Blocking Overreach Protection Application

In blocking overreach protection (BOP), each terminal normally transmits a trip permission signal, and transmits a trip block signal if the reverse looking Z4 operates and the forward overreaching element does not operate. Tripping of the local circuit breaker is performed on condition that the forward overreaching element has operated and a trip permission signal has been received. As the forward overreaching element, it is possible to use zone 2 or zone 3. If signal modulation is performed by an ON/OFF method, the signal is not normally transmitted and a trip block signal is transmitted only when the reverse looking element operates. Tripping is performed on condition that the forward overreaching element has operated and no signal has been received. In this signaling system, the signal transmitted is a trip block signal and transmission of this signal is only required in the event of an external fault. Therefore, even if power line carrier is used, there will be no failure to operate or false operation due to attenuation of signals caused by signal transmission through the fault.

The BOP receives a trip permission signal all the time. Therefore, when a forward external fault occurs, the infeed terminal on which the forward overreaching element has operated attempts to perform instantaneous tripping. At this time, at the remote outfeed terminal, the reverse looking element operates and transmits a trip block signal. This signal is received at the infeed terminal after a channel delay time. Therefore, a short delay is required for the tripping to check for the reception of a trip block signal.

The BOP performs fast tripping for any fault along the whole length of the protected line even if an open terminal exists. A strong infeed terminal operates for all internal faults even if a weak infeed terminal exists. Therefore, no echo function is required. However, since no weak infeed logic is applicable to the BOP, the weak infeed terminal cannot operate.

When a sequential fault clearance occurs for a fault on a parallel line, the direction of the current on the healthy line is reversed. The status of the forward overreaching element changes from an operating to a reset state at the terminal where the current is reversed from the inward direction to outward direction, and from a non-operating status to an operating status at the other terminal. In this process, if the operating periods of the forward overreaching element of both terminals overlap, the healthy line may be tripped erroneously. To prevent this, current reversal logic is provided. (See Section 2.4.3.6 for current reversal.)

Scheme Logic

Figure 2.4.3.4 shows the scheme logic of the BOP. The logic level of transmit signal CS and receive signal R1-CR is "1" for a trip block signal and "0" for a trip permission signal.

The transmit signal is controlled in the BOP as follows:

In the normal state, the logic level of transmit signal CS is 0, and a trip permission signal is transmitted. If the reverse looking Z4 operates and at the same time the forward overreaching element zone 2 or zone 3 selected by the scheme switch [ZONESEL] does not operate, CS becomes 1 and a trip block signal is transmitted. When this condition continues for 20 ms or more, current reversal logic is picked up and a drop-off delay time of TREBK setting is given to reset the transmission of the trip block signal.

Transmission of a trip permission signal continues for the TSBCT setting even after the local terminal is tripped, assuring command tripping of the remote terminal.

The BOP outputs single-phase tripping signal S-TRIP or three-phase tripping signal M-TRIP to the local terminal when zone 3 or zone 2 operates and at the same time the trip permission signal is received (R1-CR=0). The delayed pick-up timer TCHD is provided to allow for the transmission delay for receipt of the trip block signal from the remote terminal in the event of a forward external fault.

UVC. The phase selection logic is described in Section 2.4.3.7. & & ≥ 1 CS Z4 20ms t 0 0.00 – 10.00s 0 t M-TRIP S-TRIP Phase Selection & Z2 Z3 TCHD 0 - 50ms t 0 "Z2" "Z3" [ZONESEL] [PSB-CR] " ON " PSB NON VTF & R1-CR 1 TREBK TSBCT 0.00 – 1.00s 0 t

Figure 2.4.3.4 BOP Scheme Logic Setting

The following shows the setting elements necessary for the BOP and their setting ranges. For the settings of Z2, Z3 and UVC, refer to Section 2.4.1.

Element Range Step Default Remarks

Z4S 0.01 - 50.00Ω 0.01Ω 8.00Ω Z4 reach (0.1 – 250.0Ω 0.1Ω 40.0Ω) (*)

BRRS 0.10 - 20.00Ω 0.01Ω 5.10Ω Reverse right blinder reach (0.5 - 100.0Ω 0.1Ω 25.5Ω)

Z4G 0.01 - 100.00Ω 0.01Ω 8.00Ω Z4 reach (0.1 – 500.0Ω 0.1Ω 40.0Ω)

BRRG 0.10 - 20.00Ω 0.01Ω 5.10Ω Reverse right blinder reach (0.5 - 100.0Ω 0.1Ω 25.5Ω)

TCHD 0 - 50 ms 1 ms 12 ms Channel delay time TREBK 0.00 - 10.00s 0.01s 0.10s Current reversal block time TSBCT 0.00 – 1.00s 0.01s 0.10s

PROTECTION

SCHEME 3ZONE/Z1EXT/PUP/POP/UOP/ BOP /POP+DEF/UOP+DEF/ BOP+DEF/PUP+DEF

POP Scheme selection

ZONESEL Z2/Z3 Z2 Overreaching element selection

PSB - CR OFF/ON ON Power swing blocking

(*) Ohmic values shown in the parentheses are in the case of 1 A rating. Other ohmic values are in the case of 5 A rating.

The following elements have fixed setting values or their settings are interlinked with other elements listed above. So no setting operation is required.

Element Setting Remarks

Z4BS Fixed to 1.5Ω Z4 reverse offset reach (Fixed to 7.5Ω) (*1)

Z4S θ(*2) Interlinked with Z3S θ Characteristic angle of Z4 mho element Z4BS θ(*3) Interlinked with ZBS θ Angle of Z4 directional element BRRS θ Fixed to 75° Angle of reverse right blinder BRRS BRLS Interlinked with BRRS Reverse left blinder

BRLS θ Interlinked with BFLS θ Angle of reverse left blinder BRLS Z4G θ(*2) Interlinked with Z3G θ Characteristic angle of Z4 mho element

Element Setting Remarks

Z4BG θ(*3) Interlinked with ZBG θ Angle of Z4 directional element BRRG θ Fixed to 75° Angle of reverse right blinder BRRG BRLG Interlinked with BRRG Reverse left blinder

BRLG θ Interlinked with BFLG θ Angle of reverse left blinder BRLG

(*1) Ohmic values shown in the parentheses are in the case of 1 A rating. Other ohmic values are in the case of 5 A rating.

(*2) Valid only when mho-based characteristic is selected by ZS-C and ZG-C. (*3) Valid only when quadrilateral characteristic is selected by ZS-C and ZG-C.

The reverse looking elements Z4 (G,S), BRR (G,S) and BRL (G,S) must always operate for reverse faults for which the forward overreaching element of the remote end operates. The following setting coordination is required.

When zone 2 is selected as the forward-looking element, Z4 setting = 1.2 × (Zone 3 setting at remote end) or

Z4 setting = α × (Zone 2 setting at remote end)

Note: α should be determined in consideration of the extension of zone 2 by zero-sequence compensation.

When zone 3 is selected,

Z4 setting = 1.2 × (Zone 3 setting at remote end) In both cases,

BRR setting = 1.2 × (BFR setting at remote end)

The delayed pick-up timer TCHD is set as follows taking into account the transmission delay time of the blocking signal and a safety margin of 5 ms.

TCHD setting = maximum signal transmission delay time(*) + 5ms

(*) includes delay time of binary output and binary input for the blocking signal. 2.4.3.5 Protection for Weak Infeed Terminal

The POP and UOP are provided with an echo function and weak infeed trip function. Both functions are used for lines with weak infeed terminals.

Figure 2.4.3.5 shows the scheme logic for the echo function.

With the POP, when a trip permission signal is received (R1-CR=1) if neither forward overreaching zone 2 or zone 3 nor reverse looking Z4 have operated, the echo function sends back the received signal to the remote terminal. With the UOP, when reception of a blocking signal is stopped (R1-CR=0) if neither forward overreaching zone 2 (or zone 3) nor reverse looking Z4 have operated, the echo function stops sending the blocking signal to the remote terminal. When the circuit breaker is open (CB-OR = 0), too, the echo function sends back the trip permission signal or stops sending the blocking signal. Timer TECCB is used to set the time from CB opened to the echo logic enabled.

The terminal on which the forward overreaching element has operated can be tripped at high speed by this echoed signal.

Once the forward overreaching element or reverse looking element have operated, transmission of the echo signal is inhibited for 250 ms by delayed drop-off timer T1 even after they have reset. In order to prevent any spurious echo signal from looping round between the terminals in a healthy state, the echo signal is restricted to last for 200 ms by delayed pickup timer T2.

The echo function can be disabled by the scheme switch [ECHO] and the PLC signal ECHO_BLOCK.

The setting element necessary for the echo function and its setting range is as follows:

Element Range Step Default Remarks

TECCB 0.00 – 200.00 s 0.01 s 0.10 s Echo enable timer

ECHO OFF/ON ON Echo function

" ON " & & & ≥ 1 200ms 50ms (+) t 0 Z3 Z2 Z4 T2 T1 250ms 0 t 0 t ECH [ZONESEL] "Z3" "Z2" ECH [ECHO] 1 CB-OR ≥ 1 R1-CR 1 & TECCB 0.00 - 200.00s t 0 619:C/R_DISECHO ECHO_BLOCK 1840 1

Figure 2.4.3.5 Echo Logic

Figure 2.4.3.6 shows the scheme logic of the weak infeed trip function. Weak infeed tripping is executed on condition that a trip permission signal has been received (R1-CR=1) for the POP, and reception of a trip block signal has stopped (R1-CR=0) for the UOP, the undervoltage element UVL (UVLS or UVLG) operates and neither forward overreaching zone 2 or zone 3 nor reverse looking Z4 operates. WKIT_BLOCK 1841 WIT & WIT [ZONESEL] 250ms 0 t "Z3" "Z2" [WKIT] "ON" (+) Z4 Z3 Z2 UVL R1-CR 1 ≥ 1 & CB-OR 213:WI_TRIP 1

Figure 2.4.3.6 Weak Infeed Trip Logic

The undervoltage element responds to three phase-to-phase voltages and three phase-to-ground voltages. The undervoltage element prevents false weak infeed tripping due to spurious operation of the channel.

to the reclosing mode of the autoreclose function.

The weak infeed trip function can be disabled by the scheme switch [WKIT] and the PLC signal WKIT_BLOCK.