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

Protection

Protection

FREQUENCY

RELAY

STAGE I 52 Hz

E

STAGE 2 < 49 Hz

Pumped-storage plant protection

:11 ,1k:zing a failure of the governor control system to

Operator action only is needed for 'stator water coolant

the speed.

flow low'. A trip initiation is, however, required for

ing a onmp-turbine start-up, a protection trip

high stator cooling air temperature. The temperature

Dur

detector is set to operate at a temperature of approxi-

from the turbine control system is sent if the

r being started reaches a speed in excess of 110%.

mately 65 ° C. The inherent thermal capacity of the

machine allows for unloading before initiating a unit

Loss of pumping power

trip, i.e., the guide vanes are closed before tripping

the generator voltage circuit-breaker. Interlocking duty

power fails whilst the machine is in the pumping

by the guide vanes compares with the low forward

j c ,thC water flow reverses in the turbine, causing

power interlock on fossil-fired and nuclear

is achieved by the DC tripping system.

An immediate trip of the unit is needed to close

main inlet valve and bring it to a safe condition.

10.10 Bearing temperatures and oil levels

A relay is provided to measure output power from

g enerator: this is interlocked with an auxiliary

The guide and thrust bearing temperatures are moni-

on the inlet guide vanes such that it is only

tored and an automatic unit trip initiated, after first

when the inlet guide vanes are open. The setting

closing the guide vanes, if temperature limits are

j , s( below the power required for the condition when

exceeded.

main inlet valve is open and the runner watered.

ni [s power level is estimated to be 63 MW.

10.11 Back to back starting protection

During back to back starting, protection is required

10.6 Emergency stop pushbuttons

against three conditions:

emergency stop pushbuttons are provided, one lo-

• Generator runaway.

oito the turbine-generator and one on the control desk.

• Incorrect excitation levels on the generator-motor.

10.7 Overvoltage

• Excess heating of the stationary field winding in the

event of failure of a generator-motor to start.

ilc,:ause of the overvoltages expected if Dinorwig is

.olated from the main system, with either or both

10.11.1 Generator runaway

':[ies to the substation at Pentir still connected, and

This condition can result from:

'he capacity of the generator-motor/pump-turbine to

erspeed by 50%, an instantaneous overvoltage relay

• Incorrect excitation.

used to protect the supply transformers connected

• Excess generator accelerating torque.

J i 18 kV. The setting is 130% of nominal voltage.

Overvoltage protection for the generator is provided

• Excess motor retarding torque.

AVR and is set for 120%. It is inhibited when

le machine is synchronised to the system.

This protection is achieved by setting an overcurrent

relay operated from a CT in the tee-off to the start-

Excitation equipment protection

ing system (Fig 11.33). This is designed to trip the

unit if the current exceeds 8000 A for longer than

Protection of a sustained DC short-circuit is provided

30 seconds, or to trip instantaneously if the current

a definite time overcurrent relay operated from CTs

exceeds 21 000 A. The basis for the settings is that

3.3 kV circuit-breaker, tripping that circuit-

if the two machines 'lock' together on a back to back

- reaker only. Thyristor short-circuits are taken care

start, the current will fall below 8000 A before 30 s and

by the thyristor fuse. All cable phase faults from

will never exceed 21 000 A. So if the current exceeds

're excitation transformer to the 3.3 kV circuit-breaker

21 000 A (approximately 2 x full load), this indicates

nid all LV winding phase faults are detected by the

an electrical fault on the starting busbar system: hence

1 IV (18 kV) extremely inverse overcurrent relay. Over-

this protection deals with busbar faults. At the design

protection on the excitation transformer pro-

stage, busbar protection was considered, i.e., a bal-

'Js back-up protection of the excitation equipment.

anced system of protection, but with the added com-

Protection co-ordination must take account of the

plexity of trying to obtain a balance at low and nominal

airrent/time characteristic of the machine field, as well

frequencies it was likely to be unreliable. The instan-

1 ■ that of the fuses protecting the thyristors.

taneous overcurrent relay is more reliable although

not as discriminative. However, with the high integrity

10, 9 Stator cooling air over-temperature

designed into the starting busbars, using air insulated

and isolated connections, a busbar fault is very unlikely

Ihe stator is air cooled, with water cooling the air.

to OCCLIr.

Protection

Protection

10.17.2 Category B trips

M valve drains water away from the peripheral

Category B trips result in a controlled shutdown of

chamber to prevent water build-up between the GVs

the unit and include the following areas of protection:

from the spiral casing in order to prevent a build-up

GV out of step This protection is initiated if

of air which would otherwise be passed through

blockage of, or damage to, a single GV causes it to

the system when normal operational modes wer e

move out of step with the other GVs.

instituted. The F valve is used to bleed air out of

G V start position 2 exceeded with generator circuit-

the pump-turbine casing when making the transition

from spin mode to operational modes. If any of

breaker open If, under starting conditions, 'start

these valves are open during pumping or generation,

position 2' is exceeded after the turbine has attained

a controlled shutdown is initiated.

90 0'0 speed, the unit is shut down because turbine

acceleration would not be reduced and synchronising

M valve and P valve closed in spin in ode If either

the M or P valve is not open in the spinning

mode, an unacceptable air and/or water accumula-

tion can occur in the pump-turbine, the unit is there-

and spin modes, a unit shutdown is initiated if

the water level under blowdown conditions is too

high to permit the pump-turbine to spin in air only.

Upper/lower reservoir levels high When the water

level in the upper reservoir (pumping mode) or the

lower reservoir (generating mode) reaches the maxi-

mum permissible level, the unit is shut down.

cooling water flow to the wearing rings when

tiated for any failure of the pump start-up sequence

operating in pump start or spinning in air modes,

causes the unit to shut down to prevent damage to

to achieve a given plant state within a predetermin ed

the wearing rings and possible damage to the rotor.

ti me or number of operations. Possible causes In-

clude excessive pump priming time, excessive pump

loading time, second failure of pump synchronising

tion against damage to wearing rings.

or excessive pump start - up time.

spin pump to pump transition On-load air admis-

sion is required during generation modes but is

permitted during pumping modes. In circumstances

oil level low Operation of the unit in the pumping

where the air admission valve is open during

or generating modes requires the MIV to be fully

pumping modes, the unit is shut down.

open. fn the event of a loss of oil pressure the MIV

would start to close under the action of the counter-

• Blowdown air admission valves open in pump service

weights and a controlled shutdown is required. The

or in the spin pump to pump transition

low oil level trip backs - up the low pressure trip and

blowdown air admission valves are open when the

main pump isolator is closed and the MIV is not

pipework in the event of incorrect precharge for the

fully closed, the unit is shut down to prevent air

air pressure and a controlled shutdown must be

admission to the system during pumping.

Headgate, stopgate or tailgate closing Operation

11 DC tripping systems

with gates closed (or closing) is unacceptable so the

unit is shutdown under these circumstances.

11.1 Logic diagram

The logic diagram (Fig 11.35), which is typical of

This shutdown protection operates to prevent over-

heating of the various CW-cooled plant items. Above

stations with a generator voltage circuit-breaker, lists all

10 07o speed an alarm only is raised which requires

the unit trip-initiating devices and details which plant

has to be tripped to bring the unit into a safe condition.

The groups formed by plant items to be tripped are

explained down the left hand side of the drawing.

In normal operation the MIV must be fully open,

Group 1 and Group 3 contain the same plant items,

if it does start to close for any reason, the unit must

low forward power relay. Group 1 is

valves are used during spinning in air modes; the

Category A trip and Group 3 a Category

DC tripping systems

or de r to cater for several primary circuit con-

all CEGB power stations have used this format for

la acions an d to avoid confusion, the logic diagram

their tripping systems except Dinorwig where, because

(he following format:

of the complexity of the tripping circuits and the need

.,„,ps 1 and 3 are always Category A and Category

to reduce the number of trip relays, diodes were used

to connect trip initations to trip output relays, each

one allocated a tripping function.

Matrix tripping systems are being considered by the

and 4 arc also always Category A and

CEGB, especially where integrated generator circuit

B but apply to those stations requiring

protection systems, with built in test facilities, are being

open ing of the generator voltage circuit-breaker

offered. The matrix effectively copies the logic diagram.

c lea r the fault, leaving the generator and unit

The trip initiations are connected to horizontal bus

,Iiisforrners connected.

wiring and the output devices to the vertical buswires.

The intersections of the vertical and horizontal bus.

c arc the four basic tripping groups. Groups 5,

wires can be coupled by diode links represented by

•1r e special but always maintain the same identity

plugs on a matrix board.

All the trip relays, or any repeating relay which is

Fa ji trip initiating device has a unique number and

receiving signals from outside the relay room, must

have a minimum operating current of 50 mA to avoid

operation from induced current or capacitor discharge

currents. A 10 p.F capacitor charged up to 150 V is

discharged through the relay to check for non-operation

the turbine tripping shown on the top left hand side

on capacitor discharge currents from the negative pole

of the battery.

c. The turbine tripping sequence is described in

11.3 Trip supply and circuit supervision

Only trip supply supervision is provided on trip relay

circuits with looping of the wiring to the positive and

negative poles (Fig 11.37).

Excessive looping through multicore cables is not

used, thus avoiding the consequent large voltage drops

in the wiring which would prevent trip relay operation.

ang completed the logic diagram, the next step is to

Trip circuit supervision is provided on all turbine

and circuit-breaker trip coils because of the possibility

tripping system. Modern practice is to provide

of open-circuits on these coils. Figure 11.38 shows a

simplified circuit. Where the trip coil and associated

same. In those cases where the protective relay

B relay is duplicated (which has been done on the

majority of recent schemes), it is not possible to monitor

both coils in the 'breaker closed' position since either

of the tripping systems.

circuit will maintain a trip healthy indication. However,

the allocation of protection systems to each tripping

there are two trip coils and therefore one can fail to

operate. Furthermore, in the pre-open supervision,

h all the trip initiating devices, in groups as allocated

circuit-breaker fail and phases out of step protection

he logic diagram, down the left hand side and the

provides additional back-up protection.

. put trip relays down the right. Where possible, trip-

:1 of several trip relays are required to perform a

11.4 General comments on the tripping

".41) trip. The basis of the selection of a trip function

arrangements

a relay is to allocate all functions which have to

lri ppeçl following the operation of the low forward

Figure 11.36 represents the basic tripping scheme as

%er interlock to the main tripping relays (sometimes

used on nuclear power stations. It is easily adapted for

conventional stations with very little change in the

format.

All protection relays are ordered from this master

rile trip signals allocated to Category B trip the

tripping schematic diagram. The interfaces with the out-

put relays on the turbine and reactor are also shown.

"rd Power interlock relay, they trip the main trip

The block for reactor tripping (Group 4) shows a full

1 and 2. Pressure switches in the turbine fluid

'two from four' quadrant trip. Each of the smaller blocks

cia Operate the steam generator trip relays. To date,

Al, A2, etc., represents a 'two from three' quadrant

Protection