click on the generator graphic will bring up a more detailed view of the generator with temperature data, readings on the metering equipment and the location of the current operating point displayed on a chart of the generator operating limits.

Screens are also provided for the operator to operate the facility. For example, the SCADA allows the operator to start and stop generating units or put them into standby mode.

An alarm event brings up the most appropriate screen to reveal the nature of the problem and the controls needed to address it.

AC Station Service

The AC Station Service provides power for all the low voltage electrical loads in the generating station. It normally comprises two parallel systems to provide redundancy, so that when one system is off line, station service electrical power is provided from the other.


Each system takes power from a High Voltage bus via a 3-phase Station Service Transformer, which reduces the power to house-hold voltage. In North America, this is 120 v single-phase and 208 v three-phase. In many countries, these voltages are 230 v single-phase and 400 v three-phase.

In stations with more than one generating unit, there will be at least two station service transformers for reduncdancy.


The secondary windings of the SST are connected to a Station Service Switchboard (SSB). I stations with more than one generating unit, there will be at least two station service switchboards for redundancy. Switching is usually provided so that each stwitcboard can be supplied from either SST.

Each SSB has electrical breakers assembled according to function, such as the following: 

Station lighting – this is the system of luminaries that provide a safe level of lighting for operation and maintenance of the facility.

Convenience and welding outlets – this is the system that provides single-phase electrical power to wall outlets for computers, tools, appliances and low-power auxiliaries.

Three-phase power – Three-phase power is provided from the SSB for special applications. These include the three-phase welding outlets and motors that use phase reversal to change their direction of operation, such as motorized valves and some older powerhouse cranes.

Note: Most modern powerhouse cranes use flux vector variable frequency drives (VFD), which allow smooth operation and very slow movement of the bridge and hoist. This is very useful during precise assembly, such as lowering the generator rotor into the stator of aligning the generator mountings with the holes in the turbine shaft coupling.


A series of motor-control centres are distributed in the powerhouse for starting the motors of auxiliary equipment in the immediate vicinity of each MCC panel. Each panel has a common power bus and separately-enclosed sections for each motor control, which generally includes a starter, a fuse, a circuit breaker, metering equipment and a programmable logic controller (PLC). The PLC is a small industrial computer which connects to the SCADA system and allows remote operation and status monitoring. 

Emergency Lighting

Emergency lighting stations are normally connected to a convenience outlet and have a built-in battery and charger. The light turns on whenever there is a loss of electrical power to provide safe access until the power system is restored.

High Bay Lighting

The high bay lighting system provides illumination to the main floor of the powerhouse. The lighting level is high, since key maintenance operations take place at this level of the powerhouse, including operation of the powerhouse crane to dismantle and reassemble turbine-generator units.

While high-pressure sodium lighting has been a common choice for high bay lighting, new stations are taking advantage of recent advances in lighting technology.



Fire Detection


DC Station Service


Battery Chargers



Protection and Control Systems

Protection systems

Control systems

Communication Systems

Internet Security

Signalling and Network

Telephone switchboard and telephone booths

Annunciation System

Surveillance and Security


Intrusion alarms

Gate access controls

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