We will discuss two types of loss-of-potential (LOP) logic ... This slide shows a simple example of...

2

We will discuss two types of loss-of-potential (LOP) logic during this presentation:

• Traditional LOP, which uses traditional voltage and current elements to detect an LOP

condition.

• Advanced LOP, which uses rate-of-change of voltage and current to detect an LOP

condition.

Over time, advanced LOP has had many enhancements.

3

The purpose of LOP logic is to detect a potential transformer (PT) failure and take correct

action for the relay elements that require a healthy PT to operate.

4

This slide shows a simple example of a three-phase LOP in an event report from an

SEL311L Line Current Differential System. The distance element operates without voltage

present. It takes the distance element about 1.5 cycles to assert after the V1 voltage to the

relay begins to assert. LOP must operate faster than this to maintain distance element

security. The operating speed of the distance element during an LOP condition is a factor of

many things, including the following:

• Load magnitude

• Load angle

• Distance element reach

For more information, see “1 – Three-Phase LOP Trip.CEV” in the Event Files folder in the

supplemental material.

5

A voltage-polarized, negative-sequence directional element can be implemented by solving

for the torque-like quantity (T32Q) defined in the equation on this slide.

The torque calculation of this directional element is based on negative-sequence quantities.

T32Q is positive for forward faults.

This element requires a minimum voltage to be present for operation. A total loss of voltage

does not declare a direction.

6

To distinguish between hardware platforms that the relays are built on, we use terminology

such as 2XX and 3XX. The 2XX relays include the SEL-221 Line Protection Relay and the

SEL-2PG10. The 3XX relays include the SEL-311 series relays and the SEL-351 series

relays.

7

This slide shows a traditional LOP logic diagram in an SEL-321 Phase and Ground

Distance Relay. This logic relies on traditional overvoltage and overcurrent elements to

detect one, two, or three lost fuses.

8

This slide shows the breakdown of the logic required to assert traditional LOP.

9

This table shows a summary of recommendations and limitations for traditional LOP

conditions.

Keep the following in mind when determining set points for traditional LOP voltage

elements:

• If you have a blown fuse on one PT, V2 is 22.3 and V1 is 44.667.

• If you have blown fuses on two PTs, V2 and V1 are 22.3.

If 50M is not settable above the maximum load, then 59PL must be set below 22.3 V to

prevent the three-phase LOP from operating during a two-blown-fuse condition.

It is a good idea to set 59PL equal to 59QL so that anytime V2 is greater than V1, the

Q element takes priority for an LOP declaration. The purpose of the V1 voltage element is

to determine a three-phase LOP. In addition, 59QL should be set greater than 10% • VNOM

to prevent false assertion during system unbalance.

The advantage of setting 59PL higher is faster operation of three-phase LOP in situations

where a slowly decaying V1 causes a racing condition between LOP and a distance element.

10

Setting 50M for LOP is another consideration in traditional LOP. If 50M is set too high,

LOP can falsely assert for a close-in three-phase fault. If 50M is set too low, LOP is

blocked at all times. Therefore, 50M must be set above the maximum load.

11

If you are dealing with a two-terminal line, choose the weakest source when evaluating

guidelines 1 and 2 shown on this slide.

Once we evaluate the guidelines, we can determine if traditional LOP will work for us.

12

Recall the following guidelines, which are from the previous slide:

1. Close-in three-phase fault is less than the maximum load.

2. Remote three-phase fault is greater than the maximum load.

13

This slide shows that 50M must be set above the maximum load. When there is plenty of

pickup for a remote three-phase fault, 50M is the most reliable. It is also the most secure

when 50M is above the maximum load with a margin.

Blocking elements refer to elements that are blocked on assertion of LOP, such as 21 and

67 elements. If LOP has low security and high dependability, blocking elements have high

security and low dependability. For example, if 50M is set close to the load, the LOP

security is low. This gives blocking elements high security and low dependability.

Elements that become nondirectional overcurrent upon LOP assertion directly follow the

dependability and security of the LOP settings. A lower 50M setting is more likely to cause

an LOP assertion, which makes directional elements become nondirectional elements.

These elements are then more dependable but less secure.

This slide also shows how adjusting the 59PL element affects the operation of various

elements. A low-voltage setting is similar to a high-current setting in how it affects security

and dependability.

14

Although 50M and 59PL must not assert for an LOP condition, it is still possible to use

traditional LOP by overlapping 50M and 59PL operations as much as possible in the

protected line segment.

The more distance there is between FA and FB, the more secure the LOP logic will be. Fault

resistance will affect the distance between FA and FB.

15

In the example system shown on this slide, the system voltage is 161 kV and the load is

285 A. The source impedance and line impedance are both 1 per unit (pu).

Assume that 59PL stays equal to 14, which is about 0.2 pu. Place a bolted fault on the line,

and find the point at which Vrelay is 0.2 pu. If Ifault is greater than the maximum load, then

traditional LOP can be used. The more overlap there is between 50M and 59PL, the more

secure the LOP element.

The red line represents the pu voltage present for faults along the line. The blue line

represents the fault/maximum load. The pink line represents the logic status of 59PL when

59PL is greater than 14 V. The green line represents the logic status of 50M when the fault

is greater than the maximum load.

A bolted, close-in fault with no fault resistance is the most difficult fault to distinguish from

LOP.

16

The more overlap there is between 50M and 59PL, the better the LOP overage.

17

With a very high-resistance fault, the LOP does not assert because the voltage element

never drops out for any fault on the line. However, the question now becomes—can the

distance element still see a fault?

18

In the example on this slide, the fault levels are lower than the load levels, but the voltage is

well above the set point. Therefore, LOP does not falsely assert.

As fault resistance increases, the three-phase LOP logic becomes more secure.

19

In the example shown on this slide, a bolted, three-phase fault between 0.2 and 0.25 pu

asserts LOP, exposing a hole in the setting. This example is from the same system modeled

in the previous slides but with a higher maximum load (300 A). Because the maximum load

is higher, 50M must be set higher to allow LOP to properly operate.

20

In the example on this slide, SV1 equals !59V1 • !50P1, where 59V1 is 14 V and 50P1 is

1.5 A. The element is allowed to trip because the distance element is faster than LOP.

However, the assertion of LOP logic is misleading for this fault.

This example shows a fault placed in the hole described on the previous slide.

For more information, see “2 – Hole At 255.cev” in the Event Files folder in the


21

Calculate Ifault where V1 equals 0.2 pu with 100 MVA base.

Solve for the fault current at the point where Vrelay equals 0.2 pu using the following

equations:

• Ifault pu = 1 / (1.25 • Z1S)

• Ifault = (100 / (1.73 • KV)) • 1 / (1.25 • Z1S)

If Ifault is greater than the maximum load, traditional LOP can still be used. This is valid for

source impedance ratios (SIRs) less than 4.

Ifault is a bolted fault. This equation is valid for cases where a V1 of 0.2 pu is on the line

section being protected (SIRs of 4 or less). This equation also assumes that the overcurrent

element operates faster than the distance element.

For a 67 V nominal system, a V1 of 2 pu is about 14 V of V1. Because 14 V is the default

setting for 59PL, calculate the fault current at that point on the system. If 50M can be set

lower than the current at this point, overlap is guaranteed between 50M and 59PL.

22

Traditionally, 50M is set at 1.1 times the maximum load.

Setting the 50M at 0.75 of Ifault gives 1.33 times pu, which should get the 50M to operate

faster than 1.4 cycles in the SEL-311 series relays. This happens to be close to the actual

operate time of the distance element.

23

In the example shown on this slide, the maximum load is 238 A and the fault current at

0.2 pu voltage is 269 A (close-in fault is 448 A). The 50P1 element is set to 262 A. The

50P1 element picks up 0.25 cycles before M1P to ensure LOP does not falsely assert.

A close-in fault can challenge the traditional LOP logic speed. As long as 50P1 asserts prior

to M1P, the traditional LOP does not falsely block distance elements.

For more information, see “3 – Speed Test LOP 0.cev” in the Event Files folder in the


24

A fault detector for the distance elements must be set lower than remote faults on the end of

the line to ensure proper operation. In some cases, the fault detector of the distance element

must be set lower than the 50M element (in the overlapping scheme). With the fault detector

of the distance element set lower than the 50M element, there can be racing conditions

between the LOP logic and the distance element protection. If 50M can be set at two times

the bolted, close-in fault, it operates faster than the distance elements for a SIR of 1 and

above (such as in the SEL-311C Transmission Protection System).

25

In the example on this slide, a three-phase LOP condition is presented to an SEL-311C-1.

The load and the maximum torque angle (MTA) for a bolted three-phase fault are separated

by 50 degrees. SV1 represents the traditional LOP logic. In this example, the distance

element picks up at the same time as an SV1 assertion. The advanced LOP asserts faster

than the distance element and the traditional LOP logic. In this case, advanced LOP asserts

0.75 cycles before M1P and traditional LOP logic.

For more information, see “4 – LOP 40 Angle Load 2.cev” in the Event Files folder in the


26

In the example on this slide, a three-phase LOP condition is presented to an SEL-311C-1.

The load and the MTA for a bolted three-phase fault are separated by only 10 degrees. SV1

represents the traditional LOP logic. In this example, the distance element picks up prior to

SV1 assertion. The advanced LOP asserts faster than the distance element and the

traditional LOP logic. In this case, advanced LOP asserts 0.5 cycles before M1P and

0.75 cycles faster than traditional LOP.

The distance element operates faster during an LOP condition for a higher load and load

angle (power factor) close to the MTA of the line.

For more information, see “5 – LOP 80 Angle Load 2.cev” in the Event Files folder in the


27

With aggressive load encroachment settings, distance elements can be slowed during an

LOP condition. The example on this slide has the load encroachment set at 65 degrees, with

the initial load at 40 degrees. This reduces the resistive coverage of the distance element

drastically but allows fast operation for bolted faults and security for an LOP condition.

Traditional LOP benefits from the use of load encroachment.

For more information, see “6 – LOP 40 Angle With LE 3.cev” in the Event Files folder in

the supplemental material.

28

The yellow shaded area in the figure on this slide shows the area removed from the

coverage of a mho element to ensure that traditional LOP operates properly by using load

encroachment.

29

This slide shows the difference in resistance coverage for a 20 ohm fault down the line with

load encroachment turned on (green line) and turned off (blue line). At 0.2 pu from the

source, the load encroachment blocks the distance element from operating. With load

encroachment turned on, traditional LOP is secure, but resistance coverage is limited for

three-phase faults.

This example assumes that the source and the line are of equal impedance.

30

When the speed of LOP is the same as the protection elements, fault detectors are

employed.

For close-in faults (0 V), 59P cannot be used; 50M must be used. The more pickup there is

for a close-in fault, the faster 50M is.

The close-in fault will have a racing condition between 50M and the 21 element, especially

if 50M is set close to a fault value. Basically, a fault detector or load encroachment is

needed to block or delay the distance elements long enough for LOP logic to operate.

In legacy relays, torque-based directional elements do not allow directional decisions with

0 V for polarizing.

31

In general, one- or two-phase logic does not affect sensitivity. However, it must be delayed

to ride through transient events, such as pole scatter from closing a breaker.

32

SEL Application Guide AG2000-05, “Improvements to the Loss-of-Potential (LOP)

Function in the SEL-321,” by Jeff Roberts and Ralph Folkers (available at

http://www.selinc.com) describes conditions where traditional LOP will not work.

A slow decay in V1 will also challenge traditional LOP logic.

33

In the example on this slide, traditional LOP failed to operate for a three-way, gang-

operated switch that isolated the PT.

As can be seen from the voltage waveforms, the gang-operated switch did not open each

phase at the same time, and there was also some arcing. Because the 50P element was set

below load (this was the major issue with this operation), the relay eventually tripped on a

Zone 1 distance element. While this event shows over 14 V of V2 for the first cycle, this

condition must exist for 3 cycles for the relay to assert LOP. If the relay had been set with

50P above load, we would have relied on the three-phase LOP logic for this condition,

which would have asserted at 3.75 cycles (V1 = 13.8), just before the Zone 3 element

asserted. This condition would have latched, and no misoperation would have occurred.

Zone 3 is on a 60-cycle timer, but Zone 1 is an instantaneous trip. A Zone 1 trip would have

been prevented with a proper 50P1 setting.

In this example, CTR is 160 and PTR is 1,400. Nominal phase-to-ground voltage is 93.8 kV

(161 kV phase-to-phase voltage).

This event shows that some system LOP conditions do not provide a steep drop in V1. The

traditional LOP is not nearly as secure as the advanced LOP would be for this scenario. The

advanced LOP would have asserted when VA went to 0, which looks to have happened

prior to this event. It would be difficult to guarantee traditional LOP would have stopped

Zone 3 from asserting.

For more information, see “7 – Traditional LOP Event 1.TXT” in the Event Files folder in


34

This slide shows an unfiltered event report.

This is another example of an LOP condition that is very difficult to detect with traditional

LOP. In this example, VB voltage is very unstable and eventually leads to a Zone 1

misoperation. V1 is well above 14 V, so three-phase LOP cannot assert. V2, as shown on the

next slide, fluctuates and prevents the 3-cycle timer from picking up on one- or two-blown

fuse logic.

For more information, see “8 – Traditional LOP Event 2.TXT” in the Event Files folder in


35

The first 1.75 cycles of the event shown on this slide are when the Adaptive Multichannel

Source (AMS) first starts, so ignore the very beginning of this event.

Because the filtered event was not retrieved from the relay, the event was played back in a

new SEL-311C-1. The filtered event was then retrieved. In this event, SV2 is the traditional

LOP one- or two-blown fuse logic. Notice that SV2 asserts, but it does not time out until the

breaker has opened (SV2T). The only way to prevent this misoperation with traditional

LOP is to use a fault detector.

Notice that the advanced LOP (LOP Relay Word bit) asserts. It only deasserts after the

breaker has been opened (change in current).

The interesting item to note here is that the new SEL-311C contains delta detection logic. In

this event, M1P would not have operated because there was no change in current.

Traditional LOP may not have latched in, but it was not required to restrain M1P from

asserting in the original event.

For more information, see “9 – Traditional LOP Event 2 Playback.cev” in the Event Files

folder in the supplemental material.

36

Ultimately, the speed of traditional LOP means fault detectors or load encroachment must

be used to ensure security.

The mho element is the fastest for the following:

• Weak systems

• High loads

• Low power factor

Load encroachment and fault detectors sacrifice the sensitivity of mho elements.

37

Fault detectors and load encroachment may not need to be used on relays with delta current

supervision. The delta detection supervision inherently restrains operation of the Zone 1

element for no change in current.

38

In addition to the challenges of traditional LOP, relay design is increasing the speed and

sensitivity of elements. The negative-sequence element can operate on a 0 V condition.

39

As time has progressed, relays have become faster at detecting faults.

40

When V2 has a 0 value, the Z2 element declares forward using default settings. Traditional

torque-based elements are disabled for a 0 V condition.

41

Initial rate-of-change LOP looks at the rate of change for V1, I1, and I0. If V1 decreases

without a change of I1 or I0, LOP is declared.

I0 must be used for times when there are zero-sequence-only sources.

42

Only a 10 percent change in voltage is required to assert LOP when there is no change in

current. This makes the element very fast for LOP detection.

Relay sensitivity is not sacrificed.

Current blocking is far superior in advanced LOP logic than in traditional LOP logic. If the

fault current is 0.5 A secondary or greater than the maximum load or the fault angle is

5 degrees or more than the load angle, the logic blocks LOP regardless of the voltage drop.

43

As shown on this slide, an 8-sample-per-cycle relay samples positive-sequence current. The

k value is the present sample; k–1 is from 1 cycle ago. If the magnitude difference is greater

that 5% • INOM or the angle difference is greater than 5 degrees, LOP logic is blocked

regardless of what happens to the positive-sequence voltage. This provides much greater

sensitivity than standard LOP logic.

44

This slide shows the response of the primary relay, the backup relay, and the relay with improved LOP

logic. This is also a real LOP condition encountered in the field.

The following was taken from the technical paper “Lessons Learned Analyzing Transmission Faults” by

David Costello (available at http://www.selinc.com):

In the primary relay, an LOP is detected when negative-sequence voltage (V2) is greater than

14 V secondary and negative-sequence current (3I2) is less than 0.5 A secondary [9]. In the

backup relay, an LOP is detected when zero-sequence voltage (V0) is greater than 14 V

secondary and zero-sequence current (I0) is less than 0.083 A secondary [10]. Once asserted,

LOP blocks distance and directional elements that rely on healthy voltage signals.

This event emphasizes that early LOP logic was designed to protect distance elements from

misoperating for system faults that occurred some time after an initial LOP condition was

detected. Overcurrent fault detectors set above load were used to prevent distance element

misoperation when the LOP condition first occurred. In this event, the fault detector (50L) was

picked up during balanced load flow. Ideally, fault detectors should be set above the expected

load currents and below the minimum fault levels to ensure correct distance relay operation.

Newer relays have LOP logic that operates based on the V1 rate of change versus the rate of

change of currents. The new logic operates in less than 0.5 cycles, so distance element security

is less dependent on the fault detector settings.

In Fig. 21 [shown on this slide], the response of the original relays are shown with that of a

relay with improved LOP logic. The new relay LOP logic operates more than 1 cycle before

any distance elements assert, ensuring this misoperation will not happen again.

For more information, see “10 – LOP Speed Comparison” in the Event Files folder in the supplemental

material.

45

In addition to the heart of the advanced LOP logic, which is the rate of change of V1 and I1,

there are some other items to note, which include the following:

• The breaker must be closed—!3PO. For a very light load, a breaker status contact is

required.

• It is possible to have a temporary (transient) LOP that does not seal-in LOP (logic

point 2). The major concern here is that a no-load breaker with line-side PTs will assert

LOP until 3PO asserts (on the action of opening a breaker). To prevent a latch in, a

60cycle pickup timer is used for LOP latching.

• There is a 60-cycle dropout timer that blocks an LOP assertion for 60 cycles after any

change in I1 or I0. A breaker opening with light load may detect a ΔI1 and a rapid V1

decrease. Detecting the I1 and blocking for 60 cycles ensures that LOP does not assert

for the opening of a breaker with line-side PTs. This also blocks LOP assertion for

1 second anytime the breaker with line-side PTs is closed in to a bolted three-phase

fault.

• There is reset logic to automatically remove the LOP condition.

• This logic is not designed to detect an LOP condition when the breaker is open.

48

Over time, advanced LOP has been improved to cover more cases. The challenges

associated with advanced LOP logic include the following:

• Closing in to an LOP condition was not originally handled by advanced LOP logic.

We will highlight the new logic to cover this case in the following slides.

• Bus PTs can provide unique challenges to advanced LOP logic. We will talk about two

cases later in this presentation.

• Timing has been improved to add more security and dependability to the scheme.

49

Traditional LOP detects blown fuses on a breaker closing. SEL Application Guide

AG2004-07, “Recommended Ways to Detect Loss-of-Potential (LOP) Conditions in the

SEL-321, SEL-421, and SEL-311 Series Relays,” by Greg Hataway (available at

http://www.selinc.com) covers logic to detect LOP conditions on breaker open and close

conditions while using advanced LOP logic.

50

The logic shown on this slide is used for line-side PTs.

Breaker closing logic was originally active for 20 cycles after the breaker closed. V1 must

be greater than 10 V, so this logic does not detect all three-phase PTs blown on a breaker

close.

If the V2/V1 ratio is greater than 0.15 and the currents are balanced, the logic asserts an LOP

condition after 2 cycles. The 2-cycle time delay prevents a misoperation during pole scatter

from a breaker close.

I0/I1 and I2/I1 must be less than 0.15 to allow an LOP assertion.

Another requirement not shown on this slide is that some amount of I1 must be present for

the relay to block an LOP assertion.

A purely zero-sequence source does not block this logic. Also, closing the breaker into no

load with an LOP condition does not assert LOP.

51

How did we get an LOP condition in the first place? See the original fault in “11 – Breaker Closing

on LOP.CEV” in the Event Files folder in the supplemental material. It appears that the LOP

condition occurred during a fault.

In the example shown on this slide, the relay is applied to a distribution load. It takes the LOP logic

13.5 cycles to assert. This is because it takes a long time for the inrush condition presented to the

relay to subside. In this case, the I0/I1 ratio subsided 13.5 cycles after the breaker closing, while the

I2/I1 ratio subsided about 10 cycles after closing. Notice that the LOP dropped out after the breaker

had been closed for 20 cycles. This logic was not sealed in.

Because this is a distribution feeder, it is possible that an LOP would not lead to a misoperation.

Speed is not extremely crucial here, but latching is needed to prevent subsequent misoperations if

directional elements are being used.

Traditional logic would have performed the following actions:

• Picked up about 34 cycles after the breaker closed, with 50Q equal to 0.083 A.

• Picked up about 4.5 cycles after the breaker closed, with 50Q equal to 0.5 A.

There are two considerations with this logic:

• What is the fastest, yet secure, ratio to use? A low ratio allows very fast LOP blocking during

a fault, but it offers slow LOP assertion during a breaker close condition.

• How long should we keep this logic on? This event occurred with an SEL-421 Protection,

Automation, and Control System. The new SEL-421 close-in logic is active for 70 cycles,

which would seal in the LOP for this breaker closing event.

For more information, see “11 – Breaker Closing on LOP.CEV.”

52

An I ratio of 0.5 is the lowest we would typically expect for a phase-to-phase-to-ground

fault for I2 / I1 and I0 / I1.

An I2 / I1 of 0.15, which is implemented in the relay, is very conservative. The following

equations are used to help calculate the ratio:

• I2 = I1 • (Z0 / (Z0 + X2))

• I2 / I1 = Z0 / (Z0 + Z2)

Assume I2 / I1 is 0.15, and solve for Z2 in terms of Z0 using 5.67 • Z0 = Z2.

Therefore, Z2 must be 5.67 times larger than Z0 for a phase-to-phase-to-ground fault to

falsely assert LOP.

The advantage of this low ratio setting is that it blocks LOP faster for a legitimate fault.

53

Breaker closing logic is a great addition to the original advanced LOP logic. However, there

are still some limitations to be aware of, such as those listed on this slide.

On another note, if an LOP condition occurs during a fault, the closing logic is likely the

only way to catch the LOP condition.

54

In the figure on this slide, the original advanced LOP logic could have an issue.

If there is a fault on Bus 1 and the relay gets its polarizing voltage from the PT on Bus 1,

LOP does not assert because fault current is present (this would be a close-in reverse fault

for the line protection). When the bus is cleared, the voltage goes to zero before switching

to the new bus voltage source. If load is still fed via Breaker 2, then there is a potential for

misoperation.

55

The busbar PT logic shown on this slide is operating in parallel with the original advanced

LOP logic. When the busbar PT is enabled, the breaker closing LOP is disabled.

Busbar PT logic should be used when there are bus-side PTs. However, it is important to

note that there are some limitations with this logic. For example, the ΔI check stays asserted

for 2 cycles after a change in current. Therefore, when the bus fault in the previous example

is declared (the line relay goes from a reverse fault to a load condition), LOP is blocked for

2 cycles. Therefore, there will be a 2-cycle hole in LOP detection. If the transfer switch is

slower than 2 cycles, the relay will have no voltage present with load for this 2-cycle

window. However, the relay uses memory polarization, maintains directional stability, and

restrains from operation.

Once the healthy PT is switched over, there is a 10-cycle delay for drop out of LOP (reset

LOP time). For a very short amount of time after getting healthy voltage, the protection is

still disabled.

The logic asserts LOP anytime the bus PT goes dead. A bus differential that clears the bus

asserts LOP.

If the throw-over switch is faster than 2 cycles after the fault clears, there is the possibility

of a transient condition in which the distance element could falsely assert prior to the

assertion of LOP.

A three-phase fault that produces a V1 less than 10 V can produce a false LOP assertion

after 2 cycles (dropout time of ΔI check). However, it is likely that the three-phase fault

with this much voltage drop would be a Zone 1 fault and call for a trip prior to the LOP

assertion. If a time-delay element is used, a false LOP assertion can occur that could block

distance elements.

56

In the example shown on this slide, Bus 2 has a phase-to-ground fault, which is fed from all

three lines. The relay in question protects the leftmost line. Current transformers (CTs) are

paralleled on Breaker A and the transformer. The PT is located on the bus. When the

PhaseA-to-ground fault occurs, the magnitude change of I1 and I0 is large enough to block

LOP for 60 cycles. The bus differential relay trips all three breakers. Once all three breakers

are tripped, the voltage seen by the relay goes to zero. However, the transformer on the left

is still fed remotely and the relay sees this current. Because LOP is blocked for 60 cycles, it

is only a matter of time before this no-voltage and load-current scenario trips a distance

element.

57

This slide shows the unfiltered event report with an odd decay in the unfaulted phase

voltages.

For more information, see “12 – Unfiltered Bus PT.TXT” in the Event Files folder in the


58

At the time of the trip, V1Mag was 17.3 V secondary and V2Mag was 14 V secondary.

Traditional LOP would not have operated correctly for this event even if fault detectors could be set

using the traditional 59V1 setting. The slow decay in V1 was too slow for three-phase LOP to assert

prior to distance element operation. The one- or two-blown fuse logic would not have enough time to

assert because the pickup timer for that logic is 3 cycles. We only have about 1.625 cycles, between

I2 being low enough and V2 being high enough, to trigger the unbalance LOP logic.

Busbar PT logic would not have worked for this event because V1 at the time of the trip was greater

than 10 V.

The original advanced LOP was being used when this relay was in service.

At 5 cycles, the bus was cleared and the voltage for the bus PT approached zero. This operation

happened about 8 cycles after the initiation of the fault. Based on the voltage and current clearing, it

appears Breaker A tripped first (isolating this relay from the bus), but the other breakers cleared

slower (the bus voltage is held up for about 1.5 cycles after the local currents are reduced). The

challenge in this event is that load currents are present while fault voltages are also present.

The original advanced LOP logic blocked LOP assertion for 60 cycles after a change in current. The

latest version only blocks LOP assertion for 2 cycles after a change in current and allows the LOP to

better block for an LOP condition after a fault. However, this improved time would still not pick up

LOP prior to the distance element assertion.

The only way to block distance elements in this case is to use fault detectors. Be mindful of the bus

arrangements used.

For more information, see “13 – Filtered Bus PT.TXT” in the Event Files folder in the supplemental

material.

59

As described on the previous slide, fault detectors should be set in certain applications to

secure the distance elements when load can persist after a bus fault.

There are a few possibilities of LOP operation during a bus fault in a breaker-and-a-half

scheme using bus PT logic, including the following:

• Three-phase fault—0 V on the bus. LOP asserts after 2 cycles. After the bus clears, the

PT can be switched to a healthy source without concern. LOP remains asserted for

10 cycles in the relays. This is the LOP reset time, which in this case directly affects

how long the protection is either blocked or nondirectional after a bus fault.

• One- or two-phase fault. At the inception of the fault, LOP is blocked for 2 cycles.

– If the local breaker opens first (transition from fault current to load current), LOP

does not assert until 2 cycles after the local breaker opens.

– If the local breaker opens last, voltage and current change at the same time and

LOP is blocked for 2 cycles.

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The improvements listed on this slide secure LOP.

ΔI previously blocked LOP for 60 cycles. Previous events show the value of unblocking

LOP quickly. There can be a change in current due to the clearing of a fault, but the load

remains (bus PTs). In these cases, we want to unblock LOP as fast as possible.

ΔI now blocks LOP for 2 cycles.

Previously, LOP had to exist for 60 cycles before it was latched in. One concern for

latching the LOP was the breaker opening on no load with line-side PTs in which the 52A

contact was slower than the voltage decay. It is possible for a transient LOP assertion in this

case, but a timer of 60 cycles is not needed.

Now LOP only needs to exist for 15 cycles to latch in.

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As shown on this slide, a Phase-A-to-ground fault occurs on Line B. This line is using an

SEL-421 with CTs connected on the 3575 breaker and high-side transformer CTs. On the

following slide, we will see how the advanced LOP responds to a zero-sequence-only

source.

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In the event shown on this slide, the local breaker recloses at 4 cycles. At this instant,

breaker close LOP is enabled for 20 cycles. At 14 cycles, the remote end closes in to the

fault. The transformer becomes a zero-sequence-only source for this fault, but LOP is

asserted because V2 / V1 is met and there is no I1 present to block LOP from asserting.

Therefore, LOP falsely asserts in this example.

In this case, traditional LOP also falsely asserts unless a zero-sequence source was used to

block LOP assertion.

In bus arrangements like this, neither LOP logic would work as expected. Take care in

determining LOP conditions.

For more information, see “14 – LOP With Zero-Sequence Source.txt” in the Event Files

folder in the supplemental material.

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Take care in unique bus arrangements to evaluate LOP operation.

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Use open-breaker LOP to detect LOP conditions when the breaker is open. Use advanced

LOP when the breaker is closed. See SEL Application Guide AG2004-07 for more

information.

We will discuss two types of loss-of-potential (LOP) logic ... This slide shows a simple example of...

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