W9128A-19-R-0016_Amendment_0001_file_3_of_4.pdf

4.100

SEL-751 Relay Instruction Manual Date Code 20181207

Protection and Logic Functions Group Settings (SET Command)

Figure 4.61 Undervoltage Element Logic

When DELTA_Y := DELTA

27PP1P

|VPPmax| 3P27

When DELTA_Y := DELTA or WYE

When DELTA_Y := WYE

27PP1P

27PP1D

27PP1T

27PP2P

27PP2D

27PP2T

Relay Word Bits

|VPPmin|

|VS|

27P1P

|VPmin|

27P1D

27P1T

27P2P 27P2D

27P2T

Relay Word Bits

27P2

27P1

3P27|VPmax|

27P1P

27PP1

27PP2

27S1P 27S1D

27S1T

27S2P 27S2D

27S2T

27S1

27S2

Relay Word Bits

Note: 27S1P, 27S2P, 27P1P, 27P2P, 27PP1P, and 27PP2P are settings

VAB or VA

VBC or VB

VCA or VC

Voltage Magnitude Calculation

|VS|

|VPmin| (Minimum Phase Voltage Magnitude)

VS

(Maximum Phase Voltage Magnitude) |VPmax|

(Maximum Phase-to-Phase Voltage Magnitude) |VPPmax|

(Minimum Phase-to-Phase Voltage Magnitude) |VPPmin|

Section 00 73 00 Attachment 4 W9128A19R0016

AM-0001

4.101

Date Code 20181207 Instruction Manual SEL-751 Relay

Protection and Logic Functions Group Settings (SET Command)

Figure 4.62 Overvoltage Element Logic

When DELTA_Y := WYE

When DELTA_Y := DELTA or WYE When DELTA_Y := DELTA

59PP1P

3P59

Relay Word Bit

| VPPmin |

59P1P

|VPmax|

59P1D

59P1T

59P2P 59P2D

59P2T

Relay Word Bits

59P2

59P1

59G1P

|3V0|

59G1D

59G1T

59G2P 59G2D

59G2T

Relay Word Bits

59G2

59G1

59P1P 3P59

| VPmin |

Note: 59S1P, 59S2P, 59P1P, 59P2P, 59PP1P, 59PP2P, 59Q1P, 59Q2P, 59G1P, and 59G2P are settings

59Q1D

59Q1T

59Q2D

59Q2T

59Q1P

| 3V2 |

59Q2P

59S1D

59S1T

59S2D

59S2T

59S1P

| VS |

59S2P

59S1

59S2

59Q1

59Q2

59PP1P

|VPPmax|

59PP1D

59PP1T

59PP2P 59PP2D

59PP2T

Relay Word Bits

59PP2

59PP1

VAB or VA

VBC or VB

VCA or VC

Voltage Magnitude Calculation

|VS|

|VPmax| (Maximum Phase Voltage Magnitude)

(Maximum Phase-to-Phase Voltage Magnitude) |VPPmax|

VS

(Minimum Phase Voltage Magnitude) |VPmin|

(Minimum Phase-to-Phase Voltage Magnitude) |VPPmin|

(Negative-Sequence Voltage Magnitude) |3V2|

(Zero-Sequence Voltage Magnitude) |3V0|

Section 00 73 00 Attachment 4

4.102

Protection and Logic Functions Group Settings (SET Command)

Inverse-Time Undervoltage Protection

The SEL-751 provides two inverse-time undervoltage protection elements (27I1 and 27I2). Based on relay hardware options and settings, the 27I element offers the flexibility of using various analog quantities as operating quantities. The availability of these analog quantities is contingent on the settings DELTA_Y, VSCONN, and SINGLEV, as indicated in Table 4.36.

Figure 4.63 shows the inputs, settings and outputs of the inverse-time undervoltage element.

n = 1 or 2. q Refer to Table 4.36.

Figure 4.63 Logic Diagram of the Inverse-Time Undervoltage Element

When the fundamental frequency component of the operating quantity falls below the pickup setting (27InP), Relay Word bit 27In asserts. The timer does not start to integrate unless the operating quantity falls below 0.975 • 27InP.

The inverse-time undervoltage protection element has the characteristic defined by Equation 4.14.

Table 4.36 Operating Quantities for the 27I Element

Settings Operating Quantities Available in 27InOQ Setting Rangea

DELTA_Y VSCONN SINGLEV VAB VBC VCA VA VB VC V1 VS MINLL MINLN

DELTA 3V0 N # # # — — — # — # —

DELTA 3V0 Y # — — — — — — — — —

DELTA VS N # # # — — — # # # —

DELTA VS Y # — — — — — — # — —

WYE VS N $ $ $ # # # # # $ #

WYE VS Y — — — # — — — # — —

WYE 3V0 N $ $ $ # # # # — $ #

WYE 3V0 Y — — — # — — — — — —

# = 2.00–300.00 V $ = 2.00–520.00 V — Operating quantity is not available

The "#" and "$" signs indicate the setting range for 27InP (n = 1 or 2).

a The physical meanings of the operating quantities are described as follows:

VAB: Magnitude of A-to-B-phase voltage VC: Magnitude of C-phase voltage VBC: Magnitude of B-to-C-phase voltage V1: Magnitude of positive-sequence voltage VCA: Magnitude of C-to-A-phase voltage VS: Magnitude of Vsync voltage VA: Magnitude of A-phase voltage MINLL: Magnitude of the minimum phase-to-phase voltage VB: Magnitude of B-phase voltage MINLN: Magnitude of the minimum phase-to-neutral voltage

Settings

Relay Word Bits

27I Inverse-Time Undervoltage Element

Settings

27InP Pick up value

27InCRV Curve selection

27InCFA Coefficient A

27InCFB Coefficient B

27InCFC Coefficient C

27InTD Time dial

27InTTR Time to reset

27InTC Torque control

27In

27InT

27InRS

27InP

27InOQ

27InTC

Section 00 73 00 Attachment 4

4.103

Protection and Logic Functions Group Settings (SET Command)

Equation 4.14

The settings used are listed in Table 4.37.

The SEL-751 provides three curve options for each of the 27I elements, settable via the 27InCRV setting—CURVEA, CURVEB, and COEF (user programmable curve). CURVEA is compliant with IEC 60255-127 and is the IEC standard curve as shown in Figure 4.64. CURVEB is a non-standard curve as shown in Figure 4.64. The curve option COEF is the user programmable curve. Set the coefficient related settings 27InCFA, 27InCFB and 27InCFC to realize the curve that meets your application needs.

Table 4.38 shows the parameters of the three curves. Note that when 27InCRV is set to CURVEA or CURVEB the coefficient related settings 27InCFA, 27InCFB and 27InCFC are forced to the values shown in Table 4.38 and hidden.

When the operating quantity exceeds the pickup level, 27InP, then the output remains deasserted. If the operating quantity exceeds the pickup level for the reset time setting, 27InTTR, then the time integrator resets to 0.

Table 4.37 Inverse-Time Undervoltage Settings

Setting Prompt Setting Range Setting Name := Factory Default

27I ENABLE Y, N E27In := N

OPERATING QTY Refer to Table 4.36 27InOQ := VAB

PICKUP LVL Refer to Table 4.36 27InP := 120.00

CURVE CURVEA, CURVEB, COEF 27InCRV := CURVEA

COEFF A 0.00–3.00 27InCFA := 1

COEFF B 0.00–3.00 27InCFB := 0

COEFF C 0.01–3.00 27InCFC := 1

TIME DIAL 0.00–16.00 27InTD := 1.00

RESET TIME 0.00–1.00 sec 27InTTR := 0.01

TRQ CONTROL SELOGIC 27InTC := 1

Table 4.38 Specification Of Inverse-Time Undervoltage Protection Element

Curve Description Curve Defining Constants

27InCFA 27InCFB 27InCFC

Curve A 1 0 1

Curve B 0.98 1.28 2.171

Programmable Curve 0.00–3.00 0.00–3.00 0.01–3.00 n = 1 or 2.

TTTn 27InTD 27InCFB

27InCFA

27InOQ 27InP

27InCFC

Section 00 73 00 Attachment 4

4.104

Protection and Logic Functions Group Settings (SET Command)

Figure 4.64 Inverse Time-Undervoltage Element Curves

Inverse-Time Overvoltage Protection

There are four inverse-time overvoltage elements (59I) available. Based on relay hardware options and settings, the 59I element offers the flexibility of using various analog quantities as operating quantities. The availability of these analog quantities is contingent on the settings DELTA_Y, VSCONN, and SINGLEV, as indicated in Table 4.39.

Time Dial = 1

Time Dial = 2

Time Dial = 5

Time Dial = 10

Time Dial = 1

Time Dial = 2

Time Dial = 5

Time Dial = 10

Curve A Curve B

Multiples of Pickup Multiples of Pickup

T im e i n

S e c o n d s

T im e i n

S e c o n d s

Table 4.39 Operating Quantities for the 59I Element

Settings Operating Quantities Available in 59InOQ Setting Rangea

DELTA_Y VSCONN SINGLEV VAB VBC VCA VA VB VC VG V1 3V2 3V0 VS MAXLL MAXLN

DELTA 3V0 N # # # — — — — # # # — # —

DELTA 3V0 Y # — — — — — — — — # — — —

DELTA VS N # # # — — — — # # — # # —

DELTA VS Y # — — — — — — — — — # — —

WYE VS N $ $ $ # # # # # # — # $ #

WYE VS Y — — — # — — — — — — # — —

WYE 3V0 N $ $ $ # # # # # # # — $ #

WYE 3V0 Y — — — # — — — — — # — — —

# = 2.00–300.00 V $ = 2.00–520.00 V — Operating quantity is not available

The "#" and "$" signs indicate the setting range for 59InP (n = 1, 2, 3, or 4).

a The physical meanings of the operating quantities are described as follows:

VAB: Magnitude of A-to-B phase voltage V1: Magnitude of positive-sequence voltage VBC: Magnitude of B-to-C phase voltage 3V2: Magnitude of negative-sequence voltage VCA: Magnitude of C-to-A phase voltage 3V0: Magnitude of measured zero-sequence voltage from VS channel VA: Magnitude of A-phase voltage VS: Magnitude of Vsync voltage VB: Magnitude of B-phase voltage MAXLL: Magnitude of the maximum phase-to-phase voltage VC: Magnitude of C-phase voltage MAXLN: Magnitude of the maximum phase-to-neutral voltage VG: Magnitude of zero-sequence voltage

Section 00 73 00 Attachment 4

4.105

Protection and Logic Functions Group Settings (SET Command)

Figure 4.65 shows the inputs, settings and outputs of the inverse-time overvoltage element.

n = 1, 2, 3, or 4. q Refer to Table 4.39.

Figure 4.65 Logic Diagram of the Inverse-Time Overvoltage Element

When the fundamental frequency component of the operating quantity exceeds the pickup setting, 59InP, Relay Word bit 59In asserts. The timer won't start to integrate unless the operating quantity exceeds 1.025 • 59InP.

The inverse-time overvoltage protection element has the characteristic defined by Equation 4.15.

Equation 4.15

The settings used are listed in Table 4.40.

The SEL-751 provides four curve options for each of the 59I elements, settable via the 59InCRV setting—CURVEA, CURVEB, CURVEC, and COEF (user-programmable curve). The characteristics of Curve A, Curve B, and Curve C are shown in Figure 4.66.

Table 4.40 Inverse-Time Overvoltage Settings

Setting Prompt Setting Range Setting Name := Factory Default

59I ENABLE Y, N E59In := N

OPERATING QTY Refer to Table 4.39 59InOQ := VAB

PICKUP LVL Refer to Table 4.39 59InP := 120.00

CURVE CURVEA, CURVEB,

CURVEC, COEF

59InCRV := CURVEA

COEFF A 0.00–6.00 59InCFA := 3.88

COEFF B 0.00–3.00 59InCFB := 0.96

COEFF C 0.01–3.00 59InCFC := 2.00

TIME DIAL 0.00–16.00 59InTD := 1.00

RESET TIME 0.00–1.00 sec 59InTTR := 0.01

TRQ CONTROL SELOGIC 59InTC := 1

Settings

Relay Word Bits

59I Inverse-Time Overvoltage Element

Settings

59InP Pick up value

59InCRV Curve selection

59InCFA Coefficient A

59InCFB Coefficient B

59InCFC Coefficient C

59InTD Time dial

59InTTR Time to reset

59InTC Torque control

59In

59InT

59InRS

59InP

59InOQ

59InTC

TTTn 59InTD 59InCFB

59InCFA

59InOQ 59InP

59InCFC

1–

Section 00 73 00 Attachment 4

4.106

Protection and Logic Functions Group Settings (SET Command)

The curve option COEF is the user-programmable curve. Set the coefficient related settings 59InCFA, 59InCFB, and 59InCFC to realize the curve that meets your application needs. Table 4.41 shows the parameters of the three curves. Note that when 59InCRV is set to CURVEA, CURVEB, or CURVEC the coefficient related settings 59InCFA, 59InCFB, and 59InCFC are forced to the values shown in Table 4.41 and hidden.

When the operating quantity remains lower than the pickup level, 59InP, then the output remains deasserted. If the operating quantity gets lower than the pickup level for the reset time setting, 59InTTR, then the time integrator resets to 0.

Figure 4.66 Inverse Time-Overvoltage Element Curves

Table 4.41 Specification of Inverse-Time Overvoltage Protection Element

Curve Description Curve Defining Constants

59InCFA 59InCFB 59InCFC

Curve A 3.88 0.96 2

Curve B 5.64 0.24 2

Curve C 0.14 0 0.02

Programmable Curve 0.00–6.00 0.00–3.00 0.01–3.00 n = 1, 2, 3, or 4.

Time Dial = 1

Time Dial = 2

Time Dial = 5

Time Dial = 10

Time Dial = 1

Time Dial = 2

Time Dial = 5

Time Dial = 10

Time Dial = 1

Time Dial = 2

Time Dial = 5

Time Dial = 10

Curve A Curve B Curve C

Multiples of Pickup Multiples of Pickup Multiples of Pickup

T im e i n

S e c o n d s

Section 00 73 00 Attachment 4

4.107

Protection and Logic Functions Group Settings (SET Command)

Synchronism-Check Elements

Figure 2.23, and Figure 2.24 show examples where synchronism check can be applied. Synchronism-check voltage input VS is connected to one side of the circuit breaker, on any phase you want. The other synchronizing phase (VA, VB, or VC voltage inputs) on the other side of the circuit breaker is setting selected.

The two synchronism-check elements use the same voltage window (to assure healthy voltage), frequency window (FNOM ±5 Hz), and slip frequency settings (see Figure 4.67 and Figure 4.68). They have separate angle settings.

If the voltages are static (voltages not slipping with respect to one another) or setting TCLOSD := OFF, the two synchronism-check elements operate as shown in the top of Figure 4.68. The angle settings are checked for synchronism-check closing.

If the voltages are not static (voltages slipping with respect to one another), the two synchronism-check elements operate as shown in the bottom of Figure 4.68. The angle difference is compensated by breaker close time, and the breaker is ideally closed at a zero-degree phase angle difference, to minimize system shock.

q Figure 4.68

Figure 4.67 Synchronism-Check Voltage Window and Slip Frequency Elements

NOTE: The synchronism-check element is only available when

VSCONN := VS.

59VP

59VS

SF

to

Relay Word Bits

25VLO

Setting

25SF

Setting

BSYNCH

SELOGIC

Setting

25VHI

VP

VS

Setting

Block Synchronism Check

VP Within

"Healthy Voltage" Window

VS Within

"Healthy Voltage" Window

Slip Frequency Calculator

Enable

2 CYC

to Angle Difference Calculator—to

Slip Frequency

Maximum Slip Frequency

Absolute Value Slip Frequency Element

SF

(FNOM + 5) Hz

FREQ

(FNOM —5) Hz

VP = 25RCF*VPH

where VPH = VA, VB, VC, VAB, VBC, or VCA depending on the SYNCPH setting.

High Threshold

High Threshold

High Threshold

Low Threshold

Low Threshold

Low Threshold

Section 00 73 00 Attachment 4

4.108

Protection and Logic Functions Group Settings (SET Command) q From Figure 4.67; w see Figure 4.81.

Figure 4.68 Synchronism-Check Elements

These synchronism-check elements are explained in detail in the following text.

Angle Difference Calculator

Angle Difference Calculator

Slip Frequency Element

Maximum Angle 1

Maximum Angle 2

Maximum Angle 1

Breaker Close Time

Maximum Angle 2

Slip Frequency Element

SF

VP

VP

VS

VS

Relay Word Bits

SF

Slip Frequency q

0°

Relay Word Bits

25A1

25A2

25A1

25A2

Relay Word Bits

Angle Difference

Angle Difference

(absolute value)

(absolute value– compensated by setting TCLOSD)

Synchronism-Check Element 1

Synchronism-Check Element 2

Synchronize at Angle Difference = 0°

Operation of Synchronism-Check Elements if voltages VP and VS are static (not slipping):

slip frequency ≤ 0.005 Hz or:

Setting TCLOSD = OFF

Operation of Synchronism-Check Elements if voltages VP and VS are slipping:

0.005 Hz < slip frequency ≤ setting 25SF and:

Setting TCLOSD ≠ OFF

25ANG1

25ANG2

Settings

TCLOSD

25ANG1

25ANG2

Angle Difference Increasing

Last Check of SELOGIC Setting

79CLS (w)

Settings

Synchronism- Check Element 1

Synchronism- Check Element 2

VP = 25RCF*VPH

where VPH = VA, VB, VC, VAB, VBC, or VCA, depending on the SYNCPH setting.

VP = 25RCF*VPH

where VPH = VA, VB, VC, VAB, VBC, or VCA depending on the SYNCPH setting.

Section 00 73 00 Attachment 4

4.109

Protection and Logic Functions Group Settings (SET Command)

Voltage Input VS Connected Phase-to-Phase or Beyond Delta-Wye Transformer Sometimes synchronism-check voltage VS cannot be in phase with voltage VA, VB, or VC (wye connected PTs) or VAB, VBC, or VCA (delta-connected PTs). This happens in applications where voltage input VS is connected

➤ Phase-to-phase when using a wye-connected relay

➤ Phase-to-neutral when using a delta-connected relay

➤ Beyond a delta-wye transformer

For such applications requiring VS to be at a constant phase angle difference from any of the possible synchronizing voltages (VA, VB, or VC; VAB, VBC, or VCA), an angle setting is made with the SYNCPH setting (see Table 4.42 and Setting SYNCPH on page 4.109).

Setting SYNCPH Enable the two single-phase synchronism-check elements by setting E25 := Y.

Wye-Connected Voltages The angle setting choices (0, 30, …, 300, or 330 degrees) for setting SYNCPH are referenced to VA, and they indicate how many degrees VS constantly lags VA. In this case, voltage input VA-N has to be connected and has to meet the “healthy voltage” criteria (settings 25VHI and 25VLO—see Figure 4.68). For situations where VS cannot be in phase with VA, VB, or VC, the angle setting choices (0, 30, …, 300, or 330 degrees) are referenced to VA.

Delta-Connected Voltages The angle setting choices (0, 30, …, 300, or 330 degrees) for setting SYNCPH are referenced to VAB, and they indicate how many degrees VS constantly lags VAB. In this application, voltage input VA-VB has to be connected and has to meet the “healthy voltage” criteria (settings 25VHI and 25VLO—see

Table 4.42 Synchronism-Check Settings

Setting Prompt Setting Range Setting Name := Factory Default

SYNCH CHECK Y, N E25 := N

VS WINDOW LOW 0.00–300.00 V 25VLO := 105.00

VS WINDOW HIGH 0.00–300.00 V 25VHI := 130.00

V RATIO COR FAC 0.50–2.00 25RCF := 1.00

MAX SLIP FREQ 0.05–0.50 Hz 25SF := 0.20

MAX ANGLE 1 0–80 deg 25ANG1 := 25

MAX ANGLE 2 0–80 deg 25ANG2 := 40

SYNCH PHASE VA, VB, VC, or 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330 deg lag VAa a Range shown for DELTA_Y := WYE.

SYNCPH := VA

SYNCH PHASE VAB, VBC, VCA, or 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300, 330 deg lag VABb b Range shown for DELTA_Y := DELTA.

SYNCPH := VAB

BRKR CLOSE TIME OFF, 1-1000 ms TCLOSD := 50

BLK SYNCH CHECK SV BSYNCH := 52A

NOTE: The synchronism-check element is only available when

VSCONN := VS.

NOTE: Settings SYNCPH := 0 and SYNCPH := VA are effectively the same (voltage VS is directly synchronism checked with voltage VA; VS does not lag VA). The relay displays the setting entered (SYNCPH := VA or SYNCPH := 0).

Section 00 73 00 Attachment 4

4.110

Protection and Logic Functions Group Settings (SET Command)

Figure 4.67). For situations where VS cannot be in phase with VAB, VBC, or VCA, the angle setting choices (0, 30, …, 300, or 330 degrees) are referenced to VAB.

Figure 2.23 shows a relay wired with delta-connected phase PTs, and a C-phase-to-ground connected VS-NS input. With ABC rotation, the correct SYNCPH setting for this example is 270 degrees, the amount that VC lags VAB. However, the setting angle is 90 degrees for the ACB phase rotation.

Use the voltage ratio correction factor (setting 25RCF) to compensate magnitude of the phase voltage to match the synchronism voltage VS. Many applications require 25RCF := 1.00, however some applications may need a different setting. For example, Figure 2.23 requires 25RCF := PTR / (1.732*PTRS). This is 0.58 if the PTR and PTRS are equal.

See the Application Guide entitled Compensate for Constant Phase Angle Difference in Synchronism Check with the SEL-351 Relay Family (also applies to SEL-751) for more information on setting SYNCPH with an angle setting.

Synchronism-Check Elements Voltage Inputs The two synchronism-check elements are single-phase elements, for both of which you would use single-phase voltage inputs VP and VS:

1. VP Phase input voltage (VA, VB, or VC*25RCF for Delta_Y := Wye; VAB, VBC, or VCA*25RCF for Delta_Y := Delta), designated by setting SYNCPH (If SYNCPH is set to one of the angle settings, then VP = VA*25RCF or VAB*25RCF depending on the Delta_Y setting.)

2. VS Synchronism-check voltage, from SEL-751 rear-panel voltage input VS

For example, if the rear-panel voltage input VS-NS is connected to B-phase (or BC phase-to-phase for delta) then set SYNCPH := VB (or VBC for delta). The voltage across terminals VB-N (or VB-VC for delta) is synchronism checked with the voltage across terminals VS-NS (see Figure 2.23).

System Frequencies Determined from Voltages VA (or VAB for Delta) and VS

To determine slip frequency, first determine the system frequencies on both sides of the circuit breaker. Voltage VS determines the frequency on one side.

Voltage VP determines the frequency on the other side.

Synchronism-Check Elements Operation Refer to Figure 4.67 and Figure 4.68.

Voltage Window Refer to Figure 4.67. Single-phase voltage inputs VP and VS are compared to a voltage window, to verify that the voltages are “healthy” and lie within settable voltage limits 25VLO and 25VHI. If both voltages are within the voltage window, the following Relay Word bits assert:

59VP indicates that voltage VP is within voltage window setting limits 25VLO and 25VHI

59VS indicates that voltage VS is within voltage window setting limits 25VLO and 25VHI

NOTE: Settings SYNCPH := 0 and SYNCPH := VAB are effectively the same (voltage VS is directly synchronism checked with voltage VAB; VS does not lag VAB). The relay displays the setting entered (SYNCPH := VAB or SYNCPH := 0).

Section 00 73 00 Attachment 4

4.111

Protection and Logic Functions Group Settings (SET Command)

Other Uses for Voltage Window Elements If voltage limits 25VLO and 25VHI are applicable to other control schemes, you can use Relay Word bits 59VP and 59VS in other logic at the same time that you use them in the synchronism-check logic.

If you are not using synchronism check, you can still use Relay Word bits 59VP and 59VS in other logic, with voltage limit settings 25VLO and 25VHI set as desired. Enable the synchronism-check logic (setting E25 := Y) and make settings 25LO, 25HI, and 25RCF. Apply Relay Word bits 59VP and 59VS in the logic scheme you want, using SELOGIC control equations. Even though synchronism-check logic is enabled, you do not need to use the synchronism-check logic outputs (Relay Word bits SF, 25A1, and 25A2).

Block Synchronism-Check Conditions Refer to Figure 4.67. The synchronism-check element slip frequency calculator runs if both voltages VP and VS are healthy (59VP and 59VS asserted to logical 1) and the SELOGIC control equation setting BSYNCH (Block Synchronism Check) is deasserted (= logical 0). Setting BSYNCH is most commonly set to block synchronism-check operation when the circuit breaker is closed (synchronism check is only necessary when the circuit breaker is open):

BSYNCH := 52A (see Figure 4.80)

In addition, you can block synchronism-check operation when the relay is tripping:

BSYNCH := … OR TRIP

Slip Frequency Calculator Refer to Figure 4.67. The synchronism-check element Slip Frequency Calculator in Figure 4.67 runs if voltages VP and VS are healthy (59VP and 59VS asserted to logical 1) and the SELOGIC control equation setting BSYNCH (Block Synchronism Check) is deasserted (= logical 0). The Slip Frequency Calculator output is:

Slip Frequency = fP – fS (in units of Hz = slip cycles/second) fP = frequency of voltage VP (in units of Hz = cycles/second) fS = frequency of voltage VS (in units of Hz = cycles/second)

A complete slip cycle is one single 360-degree revolution of one voltage (e.g., VS) by another voltage (e.g., VP). Both voltages are thought of as revolving phasor-wise, so the “slipping” of VS past VP is the relative revolving of VS past VP.

For example, in Figure 4.67, if voltage VP has a frequency of 59.95 Hz and voltage VS has a frequency of 60.05 Hz, the difference between them is the slip frequency:

Slip Frequency = 59.95 Hz – 60.05 Hz = –0.10 Hz = –0.10 slip cycles/ second

The slip frequency in this example is negative, indicating that voltage VS is not “slipping” behind voltage VP, but in fact “slipping” ahead of voltage VP.

In a time period of one second, the angular distance between voltage VP and voltage VS changes by 0.10 slip cycles, which translates into:

0.10 slip cycles/second • (360°/slip cycle) • 1 second = 36°

Section 00 73 00 Attachment 4

4.112

Protection and Logic Functions Group Settings (SET Command)

Thus, in a time period of one second, the angular distance between voltage VP and voltage VS changes by 36 degrees.

The absolute value of the Slip Frequency output is run through a comparator and if the slip frequency is less than the maximum slip frequency setting, 25SF, Relay Word bit SF asserts to logical 1.

Angle Difference Calculator The synchronism-check element Angle Difference Calculator in Figure 4.68 runs if the slip frequency is less than the maximum slip frequency setting 25SF (Relay Word bit SF is asserted).

Voltages VP and VS Are “Static”. Refer to top of Figure 4.68. If the slip frequency is less than or equal to 0.005 Hz, the Angle Difference Calculator does not take into account breaker close time—it presumes voltages VP and VS are “static” (not “slipping” with respect to one another). This would usually be the case for an open breaker with voltages VP and VS that are paralleled via some other electric path in the power system. The Angle Difference Calculator calculates the angle difference between voltages VP and VS:

Angle Difference = |(VP – VS)|

For example, if SYNCPH := 90 (indicating VS constantly lags VP = VA by 90 degrees), but VS actually lags VA by 100 angular degrees on the power system at a given instant, the Angle Difference Calculator automatically accounts for the 90 degrees and:

Angle Difference = |( VP – VS)| = 10°

Also, if breaker close time setting TCLOSD := OFF, the Angle Difference Calculator does not take into account breaker close time, even if the voltages VP and VS are “slipping” with respect to one another. Thus, synchronism-check elements 25A1 or 25A2 assert to logical 1 if the Angle Difference is less than corresponding maximum angle setting 25ANG1 or 25ANG2.

Voltages VP and VS Are “Slipping”. Refer to bottom of Figure 4.68. If the slip frequency is greater than 0.005 Hz and breaker close time setting TCLOSD OFF, the Angle Difference Calculator takes the breaker close time into account with breaker close time setting TCLOSD (set in ms; see Figure 4.69).

The Angle Difference Calculator calculates the Angle Difference between voltages VP and VS, compensated with the breaker close time:

Angle Difference = |(VP – VS) + [(fP – fS) • TCLOSD • (1 / 1000) • (360°/slip cycle)]|

Angle Difference Example (Voltages VP and VS are “Slipping”). Refer to bottom of Figure 4.68. For example, if the breaker close time is 100 ms, set TCLOSD := 100. Presume the slip frequency is the example slip frequency calculated previously. The Angle Difference Calculator calculates the angle difference between voltages VP and VS, compensated with the breaker close time:

Angle Difference = |(VP – VS) + [(fP – fS) • TCLOSD • (1 / 1000) • (360°/slip cycle)]|

Intermediate calculations:

(fP – fS) = (59.95 Hz – 60.05 Hz) = –0.10 Hz = –0.10 slip cycles/second TCLOSD • (1 / 1000) = 0.1 second

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Resulting in:

Angle Difference

= |(VP – VS) + [(fP – fS) • TCLOSD • (1 / 1000) • (360°/slip cycle)]|

= |(VP – VS) + [–0.10 • 0.1 • 360°]|

= |(VP – VS) – 3.6°|

Figure 4.69 Angle Difference Between VP and VS Compensated by Breaker Close Time (fP < fS and VP Shown as Reference in This Example)

During the breaker close time (TCLOSD), the voltage angle difference between voltages VP and VS changes by 3.6 degrees. This angle compensation is applied to voltage VS, resulting in derived voltage VS*, as shown in Figure 4.69.

The top of Figure 4.69 shows the Angle Difference decreasing—VS* is approaching VP. Ideally, circuit breaker closing is initiated when VS* is in phase with VP (Angle Difference = 0 degrees). Then when the circuit breaker main contacts finally close, VS is in phase with VP, minimizing system shock.

NOTE: The angle compensation in Figure 4.69 appears much greater than 3.6 degrees. Figure 4.69 is for general illustrative purposes only.

Angle Difference

Decreasing (VS* Approaching VP)

Angle Difference

Increasing (VS* Moving Away From VP)

Angle Difference

Angle Difference

Angle Compensation for Breaker Close Time

TCLOSD

Angle Compensation for Breaker Close Time

TCLOSD

Setting

25ANG1

(or 25ANG2)

Setting

25ANG1

(or 25ANG2)

Setting

25ANG1

(or 25ANG2)

VP

VS

VS*

VS*

VP

VS

Setting

25ANG1

(or 25ANG2)

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The bottom of Figure 4.69 shows the Angle Difference increasing—VS* is moving away from VP. Ideally, circuit breaker closing is initiated when VS* is in phase with VP (Angle Difference = 0 degrees). Then when the circuit breaker main contacts finally close, VS is in phase with VP. But in this case, VS* has already moved past VP. To initiate circuit breaker closing when VS* is in phase with VP (Angle Difference = 0 degrees), VS* has to slip around another revolution, relative to VP.

Synchronism-Check Element Outputs Synchronism-check element outputs (Relay Word bits 25A1 and 25A2 in Figure 4.68) assert to logical 1 for the conditions explained in the following text.

Voltages VP and VS Are “Static” or Setting TCLOSD := OFF. To implement a simple fixed-angle synchronism-check scheme, set TCLOSD := OFF and 25SF = 0.50. With these settings, the synchronism check is performed as described in the top of Figure 4.68.

If there is the possibility of a high slip frequency, exercise caution if you use synchronism-check elements 25A1 or 25A2 to close a circuit breaker. A high slip frequency and a slow breaker close could result in closing the breaker outside the synchronism-check window. Qualify the breaker close command with a time delay, such as:

SV06 := 25A1

CL := CC and SV06T

Set SV06PU with enough pickup delay to ensure that the slip frequency is low enough for the circuit breaker to close within the synchronism-check window.

Voltages VP and VS Are “Slipping” and Setting TCLOSD OFF.

Refer to bottom of Figure 4.68. If VP and VS are “slipping” with respect to one another and breaker close time setting TCLOSD OFF, the Angle Difference (compensated by breaker close time TCLOSD) changes through time. Synchronism-check element 25A1 or 25A2 asserts to logical 1 for any one of the following three scenarios.

1. The top of Figure 4.69 shows the Angle Difference decreasing—VS* is approaching VP. When VS* is in phase with VP (Angle Difference = 0 degrees), synchronism-check elements 25A1 and 25A2 assert to logical 1.

2. The bottom of Figure 4.69 shows the Angle Difference increasing—VS* is moving away from VP. VS* was in phase with VP (Angle Difference = 0 degrees), but has now moved past VP. If the Angle Difference is increasing, but the Angle Difference is still less than maximum angle settings 25ANG1 or 25ANG2, then corresponding synchronism-check elements 25A1 or 25A2 assert to logical 1.

In this scenario of the Angle Difference increasing, but still being less than maximum angle settings 25ANG1 or 25ANG2, the operation of corresponding synchronism-check elements 25A1 and 25A2 becomes less restrictive. Synchronism-check breaker closing does not have to wait for voltage VS* to slip around again in phase with VP (Angle Difference = 0 degrees).

There might not be enough time to wait for this to happen.

Thus, the “Angle Difference = 0 degrees” restriction is eased for this scenario.

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Protection and Logic Functions Group Settings (SET Command)

3. Refer to Reclose Supervision Logic on page 4.132.

Refer to the bottom of Figure 4.81. If timer 79CLSD is set greater than zero (e.g., 79CLSD := 100 ms) and it times out without SELOGIC control equation setting 79CLS (Reclose Supervision) asserting to logical 1, the relay goes to the 89lockout state (see top of Figure 4.82).

Refer to the top of Figure 4.81. If timer 79CLSD is set to zero (79CLSD := 0.00), SELOGIC control equation setting 79CLS (Reclose Supervision) is checked only once to see if it is asserted to logical 1. If it is not asserted to logical 1, the relay goes to the lockout state.

Refer to the top of Figure 4.69. Ideally, a circuit breaker closing is initiated when VS* is in phase with VP (Angle Difference = 0 degrees). Then when the circuit breaker main contacts finally close, VS is in phase with VP, minimizing system shock. But with time limitations imposed by timer 79CLSD, this may not be possible. To try to avoid going to the lockout state, the following logic is employed.

If 79CLS has not asserted to logical 1 while timer 79CLSD is timing (or timer 79CLSD is set to zero and only one check of 79CLS is made), the synchronism-check logic at the bottom of Figure 4.68 becomes less restrictive at the “instant” timer 79CLSD is going to time out (or make the single check). It drops the requirement of waiting until the decreasing Angle Difference (VS* approaching VP) brings VS* in phase with VP (Angle Difference = 0 degrees). Instead, it just checks to see that the Angle Difference is less than angle settings 25ANG1 or

25ANG2.

If the Angle Difference is less than angle setting 25ANG1 or 25ANG2, then the corresponding Relay Word bit, 25A1 or 25A2, asserts to logical 1 for that “instant” (asserts for 1/4 cycle).

For example, if SELOGIC control equation setting 79CLS (Reclose Supervision) is set as follows:

79CLS := 25A1 OR …

and the angle difference is less than angle setting 25ANG1 at that “instant,” setting 79CLS asserts to logical 1 for 1/4 cycle, allowing the sealed-in open interval time-out to propagate on to the close logic in Figure 4.80. Element 25A2 operates similarly.

Synchronism-Check Applications for Automatic Reclosing and Manual Closing

Refer to Trip/Close Logic on page 4.129 and Reclose Supervision Logic on page 4.132.

For example, set 25ANG1 = 15 degrees and use the resultant synchronism-check element in the reclosing relay logic to supervise automatic reclosing, e.g., 79CLS := 25A1 OR … (see Figure 4.81)

Set 25ANG2 = 25° and use the resultant synchronism-check element in manual close logic to supervise manual closing (for example, assert IN301 to initiate manual close), e.g., CL := IN301 AND (25A2 OR …) (see Figure 4.80)

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In this example, the angular difference across the circuit breaker can be greater for a manual close (25 degrees) than for an automatic reclose (15 degrees).

A single output contact (e.g., OUT102 := CLOSE) can provide the close function for both automatic reclosing and manual closing (see Figure 4.80 for logic output).

Power Elements You can enable as many as two independent three-phase power elements in the SEL-751. Each enabled element can be set to detect real power or reactive power. When voltage inputs to the relay are from delta-connected PTs or when you use a single voltage input, the relay cannot account for unbalance in the voltages in calculating the power. When you use one voltage (only the VA or VAB) and set SINGLEV := Y, the relay assumes that the system voltages are balanced in both magnitude and phase angle. Power and power factor are calculated assuming balanced voltages. Take this into consideration in applying the power elements.

With SELOGIC control equations, the power elements provide a wide variety of protection and control applications. Typical applications are:

➤ Overpower and/or underpower protection/control

➤ Reverse power protection/control

➤ VAR control for capacitor banks

Table 4.43 Voltages When Setting SINGLEV := Y

DELTA_Y Setting PHROT Setting Voltages

WYE ABC VA = |Va| °

VB = |Va| –120°

VC = |Va| +120°

WYE ACB VA = |Va| VB = |Va| +120 VC = |Va| –120

DELTA ABC VAB = |Vab| VBC = |Vab| –120 VCA = |Vab| +120

DELTA ACB VAB = |Vab| VBC = |Vab| +120 VCA = |Vab| –120

Table 4.44 Power Element Settings (Sheet 1 of 2)

Setting Prompt Setting Range Setting Name := Factory Default

ENABLE PWR ELEM N, 3P1, 3P2 EPWR := N

3PH PWR ELEM PU OFF, 1.0–6500.0 VAa (sec-ondary)

3PWR1P := OFF

PWR ELEM TYPE +WATTS, –WATTS,

+VARS, –VARS

PWR1T := +VARS

PWR ELEM DELAY 0.0–240.0 s PWR1D := 0.0

3PH PWR ELEM PU OFF, 1.0–6500.0 VAa (sec-ondary)

3PWR2P := OFF

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Protection and Logic Functions Group Settings (SET Command)

EPWR := 3P1 enables one three-phase power element. Set EPWR := 3P2 if you want to use both elements.

Set the element pickup, 3PH PWR ELEM PU, to the values you want.

Figure 4.70 shows the power element logic diagram and Figure 4.71 shows the operation in the Real/Reactive power plane.

Figure 4.70 Three-Phase Power Elements Logic

The power element type settings are made in reference to the load convention:

➤ +WATTS: positive or forward real power

➤ –WATTS: negative or reverse real power

➤ +VARS: positive or forward reactive power

➤ –VARS: negative or reverse reactive power

The two power element time delay settings, PWR1D and PWR2D, can be set to have no intentional delay for testing purposes. For protection applications involving the power element Relay Word bits, SEL recommends a minimum time delay setting of 0.1 second for general applications. The classical power calculation is a product of voltage and current, to determine the real and reactive power quantities. During a system disturbance, because of the high sensitivity of the power elements, the changing system phase angles and/or frequency shifts may cause transient errors in the power calculation.

The power elements are not supervised by any relay elements other than the minimum voltage check shown in Figure 4.70. If the protection application requires overcurrent protection in addition to the power elements, there may be a race condition, during a fault, between the overcurrent element(s) and the power element(s) if the power element(s) are still receiving sufficient operating quantities. Use the power element time delay setting to avoid such race conditions.

PWR ELEM TYPE +WATTS, –WATTS,

+VARS, –VARS

PWR2T := +VARS

PWR ELEM DELAY 0.0–240.0 s PWR2D := 0.0 a The range shown is for 5 A input; range for 1 A input is OFF, 0.2—1300.0 VA.

Table 4.44 Power Element Settings (Sheet 2 of 2)

Setting Prompt Setting Range Setting Name := Factory Default

PWRnD

|VAB|

20 V

3PWRnT

Relay

Word

Bits

Switch A

Position

+WATTS

–WATTS

+VARS

–VARS

Setting

PWRnT

Switch B

Position

Where n = 1 or 2

2 Cycle

Average

•(–1)

Switch A Switch B

1 1

2 2

3PWRnP

|VBC|

3PWRnP

Setting

P

Q

* lVBCl = lVABl if setting SINGLEV := Y

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Protection and Logic Functions Group Settings (SET Command)

Figure 4.71 Power Elements Operation in the Real/Reactive Power Plane

Power Factor Elements

If the measured power factor falls below the leading or lagging level for longer than the time-delay setting, the relay can issue a warning or trip signal. The power factor elements are enabled 55DLY seconds after Relay Word 52A is asserted (breaker closed), however when 55DLY := 0 the element is always enabled irrespective of the 52A status. Figure 4.72 shows the logic diagram for the power factor elements. You can use these elements to detect synchronous motor out-of-step or loss-of-field conditions. Refer to Figure 5.1 for the relay power measurement convention.

Reactive Power

Reactive Power

Real Power

Real Power

3PWR2P

(pickup)

3PWR1P

(pickup)

3PWR1P

(pickup)

3PWR2P

(pickup)

Set as Reactive Power Elements

Reverse (Leading)

3PWR2 3PWR1 3PWR2 3PWR1

Forward (Lagging)

Set as Real Power Elements

Reverse Forward

SEL-751

SEL-751

PWR1T =

+VARS

(type)

PWR2T =

—VARS

(type)

PWR2T =

—WATTS

(type)

PWR1T =

+WATTS

(type)

Note: Highlighted area represents pickup region

Table 4.45 Power Factor Settings

Setting Prompt Setting Range Setting Name := Factory Default

PF LAG TRIP LEVL OFF, 0.05–0.99 55LGTP := OFF

PF LD TRIP LEVEL OFF, 0.05–0.99 55LDTP := OFF

PF TRIP DELAY 1–240 sec 55TD := 1

PF LAG WARN LEVL OFF, 0.05–0.99 55LGAP := OFF

PF LD WARN LEVEL OFF, 0.05–0.99 55LDAP := OFF

PF WARN DELAY 1–240 sec 55AD := 1

PF ARMING DELAY 0–5000 55DLY := 0

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Protection and Logic Functions Group Settings (SET Command)

Figure 4.72 Power Factor Elements Logic

Loss-of-Potential (LOP) Protection

The SEL-751 sets Relay Word bit LOP (loss-of-potential) upon detecting a loss of relay ac voltage input such as that caused by blown potential fuses or by the operation of molded-case circuit breakers. Because accurate relaying potentials are necessary for certain protection elements (undervoltage 27 elements, for example), you can use the LOP function to supervise these protection elements. Refer to Figure 4.73 for the LOP logic.

The relay declares an LOP when there is more than a 20 percent drop in the measured positive-sequence voltage (V1) with no corresponding magnitude or angle change (greater than a pre-determined threshold) in positive-sequence (I1), negative-sequence (I2), or zero-sequence currents (I0). If this condition persists for 1 second, then the relay latches the LOP Relay Word bit at logical 1. The relay resets the LOP Relay Word bit when the conditions of the RESET input of the latch are met, as shown in Figure 4.73.

Measured Power Factor

52A

PF Leading

55LDAP

55LGAP

55A

55AD

Relay Word Bits

Relay Word Bit

Settings

55LDTP

55LGTP

55T

55TD

55DLY

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Protection and Logic Functions Group Settings (SET Command)

Settings The LOP function is always active unless blocked by the corresponding SELOGIC control equation, LOPBLK (see Table 4.46 for the setting and Figure 4.73 for the LOP logic). The default value is LOPBLK := 0. Certain switching operations can result in LOP assertion when the drop in V1 is greater than 20 percent with no or very little change in sequence currents.

Consider using LOPBLK to avoid assertion of LOP under such conditions.

You must incorporate the LOP function in a SELOGIC control equation to supervise relay protection elements (see Example 4.16).

LOP Impact on Other Protection Elements Undervoltage and directional power elements require accurate relaying potentials for correct operation. It is critical that the relay detects an LOP condition and prevents operation of these elements. For example, when dropping a wrench on the phase-voltage input fuse holders, the relay LOP logic accurately determines that this loss of input voltages is an LOP condition and does not trip if the LOP Relay Word bit supervises selected tripping elements (see Example 4.16). If you are using voltage-determined relay elements for tripping decisions, then blocking these elements is crucial when the voltage component is no longer valid.

EXAMPLE 4.16 Supervising Voltage-Element Tripping With LOP

The factory-default setting supervises undervoltage trip by the LOP as shown by the following:

SV01 := … OR (27P1T OR 27P2T) AND NOT LOP

Similarly, if you want the additional voltage-affected elements (e.g., 55T) to act only when there are correct relaying potentials voltage, use the following in the equation:

… OR (27P1T OR 27P2T OR 55T) AND NOT LOP …

and remove 55T from TR

You can supervise each element separately or as a group when these elements occur in the trip equations, as shown in this example.

Table 4.46 Loss-of-Potential (LOP) Setting

Setting Prompt Setting Range Setting Name := Factory Default

LOP BLOCK SELOGIC LOPBLK := 0

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Protection and Logic Functions Group Settings (SET Command)

Figure 4.73 Loss-of-Potential (LOP) Logic

LOP Monitoring and Alarms You should take steps to immediately correct an LOP problem so that normal protection is rapidly re-established. Include the LOP Relay Word bit in an output contact alarm to notify operation personnel of abnormal voltage input conditions and failures that can be detrimental to the protection system performance if not quickly corrected.

Vector Shift Element The vector shift element is used to detect islanding conditions of distributed generators (DGs) or loss of mains, and disconnect these DGs from the utility network under these conditions. Failure to trip islanded generators can lead to problems relating to out-of-synchronism reclosing, equipment damage, unstable operation, degradation of power quality and personnel safety.

The vector shift element in the SEL-751 is designed for applications where a DG is connected to either the utility or other main generators that require fast disconnection upon detection of an islanding condition. The vector shift element operates within three cycles providing fast and reliable island detection; this operating time is fast enough to prevent out-of-synchronism reclosing of the network feeders avoiding generator damage and any adverse affects.

Islanding is a three-phase phenomenon: therefore, the vector shift monitoring is performed on all of the three phases of the voltage signals. Detection of a vector shift condition occurs when there are sudden phase variations on all three phases of the voltage waveforms. At the moment of islanding, the sudden change in load current causes a sudden change in the periods of the voltage signals. This element measures the difference in the present period

1 s

S

R

Q

Q

1 s

1 cyc

LOP

| V1 | > 10.5 V

Δ | V1 | > 25%

Δ | I1 | > 0.1 • INOM

Δ | I2 | > 0.1 • INOM

Δ | I0 | > 0.1 • INOM

| V2 | < 5 V

| V0 | < 5 V

| V1 | > 0.75 • VNOM/(√3)

(RESET has priority)

Note: INOM is 1 A or 5 A depending on the part number.

INOM is the phase secondary input rating.

VNOM (in secondary volts) is the nominal line-to-line voltage setting.

Relay

Word

Bit

VNOM = OFF

Δ I1 > 10°

| I1 | > 0.1 • INOM

Δ I2 > 10°

| I2 | > 0.1 • INOM

Δ I0 > 10°

| I0 | > 0.1 • INOM

LOPBLK

SELOGIC

Setting

Section 00 73 00 Attachment 4

4.122

Protection and Logic Functions Group Settings (SET Command) duration and a reference period (as explained below). This difference is then converted to degrees and compared against the user-defined setting

78VSAPU.

Vector Shift Element Logic The logic diagram of the vector shift element in Figure 4.74 displays the steps performed to detect an islanding condition:

➤ Zero-crossing based period estimation

➤ Angle shift calculation and angle shift threshold check

➤ Angle shift count calculator

➤ Blocking conditions

Figure 4.74 Logic Diagram of the Vector Shift Element

The element performs period calculations on each of the voltage inputs, VA, VB, and VC. Zero-crossing detection logic is used to perform the period calculations. The time stamps of two consecutive positive-going zero-crossings or two consecutive negative-going zero-crossings are used in determining the period. The relay establishes a reference period for each phase using the previous 32 period measurements. The initialization period for this element requires at least 16 cycles of voltage signal to establish an accurate reference period. During the initialization period, this element does not detect an islanding condition.

Zero-Crossing

Detector

Angle Shift

Calculation

Angle Shift

Calculation

Angle Shift

Calculation

Count Calculator

OUTPUT = 1

if count > 4

|VC

VA

VB

VC

Zero-Crossing

Detector

Zero-Crossing

Detector

Settings

78VSAPU

Setting

|VA

|VB

78VSBL

78VSO

Relay Word Bit

The logic diagram shown applies when DELTA_Y := WYE.

When DELTA_Y := DELTA, the quantities VA, VB, and VC are replaced by VAB, VBC, and VCA, respectively. In addition, VNOM is not divided by √3.

The element is disabled when VNOM := OFF.

VNOM := OFF

78VS59

100 √3

VNOM

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Protection and Logic Functions Group Settings (SET Command)

In each quarter-cycle, the relay calculates the difference between the present period on each phase with the corresponding reference period. This difference is expressed in degrees to determine the angle shift and compared against the setting 78VSAPU. If the calculated angle shift is greater than the angle shift threshold setting 78VSAPU, the comparator output for the corresponding phase will be one; this output is fed to the angle shift count calculator logic.

The count calculator receives angle shift detection information from all three phases and records the number of times that the angle shift threshold of all phases has been exceeded in two consecutive quarter-cycles. If the angle shift count exceeds four and no blocking conditions exist, 78VSO is asserted indicating an islanding or loss of mains condition.

Power system short circuit conditions can also cause the voltage angle change to exceed the angle shift setting threshold. To prevent possible false tripping, the vector shift element is blocked for undervoltage conditions. If any of the phase voltages fall below the voltage supervision threshold setting 78VS59, the output of the vector shift element is blocked. You can program the 78VSBL SELOGIC control equation to provide additional blocking conditions as required in your application.

Depending on the DG loading conditions, a vector shift occurs once when an islanding event happens, causing a change in two consecutive period measurements after which the voltage stabilizes. For this reason, a delayed operation of the element is not applicable. Although the vector shift element allows for fast and reliable detection of DG islanding conditions, the limitation of this element needs to be realized. This element is based on the sudden phase change in the voltage waveform. If there is no load current change between the DG and the utility at the point of common coupling, there is no vector shift and this element does not detect the islanding condition. For this element to operate properly, the load change must be at least 20 percent of the rated power of the DG.

The vector shift element (78VS) and the fast rate-of-change-of-frequency element (81RF) can be used to detect islanding conditions. The vector shift element is designed to detect islanding conditions at the moment when the islanding condition happens and typically responds within 1.5–3 cycles after the islanding condition occurs. On the other hand, 81RF is designed to detect islanding conditions during and after the voltage shift occurs; the 81RF element complements the 78VS element by providing more dependable protection.

Vector Shift Element Settings See Table 4.47 for the range and default settings for the vector shift element.

Set E78VS := Y to enable the vector shift element. Set the vector shift angle pickup threshold setting 78VSAPU to the desired angle to detect a vector shift condition. The factory-default value of this setting is 10 degrees. Determine this setting based on the generator impedance and your studies.

The vector shift voltage supervision threshold setting 78VS59 defines the minimum voltage magnitude required for this element in percentage of the nominal voltage setting VNOM. The factory-default threshold is 80 percent of the VNOM setting. If VNOM setting is OFF, then the vector shift element is turned off.

The vector shift SELOGIC control equation, 78VSBL, allows you to define additional blocking conditions of the vector shift element. For instance, block the element for a few cycles when the DG breaker is closed to keep the element secure from operating under this condition.

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Protection and Logic Functions Group Settings (SET Command)

Frequency Protection The SEL-751 provides six trip over- or underfrequency elements with independent level and time-delay settings. Table 4.48 lists the ranges and settings. When an element level setting is less than the nominal frequency setting, the element operates as an underfrequency element. When the level setting is greater than the nominal frequency setting, the element operates as an overfrequency element. Figure 4.75 shows the logic diagram for the frequency elements.

Table 4.47 Vector Shift Element Settings

Setting Prompt Setting Range Setting Name := Factory Default

EN VECTOR SHIFT Y, N E78VS := N

VS ANGLE PU THR 2.0–30.0 deg 78VSAPU := 10.0

VS VOLT SUPV THR 20.0–100.0% 78VS59 := 80

VS BLOCK SELOGIC…