reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

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GLASS POLYESTER TERMINAL BASE

MOTOR

CONNECTING

FLEXIBLE CABLE

111

PRESSURE RELIEF DIAPHRAGM

■

CABLE SPACING

BLOCK

Section Through Box

TERMINAL BOX

LID

EARTHED STEEL

FLASHPLATE

NEOPRENE

RUBBER CAP

View on Front with Lid Removed

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Transient recovery: To recover after a power supply system disturbance causing loss of voltage for 0.2 seconds, followed by sudden restoration to 60% voltage for 3 seconds,

Maximum (pull-out) 200% full-load torque torque:

Noise levels: Uo BS4999, Part 51, except that in no case shall the mean sound pressure level exceed 87dB(A) at 1 metre from the surface of the machine.

5 Power station auxiliary drives

Details of some of the more i mportant motor auriliary drives required on a modern CEGB power station with 660 MW turbine-generator (TG) units are given belmk, and also in Table 7.4.

5.1 Boiler feed pumps

The normal arrangement adopted currently by CEGS is for I x 100 070 steam turbine-driven feed pump, with

2 x 50% starting and standby motor-driven sariable. speed feed pumps. Motor-driven suction stage pumps are also required for the feed pumps.

Since the motor-driven feed pumps are required for starting and standby duty only, the choice of motor is largely dictated by first cost. Variable-speed sliprinct motors with variable rotor resistance liquid type controllers have been used. Squirrel-cage induction motors

where starting is required infrequently.

The variable-speed slipring induction motors are typically 1480 r/min (maximum), 9 MW, 11 kV, starting current 100% full load, 50 Hz, with speed variation 100% to 70% by rotor-resistance control, with liquidtype speed controllers. The feed pump is driven through speed-increasing gears at 8000 r/min (maximum). Three x 50% duty suction stage pumps are provided, driven by 1000 kW, 1480 r/min cage induction motors. Figure 7.16 shows a typical slipring induction motor reed pump drive.

With the cage induction motor drive, the motors are typically 1480 r/min, 10 MW, 11 kV, 50 MVA at start, 50 Hz, driving through hydraulic couplings with speed variation 100-70%. These are mechanically coupled to the feed pumps via speed-increasing gearboxes at 8000 r/min (maximum) pump speed. The suction stage pump is rated at 1000 kW, 1480 r/min and is coupled to a shaft extension of the main drive motor.

5.2 Coaland oil-fired boiler units

5.2.1 Draught plant

The practice is to install two induced-draught and two forced-draught fans per boiler unit, each working at full-load when the boiler is at continuous maximum

rating (CMR). It is possible to run the boiler unit at reduced load in the event of failure of one of the fans.

although it is expected that this condition would nol be allowed to continue for any length of ti me. The

iitator-fed AC commutator motors have been i[lie moments of inertia of the fans are high, being ciI ti mes those of their respective motors, which rein relatively long run-up times, typically 50 to 80 s.

proi.ided.

5 .3 Nuclear reactors — AGR

5 3. 1 Gas circulators

pical arrangement is for eight gas circulators to

Two basic configurations are in common use in the CEGB, horizontal and vertical; this description concentrates on the vertical configuration, but many of the principles apply to both. Figure 7.17 shows the location of a vertically-opposed gas circulator in a nuclear reactor at Heysham I power station.

Each gas circulator is a single-stage centrifugal type, delivering 461 kg/s of CO2 at 41 bar and a temperature of 287 ° C. The inlet gas temperature to the circulator is 278 ° C and the pressure rise across the circulator is 2.83 bar.

The circulator is driven at 3000 r/min by an 11 kV motor mounted below the impeller on the circulator shaft (see Fig 7.18). A 415 V pony motor, mounted on the circulator shaft below the main motor, drives the circulator at barring speeds below 350 r/min. The motor unit for each circulator is encapsulated and comprises the composite assembly of main motor, pony motor, rotor shaft, bearings, impeller, inlet guide vanes and isolating dome operating mechanism. This assembly is contained completely within the reactor pressure boundary. The circulator unit can be withdrawn and replaced for maintenance with the reactor depressurised.

The impeller shaft passes through a barrier plate which is provided with a labyrinth seal to minimise

LOCATION OF GAS CIRCULATOR

Fu. 7.17 Posiiion of gas circulator unit within AGR reactor

hot gas flow between the motor compartment and the reactor. The impeller operates in reactor gas at nearly 300 ° C, whereas the motor is isolated thermally to operate at temperatures of about 60 ° C. Although normal operation is in CO2 at 41 bar, the circulator must be capable of operation in air at atmospheric pressure (or reduced CO2 pressure) for commissioning, maintenance or during fault conditions.

motor

The circulator main drive motor is an 11 kV, two-pole squirrel-cage induction machine contained completely within the reactor pressure boundary. The motor, mounted on the circulator rotor shaft with the 415 V pony motor, rotates at a nominal speed of 3000 r/min and maximum continuous rating of 5.59 MW.

The motor is designed to I3S2613 Class F insulation, but operates normally within Class B temperature li mits, The motor is capable of operating continuously, at rated torque, at any frequency between 48 and 51 Hz and at any voltage between ±5% of the nominal value.

The motors are designated for 'essential duty' and are capable of continuous operation at 75% nominal volts at 50 Hz for a period of 5 minutes without injurious heating. They are also capable of recovering normal operation in the event of a system disturbance

causing temporary loss of supply voltage for periods of up to three seconds, followed by a sudden restoration initially to 80% nominal voltage.

The motor stator windings and laminations are con. tam ed within the main motor outer frame which is a mild steel cylindrical ribbed fabrication with top and bottom flanges (see Fig 7.19). The top flange aligns with the lower face of the top bearing bath and the bottom flange aligns with the bottom bearing batch assembly and incorporates holes for the motor securing bolts. The stator and outer frame complete weicth s approximately 14.5 tonnes.

The rotor is fitted on the forged carbon steel shaft together with the impeller, drive-end bearing sleeve, motor cooling fan, pony motor rotor, non-reverse clutch, pulsing disc and non-drive end thrust collar (see Fig 7.20).

Intensive research, development and full-scale t!stin g were required to establish the design of these ors, which have to operate under arduous conditio . The motor has also to withstand specified radiatio•: levels and also operate with contamination from lubricating oil mist. All these presented problems; in particular, the design of bearings under high rates of change of pressure of the ambient gas, the insulation system, the electrical design and thermal design. These are discussed in more detail (see Schwarz 1973 l7j).

A very high reliability is required, particular' the units are inside the reactor pressure cm and even apart from safety aspects, any 1: outage can be very costly.

5.4 Nuclear reactors — PWR

5.4.1 Reactor coolant pumps

A typical arrangement is four reactor cook: per reactor which are located within the re:. tainment. The pump unit consists of a vert:.

stage centrifugal-type pump, with vertical dr. .tor mounted above the pump and directly coupled to it. The motor has a drip-proof enclosure, but the cooling air outlet is water cooled. A flywheel is located at the top of the motor to provide the required coastdown capability in the event of loss of electrical power supply to the motor. This limits the transient temperatures within the reactor under such conditions. The motor must withstand specified radiation levels within the containment.

Figure 7.21 is a cutaway view of a typical reactor coolant pump and motor unit.

5.4.2 Safety-related drives

There are a number of drives, classified as safety-related category Class 1E to IEEE Standard 334, which are essential for the safety of the reactor during normal periods of shutdown or emergency periods following loss-of-coolant accidents which could involve severe

-Aliation, thermal and environmental stresses. The test -,quirements to be met in order to qualify or prove

Jc2S[oT1 and reliability of such motors are given a IEEE. 334. Examples are motors for centrifugal

Airing pumps, high head safety injection pumps and :t2st,lual heat-removal containment spray pumps.

5.5Circulating water pumps

ii c usual arrangement of the cooling water system is system, in which the cooling water is taken to a ,tommon chamber whence a number of pumps dis-

.ii.iree into a common bus-main supplying all the conrisers in the station. In view of the vital importance tit the cooling water to the station output and in preplant damage due to loss of cooling water, spare

capacity is installed.

i - }1,2 motors are usually of the cage induction type. some installations use vertically-mounted motors, ,, ii cre-as others are horizontal. Figure 7.22 shows a vertical-shaft circulating water pump motor. The

.:iips are usually of low speed (typically 500 down to r min). With pump speeds lower than about 500 - ruin, it is usual to provide a motor driving through a cd-reducing gearbox, this being the most economic

saratieement.

rite more usual arrangement is to provide a pumpsometimes with removable or sliding roof for tii. Mitenance purposes. Both drip-proof and totally-

.::,.iosed motors have been used. The tendency is the totally-enclosed type, which gives greater ittroteetion against contamination of the windings. At Power stations, the stator windings are some-

. mcs additionally of the sealed type to protect against saline conditions.

6 Testing

'2 ling can be categorised into two groups: develop-

LI1I

and works testing.

The development of new designs and techniques, new materials and component parts is a research and development activity. Verification of the properties, performance, etc., must be obtained and is usually achieved by a testing process. Simulation methods may have to be used in some cases where direct testing is not possible or practical, e.g., accelerated ageing tests on insulation systems, where, for example, a relationship between voltage, temperature and time to breakdown can be established and the tests made at elevated temperatures and voltages. Fatigue testing of bars of cage induction motors is another example, where such tests are made to establish the total number of starts that can safely be made during the life of the motor.

Works testing on the completed motor needs to be made to verify that the motor is electrically and mechanically sound and that the stated performance characteristics are achieved. Such tests are categorised into basic and routine tests. Basic tests are in general made on the first motor of each type and routine tests on all other motors. Table 7.5 lists the individual test requirements for induction motors.

7 Future trends

Motors for power station auxiliary drives are likely to continue to grow, both in power requirement and complexity. More emphasis is likely to be placed on the achievement of high reliability, due to increasing costs of unplanned outages, and on safety to personnel. This implies extension of and/or improvements to existing testing techniques and could include tests to determine the permissible number of direct-on-line starts in the life of cage induction motors. With increasing attention being given to the conservation of energy, improvement in operating efficiencies will be

sought. Improved materials will continue to be developed. These could include lubricating grease with a higher withstand-temperature and improved capability of insulation systems.

The use of motors having brushes, sliprings and commutators is likely to continue to be avoided. An increasing use of AC variable-speed converter drives is forecast, particularly with improvements in tech-

niques and reduction in cost of converter equipment. Linear motors are likely to find an increasing number

of applications where linear motion is required, such as cranes and sliding doors.