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

nising equipment, there remains a small risk that this might occur through a fault or sneak circuit. In this fistance, the transformer with the higher of the two

secondary voltages would contribute to the load of the transformer with the lower secondary voltage in the same way that power transformers share load in

parallel, lithe voltage difference is small, this condition would probably remain undetected during normal service with fuse protection. Complications may arise for protection, metering, etc., which may also involve other circuits.

Ti) reduce the voltage error in the incoming and running supply, an interposing voltage transformer (\.\ hich also pro v i des DC electrical isolation) is installed bemeen the \ I ,,e,.ondar, and the synchronising equipment, as shown in Fig 12.22. Tappings are provided to facilitate a certain amount of on-site voltage adjustment. With nominal system voltage, each interposing VT tapping is selected to indicate 63.5 V ± 1% at the synchronising equipment with the switch both open and closed. With a voltage selection scheme this includes each alternative source of running supply.

The interposing VTs have a ratio between primary and secondary windings of 110/63.5 V (63.5/63.5 V at transmission voltage) and have a minimum rating of 25 VA with a maximum limit of 50 VA, except at transmission voltage where this is reduced to 36 VA. It is, however, preferred that a single rating is used throughout the synchronising scheme for interchangeability reasons. Voltage adjustment is in steps of 0.5 V over the range 0 to + 5 V above rated secondary voltage. The tappings may be divided between the primary and secondary windings as convenient. The transformers in general comply with BS3941 [2] accuracy Class 1.0; i.e., percentage voltage error ± 1 %, phase displacement +40 minutes, at any voltage between 80% and 120% of rated voltage and with burdens of between 25% and won of rated burden at a power factor of 0.8 lagging, except that the range of voltage error is between 5% and 100% of rated burden at unity pf. To ensure that saturation does not occur during over-voltage conditions, the transformer knee point must not be less than three times the rated voltage. As an additional safety precaution, an earthed electrostatic screen is fitted between the primary and secondary windings.

6.3.3Burdens

To reduce the loading on the VTs and avoid undue voltage drop in the connecting leads, the synchronising equipment is required to have a low burden or VA rating. At transmission voltage, the total burden im-

Etc. 12.22 Simplified arrangement of interposing voltage transformers

posed on a main VT by the synchronising equipme nt must not exceed 40 VA (this includes voltage selection scheme relay burdens, etc.). Similarly, other devices connected to the main VT are required to have a low burden, due care being taken to ensure that the rated burden is not exceeded for all conditions of switching.

Typical maximum burdens under the worst condition of operation that can be imposed on an interposing VT are given below:

Manual synchronising

—Synchronising voltmeter, 0.5 VA

—Synchronising phase angle voltmeter, 0.5 VA

—Synchroscope, 5 VA (per supply)

—Check synchronising relay, 5 VA (Per supply)

Automatic synchronising

1 — Automatic synchronising relay, 5 VA (per supply)

The above burdens may be exceeded provided that the total burden from all items remains within the specified maximum.

6.3.4 Lead resistance

The principal effect of lead resistance is to reduce the magnitude of the secondary voltage resulting in a voltage error at the synchronising equipment.

With a single-phase transformer and a purely resistive burden the voltage drop would be the product of the lead loop resistance and the load current. In reality, it also depends on the power factor of the synchronising equipment burden which introduces a phase as well as a voltage error. The calculation is complicated further when applied to three-phase VTs and threephase groups of single-phase VTs connected in star, as the burden across one pair of phases affects the errors across the other two pairs of phases.

Multipair telephone type cable is used to transfer the voltage supplies from the electrical auxiliary switchgear to the control room. To limit the voltage drop produced by this type of cable which, due to its small conductor size, has a relatively high resistance, these circuits operate at the main VT secondary voltage of 110 V AC. A common set of interposing VTs is employed for all circuits which are located inside the synchronising trolley.

With generator and transmission voltage switchgear, the interposing VTs are provided for each supply. These are normally housed in cubicles located close to the switchgear or in a protection equipment room. Here, multicore control cable is used which has a lower resistance than multipair cable, the secondary circuits to the control room operating at 63.5 V AC.

The maximum lead loop resistance values per kilometer for the two cable types normally concerned are as follows:

As an example, consider an interposing VT connected to the synchronising trolley via an 800 m (loop) length multicore cable. The measured output voltage at the transformer secondary terminals is 63.5 V AC with a burden of 12.7 VA at a pf of 0.86 lagging. The percentage voltage measurement error at the synchronising

equipment would be as follows (see Fig 12.23):

Vt

0.26.04 0.86 x100

63.5

=1.6%

where V z = voltage at interposing transformer secondary terminals

V, = voltage at synchronising equipment terminals

1, = load current RL = lead resistance

Although the voltage drop can be compensated for by adjustment of the interposing VT tap, as already described this may introduce other difficulties with voltage selection schemes and where possible unduly long connections are best avoided. In exceptional circumstances it may be necessary to parallel cable cores to reduce the voltage drop.

A high lead resistance may also necessitate the fitting of ballast resistors to the synchroscope on/off switch

Flo. 12.23 Voltage drop due to lead resistance

to compensate for the reduction in circuit burden when the synchroscope is switched off.

6.4 Synchronising supplies

6.4.1 Steam turbine-generator

Transmission voltage switchgear

Voltage supplies are obtained from the transmission station. Single-phase VTs are installed, which are normally capacitor VTs as these are more economic than electromagnetic VTs. in accordance with CEGB Standard 99384 [3], present standard designs are to BS3941 and rated at not less than 100 VA with accuracy either Class 1 (voltage error ± 1 07o, phase displacement +40 minutes) or Class 3P (voltage error ±31/4, phase displacement ± 120 minutes) and cover the range of burdens between 0°70 and 100% of rated burden, at a power factor of 0.8 lagging. The incoming and running supplies are derived from the yellow phase to earth primary voltage, in accordance with Engineering Recommendation 515 Part 3 [4].

Generator voltage circuit-breaker

The incoming supply for the generator voltage circuitbreaker is obtained from VTs installed in the main connections on the generator side of the switch. A group of three single-phase units is required since the main connections are phase isolated. The VTs are of the solid insulation type, individually mounted in metalclad enclosures, a set of four being installed on each phase. The VT windings are connected in star-star with both neutrals earthed. The rated line to earth secondary voltage is 110/V3 V. The connection arrangement is in accordance with CEGB Standard 994274 [5]. The transformers comply with ESIStandard 35-5 [6], which generally specifies BS3941 except that the performance and testing requirements extend beyond the current British Standard. The ratings and limits of accuracy are as given in Section 6.3.1 of this chapter.

The running supply is obtained from the generator transformer side of the generator voltage circuitbreaker. This may be from transformers of the same design and make installed in a single bank in the main connections, or from an additional winding in the interconnected star transformer used to earth the main connections at this point (see Chapter 3, Section 2.5.5).

6.4.2 11 kV gas - turbine generators

There are two options available for obtaining the incoming supply: to use a VT at the 11 kV switchboard, or to use the gas turbine VTs directly connected to the generator terminals. The former, along with the running supply, is obtained from the 11 kV switchboard as described in Section 6.4.3 of this chapter. The latter are solid insulated single-phase units complying with ESI Standard -35-6 [7] and mounted in metalclad enclo-

sures, forming a three-phase-connected bank. The ratings and limits of accuracy are as given in Section 6.3.1 of this chapter, with the connection arrangement in accordance with CEGB Standard 994274.

6.4.3 3.3 kV and 11 kV distribution switchgear

Star-star connected VTs are located in the fixed, metal enclosed portion of the switchboard on the outboard or circuit side of each venerator or distribution cir- cuit-breaker. The transformers are of the dry-type design with ratios, ratings and accuracy complying with 1353941. The VTs have a rated line-to-line secondary vol t age of 110 V and a rating of 50 VA, accuracy class 0.2 (voltage error +0.2%, phase displacement +10 minutes) when metering supplies are taken. Otherwise a rating of 200 VA and accuracy class 1.0 (voltage error +1%, phase displacement +40 minutes) is used. Dual rating transformers may be supplied for this purpose.

The yellow phase secondary is earthed. This provides two 110 V rated single-phase to earth supplies, connection between red and blue phases not being permitted. The burdens are divided between the two supplies with the red to yellow phase supply always selected for synchronising.

6.4.4 3.3 kV diesel generators

The incoming and running supplies are derived from 3.3 kV switchgear voltage transformers as described in Section 6.4.3 of this chapter.

7 Synchronising schemes

7.1 Standard schemes

The schematic drawings for two standard synchronising schemes are shown in Figs 12.24 and 12.25.

Figure 12.24 shows the manual synchronising scheme for an 11 kV switchgear distribution circuit. Figure 12.25 shows the manual and automatic synchronising scheme for a steam turbine-generator generator voltage circuit-breaker based on CEGB Standard 993610 [8]

The equipment in the schematics is shown in the open, reset and de-energised condition, irrespective of whether the equipment in normal operation is closed or continuously energised.

The equipment is connected to 63.5 V AC, 110 V AC, 110 V DC and 48 V DC and complies with the requirements of CEGB Specification US/12/50 [9] and CEGB Specification US/76/10 [101. Electronic equipment complies with CEGB Specification EES (1980) [11]. It should be noted that the schemes are designed to be readily extendible.

Automatic synchronising relays and check synchronising relays are CEGB approved. To obtain type ap-

proval, relay manufacturers must demonstrate that the relays comply with the applicable CEGB standards by completing a series of type tests.

7.2 11 kV distribution circuit

The procedure for manual synchronising is as follows (refer to Fig 12.24):

(a)Connect the synchronising trolley to the distribution switch synchronising circuits by inserting the synchronising plug into the socket provided at the Electrical Auxiliaries Switchgear Control Panel.

(b)Select '3.3 kV/11 kV check operative' at the manual synchronising control selector switch on the synchronising trolley. This:

•Energises one coil of the OR relay.

•Closes the OR contact in the close interposing relay coil circuit which now requires both the discrepancy control switch contact and the SYN contact to close to cause operation.

•Closes the OR contact in the GRA coil circuit which now awaits SYN contact to cause operation.

(c)Check that the voltage difference and phase angle between the two supplies are satisfactory, using the synchronising trolley instruments.

(d)The check synchronising relay confirms that conditions are satisfactory by closing SYN contacts. This:

• Energises the GRA relay.

•Opens the GRA contact in the second OR coil circuit.

(e)Initiate switch closing by rotating the discrepancy switch to the 'close' position. This energises the close interposing relay coil. It is worth noting that if the operator attempts to close the switch by operation of the discrepancy switch when the permitted synchronising conditions are absent, the following will result:

•The second coil of OR will energise, causing OR relay to drop off.

•OR contact in the close interposing relay coil circuit will open.

•The 'synchroniser locked-out' indicator is displayed.

Lockout is now maintained until the energisation of the second OR coil via the SYN contact is interrupted, by selecting 'off' at the manual synchronising control selector switch.

GI 7

—0

C B

LLOC .

O—

••■

48V

DISCREPANCY

SWITCH

00 w w uJ

Z Z (fl

UJ U.1 00 CL 0_ J

000 0

L _

7.3 Steam turbine-generator — generator voltage circuit-breaker

7.3,1 Manual synchronising

The procedure for manual synchronising is as follows (refer to Fie 2.25):

(a)Connect the , ■ nchronisinv trolley to the generator synchroni.sing circuits by inserting the synchronising plug into the socket pro), ided at the Unit Desk.

(b)Select 'Generator/Transmission' (e.g., 23.5 kV/400 kV) voltage at the manual .synchronising control selector switch on the synchronising trolley.

(c)Select 'Manual Synch' at the key-operated control selector switch 1. Key-operated control selector switch 2 is already set at 'Check Operative'. This:

•Energises the GSY relay, which closes GSY-1 and GSY-2 to connect running and incoming supplies to the synchroscope and voltmeters, and closes GSY-3 and GSY-4 contacts on either side of the close interposing relay coil.

•Energises the OR relay, which closes OR-1 and OR-2 to connect running and incoming supplies to the check synchronising relay, closes OR-3 which energises HC relay and opens OR-4, removing SYN-2 contact by-pass.

•FIC relay is now energised, which opens HC-1 and HC-2, disconnecting the running and incoming supplies from the automatic synchronising relay unit, closes HC-3 and HC-5 in the 'manual synch' circuit on either side of the close interposing relay coil and opens HC-4 and HC-6 in the 'auto synch' circuit on either side of the close interposing relay coil.

•Energises one coil of OR relay, which opens GR-1, disconnecting the 'check synch monitor' lamp, closes GR-2 (which energises GAR relay if the discrepancy control switch is prematurely operated) and opens GR-3 in the SYN-2 contact circuit.

(d)The governor set point (incoming frequency) and generator transformer tap position (running voltage) are adjusted using the unit controls and the synchronising trolley instruments until satisfactory conditions are obtained.

(e)The check synchronising relay confirms that conditions are satisfactory, by closing SYN contacts:

•SYN-1 energises the second coil of GR relay, causing GR to drop off.

•GR-I closes to illuminate the 'check synch monitor' lamp.

•GR-2 opens.

•GR-3 closes which, with SYN-2 already closed, completes the closing circuit except for the discrepancy switch.

(I)Initiate switch closing by rotating the discrepancy control switch I to the 'close' position. This completes the circuit which energises the close interposing relay coil IPC. Contact (PC-I initiates circuit-breaker closure.

(g)Key-operated control selector switch I is returned to the 'off' position which restores the circuitry to the initial condition,

7.3.2Automatic synchronising

(a)Select 'Auto Synch' at the key-operated control selector switch I, this:

•Energises the GSY relay, which closes GSY-1 and GSY-2 to connect running and incoming supplies to the automatic synchronising relay and closes GSY-3 and GSY-4 contacts on either side of the close interposing relay coil.

•Energises C relay, which closes C-1.

•Energises CA relay, which closes CA-1 (110 V AC supply to the automatic synchronising relay).

•Energises NP relay, which closes NP-1 NP-2, NP-3 and NP-4.

(b)Depress the 'auto synch start' button. This energises ST relay, which closes ST-I (start hold) and ST-2, ST-3 and ST-4 (speed and voltage control).

(c)The automatic synchronising relay signals that conditions are satisfactory for synchronising. The duplicate 'auto synch' contacts close and energise the close interposing relay coil IPC.

(d)Contact [PC-1 initiates circuit-breaker closure.

(e)Key-operated control selector switch I is returned to the 'off' position, which restores the circuitry

to the initial condition.

7.4Site commissioning tests

The following lists the test sequence for commissioning a synchronising scheme. It is for guidance only and is not intended to be a complete description of all the testing that may be necessary:

(a)Wiring inspection and circuit/wiring diagram checks.

(b)If applicable, confirm that the correct number and type of synchronising keys are available and that any excess keys are removed from site.

CONTROL

SW!IC H •

= 7-

CONTROL SELEGIC.R

5WiTck

SPEEDER MOTOR

CONTROL

TAP CHANGE {

CONTROL

Synchronising schemes