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

. , :crnai connections to by-pass slip frequency mea-

..7mient for generator synchronising. The slip feature, Liter described, is only intended for the measure-

.. %: n«)t- the small slip frequencies associated with sys- •.:n nchronising. Undervoltage is the only voltage , ii dirion specified to prevent closing. As a precaution malfunction, electrical isolation is required

the load of the interposing relay over a

of two thousand operations. A simplified ".OA diagram of a check synchronising relay is shown :1 Fig 12.13.

to compare the voltage envelope produced by the

mdulated beat waveform with a DC reference voltage 11 2 12 .14 (a)).

A permissive output is obtained when the voltage waveform is less than the DC reference. The phase setting is determined by the magnitude of the DC reference. The intersection between the two voltages is symmetrical about the phase coincident position. The actual angle over which synchronising can occur is therefore twice the setting value 61, extending from where one voltage is the setting value in advance of the other to the setting value where the voltage positions are reversed. The effect of minor variations in supply voltage may be almost eliminated by arranging that the DC reference is proportional to the scalar sum of the two supplies.

Performance is further improved if the voltage envelope is full wave rectified. This assists the smoothing by effectively doubling the high frequency component as shown in Fig 12.14 (b).

A phase angle setting of 20 ° (elec) is used for all power station applications and also normally at trans- mission stations. A setting accuracy of —2 ° +0 ° (elec) is specified, although better than this is normally obtained. The total variation from the actual setting under all combinations of specified variations in AC

K 1 ®

VOLTAGE

LOCK OUT

OUTPUT STABILISED RELAY

POWER SUPPLY

TO ALL MODULES

18V 0 C

FIG. 12.13 Simplified block diagram of a typical check synchronising relay

DC REFERENCE LEVEL

1-4- 0 ---0-1-41-

PHASE COINCIDENCE

(a) Modulated beat waveform

LINK TO

INCLUDE

TIMER

6-0

OUTPUT

CONTACTS

(S-0

PHASE

COINCIDENCE

mum acceptable value. With slip measurement as described, this time delay is automatically provided by the slip measurement timer; otherwise, a separate timer is incorporated, having an accuracy better than +20%

— 0%. This timer is also by-passed for generator synchronising .

SYNCHRONISING

PERMITTED OVER rBTE0 OVER 11 (ELEC) ANGLE

ELEC) ANGLE

PHASE ANGLE SETTING 20= (ELEC)

FIG. 12.15 Limitation of slip frequency measurement

Besides resulting in a sizeable phase error, the time ailable for initiating switch closure is less than one ,2s:ond (as indicated to the operator by the 'check s\r nch monitor' lamp, where fitted). This period is llikely to be adequate for the operator to respond ,I ncl therefore this method of slip measurement is unsuitable for generator synchronising with load pick-up. The switch closing time has to be considered when ,electing the slip frequency setting. If switch closure initiated when the phase displacement is increasing at a position corresponding to the phase angle setting, tile phase error will be greater than this value by an 'mount equal to the advance angle when the switch :/lain contacts close. The maximum acceptable slip for ,s[em synchronising is 0.055 Hz (20 0 (elec)/s). Using rIlls value as an example, with a switch closing time DI 300 ms the cumulative phase error under these ,anditions (with a phase setting of 20 ° (elec)) is 26 ° [elec). To reduce this additional error, a requirement

tas been introduced that the ti me taken to traverse the phase angle setting must he greater than 10 times the

,citch, closing time or 2 seconds, whichever is the :reater. Therefore, in the example above with a switch dosing ti me of 300 ms, the minimum timer setting is

= seconds. This setting corresponds to a maximum

frequency limit of 0.037 Hz (13 ° (elec)/s) and the

amulative phase error is reduced to approximately

24° (dee).

To span the range of switch closing times, adjustable timer settings between 2 seconds (0.055 Hz) and lO . seconds .(Q.011 Hz) are specified. These settings are adjustable either continuously or in one second stages

!ID 6 seconds (0.018 Hz) with no intermediate steps !lien before the 10 seconds (0.011 Hz) setting. Accu-

. 'leY of slip measurement is better than +5%.

There is a further requirement that at least 2 seconds must elapse between the application of the 'incoming' and 'running' supplies to the relay and an output be-

. ng given. This ensures that slip frequency is correctly Fil easured, particularly if it is greater than the maxi-

The undervoltage check facility inhibits synchronising if either the incoming or the running voltage is less than a preset percentage of nominal voltage. Accurate voltage measurements can be made by separately comparing the two voltage inputs, after being rectified and smoothed, with a stabilised DC reference voltage obtained from the relay power supply. The magnitude of this reference represents the nominal system voltage. Voltage settings are adjustable either continuously or in 2.5% steps over the range 80-90% of nominal system voltage. For steam turbine-driven generators, the normal setting is 85% and for all other applications, including transmission stations, the normal setting is 80 070.

Using the above methods of measurement, check synchronising relay operation can now be described. On being switched into service, the incoming and running voltages are measured and, assuming that both are greater than the preset percentage limit of nominal system voltage, an output initiated by the phase measurement circuit starts the slip frequency timer when the phase is less than the phase angle setting, i.e., the voltage waveform is less than the DC reference. The timer energises the output relay on completion of its ti ming cycle provided the output from the phase measurement circuit persists, i.e., the voltage waveform has remained less than the DC reference. Energising the output relay signals permission to close the switch.

5.4 Automatic synchronising relay

Automatic synchronising relays monitor the voltages and frequencies of the incoming and running supplies and give out signals which are used to control the incoming voltage (or running voltage) and the incoming frequency. The relay senses voltage and frequency differences. On detecting differences the matching circuits signal corrections, the voltage corrections being signalled either prior to or simultaneously with frequency corrections. Once preset conditions are obtained, a pulse signal is sent out to energise the interposing relay which, in turn, closes the switch.

Early designs were electromechanical devices; one example used electromagnets and springs similar to a wattmetric element to produce restraining and operating torques in an aluminium disc. When the operating torque was applied for a sufficiently long period, this gradually wound-up a thread around a grooved pulley against the force of the spring until, eventually, the relay contacts closed when the voltages were equal and

phase coincided. Development progressed with the growing need for a higher degree of precision and reliability resulting from the increase in size of generators being installed and the increasing trend towards automation of power stations. This improvement continued with the introduction of static circuitry until, with present day relays, models with multifunction specifications capable of operating over a wide voltage, frequency and ',lip frequency range are available to suit most user needs.

Figures 12.16 and 12.17 show a typical automatic synchronising relay and its associated simplified block diagram respectively.

The relay is designed to fail safe and duplication of circuitry is extensively employed in order that component failure is promptly detected. The malfunction of any component prevents closure of the switch and results in lockout of the synchroniser. Lockout is an inhibit which, once initiated, remains operative until the synchroniser is switched off. Other lockout features

6, 8, 12 and 20 seconds per slip cycle being bic (with an accuracy of ± 10 070). The slip fre- ,•,:%, .etting is determined by the unit synchronising

•r;: ments.

CONTINUOUS

SIGNAL

FIG. 12. 18 Governor signals

If the slip frequency should become too slow or a point is reached where the two supplies have identical frequencies at a phase angle unacceptable for synchronising, the frequency matching circuit would give no further output signal unless one of the frequencies drifted. To overcome this problem, an urge circuit is incorporated which sends a signal to the prime-mover speed governor if a fixed time has elapsed, typically 20 seconds, since the last 'raise' or 'lower' instruction. The urge circuit sends a pulse in the same sense as the previous pulse.

With steam turbine-generators, this signal is adapted into a minimum slip frequency setting to prevent motoring. This is called low slip lockout' and is a fixed setting. A typical value is 14 seconds per slip cycle.

To synchronise a steam turbine-generator super-syn- chronously, one speed control pulse must not produce a frequency change or slip bandwidth (l c ) greater than the difference between the maximum slip frequency (fsmax) and the minimum slip frequency 0 - If the governor rate with a continuous pulse is a g , then the pulse width selected must be less than t_i max = /a g seconds.

Example A steam turbine-generator is required to be automatically synchronised with approximately 5 11 0 load pick-up and a low slip lockout of 14 s/slip cycle. With a governor rate of 0.05 Hz/s and speed droop of 4%, determine the maximum slip setting and the maximum governor pulse width. The maximum slip settings available are 4, 6, 8, 12 and 20 s/slip cycle.

100100

0.1Hz = 10 s/slip cycle

If the maximum slip setting of 8 s/slip cycle is selected from the range of settings available, the maximum slip frequency is 0.125 Hz

so, fe = 0.125 — 0.071 = 0.054 Hz

The governor pulse width must be less than

The signal pulse width is adjustable to cater for a variety of governor rates, typically 0.1 to 2.0 s. The recommended setting would be 0.8 times the maximum pulse width, i.e., 0.86 s in this example.

Figure 12.19 shows an example of the modulated beat waveform and governor pulse signals during synchronising under the above conditions.

The number of possible chances to close the switch within the slip bandwidth can also be specified, although this is normally not done by the CEGB. If it is specified, one speed control pulse must not produce a frequency change greater than:

slip bandwidth

fan

number of chances . to close switch

Example A pumped-storage generator is to be synchronised sub-synchronously in the generating mode when a slip frequency of 1 07o is attained. If the required number of chances to close is 3 and the governor rate is 0.4 Hz/s, determine the maximum pulse width.

Slip bandwidth = 50 x 1 07o = 0.5 Hz

0.5

= 0:17/0.4 = 0.425 s

With large steam turbine-generators, the high inertia prevents the speed falling quickly and therefore the minimum pause width, i.e., time between successive 'lower' pulses, is increased (instead of every beat cycle) to prevent a large discrepancy arising between the actual turbine-generator shaft speed and the governor set point.

The equation for rotational motion with a constant angular acceleration is given by:

rearranging

(w2) 2 - (.1) 2

—

2ce

where a = angle of rotation, radians

angular velocity

– frequency

27r

therefore, converting the equation to frequency:

Rate of change of frequency = — ( f 2 1. 21 )

2 —

The maximum allowable rate of change in frequency is brought about by a deceleration of the rotating shaft from the maximum to the minimum slip frequency setting over one beat cycle, i.e., a = 27r. This is also the maximum allowable governor rate if the two are to remain in step. Therefore:

where fsmax = f1 (neglecting the negative sign as

fsmin = f2 this is a deceleration)

A governor rate less than the maximum allowable is recommended, i.e., recommended governor rate = 0.8 x maximum allowable governor rate.

With pulsed signals, the governor rate is reduced in proportion to the ratio of pulse width to pulse cycle.

i.e., actual governor rate =

pulse width

governor rate x

pulse cycle

In this case, the actual governor rate is the recommended governor rate.

The minimum pause width can be calculated using the above equations. A typical automatic synchroniser has adjustable pause widths between 0.5 to 10 s.

Example A 660 MW steam turbine-generator has a governor rate of 0.1 Hz/s. The maximum slip setting is 8 s/slip cycle with a low slip lockout of 14 s/slip cycle. With a pulse width of 0.2 s, determine the minimum pause width setting.

(0.125) 2 - (0.071) 2

2

= 0.005 Hz/s

Recommended = 0.8 x 0.005 = 0.004 Hz/s governor rate

The switch close signal must only be sent out when both supplies are within the specified limits. The phase angle sensing elements, which initiate switch closure, are therefore duplicated and the output contacts are connected in the positive and negative sides of the interposing relay. The required duration of the closing pulse is for a period of -t 0.55 s and /. 1.0 s and is typically set at 0.75 s. The output contacts are rated to switch the load of the interposing relay and its associated wiring over many operations. The relay is a single-shot device and therefore both elements must signal in step, otherwise lockout facilities are initiated which remain operative until re-primed. The relay can only be reprimed by being switched off. This occurs automatically after successful operation of the relay.

The relay is set to the closest available setting to the measured switch closing time during commissioning. A range of steps with 25 ms intervals between 50 ms and 500 ms is typically provided. In this respect, it is important that the switch closing times are consistent if the relay is to remain correctly matched. Under these conditions, accuracy of synchronising with a phase error less than 8 ° (elec) is specified. However, better performance with a phase error less than 5 ° (elec) is normally obtained. After synchronisation, the incoming and running voltages are disconnected and remote indications are given out that synchronising is complete and that the synchroniser is locked out.

5.41 Steam turbine- generator synchronising

Automatic synchronising relays for steam turbine-gen- erators can be used for synchronising across generator voltage or transmission voltage circuit-breakers. Steam turbine-generators are run-up and operated either manually or under the supervision of an auto-

matic sequence control system. At a suitable point, the automatic voltage regulator is switched in to establish open-circuit voltage and control of voltage (running) and speed is passed to the automatic synchroniser. Automatic synchronisation is required to be achieved super synchronously within the following operational li mits:

The voltage difference across the circuit-breaker is reduced in steps by operation of the generator transformer on-load tapchanger until voltage matching better than 4 07o error setting is attained. With an initial speed of between 46.5 Hz ( - 7%) and 51.5 Hz ( + 3 010, the governor set point is adjusted to cause the unit to approach synchronous speed from a higher speed. The action continues until the speed has fallen below the maximum slip setting of 0.2%. With a speed droop of 4 07o, this corresponds to a load pick-up within 1-5 010 of full-load. Provided these conditions do not change, the phase angle sensing elements at the first chance initiate the closing of the switch at a point which will result in the main contacts closing with a phase error less than 8 ° (elec).

Lockout facilities operate if:

•Either incoming or running voltage drops below 85 0/o of nominal.

•The steam turbine-generator speed is no longer fast, i.e., a low slip condition which could result in the generator motoring.

•Component failure occurs.

•Synchronising is complete.

Indications of 'synchronising complete' or 'synchroniser locked out' inform the operator of the end result. A buzzer also sounds when the switch has closed.

5.4.2 Gas- turbine generator synchronising

Gas-turbine generators are run-up and operated under the supervision of the automatic sequence control system. The synchronising operation forms part of this control sequence and the synchronising unit is automatically switched in at a speed which will ensure that synchronising can take place at the minimum specified system frequency, while permitting the speed to be adjusted as rapidly as the machine and control characteristics permit. Having taken control of voltage and speed, the automatic synchroniser is required to achieve synchronisation within the following operational limits:

Speed adjustment continues sub-synchronously until the maximum slip setting of 107 0 is reached, while the voltage is regulated by adjustment of the automatic voltage regulator for emergency gas turbines (incoming) or by adjustment of the generator transformer on-load tap-changer (running) for peak lopping gas turbines, until the voltage difference is within the voltage error setting of' 4%. Providing these conditions do not change, the phase angle sensing elements at the first chance initiate the closing of the switch at a point which will result in the main contacts closing with a phase error less than 8 ° (elec). To take account of inherent variations in turbine governor characteristics, which can cause some change of slip at the point of synchronising as well as changes in DC auxiliary supply voltage which can result in a variation in normal switch closing time, a maximum phase error of 16 ° (elec) is permissible.

If, for operational reasons, it is required to synchronise the machine with the incoming supply frequency greater than the running supply frequency, then the synchroniser reduces the speed to within the maximum slip setting value and synchronises the unit within the voltage and phase error settings given.

Lockout facilities operate if:

•Either incoming or running voltage drops below 80 070 of nominal.

•Any auxiliary supply is absent.

•Component failure occurs.

•Synchronising is complete.

Indications of 'synchronising complete' and 'synchroniser locked out' or 'synchroniser failed and locked out' inform the operator of the outcome.

5.4.3Diesel generator synchronising

Diesel generators are run-up and operated under the supervision of an automatic sequence control system. The automatic synchroniser is manually or automatically switched-in as part of the control sequence, at a speed at which the synchroniser can take over control of voltage and speed and achieve synchronisation supersynchronously within the following operational limits:

Speed control is initiated after voltage matching has been achieved. The voltage is regulated by adjustment of the automatic voltage regulator for emergency diesel generators until the voltage difference between the incoming and running voltage is within the voltage error setting of 4%. From an initial speed of between 46.5 Hz ( 7%) and 51.5 Hz (± 3%), the governor set point is adjusted to cause the unit to approach synchronous speed from a higher speed. This action continues, making further adjustments to voltage, if necessary, until the speed has fallen just below the maximum slip setting of 0.2%. With a speed droop of 4%, this corresponds to a load pick-up within 5 1170 of full-load. Provided these conditions do not change, the phase angle sensing elements at the first chance initiate the closing of the switch at a point which will result in the main contacts closing with a phase error of 8 ° (elec).

As for gas turbines, to take account of inherent variations in engine governor characteristics which can cause some change of slip at the point of synchronising, and changes in DC auxiliary supply voltage which can result in a variation in normal switch closing time, a maximum phase error of 16 ° (elec) is permissible.

Lockout facilities operate if:

•Either incoming or running voltage drops below 85% of nominal.

•The diesel generator speed is no longer fast.

•Component failure occurs.

•Synchronising is complete.

Indications of 'auto synch locked out' when a close signal has been given and 'auto synch complete' when the switch has closed, inform the operator of the end results. A buzzer also sounds when the switch has closed.

6 Derivation of synchronising supplies

6.1 Secondary supplies

Synchronising is performed with incoming and running supplies which simulate the primary circuit conditions. These are obtained from voltage transformers (VTs) and transferred to the synchronising equipment. Clearly the secondary voltages must be derived in a manner satisfactory for synchronising purposes. This requires consideration of the selection of the supplies that are measured and the degree of accuracy within which measurements are made.

The measuring circuits operate at a nominal voltage of 63.5 V AC. The connections between the VTs and synchronising equipment are direct wired (transducers

are used at transmission stations controlled by a telecontrol system, however, this is beyond the scope of th i s c hapter). Since this leaves frequency unchanged, only the measurement of voltage and phase needs to

be considered.

0%erall measurement to within an accuracy of 2% primary voltage and approximately 2 ° (elec) of phase ar e obtained with correct circuit adjustment under normal system conditions.

6.2Selection of voltage transformer supplies

6.2.1Single voltage supply

I is first necessary to consider a distribution switch in he process of being commissioned which is about to be used to system synchronise for the first time. The r unning supply is obtained from a voltage transformer (VT) associated with a switch that has already been c ommissioned with phase connections known to be correct. The running supply can therefore be used as a reference for checking the phase connections of the lacoming supply. Assume that three-phase VTs are available, enabling the voltage magnitude on each phase and the voltage difference across the three pairs of c orresponding phases (i.e., R-R B-B Y-Y) to be rnea- )ured at secondary terminals.

The number of connection configurations for the incoming supply is six. Only one of these is correct, he other five being obtained by crossing the phases, as tabulated below:

Incoming supply phasing

phase connections, as in (1), are confirmed

after the measurement of a voltage of equal magnitude (approximately) on each phase and a voltage differ-

ence of zero (approximately) across the three pairs of corresponding phases. In (2) and (3), the phase rotation is correct but the phase relationship is wrong and, in

(4) (5) and (6), both the phase rotation and phase relationship are wrong.

It can be seen that in addition to voltage magnitude a minimum of two, and preferably three, voltage diflerence measurements are needed to system synchronise correctly. The same applies with generator synchronising, but here it is also possible, though not correct, to synchronise with (2) and (3), the generator phase identification, in effect, being rotated.

The purpose of commissioning tests, in part, is to find any errors or mistakes in the design, manufacture

Derivation of synchronising supplies

and installation which have escaped earlier detection, although it must be added that many prior steps are taken to check that this does not happen. Once the phasing out procedure described has been properly carried out and the circuit commissioned, it is reasonable to assume that the primary connections and the direction in which prime-movers rotate will remain unchanged. On this basis, synchronising can safely be performed using a single representative voltage supply since, with balanced symmetrical three-phase supplies, the synchronising measurements will be identical for all three phases.

To summarise, three-phase VT secondary supplies are needed for commissioning tests to establish that the phase connections are correct. In certain instances, it is necessary for commissioning tests to be carried out using an adjacent circuit three-phase VT if the circuit being commissioned has only a single-phase VT installed. After it has been established that the phase rotation and phase relationships are correct, each threephase system is represented for synchronising purposes by a single supply voltage which may be either a singlephase or line voltage.

6.2.2 Incoming and running voltage

To be able to synchronise using the secondary voltages obtained from the incoming and running VTs, these, as shown in Fig 12.20, must:

•Measure the same primary circuit voltage (e.g., red phase or red/yellow line voltage).

•Have the same primary/secondary voltage transformation ratios and angular displacements.

To meet the second requirement, VTs of the same design and make are usually provided for both incoming and running supplies.

To obtain a proper and accurate measurement of the primary voltage, voltage transformers are preferably connected to the busbar immediately adjacent to the switch. This applies at the main generator circuitbreaker and to the incoming supply at a switchboard

V, ELLOW

V rELLCW

NOT 70 SCALE

FIG. 12.20 'Incoming' and 'running' secondary supplies

where the VT is positioned on the outboard or circuit side of the switch. However, it is not normal practice to obtain the running supply at a switchboard (or transmission substation) by connecting a VT to the main busbars as it is considered that this would reduce busbar integrity by introducing a potential source of failure. The running supply is therefore obtained from a VT on the outboard side of a switch already connected to the main busbars. Where several VTs may be connected to the main busbar at the same ti me, a voltage selection scheme is provided to choose the running voltage.

Voltage selection schemes discriminate in favour of the VT associated with the circuit allocated the highest priority and also prevent the VT secondaries from being paralleled. If the VT associated

circuit with the highest priority is open, the VT associated with the circuit with second priority is selected, and so on. Main incomers are normally allocated highest priority, followed by interconnectors with generator circuits last.

Voltage selection and repeat relays are used to produce the required logic, the scheme being designed to ensure that under all conditions of voltage selection the required voltage accuracy at the synchronising equipment is still obtained.

The different scheme arrangements for a 3.3 kV or

II kV switchboard are set out in a series of standardised drawings, a typical example being given in Fig 12.21. This is for a switchboard with three incoming circuits.

From examination of the relay logic, it can be seen the RYB bus wires

6.3.1Voltage transformers

Voltage transformers (VTs) are divided into different limits of percentage

voltage ratio error and phase displacement error for a given range of operating conditions. The actual error for a given accuracy class and under normal operating conditions, is determined by the size and power factor of the secondary load or burden. Burden is defined as the value of the impedance of the external secondary circuit, expressed in ohms (or in volt-amperes at rated secondary voltage), at the relevant power factor.

Voltage transformers on most circuits are provided for more than one purpose. These include supplies for other voltage measuring and recording instruments, e.g., busbar voltage and system frequency, metering, protection, automatic voltage regulators, etc., as well as for synchronising equipment. The individual burdens may vary over a wide range of volt-amperes and power

factors.

The VT class of accuracy required is determined by the secondary supply with the highest accuracy requirement.

For interchangeability reasons, dual rating VTs are installed for a number of applications, with more than one VT per phase if needed to accommodate the number of supplies required. These combine the different accuracy requirements for metering, measurement, protection, etc., in one VT, the limit of voltage and phase error depending on the size of the output burden and power factor for a given range of primary voltage

50/150 VA unit with three limits of accuracy are:

0.05-0.8 p.u. rated primary voltage and

1.2-1.9 p.u. rated primary voltage

The different VT secondary supplies are separated or fused either individually or in small logical groups. This approach is essential with capacitor VIs used at transmission voltages, where it is not always possible to obtain satisfactory discrimination with fuses in series owing to the low short-circuit currents available. The factors which determine the distribution of the supplies are the importance, accuracy, burden requirements and security against protection or equipment failure due to loss of supply. With the exception of 3.3 kV and 11 kV switchgear, the supply distribution arrangements are detailed on standard diagrams. A fused supply is provided for the exclusive use of the synchronising equipment.

6.3.2 Interposing voltage transformers

Voltage transformer accuracy is not the only source of secondary voltage error. This also occurs due to lead resistance (see Section 6.3.4 of this chapter). The sum of these two errors in the incoming and running supply will not be the same at the synchronising equipment, particularly if the lengths of the connecting cables and hence lead resistances are significantly different. Clearly it is important for synchronising purposes that the errors in the measured voltages are as small as practicable. However, there is a further reason why this is important should the two supplies become electrically connected. Although the direct interconnection of VT secondaries is not permitted, with preventive steps taken internally and externally to the synehro-