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

GENERATOR

EXTRACTION

PUMP

GLAND

STEAM

CONDENSER

Vici, 11.6 A simplified feedlicaling sysiem

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On detection of loss of feedwater from either one or two of the electric feed pumps, a conditional 'l oss of boiler water' trip is automatically initiated and will be effected, unless the turbine feed pump is run-up and delivering within t1 seconds.

•One or two electric feed pumps in service — load not in excess of that met by one electric feed pump capacity. On detection of loss of feedwater from one electric feed pump, a conditional 'loss of boiler water' trip is automatically initiated and will be

effected, unless the other electric feed pump is delivering rated flow, or the turbine feed - pump is trailing and is brought manually to rated delivery within t2 seconds.

During start-up, the boiler is protected against a low drum water level by the 'drum level' low trip and the feeciwater trip is vetoed for the following reason.

On initial light-up, the electric feed pump may not be required to deliver feedwater to the drum and will not be put into service until it is required later on. This is so because some units are able, during initial light-up, to generate sufficient steam for a supply to the turbine feed pump to enable it to meet feedheater requirements. In these circumstances, it is necessary to veto the trip initiation from loss of feedwater in order to avoid a permanent boiler firing trip being established.

The 'loss of feedwater' trip is therefore conditioned by the generator HV circuit-breaker, as shown in Figs 11.7 and 11.8, to remove the trip whenever the generator is on open-circuit. Under these circumstances, the boiler is protected against a low drum level by the low drum level' protection.

4.5.3 Boiler circulating pumps — unconditional signal

On boilers which require forced feedwater circulation between the drum and the boiler furnace, all available boiler circulating pumps are operated at all loads. The boiler system is designed such that full load can be met using three out of four circulating water pumps (Fig 11.9). Therefore the minimum number for avoidance of a trip is dependent on whether the unit load is above that at which at least three pumps are required.

When the boiler is operating at loads above this level with only three pumps in operation, the loss of a further pump will automatically initiate an unconditional 'loss of boiler water' trip. When the boiler is operating at or below this load with only two pumps running, the loss of one of these pumps automatically initiates an unconditional 'loss of boiler water' trip. The scheme shown in Fig 11.9 satisfies these trip requirements:

HV CB

AUX SWITCH

..■■■■

B

0

BOILER

FIRING

TRIP

RELAY

TIME DELAY

RELAY

TIME DELAY

RELAY

EMERGENCY FEED PUMP

EMERGENCY START RELAYS

4.5.4 Sudden loss of steam demand (turbine trip}

This form of protection applies equally to conventional and nuclear plant.

The control of the turbine steam valves is by a fluid pressure system which holds the valves open against a Spring load. Release of this pressure operates pressure switches in the fluid system. Referring forward to Fig 11.36, which shows a tripping scheme for nuclear plant; operation of the pressure switches via time delay relay TDR1 contacts, operates trip relay 7, which sends a Signal to trip the steam generator. If any of the 400 kV disconnectors or circuit-breakers are open, then TDR1 is de-energised and prevents a trip of the steam generator when the turbine-generator is not connected to the transmission system. This allows for a turbine trip when the generator is off-load without tripping the steam generator. The time delay provided by TDR1

ensures that tripping of the steam generator is not prevented by opening of the circuit-breaker from a unit trip, which sends a trip to the turbine and the circuit-breaker at the same time.

By tripping the steam generator following a turbine trip when the generator is on-load, its tubes are protected against overheating. Excess steam will exhaust to atmosphere. It is to be noted that if the generator is deloaded and a steam generator trip is not required, the operator must open the generator circuit-breaker first before tripping the turbine.

5 Turbine protection

As with the boiler, faults can occur on the turbine and its auxiliaries which can be isolated without tripping the turbine: Volume C, Chapter 2 'Turbine plant systems'

valve actuators, and diverting the fluid remaining i n the actuators to flow to drain. Volume C, Chapter 2 describes the system in detail, but the basic operation of the hydraulic control system is as follows.

Fluid pressure is required to hold the turbine sl op and governor valves, which control the steam suppl y to the turbine, open against spring pressure, in th e

absence of fluid pressure, the springs close the steam valves. Hydraulic control fluid is supplied to the ste am

valves via two emergency trip valves, and the turbine is tripped by the operation of either of these valve s diverting the hydraulic fluid to drain. The trip plunge rs in the emergency trip valves are held in the clo sed position by means of a spring which is held corn. pressed by a trip latch. This latch is released either by energising the trip solenoid or by acting directly on the trip latch from the overspeed bolts or the manual trip lever. This causes the emergency trip valve to op en , isolating the supply of hydraulic fluid to the turbine valve operating gear, and putting the hydraulic fluid to drain, with consequent loss of fluid pressure and closure of the main steam valves.

5.1 Turbine trips

The following turbine protection devices trip the turbine;

•Loss of lubricating oil.

•Condenser vacuum low (exhaust pressure high).

•Condensate conductivity high.

•Manual trip lever.

•Overspeed trip.

•LP exhaust steam temperature high.

•Loss of electric governor.

•Low steam inlet temperature and pressure.

They are typical of modern plant. A full description of the systems is given in Volume C, Chapter 2.

The turbine trip systems are designed to give full redundancy of trip initiating devices (Design criteria, Section 2 of this chapter) and on-load testing up to the emergency trip valves. Weekly on-load testing of the turbine emergency trip valves is essential as part of the procedure to avoid an uncontrolled overspeed of the turbine for failure to close the main turbine stop valves. On-load testing is done by isolating each emergency trip valve solenoid in turn and using the protection devices to operate the isolated trip valve.

5.2 Loss of lubricating oil pressure

The fluid for operating the turbine valves, forms a separate system from the lubricating oil system. The oil in the lubricating system is circulated by a centrifugal type pump, directly driven from the turbinegenerator-

shaft (Fig 11.11). This pump receives oil under pressure from an oil-turbine-driven boost pump and delivers

oil by way of the oil turbine to the turbine-generator bearings. The main oil pump also delivers oil direct to the generator hydrogen seal oil system.

Pressure drop through the oil turbine reduces the oil pressure to that required for the turbine bearings. The oil turbine is mechanically coupled to the boost

pump, which has its suction flooded in the main oil tank and. delivers ail to the main oil pump inlet. An oil tank ountedm AC motor-driven lubricating oil pump

is also provided to supply oil to the turbine-generator bearings during run-up and rundown of the unit.

Falling lubricating oil pressure to the turbine-gen- erator bearings automatically starts the AC motordriven lubricating oil pump. If this pump fails to start,

or completely loses its pumping, further reduction in oil Pressure to the bearings will cause a DC motor-driven

oil pump to start automatically and, at the same time, a turbine-generator trip is initiated.

Turbine tripping is initiated directly by loss of lubricating oil pressure. Duplicated spring-loaded trip cylinders in the front pedestal are pressurised by the lubricating oil pressure. On loss of bearing oil pressure, oil is released from the trip cylinders, and mechanical linkages displace the trip latches of the duplicate trip gear. This causes the emergency trip valves to operate. Tripping is delayed by the action of a deadweight accumulator which maintains minimum supply pressure during transient pressure fluctuations. Each trip can be on-load tested.

5.3 Condenser vacuum low (exhaust pressure high)

Each of the two direct-acting vacuum trip units consists of two sensing elements. Each of the four elements compares condenser vacuum with absolute vacuum, and

both elements of either unit are required to sense low vacuum before direct tripping of the turbine can occur. The turbine tripping is initiated when either of the low vacuum trip units senses tow vacuum and, by direct mechanical action, causes lubricating oil to be released from a corresponding trip cylinder (same one as for lubricating oil). This then causes the corresponding emergency trip valve to operate. The system can be on-load tested by admitting air to its sensing elements: this includes operation of the trip valve. Operation of a by-pass interlocking valve prevents operation of the main stop valves. This valve has to be in the by-pass position before on-load testing can. be carried out.

The above arrangement causes a trip of the unit by fluid pressure switches in the hydraulic control fluid supply to the emergency trip valves. This trip is routed via the low forward power relay and is therefore time delayed. Motoring of the set under low vacuum conditions would cause rapid overheating of the turbine blades. This situation could arise if the low forward power relay interlock fails to close. A direct electrical trip from the low vacuum to the generator circuitbreaker is therefore provided: this has to be done electrically since operation of the power fluid pressure switches is a delayed trip. The following explains how the electrical trip for low vacuum on a modern 660 MW turbine-generator is achieved.

Turbine tripping is effected electrically by pressure switches which monitor the vacuum of the three

condensers. The two vacuum lines are brought out downstream of an arrangement of a ball shuttle valve, which automatically selects the condenser with the poorest vacuum and connects it to each of the two vacuum lines. The three condensers are interconnected by balance pipes, and hence trip initiation occurs if the vacuum in any condenser falls below the trip setting. Four pressure switches are provided, arranged in a 'two from two' logic configuration per trip channel.

5.4 Condensate conductivity high

Turbine tripping is initiated by conductivity transmitters used to monitor condensate contamination at the outlet from the condensate polishing plant and from the condensate extraction pumps discharge pipework. A set of four transmitters is provided corresponding to each location and these will operate in a 'two from two' logic configuration per tripping channel for each set of transmitters. Each transmitter can be checked on-load, in turn, by adjustment of its set point.

5.5 Manual trip lever

The turbine manual trip lever mechanically forces both of the trip linkages connected to the two emergency trip valves into the trip position. The trip latches become displaced and cause the associated emergency trip valves to operate.

valve has nominally closed could be sufficient to cancel the motoring power, leaving the machine to float or run generating at low power, which is why a reverse power relay would not operate and is not suitable. The operating limits chosen are 0.2% to 0.7% forward power.

Sensitive low forward power relays, such as the Brown-Boveri type PPX 110/111, have been specifically designed and approved for this purpose. Power measurement is by three-phase power measurement at rated voltage. The principle of operationis explained with the aid of a block diagram (Fig 11.13). The voltage and current transformers of the generator are connected to the interposing transformers A or B, respectively. These transform the input values to the necessary level for the relay electronics. The current signal is converted into a squared voltage signal and during the negative half-cycles it is switched to the low pass filter by means of the field-effect transistor switch. This determines the linear average value and generates a DC voltage proportional to the active power. The phase correction network serves to compensate the phase angle errors of the measurement transformers. For three-phase connection the changeover switch on the summing amplifier must be set in position 3WM, and for two-phase connection (2 wattmeter method) in position 2WM.

The summing amplifier adds the voltages proportional to the phase loads. The voltage at its output,

proportional to the three-phase power, is fed to the trigger which has an adjustable setting. Load selection is by means of the max/min switch located immediately after the trigger device.

Blocking gates are provided to inhibit relay operation

if:

•The power supply is too low.

•The relay is blocked externally.

For relay type PPX110 the operating signal passes via the timing element and, if required, via bridging link (2) to an auxiliary relay. A second parallel route for the signal is via an inverter to the output terminal.

On the type PPX110 relay, tripping can be delayed by 0.5-5 s and indicated by an LED; the setting normally chosen is 2 s. The auxiliary relay can be applied to supervise the 15 V stabilised power supply by closing the bridging link (3).

Two timing elements are supplied with the type PPX1 11 relay, one delays the operating signal by 0.5- 5 s and the other by 5-50 s.

The following protective devices operate through the low forward power relay:

•All turbine trips, except the low vacuum electrical trip of the trip valves.

CONNECTIONS

TO MAIN

GENERATOR VOLTAGE AND CURRENT TRANSFORMERS

INTERPOSIWG

TRANSFORMERS

PHASE

CORRECTION —0--

NETWORK

FIELD-EFFECT

TRANSISTOR

SQUARE SWITCH

WAVE

CONVERTER

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•All generator mechanical trips, plus the generator electrical trips, which are described in Section 6 of this chapter.

5.7 LP exhaust steam temperature high

If the vacuum is low and the spraywater system is being used to keep the turbine blades cool, a measurement of exhaust steam temperature is an indication of the failure of the spraywater system. This is not always a unit trip and depends on the turbine manufacturer. The spraywater system has its own protection; if it fails, the turbine is tripped and this will trip the unit through low hydraulic fluid pressure.

5.8 Loss of electric governor

The latest turbines are fitted with electronic governors employing a redundancy system of electrical control channels in order to provide adequate security. As a final protection, in the event of failure of a sufficiently large number of channels to invalidate the redundancy system, or of any other failure which renders the governor inoperative, a signal will be given to trip the unit via the low forward power interlock relay.

5.9 Low steam inlet temperature and pressure

Consideration of an incident in 1960, where a turbine oversped out of control, suggested that a loss of firing occurred and the feedwater regulation system was unable to cope with the transient. The result was water carryover that caused the turbine to overspeed. In that combination of circumstances, it was reasonable to contend that the major fault was a loss of firing and that the best available indication of this condition was low steam inlet pressure at the turbine. An unloading system had been evolved in 1954 for application to 60 MW and higher rated turbine-generators. The principle was to commence deloading at 90 07o of normal operating pressure and to reduce the load to 10 07o MCR by the time pressure had fallen to 85% of normal. Hence the unloading gear offered reasonable protection against water carryover, since the most probable cause of carryover was the combination of feedwater regulation or partial firing. Later, in 1969, a form of tripping from low steam inlet pressure was recommended. However, due to the low reliability of the system, which caused a number of spurious trips, the equipment was taken out of service in the mid-1970s.

The principle of low steam inlet pressure protection, whilst being acceptable when the grid was supported by a large number of small units which operated at fixed pressure in the 70% to 100% MCR range, was not suitable for newer modes of operation, which included sliding pressure operation. To continue with its application therefore required an updating of the design,

refurbishment of the hardware and the application of sophisticated veto facilities. This was considered in . appropriate and expensive and therefore the discon_ tinuation of the application of low steam pressure deloading and trip equipment on high pressure fossi1. fired plant was recommended on all 500 MW and 660 MW units, the main grid supporting stations.

This change in protection philosophy left the turbine at risk to water carryover in the event of a total los s of boiler firing. It is now standard practice on all the 660 MW units at conventional power stations to design on the basis of the operators taking corrective action, which must be done within I minute. To facilitate this, the following features are provided:

•A suitable alarm is initiated from a total loss of firing.

•The status of the flames in the furnace is displayed.

•The pertinent steam pressures and temperatures are displayed.

•An emergency trip pushbutton is provided.

All the above facilities are mounted on the appropriate unit control panel/desk within a single viewing angle.

Nuclear power stations, with once-through boilers, have a short time constant for the passage of steam from the boiler to turbine, and there is insufficient ti me for the operator to be sure of preventing water carryover to the turbine for abnormal boiler conditions, as in conventional stations. There was therefore no alternative but to provide an automatic trip by the inferred measurement of steam saturation from readings of steam temperature and pressure.

6 Generator protection

The protection arrangements for the generator necessarily include the main connections and transformer windings connected at the generator terminal voltage. The generator transformer protection arrangements will overlap with generator protection, especially where the generator is directly connected to its transformer. The generator protective systems are listed on Fig 11.35.

6.1 Stator earth faults (low impedance earthing)

Prior to 1950, established practice was to earth the generator star point using a voltage transformer, the secondary of which generally operated an alarm. Following a number of serious breakdowns in the gen - erator windings during 1950 and 1951, this practice was discontinued and replaced by low resistance earthing using a liquid earthing resistor. The aim was to li mit the fault current to 300 A for all sizes of gen - erators. The time rating of the resistor was 30 seconds , although 10 seconds was allowed if difficulty in ae-