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

neural, self-contained, free-standing unit and is , ormally mounted on the roof of the diesel generator with maximum efficiency of heat transfer to :iio , phere. Radiators are constructed from a number

, (. parate cooler elements, any one of which is capable iq being removed without disturbing the others, inihe provision of isolating and drain valves for element. The elements are constructed from non- •.rrous materials in separate sections to accommodate :ne , eparate cooling circuits. A number of elements can he iked on any cooling circuit, but the use of one

...1,:inent on two separate cooling circuits by division of he header is avoided.

rwo nominal 100% duty, thermostatically controlled 7,1,1E xur cooling fan groups are provided, so that each

!,111 group on its own can provide 100% diesel gencooling at an ambient temperature of 15 ° C. i he Ian groups are arranged on a duty/standby basis,

li h either group being capable of selection as the duty

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group. The second fan group is arranged to act atomatically as standby in case of failure of the duty :2 roup and to supplement the supply of cooling air dur-

ing periods of ambient temperature in excess of 15°C. The fans are electrically-driven with a preference for direct drive. Multi 'vee' belt drives are acceptable, so long as twice the number of belts required for the duty is provided, together with belt adjustment and

motor/fan alignment facilities.

Water pipework and valves

All interconnecting pipework and valves are designed and installed in accordance with CEGB Standard 239903

— Piping Systems — Low Pressure Water Services'. Compression type pipe couplings are not used where failure could result in a safety hazard to personnel or plant. Where necessary, expansion joints are fitted, e.g., on long runs. Provision for drainage and collection

of the water is also provided.

5.2.5 Fuel oil systems

Each diesel generator fuel oil system basically comprises bulk storage tanks, transfer pumps, daily service tanks, line filters, recovery tanks, etc.

The capacity of the bulk storage tanks is calculated to provide a minimum of 80 hours full-load running of the associated diesel generator.

The daily service tanks are sized for 8 hours fullload running.

A eravity fuel supply to the engine from the daily service tank is preferred for all states of diesel generator loading, to avoid the use of additional auxiliary equipment ,‘Iiich could jeopardise the fuel supply to the engine under emergency conditions. At some installations this cannot be achieved and, in these instances, duplicate pressurising pumps (engine-driven and electrically-driven standby) are provided. In any circumstances, the diesel generator must be capable of a black start with at least 10% of rated output by gravity fuel feed, with the pressurising pumps by-passed if necessary.

To enable each daily service tank to be replenished automatically from the bulk storage tanks, duplicate fuel oil transfer pumps are provided and operate on a duty/standby basis. These are of sufficient capacity to make up the fuel used from the daily service tank by running for not more than half of the engine running time.

A deadweight type fire valve with fusible link is provided to drain the daily service tank to the bulk storage tank automatically in the event of a fire. Operation of the fire protection system will inhibit operation of the transfer pump to the tank concerned.

The fuel oil is arranged to reach the engine at the correct temperature for atomisation, irrespective of ambient temperature. Where necessary, thermostatically controlled fuel oil heaters are installed to achieve this.

Daily service tank and associated equipment

Tanks are constructed in accordance with BS799 Part 5, using mild steel coated with an oil resistant treatment, approved by CEGB.

They are vented to atmosphere via a flame trap. An overflow is provided with pipework back to the recovery tank. An easily cleanable sludge trap is fitted. Because of the height at which these tanks are likely to be mounted, ready access for cleaning in the form of galleries and ladders is provided.

A calibrated level indicator, visible from ground level, is provided.

Bulk storage tanks

The tanks are constructed in accordance with CEGB Standard 20752 — 'Fuel Oil Storage Tanks' and BS2594. They are located in pits in the diesel house. After manufacture, they are given an oil resistant treatment approved by the CEGB.

Additional capacity is provided in the bulk storage tank above the normal high level control which stops the transfer pump, to accommodate the contents of the recovery tank and the daily service tank when emptied by the operation of the fire protection.

A tank level indicator and local high level visual and audible alarms are provided. A temperature monitor in the gas space above the fuel oil level provides a high temperature alarm for fire prevention.

Because the tank is located in a pit, permanent

galleries and ladders are provided for cleaning and for access to valves and the sludge trap

5.1.4 of this chapter for fire protection).

Recovery system

in order to collect fuel oil leakage, blowclown, etc,, from the engines and overflow from the daily servi ce tank, a recovery system is provided including a recovery tank. From the latter, oil is automatically returned to the bulk storage tank.

Dependent on the relative heights of recovery tank and bulk storage tank, automatic duty/standby elec- trically-driven pumps are sometimes fitted to return the oil to the bulk storage tank. A visual level indicator is fitted.

The construction of the recovery tank is generally si milar to that of the daily service tank.

Oil pipework and valves

All interconnecting pipework and valves are designed and installed in accordance with CEGB Standard 23992

— 'Piping Systems — Oil Services'.

Pipework is not routed over the engine exhaust manifolds or in any position where it is at risk from fire in the event of leakage from pipe joints, etc. Provision for drainage by suitable trays and pipework is provided. Compression type couplings are not used where their failure constitutes a fire risk or a hazard to personnel or plant.

Adequate supports and flexible pipework are used to minimise the dangers of pipeline fatigue fractures due to vibration.

I mmediately after testing, the fuel oil system is treated with an inhibitor to ensure that the fuel lines are protected from rust, corrosion, scaling or any form of deterioration.

5.2.6 Inlet and exhaust air pipework, turbochargers and silencers

Aspiration

The aspiration system draws air from outside the engine room via duplicate filters with an air inlet designed such that 100% air flow to the engine at the design pressure drop can be achieved with one filter panel blocked or removed for cleaning. The design of the intake and the air approach velocity are arranged to avoid the ingress of moisture from any source as well as the draw-in of snow. -

The CEGB usually prefers engines with exhaust driven turbochargers to supply air for complete fuel combustion at maximum loading to achieve the full

The overspeed trip device is separately driven off the camshaft gear train. It operates independently of the fuel pump control rack, by actuating the fuel pump blowdown system, which clears the fuel and air locks the pumps.

The setting of the overspeed trip is arranged so that a temporary rise in speed due to instantaneous loss of full load, when operating with either governor setting, w ill not cause a trip condition.

5.3Generator and electrical equipment

5.3.1Generator design and construction

Reliability being a prime requirement, manufacturer's standard units are preferred which can be demonstrated to have a proven record of reliability. At times, it is preferable to install a standard unit, even if it does not fully comply with every detail of the specification, so long as these departures are identified at the tender stage.

The generator unit is designed to comply with BS5000 Part 99 and BS4999 Parts 3 and 51. The type of environmental protection provided, i.e., to BS4999 Part 20, IPW55, enables the machine to operate satisfactorily under fire protection deluge conditions (see Section 5.1.4 of this chapter).

The method of cooling preferred by the CEGB is that covered by BS4999, Part 21, 1C31, utilising a fan mounted on the generator shaft. This avoids reliance on separately driven auxiliaries such as fans and pumps. Cooling air is taken from and returned to the outside of the diesel generator house via inlet air filters and outlet air protective louvres.

For very large machines the above method of cooling may be inadequate. Under these conditions, a forced air, water cooled design to IC81 may have to be accepted, so long as the constructional requirements comply with ESI Standard 44-3 — 'Electric Motors

Specification (3300 V and above)'.

Insulation is specified to comply with BS2757 Class F. However, to achieve a long insulation life, temperature rises are limited to the lower Class B temperature.

Since the generator unit is required to operate suddenly after long periods at standstill, off-load heaters for anti-condensation protection, anti - corrosion protection, and the protection of bearings against transmitted vibration are provided.

The .generator unit is capable of continuously withstanding levels of imposed vibration from the engine of 0.25 mm peak-to-peak amplitude between 5 and 10 Hz and levels of imposed vibration velocity from the generator core in accordance with BS4675, Classification 11.2, between 10 and 200 Hz. These levels can be measured at any point of the frames or bearing pedestal.

Generator mechanical design

Generators are salient pole machines, with a revolving

field of the brushless variety, i.e., with shaft-mounted diodes as rectifiers for the provision of DC excitatio n current. The machirws are suitable for direct couplin g to the diesel engine and have their own self-lubricated bearings in accordance with ESI Standard -14-3, Sec. tion 5.4.1. The generators may share one bearing with the diesel engine. The bearings are split horizontally for ease of maintenance.

The rotor shaft is a single forging, machined all me r with an integral coupling flange. Because of the cycli c variation in the torque of a diesel engine, the rotor is capable of withstanding continuously a N. ibratory torque of +2.5 times the rated full-load torque over the range of 95 to 110% rated speed. It is also capable of withstanding +6 times rated full load torque when passing through critical speeds.

Means are provided to prevent the circulation or shaft currents through the rotor and bearings by the use of an insulated bearing pedestal.

On-site balancing facilities are provided.

Generator electrical design

The generator is capable of supplying, for one hour during any period of twelve consecutive hours' running, a load 10% greater than the nominal continuous rating.

As stated previously in Section 5.1.2, the generators are not normally required to operate in parallel with each other, but must be capable of operating satis. factorily in parallel with the station electrical auxiliary system for the purpose of full - load testing. Reactive kVA sharing equipment is therefore provided (see next Section 5.3.2). When operating in this mode, harmonic currents produced by the generator are circulated through the machine and other equipment. Such currents must be within the temperature rise capabilities of such equipment.

The 3.3 kV stator windings are star-connected, with the neutral bar connected to earth via a resistance to li mit the phase to earth fault current to 1000 A, to minimise damage under fault conditions.

Separate line and neutral terminal boxes mounted on opposite sides of the stator casing are usually provided, designed to limit damage to other plant and injury to personnel in the event of a through or internal fault.

The length of the stator may at times prohibit the fitting of such boxes direct to the stator casing because of their size. In these circumstances,

standing line and neutral terminal boxes are used. These are located adjacent to the stator and linked to it by

means of connections in ducts.

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If the stator is not solidly bolted to its foundations i.e., it is located on anti-vibration mountings, care is taken to incorporate the necessary flexibility into the cable connections to the terminal boxes, or to the ducted connections when separate free-standing terminal boxes are provided.

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•The operator manually initiates starting, run-up and excitation to 3.3 kV, using the appropriate facilities at the diesel generator local control panel.

•When the above sequence is complete, the speed and voltage controls available at the local panel or in the station central control room can be used prior

to synchronising, with the 3.3 kV system.

• Nfter the operator has satisfied himself that the diesel generator is ready for returning to emergency service, it is shut down manually and the selector switch is returned to position

Manual and automatic synchronising facilities are available in the station central control room and at the local diesel generator control panel.

Protection

The provision of initiating devices for automatically shutting down the diesel engine and tripping the diesel generator 3.3 kV circuit-breaker are restricted to those considered to be essential, bearing in mind the duty of the equipment and the requirement for high reliability. Wherever possible, the operator is given prior warning of an impending trip to enable him to rectify the fault before a trip occurs. A typical example is the lubricating oil temperature. Two temperature sensors are provided; the first and lower-set device provides an alarm only for high temperature and the second device operates on extremely-high temperature only to trip the diesel generator.

The following is a typical list of protection trips:

•Circulating current (generator stator phase faults and earth faults).

•Extremely inverse overcurrent (back-up protection).

•Excitation diode short-circuit.

•Excitation overcurrent (time delayed trip).

•Undervoltage (time delayed trip).

•Overvoltage (time delayed trip).

•Jacket water temperature high.

•Lubricating oil temperature high.

•Lubricating oil temperature low.

To shut down the engine, all protective devices act directly to shut-off the fuel supply to the engine by means of a solenoid valve. This is energised to shut-off the fuel and requires local manual resetting following operation.

Interlocks

Interlocks are provided on each diesel generator to protect plant and personnel. They are grouped in two categories as follows:

•Operational interlocks — those which operate in the automatic control equipment to maintain a cor-

rect relationship between various automatically con, trolled functions. They are sometimes known as 'sequence' interlocks.

•Essential interlocks — these are permanently i n service to protect individual items of plant. They generally trip the 415 V or 3.3 kV switchgear supply.

ing the devices they protect. They are sometimes known as 'safety' interlocks.

A l arm system

Each diesel generator local control panel is equipp e d with alarm annunciator equipment to draw the Op_ erator's attention to fault or abnormal operation of the diesel generator unit and its auxiliary systems. This provides local visual and audible alarms for the safe and efficient operation of the plant.

This alarm system is independent of the station service alarm system in the station central control room and self-contained in its operation. However, a limited number of important alarms are repeated to the control room. The remainder, i.e., alarms that require the control room staff to despatch an auxiliary plant a ■ tendant to the normally unmanned diesel generato, room to take the necessary remedial action, are grouped as repeat alarms to the control room.

The alarm system obtains its electrical supply from the 1. 10 V AC guaranteed essential instrument and control supply (uninterruptable supply).

Emergency stop

Each diesel generator is provided with an emergency pushbutton switch mounted on the local electrical panel. The pushbutton switch operates into the engine shutdown system described above, including tripping of the 3.3 kV circuit-breaker. The pushbutton switch is of the 'stay put' type, i.e., after depressing, the contacts stay closed until the switch is reset by means of a key normally held by senior authorised personnel. This ensures that operation of the pushbutton switch is fully investigated by management before the diesel generator is returned to service.

5.3.4 Control of auxiliary systems

Lubricating oil priming system

The priming pump automatic control system incorporates adjustable timing equipment to start and stop the duty priming pump, to achieve the engine manufacturer's recommended priming cycle. In some cases, priming is continuous and, if the duty pump fails, the ti ming equipment signals are automatically transferred to the standby pump.

Total failure of the lubricating oil priming system is detected by a pressure switch, which initiates a local

audible and visual alarm. This alarm is interrupted by the ti ming equipment when priming is not in operation.

Where intermittent lubricating oil priming is pro - vided, it is interlocked with the diesel generator auto-

Lubricating oil and diesel engine water jacket heaters

Where such heaters are provided for use during standstill periods of the diesel generator (see Sections 5.2.2 and 5.2.4 of this chapter), they are controlled by thermostats. Usually three thermostats are provided for each heater or group of heaters, i.e., control, safety (if the control thermostat fails to switch off the heater) and low temperature alarm (if the heater or its associated control thermostat fails).

5.4 Testing

Throughout works, site and operational testing, emphasis is placed on tests that show that the plant complies with the specification and is capable of performing its emergency role under all conditions of operation.

5.4.1 Tests in manufacturer's works

In general, the object of the works tests is to show that the plant complies with the specification, schedules and previously approved drawings.

Prior to tests taking place, detailed test schedules are agreed between the manufacturer and CEGB staff. The latter usually witness the principal tests, whilst other tests can be carried out by the manufacturer, so long as the results are submitted to the CEGB for scrutiny.

.1u.k-iliary plant

As far as is practical, all items of auxiliary plant are fully tested in the respective manufacturer's works before assembling them with the diesel generator. Such tests include hydraulic pressure tests on oil coolers, valves and piping, water coolers and tanks, air compressors and their auxiliaries. All rotating plant, such as air compressors, and all types of pumps are subjected to running tests over the full specified range of duty. Tests on motors prior to assembly with their driven items are covered in Chapter 7.

Generators and exciters

The following tests are carried out on each generator assembled for running with its exciter and driven by a temporary motor, in the manufacturer's works, before coupling to the diesel engine:

•Demonstration of compliance with vibration requirements.

•Determination of generator magnetisation and loss curves, using exciter field current as a variable parameter.

•Resistance and insulation resistance of all windings.

•High voltage test on all windings.

•Insulation and high voltage tests on all wiring and equipment.

•Heat runs on open-circuit, short-circuit and full

kVAr loading to determine temperature rises of ge n , erator and exciter.

To prove the design fully, the first generator of each size in a particular contract is subjected to the following additional tests:

•Using test slip rings, determination of generator and exciter open-circuit and short-circuit and loss curves.

•Oscillograph and analyser measurement of the gen,

erator phase and line voltage at no-load, no rma l voltage.

•Sudden three-phase short-circuit applied to generator

terminals with generator running at no-load, normal speed and 100 07o voltage. From this test reactances

are calculated from the current envelopes.

•Check of radio interference suppression.

Combined tests on complete diesel generators

After checking such details as cylinder head nut torques, tappet clearances, pump drive chain tensions, crankshaft deflections, correct charging with oil and correct functioning, charging and priming of all auxiliary systems; each diesel engine, fully assembled with its generator, exciter, governor, AVR and auxiliary equipment, is then subjected to the following combined tests:

•Alignment check of complete units.

•Cold, standby and hot starting tests. At least five starts are performed for each condition, defined as follows:

Cold Engine at ambient temperature with no auxiliary oil and water heaters in operation

Standby Engine at temperature attained by operation of oil and jacket water heaters at normal thermostat settings and normal ambient conditions

Hot Engine at a temperature as attained not more than five minutes after shutdown from a full-load run with stabilised temperature rises

•Run at normal speed and highest overspeed permitted by the manufacturer for five minutes to check the balance of rotating parts and compliance with vibration requirements. A typical figure for highest permissible overspeed for 5 minutes is 7/7o.

•Run at full rated output by use of artificial electrical loading facilities for a sufficient length of time to

ensure that steady working conditions have been reached, followed by a one hour run at 10 010 over-

load. Records of temperature attained on the unit are taken.

•Fuel consumption tests.

•Governor tests, including load acceptance and rejection tests.

• lnsulation and high voltage tests on all engine- mounted wiring and equipment.

I: should be noted that the full output, fuel consump- , ii, waveform and no load endurance tests are carried Lt on the first unit of each type and size only.

Dependent on the environmental conditions specified

:or J particular station, the diesel generator tests are

.1 , o sometimes carried out to ensure compliance in

. :ie following areas:

•Seismic withstand.

•I Iiirh wind withstand.

•Proof against fire protection sprays.

5 4.2 Tests at site

i! miunie loads complies with manufacturer's guaran- --.,. Test results are compared with those obtained H.'Lng. works tests and any significant deviations are

... .e , tigated and rectified.

Prior to the site testing phase, the manufacturer

_! 7', e‘i the format of the detailed test schedules with CEGB, All testing is carried out in accordance with

schedules, which are completed and witnessed

he manufacturer and the CEGB site construction

.- ;resentatives at the time of the test. These schedules

.ice subsequently made available for scrutiny by the f( iB operating staff, the Nuclear Installations In-

.[Orate or other bodies, as appropriate. ')Lte testing is subdivided into three stages:

•Test the insulation resistance of power supply cables, motors and small wiring, together with associated control and alarm devices.

•Check the direction of motor rotation.

Earthing:

•Physical inspection of all earthing connections to electrical equipment metalwork.

•Ensure that the diesel generator unit earthing system is correct to drawings and connected to the main station earthing system.

Pipework:

•Operation of valves.

•Pressure testing.

•Comprehensive purging to remove dirt and debris that may have entered during transport to site and erection.

Pumps, compressors, fans, etc.:

•Check the duration and magnitude of the starting current.

•Check for correct operation and setting of control safety and alarm devices.

•Raise system to normal pressure and check for leakages.

•Recheck operation of control, safety and alarm devices.

Tanks and air receivers:

•Carry out an internal inspection for cleanliness and absence of damage to internal linings.

•Check the overflow/blowdown arrangements.

•Check for the correct operation of control, safety and alarm devices, in conjunction with associated pumps/compressors.

•Cheek level/pressure gauges, both local and remote.

Engine speed controls:

•Check the operation of speed setting motor.

•Set operation of electrical and mechanical governor for service.

•Check the operation and setting of the overspeed trip switch.

Engine mechanical checks:

•Recheck cylinder head nut torques, tappet clearances, pump drive chain tensions, crankshaft deflections, engine and generator shaft alignment, correct charging with oil, etc,

•Charging, priming and venting of auxiliary systems.

•Check engine pressure/temperature sensors.

Generator:

•High voltage tests.

•Retest insulation resistance and check polarisation index of stator winding.

•Check anti-condensation heaters and control circuits.

•Set and adjust AVR for service.

Local control and alarm panels:

•Check AC and DC auxiliary supply voltages and polarities.

•Check the pick-up and drop-off values of relays.

•Check sequence functions of relays.

•Check functioning of alarm annunciator equipment, including accept, reset, lamp test facilities and the audible alarm device.

Stage 2 — Isolated system tests

Again, this is only a brief summary of a detailed check list agreed hetween the CEGB and a manufacturer.

Local alarms:

•Check operation of all alarm initiating devices around the plant in conjunction with local alarm annunciator equipment and separate or grouped alarm outputs (as appropriate) to the station central control room alarm system.

•Check correct operation of generator circuit-breaker,

field suppression switch and fuel shut-off valve f rom all trip devices on the diesel generator.

Controls:

•Check diesel engine remote start and stop controls, including automatic starting circuit.

•Check governor and AVR remote 'raise' and lo+Aer, `manual' and 'automatic' controls.

•Carry out secondary voltage injection on AVR,

Engine first run:

•This generally follows the manufacturer's standard commissioning and setting - up procedures for no-load running.

•Record a series of measurements during engine starts to establish the cranking speed at which firing occurs.

•Monitor and record various air, water and oil temperatures around the unit until all parameters have stabilised.

Governor tests:

•Check engine run-up and shutdown on both the electronic and mechanical governors.

•Check engine behaviour on simulated failure of the electronic governor.

•Record the rate of rise of speed and governor action during the above tests.

•Carry out tests to establish the time taken to vary the engine speed over the range of ±5% of synchronous speed with increasing and decreasing speed.

•Take engine to overspeed and check the overspeed trip setting.

•Check range of speed control from the control desk.

AVR tests:

•Run-up and shutdown the unit to record cut-in and cut-out points for excitation, and check rate of rise of generator output voltage.

•Run-up the unit with excitation diode short-circuited to prove protection.

•Run-up the unit with excitation diode open-circuited to prove alarm.

•Check excitation failure trips.

•Check range of AVR settings.

Loading tests:

Protection:

•Primary inject current transformer circuits to check balanced current and earth fault protection, including setting of associated relays and AVR current transformer circuit for reactive kVA sharing.

During commissioning of diesel generators in a new power station, it is unlikely that sufficient load will be available to fully load the unit. It is therefore usual to bring to site the artificial electrical loading facilities used during works testing. This also provides greater