Control Systems

All alternators require switchgear to link them to their loads. Smaller machines will have a mechanically operated switch (the "isolator"), an electrically operated switch (the "contactor"), a set of fuses and an overload trip device.

Larger machines will have fault detection devices e.g. thermistors, phase fault relays, these are linked to a trip relay which will trip the drive, reject the load and de-energise the rotor coils. The control equipment is generally located in a separate cabinet or cabinets. One part of the system monitors the load being taken and checks for faults in the power circuits. If the alternator is operated in synchronised mode, this system will control the load via the drive engine control system. The system controls the exciter to generate the appropriate voltage and current to meet the demand.

Another part of the system monitors the frequency and phase of the generated current. When the alternator is operating alone, this system serves only to raise alarm or trip signals if values go outside the acceptable range. When the alternator is to be synchronised with an existing supply, the control system signals the engine control until there is a match, when the main contactor is closed.

Note that speed control, when required, is done by the engine / driver control system, which is responsible for overspeed control and trip actions.

4.14.7 INTEGRATION ASPECTS

Industrial alternators are built to order. Generally the supplier will select the appropriate unit for the duty, but it is much better if the client or contractor's rotating equipment and electrical engineers are consulted. Hazardous area requirements are fundamental, as these safety features cannot be added afterwards.

The range of loads to be fed must be considered, particularly for a unit which will operate in “stand-alone” mode. For example, reciprocating compressors can have a rapidly fluctuating torque characteristic that may cause problems with other machines on the same power net. The electrical characteristics of the loads may require additional electrical components to prevent excessive “reactive” currents, which waste energy.

The design of the power network is a skill in its own right, particularly if large drives must be started, or if parts of the system are to be dropped or “shed” in emergency.

Inverter drives can produce harmonic signals that can impact on control systems or damage electrical equipment. The larger the drive, the bigger the potential problem. Use of high voltage inverter drives requires the active input of a competent electrical engineer.

Specialist contractors who apply considerable experience of previous installations generally design emergency generator packages. The designs must be tested against the needs of the client, but the design philosophy differs from normal process plant design.

4.14.7.1 Cleanliness

Water cooled alternators are insensitive to dirt, unless it actually gets into the bearings or inside the casing. Internal cleanliness is important, otherwise dirt will foul heat exchanger surfaces. Oil contamination inside the machine poses a fire risk.

Air cooled units are sensitive to dirt in the surroundings, particularly if in an open ventilated enclosure and with no air inlet filters.

4.14.7.2 Mechanical Integrity

Alternator rotors turn at relatively low speeds and are contained within massive housings. Ejection of parts is very unlikely. Alternators are heavy and require a rigid mounting structure, poor mounting can distort the frame, causing vibration and potential damage.

4.14.7.3 Condition Monitoring

Vibration monitoring, in particular by trending, can identify a range of machine problems. For details of Condition Monitoring Systems and related hazards see Section 5.12.

Modern alternator control systems have a number of integral monitoring functions for protection purposes, but do not usually record and trend. Specialist electrical test equipment can be connected to alternators, and can be used to detect a range of electrical and mechanical problems. A high level of interpretative skill is required. Routine use of this equipment cannot usually be justified, unless several alternators are installed, or equipment is shared with other installations. Symptoms like electrical faults, overheating or strange noises will prompt further study.

4.14.7.4 Protective Systems

As well as the protective functions already described in the alternator control systems, normally there will be temperature sensing devices in the bearings, coils and internal cooling air circuit. These may display locally or in the control room, or simply operate protective devices. It is normal to have pre-alarm and trip on each temperature.

It is important that the protective systems are integrated with those in the driver and ancillaries. 4.14.7.5 Bearings

Where the bearings are oil fed, the oil supply may be derived from the driver, or from a small dedicated oil console. Diesel engines in particular suffer from fuel and carbon contamination of lubricating oil, which is best confined to the engine itself. The driver must be interlocked to the supply oil pressure, and tripped if the pressure falls.

4.14.7.6 Sealing

Alternators have simple labyrinth shaft seals on the air circuit, they should be reasonably dust tight and hose proof (IP55) but are not gas tight. It is good practice to set up terminal boxes and cable glanding to avoid water ingress down the cable. Terminal boxes are not isolated from the internal air space.

After maintenance, the dust and water tightness will be lost if the panel seals are damaged or ill- fitting.

4.14.8 CONTROL

x The alternator control must be integrated with the driver control system.

The alternator control must be integrated with the control of the driving machine, in particular to ensure that trips, speed and load controls work properly. If there are different control stations e.g. engine governor, alternator panel, control room DCS, it must be clear which system has control under any particular circumstance, and that controls do not conflict.