Untitled Ts02138pdf

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Applying the Directional Neutral, 67N, Function in Microprocessor Multifunction Relays

Felix Nepveux

Jacobs Engineering

Greenville, SC 29607

864-676-6303

[email protected]

ABSTRACT

This paper is a case study of the application of the 67N function in a digital multifunction relay. The directional

neutral current relay, function number 67N, is often found to be critical to the proper operation of a protective

scheme. Furthermore, a false trip of a 67N will usually be devastating. The 67N is a complex function that always

required more wiring than almost any other relay. The 67N is now available as one of many functions in digital

multi-function relays. However, in the digital relay the 67N may be even more difficult to set and test than it was as

a single function in an old electromechanical relay.

INTRODUCTION

The proper operation of the 67N function is critical because of the location in a system where these relay functionsare normally found, as shown in Figure 1. They will usually be found in central locations in a power system, such as

in the tie circuits to a synchronizing bus, or right at the utility tie transformer as shown in Figure 1. They will usually

have the lowest ampere, or tap, setting of any relay. If improperly applied the 67N will have the opportunity to clear

the entire system when no fault has occurred. The system designer must be certain that the 67N function will operate

in the intended direction, at the intended setting, for the intended condition.

The wiring of the directional neutral current relay, function number 67N, has always been complex. The wiring

instructions were long, but there was a simple arrow on the wiring diagram indicating the tripping direction. If the

wiring was correct, you could expect the relay to function properly.

The 67N function is now one of many functions in a microprocessor based multifunction relay. A typical

multifunction relay is shown in Figure 2. There are now many selections, settings, and programming steps required

to activate all of the trip functions. In addition, to calculate the trip settings, the relation between direction settings

and wiring must also be clearly understood. There is no longer a single direction of tripping related to the wiring,

with no simple arrow on the relay wiring drawing. The instructions on testing can be difficult to understand. Also,

the test procedures no longer provide positive proof of operation of the trip circuit.

Knowledge of power system theory is now more important than ever before. In defense of the relay vendors, relayinstructions cannot be expected to be a course in power system concepts. The experiences related in this paper,

about the use of an excellent multifunction relay, illustrate this point. It was found that great care must be taken in

the application of the 67N function. The tripping angle settings for three phase, 67, and single phase, 67N, faults

were different due to differences in the impedance and sequence voltages of phase and ground faults. The

polarization source was found to require special consideration. There was no explanation in the relay instructions on

how to select the proper polarization source required for the low resistance grounding system. There was also no test


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A fault in the cable from the main transformer secondary lugs to the switchgear could not be isolated with bi-

directional relays. The trip settings of the 51N functions in the relay for AB and LM, at both ends of the system, are

identical. A ground fault in the cable connection between the secondary of either transformer and the switchgear

would result in a trip of 51N functions at both ends of system. Breakers A, B, C, and D would all trip, completely

deenergizing the system, and plant. What is interesting in this situation is that the neutral current in the T1 51N is

the neutral current supplied by T2.

It was logical to consider a directional neutral function to determine the location of the fault and the appropriate

relays to trip. The purpose of the 67N-AB is to trip on neutral current flowing toward T1, from A and B, earlier than

the 51Ns at A and B, and at L and M. If T2 is down, and L and M open, then only the 51G functions will clear the

system. With both transformers in service, the 67N scheme should clear A and B, leaving the system powered by

T2. Note that the 51G function is set high for a last level of protection for the ground resistor if all else has failed to

clear a ground fault.

DIRECTIONAL RELAY OPERATION

To understand the problem the method of operation of the directional relay must be understood. Current direction is

a difficult concept to define in an alternating current system as current and voltage are always changing direction.

The concept of direction of current is actually an assignment of importance to the relation of the angle of the current

to the angle of a current or voltage reference. This may sound like the sign, + or -, of power factor, and it is.

Figure 4 shows how the current direction relative to the voltage varies for different locations of the ground fault.This is the basis for direction in a neutral directional relay. The challenge is to properly set the trip angle for the

directions the current and potential transformers are connected.

Another detail to remember is that in a resistance grounded system the impedance in the fault is almost entirely

resistive. Therefore, the angle between current and polarizing voltage for tripping must be either 0 or 180 for

maximum sensitivity. Note that 135 (+0 or 180) would be an appropriate setting for the 67N function in a solidly

grounded system where the impedance is mostly reactive. The effects of differences in impedances in a solid and

resistive grounded circuit are not mentioned in the relay manual.

SELECTION OF POLARIZING SOURCE

The first question to be answered is what is the best polarizing source. The relay in this paper allowed only voltage

polarization, with a choice between negative sequence voltage, V2, and zero sequence voltage, V0. There is an

implication that either source works for any type of system. There was no caution concerning the differences

between resistance and solidly grounded systems. It appears that V2 polarization may be selected, and that the cost

of the extra V0 deriving PT can be avoided. However, this is not the case. In a resistance grounded system the

component of V2 in a ground fault is essentially zero. This is why the YD connected potential transformers were

always required for an electromechanical 67N. The magic of the computer can not repeal this law of nature.

Figure 5 shows the sequence components of a ground fault in a resistance grounded system. When a ground occurs

in a resistance grounded system the grounded phase goes to 0 volts to ground, and the two other phases go to the

square root of 3 times normal line to ground voltage. In this condition the positive sequence set is the same as in an

unfaulted system . The zero sequence voltages in each phase offset the start of the positive sequence vectors by an

amount equal to the negative of the normal line to ground voltage of the faulted phase. Negative sequence


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In both of these cases, resistance and solidly grounded systems, the polarizing voltage is 180 out of phase with the

neutral current. It is critical to determine if the design of the relay takes this angle into account, or if it has to be

accommodated in the settings. In the relay used in this system, the settings for the same direction of tripping for a 67

and 67N have values of trip angle that are essentially opposite. This is non-intuitive to say the least.

TESTING PROCEDURES

The test procedure for the 67N function specifies wiring, settings, voltages, and currents to be used for the test, as

well as the criteria for success of the test. It also specifies the current to be used for the test. The relay instructions

call for a test at a current setting that is not the permanent setting value. The relay must be disconnected from CTs,

reset to the test value, then, after passing the test, reset and reconnected. Errors can be made in the reconnection and

resetting of the relay.

The procedure of testing a relay with settings other than the permanent settings is a violation of traditional relay

testing concepts. Tests have always been used to show that a relay will trip when expected at the specified settings.

After a successful test, any change in the settings or wiring meant you had to retest the relay. This now means that

two tests should always be performed on computer based relays. First, the vendor recommended tests at special

values. Then, a second set of tests with the permanent settings that will not be changed. It is not easy to have faith

that a successful test of any one function implies that all will function. There is much programming specific to each

function that can not be proven by any one test. It should also be mentioned that the purpose of the testing is not

only to show that the relay functions, but that the desired settings are in fact set in the relay. It would be very easy to

fail to completely reset a relay after a test, or to download the wrong setting file after a test.

The procedure for the test of the 67N function in the relay in this paper is actually a test for tripping based on

negative sequence voltage polarization. The setup procedure forces you to select negative sequence polarization for

the test. There was no statement that the polarization source selection is being manipulated, only a number of times

to push a button. There was no warning after the test that zero sequence polarization should be reselected, if this is

what you had originally set. And there was no alternate test provided for zero sequence voltage polarization.

The vendor recommended test is set up with an assumption of the following voltages prior to the fault. The

following are the phase voltages, and the sequence components of the phase A to neutral voltage. Neutral voltage is0 volts with respect to remote earth. So far all seems fine.

Van = 120 @0 Va0 = 0@0

Vbn = 120@240 Va1 = 120@0

Vcn = 120@120 Va2 = 0@0

The relay manual then lists the following voltages for the three voltage inputs during the fault condition to be tested,

with the sequence voltages for phase a:

Van = 10 @0 Va0 = 36.7@180

Vbn = 120@240 Va1 = 83.3@0Vcn = 120@120 Va2 = 36.7@180

These voltages, and their components, imply that the system in which the relay is to be used is solidly grounded.

There is a significant negative sequence voltage component that could be used for polarization. However, these

voltages will never occur in a resistance grounded system.


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If the test is performed as specified, but the resistance grounded system voltages are used instead of the

recommended voltages, the relay will fail to trip. If the testing personnel are smart enough to vary the procedure so

that zero sequence polarization is selected, the test will work, but then there will be no vendor documentation

showing that a proper procedure has been used. There could be much discussion if damages were related to an

improper trip of the 67N function if a faulty test procedure had been used.

NOTES ON ANGLES IN MICROCOMPUTER RELAYS AND TEST EQUIPMENT

After all of the other problems are solved, there is still one more problem to overcome. The conventions for angle

direction is sometime different between relay setting values and test set dials. The normal method of graphing

vectors includes line to neutral phase A, B and C vectors rotating counterclockwise, with angles considered positive

also in a counterclockwise direction about the origin.

This is also confusion in the way angles are probably entered in the relay. In this convention a positive value of

angle is an angle leading the referenced vector. However, most test sets have angle entry screens which consider

positive values of angle as an angle lagging the referenced quantity. This can add a lot of time to setting up a test. A

successful test at the wrong angles will mean that the wiring may have been reconnected to make the test work, with

the wiring left as changed, and marked up as changed on the as-built drawings. The only problem is that the relay

now does not work, although there is a successful test report on the unit. The conventions in the relay and the test set

must be positively understood before setting and testing the relay.

CONCLUSION

There are many advantages of processor based multiphase, multifunction relays. Ease of use and testing do not yet

seem to be some of these advantages. The difficulty in use and testing could actually result both a belief of

successful application and a guarantee of failure, eliminating all the benefits of their use. Even with better

instructions, technicians and engineers will be required to have a better understanding power system concepts to

successfully apply the new relays.


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TAPPI Paper Summit 2002

LOCATION OF 67N

67N

Whole system

Figure 1

Power Source

Y

Y


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51, 50, 67, 67N

32, 81, 62,

Metering, displays,History data storage

Multifunction Relay

Figure 2

Common CT, PT inputs Single trip to breaker

Communications


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51, 67,

51N, 67N,

51G

Single Phase

Ground Fault

B

AC

D

E

F

G

H

I

J

K

L

M

2PTs,

OD,OD

3 PTs,

Yg,Yg

2PTs,

OD,OD

3 PTs,

Yg,YgH1 H2

51, 67,

51N, 67N,

51G

T1 T2

Relay AB Relay LM

Figure 3


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Single Phase

Ground Fault at A

Single Phase

Ground Fault at B

Voltage SourceVoltage Sourcex

x

ia

ib

When voltage is in positive half cycle

the current in the CT will be entering

the polarity terminal for a fault at A,

but leaving the polarity terminal for a fault at B.Note that there is a voltage source at both ends.

Voltage vector, VLG,

for Fault at either A or B

Figure 4


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Sequence Components Of Solid Grounded

System With A Ground Fault On A

Seq Components, polar MAG ANG RAD

Va0 = 33.3 AT 180.0 DEG A1+A2+A0= 0.00 0.00 0.00

Va1 = 66.7 AT 0.0 DEG B1+B2+B0= 100.00 240.00 4.19

Va2 = 33.3 AT 180.0 DEG C1+C2+C0= 100.00 120.00 2.09

Vb0 = 33.3 AT 180.0 DEG

Vb1 = 66.7 AT 240.0 DEG

Vb2 = 33.3 AT 300.0 DEG

Vc0 = 33.3 AT 180.0 DEG

Vc1 = 66.7 AT 120.0 DEG

Vc2 = 33.3 AT 60.0 DEG

-100

-80

-60

-40

-20

0

20

40

60

80

100

-80 -60 -40 -20 0 20 40

Va2 AT 180

Ia AT 0

Figure 5


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Sequence Components Of Resistance Grounded

System With A Ground Fault On A

Seq Components, polar MAG ANG RAD

Va0 = 100.0 AT 180.0 DEG A1+A2+A0 0.00 0.00 0.00

Va1 = 100.0 AT 0.0 DEG B1+B2+B0 173.20 210.00 3.67

Va2 = 0.0 AT 0.0 DEG C1+C2+C0 173.20 150.00 2.62

Vb0 = 100.0 AT 180.0 DEG

Vb1 = 100.0 AT 240.0 DEG

Vb2 = 0.0 AT 120.0 DEG

Vc0 = 100.0 AT 180.0 DEG

Vc1 = 100.0 AT 120.0 DEG

Vc2 = 0.0 AT 240.0 DEG

-100

-80

-60

-40

-20

0

20

40

60

80

100

-200 -150 -100 -50 0

V2s ALL = 0

V0s AT 180

Ia AT 0

0,0

Figure 6

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