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

•Instruments required are relatively basic and unlikely to fail.

•Common mode failure cannot occur when manual synchronising is used as the back-up to automatic synchronising (as may happen if back-up is provided by a second automatic synchronising relay).

•Low capital cost of the equipment, if considered in relation to its wide application.

In summary, manual synchronising supervised by a Lheek synchronising relay is appropriate for the less demanding requirements of system synchronising, as called for in periodic maintenance switching, etc. Manual synchronising has an advantage as back-up for generator synchronising where automatic synchronising is the normal method and also for use at switches where generator synchronising is expected to be in-

TABLE 12.1

Synchronising method for each type of generator

(i)May be done super-synchronously if required for operational reasons

(ii)Using starting equipment (automatic synchronising relay does not control the machine speed)

matic synchronising relay may be used for diesel generators. As described later, this is a non-essential duty.

The advantages of using an automatic synchronising relay are as follows:

•The complete synchronising operation is carried out automatically.

•Synchronising is performed consistently, with a degree of precision and final accuracy which cannot be approached by manual means.

•Synchronism is achieved rapidly, minimising the synchronising time.

•The resultant disturbance to the generator and system after the switch closes is kept to a minimum.

•Synchronising can be incorporated into a comprehensive unit automatic start-up sequence.

•Operator work load is reduced when synchronising has to be performed at regular intervals or a number of generators need to be synchronised in succession over a short period of time.

•It is capable of operating with falling voltage and frequency.

•The risk of human error is largely removed.

•The demand for skilled operating staff is reduced.

•It permits the synchronising of remotely operated plant at unmanned generating stations.

For the above reasons, especially accuracy, automatic synchronising is the normal method of steam turbinegenerator synchronising. Usually, reasonable time is available for this operation, but it may have to be done quickly when system conditions demand rapid connection of a generator for system security or load despatch reasons.

Automatic synchronising is essential for rapid synchronisation of the gas turbines used for emergency generation, which are required to be brought from rest to full-load in less than two minutes. In the event of a transmission system disturbance causing a fall in frequency, a low frequency relay initiates starting at a preset value. As a design basis for operation, the severest system conditions are assumed to be a frequency fall of I cycle every 25 seconds down to 40 Hz and a voltage fall of 1.5% every 25 seconds.

The ability to synchronise with a falling frequency is also used at pumped-storage power stations when synchronising in the pumping mode, as the machine speed falls too quickly for an operator to respond manually.

4 Synchronising facilities and controls

4.1 Synchronising facilities

The major synchronising facilities in a power station are remote from the switchgear and are situated in the

central control room at the unit control desk and at other control panels, i.e., the electrical auxiliary and transmission system control panels and gas-turbine remote control panel, if provided. Gas turbines are also provided with synchronising facilities at the local control panel as is switchgear in switchrooms or at plant locations if required.

The synchronising facilities at each of these locations for three standardised generator/transmission voltage switchgear schemes and the electrical auxiliaries system, including gas turbine and diesel generator switchgear, if provided, are shown in Table 12.2. This does not include the synchronising facilities for the transmission station switchgear at the transmission control panel, as these extend beyond the scope of this chapter.

The synchronising facilities for the steam turbine. generator are located at the unit control desk, irrespective of whether synchronising is with generator or transmission voltage switchgear. For generator voltage switchgear (Table 12.2, scheme (a)), synchronising normally takes place across the generator voltage circuitbreaker and automatic and manual synchronising facilities are provided. Automatic synchronising is the normal mode, backed up by manual synchronising with check operative. Manual synchronising with check inoperative is also provided when it is necessary to bring a generator into service with the transmission supply absent. In addition, manual synchronising facilities, with check operative and check inoperative, are provided for the associated transmission voltage circuitbreaker. Manual synchronising with check operative is provided for re-synchronising the steam turbinegenerator on restoration of transmission supplies following loss or disconnection. Manual synchronising with check inoperative is prevented, except when the generator voltage circuit-breaker is open.

In both transmission voltage switchgear schemes (Table 12.2, schemes (b) and (c)), the steam turbinegenerator is synchronised across the transmission voltage circuit-breaker and switch disconnector, respectively. Automatic and manual synchronising facilities are provided for both schemes. Automatic synchronising is the normal mode while manual synchronising with check operative is available as back-up. Manual synchronising with check inoperative is also provided for the circumstances referred to in scheme (a).

Each steam turbine-generator switch is equipped with a fixed set of synchronising equipment. This includes the automatic synchronising relay, where applicable, the check synchronising relay and guard relay plus the associated control circuitry. The one exception is for the manual synchronising instruments mounted on the synchronising trolley, which are common.

The main 3.3 kV and 11 kV electrical auxiliaries system is set out as a mimic diagram on the electrical auxiliaries control panel. This consists of standard symbols, signs and lines, etc., arranged in such away- that they represent the circuit layout of the switch gear, transformers, generator main plant, etc. Different

Synchronising Controls

gaS

3 3kV and

• •kV oistn. cvgevr

33kV evlor

colours identify each system voltage and limited opera- [Lona] information is displayed. To remotely open and each circuit switch, a control switch is fitted at circuit switch position. A synchronising socket is Installed near each circuit control switch where manual nchronising facilities are required. A common set of , ..nchronising equipment is provided including check

)ynchronising relay, guard relay and associated control , :ircuitry mounted inside the synchronising trolley.

Manual synchronising with check operative is in- )tailed for system synchronising, with manual synchronising and check inoperative normally available in the absence of one or both supplies. identical facilities,

.ocated in a 3.3/11 kV synchronising trolley, are proicled in switchrooms, etc., if these are required.

Synchronising facilities for 11 kV gas turbines are based on the requirements of Design Memorandum

066/1 11]. The main facilities are located at the gas turbine remote control panel and automatic and manual synchronising facilities are installed on the same operating basis as already described in scheme (a). The common synchronising trolley equipment is used for manual synchronising.

Identical automatic synchronising facilities are provided at the gas turbine local control panel, which is required to contain all the controls and instrumentation including the automatic synchronising relay necessary to operate the plant. Although used primarily for commissioning, local facilities permit plant operation should the remote facilities fail.

Diesel generators at 3.3 kV are not used as sources of generation to meet system demand but to provide essential power station supplies in an emergency following the loss of external supplies. There is no set

standard for the application of diesel generator synchronising facilities, so two recent designs at nuclear power stations are described. Automatic synchronising facilities are provided, although they are not used in an emergency. They are included for operator convenience during the regular 'on load' testing necessary to demonstrate the high level of availability required or such plant. With the first scheme, two additional synchronising [Folio s, equipped for manual and automatic synchronising, are provided for synchronising at the electrical auxiliaries control panel. In the second scheme, automatic synchronising facilities are provided for each diesel generator at the electrical auxiliaries control panel, and automatic and manual synchronising facilities are provided at each local control panel. Manual facilities, with check operative and check inoperative, are provided on the same operating basis as already described.

4.2 Synchronising controls

Many precautions need to be incorporated into the design of a synchronising scheme to prevent error or incorrect operation. Of particular concern is that part of the operation done manually; therefore, the synchronising procedure includes measures designed to minimise the risk of human error by making each manual step in the operation the end result of considered action on the part of the operator. Several of these measures, which are concerned with the operation of the synchronising equipment, have already been described in Section 3 of this chapter. Further measures are included in the preparatory actions performed prior to starting the actual synchronising operation and, for manual synchronising, extend to the operation of the circuit control switch.

The measures included in the preparatory actions, which apply to each circuit equipped for synchronising, ensure that:

•The same basic procedure is followed to prepare each circuit for synchronising.

•The circuit switch cannot be closed without first following the procedure of actuating a control switch to select the synchronising mode, i.e., automatic, manual with check operative or inoperative.

•Secondary voltage circuits are not established until the control selector switch is actuated.

At generator and transmission voltages, certain 'auto synch/manual synch' control selector switches are made key-operated and a separate key-operated control selector switch is installed for the 'check operative/check inoperative' facility. In addition, no synchronising operation on a transmission voltage switch can take place at the unit control desk without first actuating a keyoperated 'generator unit desk' control selector switch located on the transmission control panel.

fitted for the key-operated control selector switches, which can

only be operated with the key in position. The key i s free to be removed in the 'off' position and is trapped in all other positions: when not in use, the keys are normally kept in a separate key parking socket. Th e key patterns produced are coded by the manufacture r and a small number of these have been selected by the CEGE3 to form different key groups. Each key group is dedicated to a particular control selector switch function.

Two key groups are used at the unit control desk. One key group (ov) is fitted to the 'check operative/ check inoperative' control selector switch and the other key group (k) is fitted to the synchronising mode control selector switch in the generator voltage switchgear scheme (a) (key-operated control selector switches may be fitted with the other two schemes). The third key group (s), located at the transmission control panel, is used to synchronise a transmission voltage switch. The purpose of this key group is to ensure that synchronising can only take place on one circuit at a time at the transmission station.

To enable synchronising to be carried out in the central control room, the 'local/remote/test' control selector switch at the circuit switch local control panel or switchboard must be selected to the 'remote' position. This transfers operation control from the local 'open/ neutral/close' control switch to a discrepancy control switch, which is the final initiating control whose design features need to be described.

Discrepancy control switches inform the operator of the circuit switch position while going through the motion of actuating the switch. On being rotated through a quarter of a turn in the appropriate direction, an internal discrepancy light flashes to alert the operator that the switch indicates (by a coloured line or arrow on a white background) the wrong sense. The switch will continue to flash if left in the wrong sense. Having cautioned the operator, with further rotation the 'open' or 'close' signal is initiated, and as soon as the circuit switch has operated, the discrepancy light extinguishes to inform the operator that the switching operation is now complete. On release, the control switch has an inbuilt return action to bring the switch to rest indicating the new circuit switch position.

The operation of the synchronising controls can now be described.

4.2.1 Steam turbine-generator

Figure 12.8 shows the steam turbine-generator synchronising controls on the unit desk.

Voltage and speed controls

The voltage at the low voltage terminals of generator transformers must be kept at 100 0/o of rated voltage at all times to maintain the voltage constant at the auxiliaries connected via unit transformers to this point.

Steam turbine-generators are therefore fitted with automatic voltage regulators which are normally set to maintain the terminal voltage at its nominal value. Voltage matching is therefore carried out using the een crator transformer on-load tapchanger by raising and lowering the tap position.

The tapchanger alters the transformer open-circuit %oltage by adjusting the effective number of turns in he high voltage winding. In total, there are nineteen ;ap positions, with a voltage step of 1.11% nominal between each tap position. 'Raise' and 'lower' refers to changing to a higher or lower tap position num-

ber, respectively. If synchronising at the generator oltage circuit-breaker, this is by adjustment of the

running voltage and, at the transmission voltage switch,

b!, adjustment of the incoming voltage. With the lat- er, the 'raise' tapchange operation reduces the trans-

ormer open-circuit voltage and, similarly, the 'lower'

iapchange operation increases the transformer open- circuit voltage.

speed governor set point controller (as described in Section 2.1 of this chapter) is used to raise or lower the generator incoming frequency.

Generator voltage circuit-breaker (with transmission voltage circuit-breaker) scheme

cManual synchronising of the steam turbine-generator

ommences with the connection of the synchronising trolley plug to the generatorvoltage circuit - breaker

synchronising socket. The selection of manual synchronising is made by key operation (k) of the 'off/auto synch/manual synch' control selector switch to the 'manual synch' position. Actuation connects the incoming and running supplies to the synchronising instruments. The supplies are also connected to the check synchronising relay with key-operated (ov) 'check inoperative/check operative' control selector switch selected to the 'check operative' position. The auxiliary power supply is permanently connected to the check synchronising relay.

The generator voltage circuit-breaker is closed by operation of the discrepancy control switch, provided that the check synchronising relay output contacts have previously closed. Satisfactory conditions are indicated by the illumination of a check synchronising relay monitor lamp. The guard relay inhibits the close signal and the display of the check synchronising monitor lamp if the discrepancy control switch is prematurely operated.

Actuation of the key-operated (ov) selector to the 'check inoperative' position energises an auxiliary relay which by-passes the check synchronising relay output contacts. Manual synchronising in this mode is normally restricted to closing onto a dead circuit. Selection of automatic synchronising by key operation

(k) of the control selector switch to the 'auto synch' position, energises auxiliary relays which connect the incoming and running supplies and the auxiliary power supply to the automatic synchronising relay. With the

equipment prepared and the steam turbine-generator running, manual initiation of the 'auto synch' start button instructs the automatic synchronising relay to begin. synchronising.

Manual synchronising facilities for the transmission 1,olta. ,2e circuit-breaker are available at the transmission control panel and the unit control desk. To establish manual synchronising at the unit control desk, the key-operated (s) control selector switch at the transmission control panel is selected to the 'generator unit desk' position. This illuminates a `HV synchronise available' lamp at the unit control desk.

The initial preparation for synchronising at the unit control desk is by plug connection of the trolley. Selection of manual synchronising is by actuation of key-operated (k) 'off/manual synch' control selector switch to the 'manual synch' position. This connects the incoming and running supplies to the synchronising instruments and to the check synchronising relay provided the key-operated (ov) 'check operative/check inoperative' control selector switch is selected to the 'check operative' position. The auxiliary power supply to the check synchronising relay is permanently energised. The circuit-breaker is closed by operating the discrepancy control switch, with the conditions the same as described for the generator circuit-breaker.

NIanual synchronising without check synchronising is by operation of the key-operated selector (ov) to the 'check inoperative' position. Manual synchronising in this mode is inhibited, except when the generator voltage circuit-breaker is open.

Transmission voltage circuit-breaker (with no generator voltage circuit-breaker) scheme

To establish either manual or automatic synchronising at the unit control desk, the key-operated (s) control selector switch at the transmission control panel is selected to the 'generator unit desk' position. Synchronising is then carried out as for the generator voltage circuit-breaker except that the transmission voltage circuit-breaker discrepancy control switch is used.

Transmission voltage switch disconnector (with transmission voltage circuit-breaker) scheme

The synchronising controls are transferred to the unit control desk by selection of 'generator unit desk' on the key-operated (s) control selector switch at the transmission control panel. Synchronising is again carried out as for the generator voltage circuit-breaker except that the transmission voltage switch disconnector discrepancy control switch is used.

4.2.2 11 kV gas-turbine generators

Gas turbine synchronising facilities are available at the gas turbine remote control panel after the selection of 'remote' at the gas turbine local control panel.

To synchronise manually `no-load' is selected at the control selector switch and to synchronise automatically either 'peak' or 'standby' is selected.

Manual synchronising begins after the gas turbine has been started and run-up under the supervision of the automatic sequence control system. It commences with the insertion of the plug into the synchronising socket which connects the auxiliary power supply to th e check synchronising relay. Synchronising is with 'check operative' selected at the control selector switch o n the synchronising trolley which connects the running voltage and the incoming voltage to the synchronising equipment. The voltage is raised and lowered by adjustment of the automatic voltage regulator (this may be by adjustment of the generator transformer on-load tapchanger with peak-lopping gas turbine schemes).

The speed is altered by adjusting the gas turbine speed governor set point controller. Closure of the II kV gas turbine circuit-breaker, which is initiated by operating the discrepancy control switch, cannot be carried out until the check synchronising relay output contacts have closed, confirming that the synchronising conditions are correct. The guard relay prevents the circuit-breaker being closed by premature operation of the discrepancy control switch.

Selection of either 'peak' or 'standby' energises a relay which connects the auxiliary power supply to the automatic synchronising relay. When the gas turbine is running up in speed, a further relay energises which connects both the running voltage and the incoming voltage to the automatic synchronising relay, causing the synchronising operation to begin. Circuit-breaker closure disconnects both the running and incoming voltage from the automatic synchronising relay.

4.2.3 3.3 kV and 11 kV distribution switchgear

Manual synchronising facilities only are available at the electrical auxiliaries control panel. The operation commences with plug connection of the synchronising trolley to the appropriate circuit, which connects the auxiliary power supply to the check synchronising relay. With 'check operative' selected and the 3.3 kV or 11 kV circuit-breaker open (to connect running voltage) both the running voltage and incoming voltage are connected to the synchronising equipment. Voltage controls are not provided since with normal voltage regulation the voltage difference should be slight. The

3.3 kV or 11 kV circuit-breaker is closed by operating the discrepancy control switch, initiation being permitted by the check synchronising relay and guard relay if synchronising conditions are correct.

Manual switching with 'check inoperative' selected is normally restricted to closing the circuit-breaker onto a dead busbar.

The above also applies to manual synchronising in switchrooms or other plant locations, if this facility is installed, except that switch closing is initiated by operating the local 'open/neutral/close' control switch.

roor facilities are otherwise used for closing onto a

busbar.

(scheme (a) and scheme

re desci ibed in the following paragraphs.

i

s,,,, tque (a)

.:,:nerator synchronising trolley is used for synchro-

-ni ,i n g, Both manual and automatic synchronising

operations begin after the diesel generator has been

,[arted and run-up under the supervision of the auto-

• ane sequence control system, and commence with ole insertion of the synchronising plug into the ap-

propriate circuit socket.

For manual synchronising, 'check operative' is se-

1,:cied at the control selector switch on the synchronis- m , trolley which connects the incoming and running whage to the synchronising equipment. Voltage and

speed control are by adjustment of the automatic volt2e regulator and speed governor controller, respective-

Closure of the 3.3 kV diesel generator circuit-

breaker is achieved in the same manner as previously described. Manual synchronising with check inoperative is again normally used for closing onto a dead circuit,

as happens following the loss of external supplies. Nloving the control selector switch to the 'automatic

-,richronising' position connects the incoming and running voltage to the relay. With the equipment prepared, manual operation of the 'auto synch initiate' pushbutton on the synchronising trolley prepares the DC output circuits which control the voltage, speed and circuit-breaker closing.

Scheme (b)

Only automatic synchronising facilities are available in he central control room at the electrical auxiliaries

control panel after the selection of remote at the local control panel. This removes the need for the additional

3.3 kV diesel generator synchronising trolleys in the central control room. Synchronising is achieved, as before, by depressing the 'auto synch initiate' pushbutton located on the control panel.

Automatic and manual synchronising facilities are available at the local control panel, the synchronising operation being as already described.

5 Synchronising equipment

5,1 Synchronising trolley

Figure 12.9 shows a typical control room synchronising trolley which is a compact floor-standing unit mounted

Synchronising equipment

on castors for ease of mobility. The housing is a sheet steel fabrication which matches the design of the control desk in profile and colour. The trolley contains two sets of instruments, one for synchronising 3.3/11 kV circuits and the other for synchronising generator/grid voltage circuits. The check synchronising relay, guard relay and interposing transformers for synchronising 3.3/11 kV circuits only are internally mounted. Accommodation for the two flexible trailing cables is located on either side of the trolley. Each cable is for one set of synchronising instruments and is approximately 3 m long. An approved design of synchronising plug ter- minates the free cable end. This, as described earlier, fits into mating sockets mounted on the mimic panels and control desk. The plug contains male pins and the sockets have shrouded female connectors. Parking sockets on the trolley locate the plugs when not in use.

When viewed from the front, the 3.3/11 kV instruments are located on the left hand side and the generator/grid voltage instruments are located on the right hand side of the trolley. The voltmeters, phase angle voltmeters and synchroscopes are mounted on the steeply inclined front face and the selector switches and 'synchroniser locked out' indicator are mounted below on the slightly inclined desk top. Centrally mounted on the desk is a four-position system voltage switch, for the selection of the various manual synchronising facilities.

The four control selector switch positions are:

1 3.3/11 kV, check inoperative

2 3.3/11 kV, check operative

3Off

4Generator/transmission voltage (e.g., 23.5/400 kV)

The synchronising instruments provided for duties associated with selector switch positions 1 and 2 are:

•Running voltmeter

•Incoming voltmeter

•Phase angle voltmeter

•Synchroscope

•Phase angle voltmeter switch (black scale/red scale/off)

•Slip feature switch (in/out)

•Synchroscope switch (on/off)

•'Synchroniser locked out' indicator

The synchronising instruments provided for duties associated with selector switch position 4 are:

•Running voltmeter

•Incoming voltmeter

•Phase angle voltmeter

•Synchroscope

•Phase angle voltmeter switch (black scale/red scale/off)

•Synchroseope switch (on/off)

The incoming and running voltmeters are of the same size and have the same scales. The scale shapes are

,ubstantialty linear and match to within 2 ° at all major marks on the scale. The phase angle voltmeters are the same size as the incoming and running voltmeters and the synchroscopes are of the same type and make. The two scales marked on the voltmeter and phase angle voltmeter, the maximum number permitted on an instrument, are marked in different colours for clarity.

to close the switch, but also indicates any adjustments that need to be made to the speed governor set point to reduce the frequency difference.

Zero phase displacement and slip frequency are indicated when the pointer coincides with the synchronising mark at the 12 o'clock position and is stationary. If the two frequencies are exactly equal but have a phase displacement, the pointer assumes a position with respect to the synchronising mark corresponding to the phase angle. If the frequencies are different, the speed of rotation of the pointer corresponds to the slip frequency, i.e., the slower the pointer revolves, the smaller the frequency difference. When moving slowly, the phase angle is likewise indicated between the position of the pointer and the synchronising mark. The direction of pointer rotation indicates which of the two frequencies is higher. This is clockwise if the incoming frequency is higher, indicated by an arrow and the word 'fast' or a sign, and anticlockwise if the incoming frequency is slower, indicated by an arrow and the word 'slow' or a ' sign. Indications are displayed at the right and left upper quadrants of the dial face (Fig 12.11).

Synchroscopes typically operate over the frequency range 47-51 Hz and have an accuracy for practical purposes of +2 ° (elec) When not in use, or with one circuit de-energised, the pointer is biased away from the synchronising mark by an angle exceeding 45 ° . The synchroscope is normally brought into service when the two voltages have been equalised and the frequency difference is s 1.5 Hz to ensure that the pointer starts to rotate in the correct direction. When manually synchronising a steam turbine-generator, the speed gov-

FiG. 12.11 Synchroscope dial face

ernor set point is adjusted until the synchroscope pointer is moving slowly or creeping in the 'fast' (clockwise) direction at, say, 30 ° (elec) per second. Switch closure is initiated with the pointer on the 'slow' side of the instrument at the moment the correct advance angle in front of the synchronising mark is reached. For a switch with a closing time of, say, 300 ms, the correct advance angle would be about 90 (elec). While the angle is indicated by the synchroscope pointer, the dial face is not calibrated and therefore the phase angle must be estimated.

5.2 Guard relay

The function of a guard relay, as described earlier, is to ensure that the operator follows the correct synchronising procedure by preventing operation of the switch closing circuit if the 'close' signal is initiated prior to the check synchronising relay, indicating that conclitions are satisfactory for synchronising. There are a number of ways this control feature can be incorporated into the closing circuit logic and a detailed description of guard relay operation as employed by the CEGB is given in Section 7 of this chapter. En brief, the guard relay has two coils, both of which control relay operation. At 3.3/11 kV, only one coil is energised and this operates the relay allowing closing circuit operation if the close signal is correctly initiated. However, if the close signal is prematurely initiated, this energises the second relay coil which de-energises the relay thus preventing closing circuit operation. This also causes the 'synchroniser locked out' indication to be displayed on the synchronising trolley. The control selector switch must be switched to the 'off' position to reset the relay.

5.3 Check synchronising relay

This, as mentioned earlier, is a device for checking that the voltage, phase and, when required, slip frequency of the two supplies being manually synchronised are within preset limits. Figure 12.12 shows a typical check synchronising relay.

The relay is connected into the switch closing circuit in series with the discrepancy control switch. Initiation of switch closure via the interposing relay is permitted by the check synchronising relay output contacts if the above measurements are within preset limits, otherwise the closing signal is inhibited.

Check synchronising relays have been available for many years. Following various stages of development, in which electronic circuitry superseded electromechanical means, present day units are compact static devices (except for the output relay) and are virtually maintenance free. Several types of relay are available, offering one or more measuring functions. All include phase measurement, with the addition of slip frequency and voltage measurements, if required by the user. The CEGB specifies a unit having all three functions, with