ENGINEERING ARTICLES

Under normal operation of a synchronous generator, the field created by the rotor windings locks in with revolving mmf of the stator windings and the rotor moves at synchronous speed, at least in the steady state. When excitation is lost, the rotor field suddenly loses its mmf and the rotor begins to move away from synchronism, having lost its strong magnetic coupling with the stator mmf. During this time, the governor is still set to deliver a given amount of power to the generator, so the generator will accelerate, inducing large slip frequency currents in the rotor in order to maintain the power output as an induction generator. Actually, the power requirement will be reduced as the slip increases due to the governor characteristic, the increase in stator current, and possibly the lowering of terminal voltage, but the total power is still quite large. Also, since the excitation has collapsed, the generator begins to absorb reactive power from the system in very large amounts, which ...

Some of the larger generators have stator windings that are cooled by water circulating through the windings. If the flow of cooling water is impeded or stopped for some reason, the winding will quickly overheat. The cooling water is circulated by water pumps, which are backed up by standby pumps. Flow detectors placed in the water line monitor the adequacy of the coolant flow. If the flow is reduced for any reason, these detectors can start the standby pump. If this restores the normal flow, no further action is taken, but if not restored within I minute, the turbine and generator are tripped sequentially, starting with the turbine and then the generator.

Special fire protection equipment is sometimes used to quickly extinguish fires within the generator housing. One method that has been used releases water into the generator enclosure on detection of a fire. A better system injects carbon dioxide into the generator rather than water. This is safer and does not add to the damage caused by the fire. Many large machines operate in a hydrogen cooled environment. Since hydrogen will not support combustion, additional fire protection is often not considered necessary for these units. Instead, a method is required to monitor the purity of the hydrogen to make sure no air is mixed with the hydrogen to form an explosive mixture. Most generator shells are designed to withstand such an explosion of the maximum intensity possible from a hydrogen and air mixture.

Vibration protection is also a part of the turbine protection system and is not normally supplied or specified by the electrical protection engineer. Still, as noted above, vibration detection is the backup for faults that cause unbalanced rotor heating due to rotor faults or negative-sequence rotor heating. It is important that there be coordination between the electrical protections and the mechanical backup. Vibration detection equipment should always be in service on a turbine-generator system. It is not adequate, for example, to manually add the vibration detection recorder after having detected a field ground. The reason is that the second field ground may occur quite soon after the first one and before any manual action is possible. The vibration detector should at least provide an alarm, but preferably should issue a time-delayed trip of the generator and field breakers. As noted previously, it is preferable that the vibration detector not trip the turbine, but only the gener...

Motoring protection is provided for the steam or hydraulic turbine, or for the power system, and not for the generator. Motoring is not harmful to the generator in any way. The protection is usually considered as part of the generator protective system since it uses electrical quantities, usually in the form of sensitive power relays. Steam turbines have the tendency to overheat when the steam supply is cut off for some reason and the turbine generator system begins motoring. The overheating is due to the loss of steam, which normally keeps the turbine blading at a stable temperature. When the steam flow is interrupted, the blade temperature rises. Most modem steam turbines will overheat when the steam flow is less than about 10% of full load. The time constant is large and varies from 30 seconds to 30 minutes, depending on the type of turbine and its design. The practice varies regarding the need for protection and the turbine manufacturer should be consulted regarding any requireme...

It has been noted above that over-speed causes overvoltage, which may be protected against using overvoltage relays. Basically, however, over-speed control is part of the turbine control system. Most large steam turbine controls have two or three separate speed control units, with one of these being strictly a mechanical centrifugal device that will close the turbine control valves even if electrical power is lost to the controls that require electric input. Over-speed protection must be selective and must not shut the unit down due to a temporary loss of load, even if the cause is serious, for example, a short circuit. Short circuits anywhere near a generator will collapse the voltage and the generator experiences a loss of load. Since the turbine power is unchanged, the turbine-generator unit will over-speed until the governor throttles the turbine input back. Faults are usually temporary, however, and there is no need to shut the unit down unless the fault is on the generator or G...

It is astonishing to realize that there is hardly a home, office block, factory, car, sports hall, hospital or theatre without an application, and sometimes many applications, of power electronic equipment. Some typical applications are given below: Industrial processes in the chemical, paper and steel industries; Domestic and theatre lighting; Motor drives from food mixers and washing machines through to lifts and locomotives such as 'Eurostar'; Power supplies for laboratories and uninterruptible power for vital loads; Generation and transmission control; Heating and ventilating of homes and office blocks.