I 1.1 This practice covers the design of the control system for incoming line (or transformer secondary) circuit breakers, and bus tie circuit breakers, of low-voltage and medium-voltage secondary selective substations with automatic transfer.
I V 1.2 Control of secondary selective substations with manual transfer. Modifications to the control system design given herein will be specified.
I 1.3 An asterisk (V) indicates that a decision by the purchaser is required or that additional information is furnished by the purchaser.
I 2.1 Secondary selective substations, as referred to in this practice, are defined as substations having two busses, each supplied by a normally-closed incoming line circuit breaker and connected together by a normally-open bus tie circuit breaker. Although the name is derived from designs utilizing two transformers, the design is applicable (with appropriate functional deletions) to dual-fed substations without transformers.
I 2.2 Transformer protection consists of any of the following, individually or in combination:
a. Three phase overcurrent relays energized from current transformers in the transformer primary bushings or cable ends.
b. Fault pressure or Buchholz relays.
c. Differential relays.
I 2.3 Transformer secondary protection consists of a ground fault relay in the transformer neutral connection or restricted ground fault relaying.
I 2.4 A series type lockout relay is an auxiliary relay, usually hand reset, whose operating coil is in series with the operating coil of an associated device.
Operation of protective relays in the circuit causes the lockout relay and its associated device to operate simultaneously. Contacts of the lockout relay may perform other functions in the circuit including locking out the closing circuits of one or more circuit breakers or contactors.
I 2.5 A shunt type lockout relay is an auxiliary relay, usually hand reset, whose operating coil is connected in shunt (parallel) with its control power source and is associated with another device whose coil is similarly connected to the same or a separate control power source.
Operation of protective relays in the circuit causes the lockout relay to operate, contacts of which then operate its associated device. Other contacts of the lockout relay may perform other functions in the circuit including locking out the closing circuits of one or more circuit breakers or contactors.
O 3.1 The control scheme shall provide automatic transfer of the supply to each bus from its incoming line to the other bus. The sequence of the automatic transfer operation shall be as shown in block diagram Figure 1.
O,M 3.2 The control scheme shall permit manua 11411l116l l switching to remove an incoming line from service or to return an incoming line to service after a transfer. The sequence of the manual switching operation shall be as shown in applicable block diagram Figure 2 or 3.
S V 3.3 The sequence of manual switching for substations supplied from unsynchronized sources is shown in Figure 3. The control scheme shall not permit paralleling the incoming lines during switching.
O,M V 3.4 Synchronized Sources. Figure 2 shows the sequence of manual switching for substations supplied from sources which are always in synchronism or which can be synchronized during the switching operation. A trip-selector switch shall be provided having the following features:
a. The switch shall have only 3 positions, one position for the bus tie breaker and one position for each incoming line breaker.
b. The switch shall be arranged so that the selected breaker trips after all three breakers are closed. Incoming lines may thus be momentarily paralleled during switching to prevent interruption of service.
O,R 3.5 The arrangement of instrument transformers, relays, and meters shall be per the applicable parts of Figure 4. Figure 4 shows the single-line diagram of a secondary selective substation with automatic transfer having transfer initiation from the source substation and with transformer secondary protection.
O 3.6 Intertripping with the source substation initiates automatic transfer of the secondary selective substation whenever one of the supply breakers in the source substation trips. This arrangement eliminates the time delay associated with transfer initiation by undervoltage. Separate 94 auxiliary relays shall be provided for each substation when two or more substations are supplied in parallel from a pair of feeders.
C 3.7 The intertripping shall be combined with the circuit used to trip the primary supply breaker, for substations having transformer protection or transformer secondary protection.
O,R V 3.8 If transformer protection or transformer secondary protection is specified, the protection relays shall actuate a hand-reset lockout relay. Operation of the lockout relay shall lock out the closing circuits of the source breaker and of the incoming line breaker, initiate automatic transfer, and, if the relay is shunt type, initiate tripping of the source breaker.
If transformer protection utilizes rate-of-rise fault pressure relays equal to GE Model 900-1 or ABB Type SPR, shunt type lockout relays shall be used.
If the transformer uses surge fault pressure relays of the Buchholz type, series or shunt type lockout relays may be used, but series type is preferred.
O 3.9 Series type lockout relay shall have the following features:
a. Manually reset from exterior of front panel.
b. Trip indication, mechanical target type.
c. It is preferred that a "healthy trip" pilot light provide simultaneous supervision of the trip initiation circuit as well as of the lockout relay coil, provided there are not sufficient substations supplied by the one source breaker so that total pilot light current could cause false tripping.
O 3.10 For shunt type lockout relays, relay coil supervision shall be provided by a "healthy trip" pilot light. When two or more substations at one location are supplied in parallel from a pair of feeders, only one shunt lockout relay per source breaker shall be provided at that location. See Figure 6. If these substations operate at different voltages, the lockout relays shall be located in the higher voltage switchgear.
S 3.11 For either shunt or series type lockout relay, mechanical or electrical means shall be provided and suitably marked to operate the relay manually (thereby tripping the supply breaker and initiating transfer), unless the supply breaker is in the same room.
R 3.12 Arrangement of substation control power busses shall be as shown in Figure 7 or 8 for substations with shunt type lockout relays. For substations with series type lockout relays, bussing shall be per Figure 7 with only 3 branches off bus 3, except a fourth branch is required when synchronizing check facilities are provided.
O 3.13 Voltage for control power busses shall be supervised by normally energized auxiliary relays located inside the switchgear assembly. Relays shall be suitable for continuous operation at control power bus voltage without tendency to stick in the energized position and shall be time delay on drop-out. Time delay shall be long enough to prevent operation on transient voltage dips. One relay shall be provided for each control power bus, and its coil shall be connected to the bus at the furthest point from the supply connection to the bus. A normally open contact from each relay shall be wired in series and used to actuate a "Loss of Control Power" alarm in the substation alarm panel.
R,I 3.14 Control circuits
meeting the intent of this practice are shown in Figure 5 and 6. These schemes are based on the design of switchgear
as manufactured in the
V The control circuit of Figure 5 is suitable for substations supplied from sources which are always in synchronism or from sources which can be synchronized during manual switching. Modifications to the Figure 5 circuit for substations supplied from sources which cannot be synchronized will be specified.
O 3.15 Each incoming line shall be provided with a voltmeter to indicate phase-to-phase voltage of one phase.
O 3.16 Each bus shall be provided with a voltmeter with switch to indicate phase-to-phase voltage of each phase.
O 3.17 For ungrounded neutral or high impedance grounded neutral substations, the bus voltmeters shall also be switched to indicate the phase-to-ground voltage of each phase to allow detection of grounds on the system.
S 3.18 If summation CTs are approved for watthour demand metering, the main CTs must have a warning nameplate engraved as follows (white letters on red background).
WARNING - BACKFEED PRESENT WHEN OTHER INCOMER IS CLOSED |
O 3.19 In addition to standard devices (such as circuit breaker indicating lights and targets), supervisory devices described in Pars. 3.9-3.10, and 3.20-3.23 shall be provided with the switchgear.
O 3.20 A white "transfer ready" light shall be provided for each incoming line. The light shall be connected across the contacts of the device or devices which initiate automatic transfer. When on, the light should indicate that the circuit which initiates transfer is ready to operate.
O 3.21 A latch-check switch, if available, shall be provided on each circuit breaker element used for incoming line and bus tie service. The switch shall indicate that the breaker operating mechanism has reset properly after tripping and will "latch in" when the breaker is closed.
O 3.22 Latch-check switches on incoming line breakers shall de-energize the breaker "open" indicating light when the breaker mechanism has not reset properly. The tie breaker latch-check switch shall de-energize each "transfer ready" light when the breaker mechanism has not reset properly.
O 3.23 Latch-check switches on interchangeable breaker elements. When circuit breaker elements for outgoing feeders are interchangeable with incoming line or bus tie breakers, latch-check switches shall be provided on all interchangeable breaker elements. With the breaker element in a feeder position, the switch shall de-energize the breaker "open" indicating light when the breaker mechanism has not reset properly.
O,R 3.24 D-c control power for intertrip circuits shall be monitored by a white light or under voltage relay if this loss-of-voltage is not being monitored by other lights or relays in the scheme.
O 3.25 Arrangements of relays, control devices, and nameplates on switchgear front panels for incoming and bus tie breakers are shown in Figures 10 and 11. These arrangements shall be followed as closely as available panel space allows. All devices shown shall be located on the upper half section of the front panel. Switches and similar devices shall be located at a height convenient for personnel operation.
O 3.26 A nameplate shall be provided showing instructions for manual switching to remove an incoming line from service, and to return to normal after an automatic transfer or manual switching. Figure 9 shows a suitable nameplate for substations supplied from synchronized sources.
O 3.27 Other nameplates shall be mounted on the front of the switchgear panels.
O 3.28 All nameplates shall be mounted on the front of the switchgear panels.
O 3.29 In addition to the nameplates specified above, small nameplates shall be provided on the front of the switchgear panels, mounted adjacent to each relay, showing the device function number such as 51-1, 27R-2.
Notes:
(1) Transfer initiation on incoming-line undervoltage shall always be provided. One or more of the other initiating conditions may be used as specified.
(2) Low voltage is 70-80% or less of normal voltage. Relay time at zero voltage usually 0.8-1.5 sec.
(3) Auto transfer shall be blocked when any of the three breakers is in the test or withdrawn position to prevent:
(a) An incomplete transfer as might occur if the tie breaker were at test or withdrawn.
(b) A useless operation transferring one bus to the other whose incoming line breaker is at test or withdrawn.
(c) A dangerous transfer which would energize a bus which had been deliberately deenergized and whose incoming line breaker is at test or withdrawn.
(d) Automatic operation of a breaker when in test position which might endanger maintenance personnel.
(4) Auto transfer shall be blocked when either incoming line breaker is open to prevent:
(a) Auto transfer after an incoming line breaker has tripped on overcurrent, to avoid connecting a faulted bus or feeder to the other bus.
(b) A dangerous transfer which would energize a bus which had been deliberately deenergized by opening its incoming line breaker.
(c) A useless operation transferring one bus to the other whose incoming line breaker is open.
If bus (instead of incoming line) potential transformers supply undervoltage relays used to initiate transfer, it must be assured that, when re-energizing a bus, transfer remains blocked long enough for undervoltage relay reset.
(5) Auto transfer shall be blocked whenever an incoming line breaker is carrying fault current. Low voltage during fault could initiate a transfer which would connect the faulted bus or feeder to the other bus. "50" should be set above the motor backfeed into an incoming line fault, allowing for a-c and d-c decrement; usually 12.5-18A. "50N" should be set below arcing ground fault current, usually 0.5-2A, but above any legitimate neutral current (unless 4CTs used).
(6) Auto transfer shall be blocked whenever devices on the supply side of an incoming line breaker clear currents caused by faults on the load side of that breaker, to prevent transferring the faulted bus or feeder to the other bus. Transfer shall be blocked until approximately 90% or more of normal voltage is restored after flow of fault current through the incoming breaker stops.
(7) Auto transfer shall be blocked if supply voltage on the other incoming line at time of transfer is not approximately 90% or more of normal voltage. System disturbances may cause voltage dips on both incoming lines. Since voltage may be restored to one line before the other after a simultaneous dip, transfer shall be blocked for 3 sec to wait for restoration of voltage to other line. This prevents unnecessary transfers when voltage returns to both lines within 3 sec.
(8) Tie breaker shall not be closed until residual voltage on bus 1 has decayed to or below a specified value, often 25%, or 40% if all motors are suitable for star-delta switching.
(9) Sequence shown is a transfer of supply of bus 1 from incoming line 1 to bus 2. Substitute 1 for 2 and 2 for 1 to transfer bus 2 to bus 1.
Notes For Figures 2 and 3:
(1) Provide a hand-reset lockout relay for each incoming line breaker actuated by transformer fault pressure, Buchholz, or differential relays, and by ground relays protecting transformer secondary windings and connections. In addition to tripping source substation supply breaker and initiating transfer, contacts of these relays should lock out incoming breaker closing circuits. Provide bypass contacts around lockout relay contacts in incoming breaker closing circuits which lose when breakers are in test or withdrawn position.
(2) Close switch of breaker that is open.
(3) Provide synchronizing check relay, or other means, to prevent paralleling lines 1 and 2 when their sources are not in synchronism (not required if sources supplying lines 1 and 2 are always in synchronism).
Notes
(1) Provide synchronizing check relay only if normally synchronized sources, or sources which can be synchronized, may be unsynchronized on occasion.
(2) Relays 27, 27I, and 27R (if 1- ) shall be connected line-to-line across the same two lines.
(3) In normal operation, breakers 52-1 and 52-2 are closed, with breaker 24 open:
(4) Neutral grounding resistor if medium voltage substation.
(5) For ungrounded or high resistance grounded substations provide 3 wye-wye connected potential transformers with grounded neutrals.
Notes for Figure 5 Incoming and Bus Tie Breaker Circuits (see Pages 9, 10)
(1) Paralleled closing and tripping circuit fuses are shown for illustration. See IP 16-12-1 for other acceptable schemes. If tripping circuit is not protected, supply green pilot light circuit of 52-1 and 52-2 through closing circuit fuses.
(2) Omit if sources are always in synchronism.
(3) Omit contacts 52LS-2 and 24LS at (3) if contacts 52-2/a, 24/a, and 24/b at (3) are mounted on removable breaker elements and are disconnected when breakers 52-2 and 24 are the "test" position or are withdrawn. See Note 13.
(4) Omit contact 52LS-1 and 52LS-2 at (4) if contacts 52-1/a and 52-2/a at (4) are mounted on removable breaker elements and are disconnected when breakers 52-1 and 52- 2 are at "test" position or are withdrawn. See Note 13.
(5) Omit contacts 52LS-1 and 24LS at (5) if contact 52-1/a, 24/a, and 24/b at (5) are mounted on removable breaker elements and are disconnected when breakers 52-1 and 24 are at "test" position or are withdrawn. See Note 13.
(6) Omit contact 24LS at (6) if contact 24/b at (6) is mounted on removable breaker element and is disconnected when breaker 24 is at test position or withdrawn. See Note 13.
(7) Reduced voltage coil; size resistor to limit current through relay coil to value within continuous rating of coil.
(8) Contacts 52-1/a, 52-2/a and 24/a at these locations shall be mechanically operated auxiliary contacts of the breaker (not contacts of auxiliary relays used to multiply breaker auxiliary contacts).
(9) Resistance of coil of relay 25Y shall be low enough to permit pickup of target/seal-in coil of relay 25.
(10) Circuit is for substations supplied from sources which are always in synchronism or from sources which can be synchronized during manual switching.
(11) Contact of normally energized auxiliary relay. Provide one relay for each control power bus.
(12) Omit 27I contacts in 52-1 and 52-2 closing circuit if device 25 is supplied.
(13) In Notes 3-6 above: "a" contacts of "stationary switches," such as used in GE Type AM switchgear, are not considered disconnected with breaker at "test," since a link is or can be furnished to operate switch with breaker at "test" position; LS contacts shall not be omitted in this case.
Notes:
(1) Resistor, size as required to protect 63 and 63X relay contacts in the event of flashover in 63 relay.
(2) Omit PB if 86T can be operated manually.
NOTE: Relay ratings are based on potential transformers with 120-volt secondaries and current transformers with 5 amp secondaries.
8 2-pole control power disconnect switch.
10 3-position switch, non-spring return, positions marked TRIP 1 - TRIP TIE - TRIP 2, with contacts closed as shown by subscripts 1, T, and 2, equal to GE Type SB-1 or Electro-Switch Type W.
24 Bus tie breaker, interchangeable with other breakers of same rating.
24/a, 24/b, See 52/a, 52/b, 52/CC, 52/CS, 52LC, 52LS, 52/TC.
24/CC, 24/CS,
24/LC, 24LS,
24/TC
25 Synchronizing check relay, equal to GE Type IJS51A or ABB Type CVX.
25X, 25Y Auxiliary relay, self reset, hinged armature type, equal to GE Type HGA11 or ABB Type SG
27-1, 27-2 Induction undervoltage relay, equal to GE Type IAV54EIA or ABB Type CV-2 short time. Normally closed contact circuit shall be open when relay is withdrawn.
27I-1, 27I-2 Instantaneous undervoltage relay 70-100 or 60-140 volt range, 90% minimum dropout to pickup ratio, calibrated for dropout, drawout semiflush case, normally closed contact circuits shall open when relay is withdrawn, equal to GE Type NGV13 B or ABB Type SSV-T except with two normally closed electrically independent contacts. After setting, check that the pickup voltage for the dropout setting selected does not exceed the lowest expected sustained value of normal voltage considering transformer and supply source voltage regulation.
27R-1, 27R-2 Residual voltage relay, 3-phase, dropout adjustable from 30 to 45 volts, suitable for continuous operation at 120 volts without tendency to stick in the energized position. For any dropout voltage setting:
a. Dropout shall not vary more than 2 volts for frequencies between 25 and 60 cycles.
b. Pickup shall not exceed 95 volts.
c. Hot dropout (after continuous operation at 120 volts) shall be within 4 volts of cold dropout (after prolonged de-energization and then momentary application of 120 volts).
d. Dropout voltage shall be independent of prior applied voltage or the relay shall be prominently tagged "Raise voltage to 115V before calibrating dropout."
e. The relay shall be complete with a semi-flush mounting case equivalent to GE drawout type or ABB Flexitest. Normally-closed contact circuit shall remain closed with relay withdrawn.
f. Electrolytic capacitors shall not be used.
g. Equivalent to ABB Type SVF.
50-1/51-1, Induction overcurrent relay 1.5-12, 10-80 or 4-12 amp time, 10-80 or 10-40 amp inst, 5.5 amp min inst element dropout. 51
50-2/51-2 characteristic shall be chosen to provide best compromise between coordination and medium-range fault protection. If governing outgoing circuit is a relayed motor feeder or molded-case breaker, best choice probably is equal to GE Type IAC51 (inverse) or ABB Type CO-6 (definite minimum time). If governing outgoing circuit is fused, best choice is probably extremely inverse equal to GE Type IAC77 or ABB Type CO-11. If no single load governs coordination, best choice is probably very inverse equal to GE Type IAC53 or ABB Type CO-9.
50N-1/51N-1, Induction overcurrent relay, inverse, 0.5-4 or 0.5-2.5 amp time, 0.5-4 or 1-4 amp inst, or 2-16 or 1-12 amp time, 2-16 or 2-48
50N-2/51N-2 amp inst, equal to GE Type IAC51, or ABB Type CO-6 Definite Minimum Time. Induction element tap range shall be chosen to provide coordination between 51G and the following:
a. For solidly-grounded low-voltage systems, to permit pickup at about 15% of maximum 3-phase fault current, or
b. For resistance-grounded medium-voltage systems, to permit pickup at 10-20% of maximum ground fault current.
51G-1/51G-2 Induction overcurrent relay, inverse, 0.5.4 or 0.5-2.5 amp time or 2-16 or 1-12 amp time (no inst), equal to GE Type IAC51 or ABB Type CO-6 Definite Minimum Time.
52-1, 52-2 Incoming line circuit breaker, interchangeable with other breakers of same rating.
52/a, 52/b, Auxiliary contact on circuit breakers 52 and 24. "a" contact is open when breaker is open and closed when breaker is closed.
24/a, 24/b "b" contact is open when breaker is closed and closed when breaker is open. If auxiliary relays are used to multiply contacts, they shall be energized through "a" contacts. Relays shall be self reset, hinged armature type, equal to GE Type HGA11 or ABB Type SG.
52/CC, 24/CC Anti-pump closing circuit of breakers 52 and 24, with seal-in for momentary energization.
52CS-1, Circuit breaker control switch for breakers 52-1, 52-2, and 24, Close and Trip positions with spring return to neutral, with
52CS-2, 24CS contacts closed as shown by subscripts C and T, equal to GE Type SB-1 or Electro-Switch Type W.
52LC, 24LC Latch-check switch on circuit breaker element, contacts closed indicate breaker operating mechanism has reset properly after tripping operation and will latch-in on closing operation.
52LS, 24LS Contact closed or open, as shown, only when breaker indicated (by suffix 1: 52-1; by suffix 2: 52-2; by 24LS: 24) is in operating position. With the breaker at "test" or withdrawn, the contact opens if shown closed, or closes if shown open. It is preferred that contacts 52LS and 24LS consist of suitable connections to appropriate fingers of the secondary disconnect device; if this is not possible, limit switches responsive to breaker position are acceptable. If relays are used to multiply contacts, they shall indicate "breaker withdrawn" when de-energized, and be energized through switches "8" of breaker monitored.
52/TC, 24/TC Trip coil of breaker 52 and 24.
63 Transformer fault pressure relay, rate-of-rise type, equal to GE Model 900-1 or ABB Type SPR, or Buchholz surge element.
63X Auxiliary relay, internal in ABB Type SPR; external for GE Model 900-1 and Buchholz and equal to GE Type HFA or ABB Type MG.
83 Auxiliary relay, reduced voltage coil to permit pickup of 27 relay target, self reset, equal to GE Type HFA or ABB Type MG.
86T Hand-reset lockout relay, equal to GE Type HEA or Electro-Switch Type WL. Subscript HR indicates hand reset.
94 Auxiliary relay, self reset, equal to GE Type HGA11 or ABB Type SG.
96, 97 Auxiliary relay, inst pickup, adjustable time-delay dropout, equal to AGASTAT Type 7022. Set relay 96 for 3 sec delay and relay 97 for 1 sec delay. Subscripts show TDDO: time-delay dropout; TDC: time delay closing; TDO: time-delay opening.
GL, RL, WL Green, red, or white indicating light.
PB Pushbutton, momentary contact, normally closed or normally open contact as shown.
V Voltmeter, indicating type.
VS Voltmeter switch, equal to GE Type SB-1 or Electro-Switch Type W. For grounded neutral system with 2 bus potential transformers connected open delta, switch to have 4 positions marked 1-2, 2-3, 3-1, OFF. For underground neutral or high-resistance-grounded system with 3 bus potential transformers connected Y-Y, switch to have 7 (or 8) positions marked 1, 2, 3, 1-2, 2-3, 3- 1, OFF (or two OFF positions).
27I Relays 27I prevent an automatic transfer from occurring upon simultaneous loss of both sources, or loss of one source and low voltage on the other. Relays 27I operate through relays 96. Because relays 96 are time delay type, an automatic transfer is also prevented upon simultaneous restoration of both sources after a double outage, provided the two sources attain normal voltage within three seconds of each other. Relays 27I also seal in relays 97 to prevent transfer if the supply breaker should trip first on overcurrent faults in the secondary selective substation.
27R Relays 27R protect those motors, which carry through automatic transfer, against overvoltage and instantaneous tripping of their circuit breakers due to closing in out-of-phase with residual voltage. Protection is achieved by delaying transfer until residual voltage drops to a safe level. "Carrying through transfer" is used here to mean motors with controllers arranged either to keep the motors connected to the bus during transfer or else to reconnect the motors immediately upon reapplication of voltage through the bus tie connection; motors with controllers arranged for staggered reclosing after transfer will usually have insignificant residual voltage by the time of reclosure. Motors will generate a decaying residual voltage if source power is interrupted under load with no fault to dissipate magnetic energy stored in the motors. Normal setting of 27R is 25% of normal line to line voltage dropout; if this setting causes excessively long transfer the setting can be raised as high as 40% depending on system impedance and degree of motor protection desired.
50-1, Relays 50 and 50N, operating through relays 97, are used to block transfer during overcurrent faults until fault is cleared by
50-2, feeder breaker or incoming line breaker; when properly set they permit relay 27 to time out faster under fault conditions than
50N-1, relays 51-1 or 51-2 without causing a transfer. This permits low time settings and high voltage settings for relays 27. Relays
50N-2 50 should be set above the maximum non-fault current expected to occur before a transfer, such as during a period of reduced voltage, and also above motor contribution to a fault on the incoming line or transformer.
Note: If 50 cannot be set above motor contribution, add TDPU auxiliary relay set at 0.3-0.5 sec to ride over backfeed. Operate auxiliary relay with 50 contact. Replace 50 contacts in 97 relay coil circuit of Fig. 5A with TDC contacts of auxiliary relay.
Revision Memo
9/68 Original Issue of Basic Practice
6/70 Revision 1
1/75 Revision 2
1/76 Revision 3
1/77 Revision 4
1/80 Revision 5
12/92 Revision 6
6/97 Revision 0 - Original Issue of International Practice
This International Practice Revision 0 is a reissue of BP16-12-2 without revisions, except for minor editorial changes. Purpose Codes added throughout.
© Exxon Research and Engineering Company, 1992, 1997
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