evaluation of weak in-feed tripping technique on the eskom transmission network

8/13/2019 Evaluation of Weak in-feed Tripping Technique on the Eskom Transmission Network

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Study Committee B5 Colloquium2005 September 14-16

Calgary, CANADA

EVALUATION OF WEAK INFEED TRIPPING TECHNIQUE ON THE ESKOM

TRANSMISSION NETWORK

Adam Bartylak*

ABSTRACT

To enable fast operation of the impedance relay in case of lack or insufficient fault current at the supplying

substation, a "weak infeed" function has been developed on feeder protection schemes which operate in permissive intertripping mode. Since the introduction of this functionality to modern protection relays, ESKOM

has experienced some high-risk incorrect operations, resulting in the overtripping of parallel lines. Theinvestigation identified two areas of possible lack of proper co-ordination. Timing co-ordination due tounforeseen drop off delays of the relays, and impedance co-ordination due to lack of required overlapping ofrelays' characteristics as a result of different measuring principals, different polarising, sometimes contradictory

requirements and some limitations of the relays. Limited benefits of weak infeed functionality as applied on theESKOM Transmission system are indicated. The risk analysis is then provided with conclusion to either disable

weak infeed function or make sure that very careful studies are provided in order to evaluate all possiblescenarios that could lead to incorrect operations.

KEY WORDS: Transmission - Protection - Performance - Weak Infeed

1 INTRODUCTION

In line with technological and scientific advancements, more and more sophisticated protection relays arereaching the market every year and new features and enhancements are available to protection engineers with theaim of improving the performance of electrical power systems and attracting new customers. An introduction ofelectronic (static) relays in the early eighties opened an era of continuous improvements in protection algorithmsand additional functions. Weak infeed tripping logic is an example of such advancement designed to speed up

the operation of protection schemes in situations where there is lack or insufficient current for the relay tooperate reliably. This feature is well recognised and used wherever permissive transfer tripping scheme isutilised instead of blocking. On the ESKOM Transmission network, the weak infeed tripping has been

implemented but some incorrect operations dangerous to system integrity have occurred over the last few years,which prompted an in-depth investigation.

The Transmission network in South Africa consists of 160 substations interconnected via 27000km of 220 -765kV lines and is operated by a vertically integrated organisation - ESKOM. The South African Transmissionnetwork is exposed to large numbers of primary faults, as indicated in Figure 1.1, that can be classified into fourmajor groups in terms of their cause as shown in Figure 1.2 below.

* Power System Operations Performance Manager, ESKOM Transmission, South Africa

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Figure 1.1. Number of Transmission Faults

400

500

600

700

800

900

1000

1 9 9 8

1 9 9 9

2 0 0 0

2 0 0 1

2 0 0 2

2 0 0 3

2 0 0 4

Figure 1.2 Transmission fault reasons

Fires

27%

Lightning

25%

Bird Polution

30%

Other

18%

Figure 1.3. Histogram of fault resistance

1

10

100

1000

10000

0 5 10 15 20 25 30 35 40

Fault Resistance [primary ohms]

N

u m b e r o f o c c u r r e n c e s

Over a quarter of all faults on the ESKOM Transmission network are caused by fires under the lines which often

result in very high resistance faults when a flash-over occurs in the mid-span of the line. A very dry winter inmost of the country in South Africa results in difficulties in maintaining tower footage resistance within requiredlimits and additional fault resistance at the entry point of the fault arc to ground. A histogram of fault resistances

measured over four years on the ESKOM network is shownin Figure 1.3. 16% of all recorded faults have resistanceabove 10 primary ohms, which is a significant value.The dynamic behaviour of a long arc may also result insubstantial changes in arc resistance during the fault. Forsuch faults the impedance measured by the relay changes

with time during the fault. As a result, a high enough faultresistance may cause the fault impedance locus not to enterthe tripping characteristic of the relay or to leave thecharacteristic during the fault. Such faults are difficult to

detect and isolate properly and pose many challenges to protection equipment. One of such problem, which isrelevant to weak infeed tripping, is possibility of impedance

locus entering areas of relay characteristics whereappropriate impedance co-ordination is not available. Thisissue will be described in more details later in this paper.

As a result of changes in protection technology over the last few decades a great variety of protection relays are

installed on the ESKOM Transmission system from electromechanical, through electronic, digital to highlysophisticated programmable Intelligent Electronic Devices.Feeders are equipped with over twenty different types ofimpedance relays, each with different options available,

different shapes of characteristics, type of polarisation,sequence components used, measuring algorithms, starting

quantities etc. As long as both ends of the line are equippedwith the same vintage of impedance relay the co-ordinationof different functions is fairly straightforward. Problems are becoming more challenging where different types of relayshave to be co-ordinated with each other. A composition ofdifferent protection technologies applied on ESKOM's

network is presented in Figure 1.4.

Figure 1.4. Comprosition of protection

technologies on the ESKOM

Transmission system

Electromech.

48%

Electronic

32%

Digital

17%

Programable

3%

Since the introduction of electronic relays in the earlyeighties, ESKOM adopted permissive overreach transfertripping scheme as most beneficial for local conditions andweak infeed functionality is utilised since then. Another

words, over half of ESKOM Transmission feeders are presently set to operate using weak infeed facility.

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2 WEAK INFEED TRIPPING BACKGROUND

The weak infeed function is an addition to the distance protection scheme set to operate in permissive trippingmode to improve its performance in situations where there is insufficient fault current to operate the protectionrelays (low fault current substations or open remote breaker). Figure 2.1 below illustrates such a situation wherethe source impedance ZsA behind substation A is very high or there is no source behind it. The fault current IfA is

therefore very low and may be insufficient to operate the starting elements of the relays.

Fault

BA

ZsA >>

~IfB >> ZsB <<

~IfA ~ 0

Figure 2.1. Weak infeed conditions during line fault

In this case the fault will be cleared at the substation B which has sufficient current to operate the protectionrelays but at the substation A, the current may be lower than that required to operate the protection.

To ensure correct tripping of the circuit breaker at substation A in the above conditions the following logic has been applied:

• With insufficient current at the substation A to operate the distance relays for fault in the forward directionthe forward “looking” measuring elements will not pick-up.

• If there is a genuine fault on the line in forward direction then at strong substation B, the forward elements ofthe distance relays will operate and initiate transmission of permissive carrier to the substation A.

• Substation A will receive the permissive carrier with no measuring elements picked up. This could besufficient information to trip the breaker in permissive underreach scheme where the underreaching zone

sends permissive signal.

• In permissive overreach mode of operation the protection at strong end B will send permissive carrier also forfaults beyond substation A as indicated in Figure 2.2.

• To prevent incorrect tripping at end A in case of reverse faults, additional, reverse “looking” impedanceelements are employed to block unnecessary weak infeed operation in case of reverse faults, in the same wayas for blocking inter trip scheme.

During reverse faults there is sufficient fault current at substation A supplied from strong substation B (seeFigure 2.2) and reverse, blocking elements will operate reliably.

Fault Z relay

A

ZsA >>

~

IfA >> IfB >>

B

Forward elements picked-up

Permissivesignal

Z relay

ZsB <<

~

Reverse elements picked-up

Figure 2.2. Blocking of weak infeed tripping for reverse faults

Once the weak infeed conditions are detected, the protection trips the breaker, initiates autoreclose cycle andsends permissive signal back to the substation B. This so-called "echo carrier" is necessary to speed up the protection operation at the substation B in case the fault is close to the substation A.

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The simplified logic diagram for activation of the weak infeed tripping is shown on Figure 2.3 below.

Reverse “looking” measuring

elements are NOT operated

Trip local breaker

Initiate ARC

Send permissive

carrier (Echo)

Forward “looking” measuringelements are NOT operated

Permissive carrier is being received

Additional security features e.g.undervoltage/overcurrent monitoring,

contact racing timers, breaker status

monitoring etc.

AND

Figure 2.3. Simplified weak infeed tripping logic diagram

Protection manufacturers often include additional security features such as undervoltage, undercurrentmonitoring, contact racing timers, breaker status monitoring etc. to prevent weak infeed function fromunnecessary operation. These are indicated by dotted line in Figure 2.3 above.

3 APPLICATION AND BENEFITS OF WEAK INFEED TRIPPING

To evaluate benefits of the weak infeed tripping sequence many local aspects of fault environment, protection philosophies, automatic re-closure (ARC) policies, available relays, teleprotection equipment, installed circuit breakers and application of switch-onto-fault (SOTF) tripping function have to be considered.

Weak infeed tripping may contribute to faster fault clearance in two different scenarios on the power system:

• During the first occurrence of a fault on the Transmission line when breakers at both ends of the line are

closed and one substation does not have sufficient fault current to operate relays;• during ARC cycle or manual closing when dead line charging breaker is closed onto a fault and the remote

end breaker is opened.

The discussion below evaluates expected benefits of weak infeed tripping as applied on the ESKOMTransmission network in these different scenarios and indicates possible alternative options to ensure fast faultclearance.

3.1 Genuine Weak Infeed Conditions

Substations with fault level too low to operate protection relays are not common on the ESKOM Transmissionsystem since most of the network is well meshed with very few radial connections.

Figure 3.1. Fault Level Histogram

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60

Fault Level [kA]

N u m b e r o f b u s b a r s

The fault level histogram shown in Figure 3.1 indicates only 4 busbars out of 190 with fault level below 2000A. Weak infeedcould be expected practically only at these four substations buteven there, with sufficiently low CT ratio (e.g. 600/1A or less),the secondary current required to operate any electronic or

digital impedance relay would be available. Some of oldelectromechanical relays require currents as high as 250mA on

the secondary side to ensure reliable operation but on theserelays weak infeed functionality is not available. With acurrent sensitivity setting of 10%, which is common practice inESKOM, fault currents as low as 60A primary can be detected

with a CT ratio of 600/1. Fault resistance, however, can limit

the magnitude of fault current significantly even at high faultlevel substations, which can not be ignored. Figure 3.2

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indicates the impact of fault resistance on themagnitude of fault current for a single phase to ground

fault on a line terminating at a 2000A fault levelsubstation. Even with very high fault resistance at theend of a 300km long 275kV standard Transmissionline, the fault current does not decrease below 400A

which is far above the required minimum of 60A. Onradial feeds where three phase fault level of the

supplied substation may be equal to zero, ifcontribution of customers' rotating machines isignored, most of the faults - single phase to groundfaults, will still be detected due to zero sequencecontribution of remote transformers. In the case ofmultiphase or three phase faults the lack of fault

contribution does not jeopardise the network orequipment as fault will be cleared once strong end breaker opens. The only disadvantage of tripping only

one breaker in such situations will be more complicated faultfinding.

Figure 3.2. Line fault currents for different fault resistan

400

600

800

10001200

1400

1600

1800

2000

0 100 200 300

Fault distance [km]

F a u l t c u

r r e n t [ A ] 0

50

100

150

Fault

Resistance

[ohm]

On the ESKOM Transmission network weak infeed substations that would not be able to deliver fault current ofat least 10% of maximum load of the line for most common, single phase to ground faults, practically do notexist.

3.2 Weak Infeed During ARC Cycle or manual closing

Weak infeed tripping can play a role during ARC cycle when one breaker of the opened line is being closed ontothe remote fault (beyond zone 1 reach) as indicated on the Figure 3.3 below.

IfA = 0IfB

BZsBZsA

A

Fault

Zone 2

Zone 1

~ ~

Figure 3.3. Fault beyond zone 1 reach during ARC cycle when remote breaker is open.

In such case the fault can not be rapidly cleared at substation B as it is beyond zone 1 reach. Assuming that

permissive overreach intertripping scheme is employed, weak infeed logic would result in "echo carrier"transmission from end A to end B speeding up protection operation (ref. Figure 2.3 Simplified weak infeedtripping logic diagram).

In such a scenario, a very clear benefit could be obtained fromthe application of weak infeed logic i.e. instantaneous or slightlydelayed fault clearance, as only small co-ordinating timer in

range of 50 milliseconds is necessary to accommodate contactracing. Another benefit is better resistive coverage of the protection scheme. The fault beyond zone 1 due to its highresistive nature, as illustrated in Figure 3.4, will still be clearedinstantaneously with "echo carrier" transmission. This benefit is particularly important on short lines where resistive coverage of

zone 1 is often limited unless independent settings of resistive part of the impedance characteristics are available on the relays

(e.g. quadrilateral characteristics). With limited resistivecoverage of zone 1 even high current resistive faults, on linesclose to power stations, would have to be cleared in zone 2 time.Instantaneous trip in ARC cycle, when the breaker is closed onto

a sustained fault is very important also from another perspective.

Once the breaker is open after the initial fault clearance, therelay has lost its memory voltages often used to enhance

polarisation. For a very close up fault, where secondary voltages are very low, there is also a possibility that the

R

Figure 3.4. High resistance fault beyond

zone 1 characteristic.

Zline

R f

Zone 2

Zone 1

X

318 - 5



relay may not detect the fault in directional zone 1 on closing with no fault resistance. Such situations couldhave a detrimental impact on the stability margins of the Transmission network as well as stability of auxiliary

plant at the power stations which is not acceptable on highly stressed Transmission networks.

For these reasons, however, a "line check" or "switch-on-to-fault" (SOTF) protection function has beendeveloped and is successfully utilised world-wide. The SOTF function provides instantaneous operation of the

relay on closing of the breaker by insertion for a short duration either a simple overcurrent relay or aunidirectional fault detector (starter) to enhance the reliability of tripping on closing onto the fault. On many

relays this functionality can also be inserted during ARC cycle providing more reliable instantaneous faultclearance than that provided by weak infeed logic, as communication channels are not require for SOTF.

The benefits of weak infeed tripping during ARC cycle are covered by SOTF function if the relay settings allowto insert this function during short ARC dead time. During manual closing SOFT function is always activated.

4 CO-ORDINATION OF WEAK INFEED TRIPPING FUNCTION

Weak infeed tripping function requires quite sophisticated co-ordination to ensure reliable and, moreimportantly, secure operation in all possible, practical system conditions. Focus of this co-ordination should be

on security more than reliability as unnecessary operation of weak infeed function results in the simultaneousloss of two Transmission circuits instead of one, which in many cases may result in cascading tripping leading toa major incident with serious, if not catastrophic, consequences. Failure to operate on the other hand may resultonly in prolonged fault duration (zone 2 tripping time) for low fault current faults assuming failure of SOFTfunction, which in most practical cases will not jeopardise equipment, safety or system stability.

There are three aspects of weak infeed co-ordination that have to be taken care of to ensure adequate

performance:

• timing co-ordination;

• co-ordination of tripping and blocking impedance polygons;

• co-ordination of undervoltage and undercurrent guards.

4.1 Timing Co-ordination

The most important role of timing co-ordination in weak infeed tripping logic is to prevent possible incorrecttripping for faults on parallel feeder as illustrated on Figure 4.1 below.

ZsA

~Zone 2 +

Carrier SentZsB

A B

2

1Blocking Zone +

Carrier Receive

~

Fault

Figure 4.1. Possible incorrect tripping due to lack of timer co-ordination.

During fault on feeder 2 as indicated in Figure 4.1 above, at substation B the zone 2 will operate on healthyfeeder 1, and send permissive inter trip signal to substation A. At substation A this signal will be received but

reverse "looking" blocking elements will prevent any unwanted operation. As soon as the breakers open onfaulty feeder to clear the fault, all measuring elements on healthy feeder have to drop off. If the blockingelement at substation A drops off faster than zone 2 at substation B then there will be a situation when all therequirements of weak infeed tripping are met resulting in incorrect operation and sympathy trip. The weak

infeed tripping has to be therefore delayed by the difference of drop-off times between them. Moreover after thezone 2 at substation B drops off, the transmission of permissive signal will be maintained for a certain time until

relevant relays in teleprotection equipment drop off at both ends of the telecommunication channel. Finally the propagation delay of the channel has to be considered plus some safety margin.

318 - 6



The effective time delay that has to be introduced to prevent overtripping is equal to:

Td = (Td/o Zone 2 - Td/o Blocking Zone) + TTeleprotection + T propagation + Tsafety margin

Where: Td - required delay setting;

Td/o Zone 2 - zone 2 drop delay off at substation B;Td/o Blocking Zone - blocking zone drop off delay at substation A;

TTeleprotection - delay of teleprotection equipment at both substations A and B;T propagation - propagation delay;Tsafety margin - safety margin.

As long as relays at both ends of the line are of the same manufacturers their drop off delays should becomparable and the first part of the equation (Td/o Zone 2 - Td/o Blocking Zone) becomes negligible. In such cases delay

of 50 - 100 milliseconds seems to be sufficient. Where different relays are installed, however, the drop offdelays have to be carefully evaluated from records of past performance or tested, as from ESKOM's experiencethey can be surprisingly long on certain relays. Where high differences are detected, the Td delay has to beincreased accordingly reaching possibly 150 - 200 milliseconds. If such delays are necessary, the wholeapplication of weak infeed has to be carefully thought through as reduction of zone 2 timer to 200 milliseconds

would give the same result.

The selection of this delay is critical for the security of weak infeed tripping. In cases where the difference in thedrop off delays of the relays at substations A and B are substantial, the application of weak infeed may jeopardise security of the entire protection scheme unless long co-ordination delays are selected. Long delays onthe other hand make the application of weak infeed function questionable as same results can be achieved byreduction of zone 2 time delay with much lesser risk.

4.2 Co-ordination of Tripping and Blocking Impedance Characteristics

Co-ordination of tripping and blockingcharacteristics of the relays on the X/R impedance

plain is the most critical to avoid incorrect tripping.Many factors have to be considered such as shapes ofthe characteristics, types and depths of polarisation,algorithms used in digital relays, particularly thedynamic expansion of the resistive part of thecharacteristic during earthfaults and impact of seriescompensation of the line. There is usually little

explanation in manufacturers' manuals how this co-ordination should be provided with a few guidelinesrecommending that the reverse blocking zone mustoverreach the remote end forward zone 2 as indicatedin Figure 4.2. This is to ensure effective blocking forevery reverse fault (or fault on parallel line) that can

be measured by the remote end zone 2. Such co-ordination can easily be achieved for the reactive parts of the characteristics. Where fault resistancereaches significant values, however, the co-ordination can be lost. A very simplified illustrationof such a situation on relays with pure mho

characteristics is shown in Figure 4.3. In thisexample, due to significant value of fault resistance

R f , the impedance locus falls outside the blockingcharacteristic of the relay at substation A but insidethe forward zone 2 characteristic of the relay atsubstation B. In this case the weak infeed function

will result in "sympathy trip" of the line AB for an

external fault. The area of lack of co-ordination has been shaded in Figure 4.3. To ensure that proper co-ordination is maintained for high resistance faults, the blocking characteristic of the relay at end A must cover the entire part of the forward zone 2 at end B that can

BA

Reverse

Blocking

Zone

Forward Zone 2

Figure 4.2 Simple example of tripping and blocking

elements co-ordination.

BA

Reverse

BlockingZone

R f

Forward Zone 2

Figure 4.3 Simplified example of possible lack of co-

ordination during high resistance fault.

318 - 7



operate for faults beyond substation A. This part of forward zone 2 characteristic is indicated with bold line inFigure 4.3.

In practice, many factors influence shape of the areawhere co-ordination may be lost. Let's consider themore realistic situation with cross-polarised relays

as indicated on Figure 4.4 below.In order to ensure appropriate co-ordination, the

polarisation of the blocking characteristic atsubstation A should be much stronger than that ofthe tripping characteristics at substation B.Assuming that the same relays are installed at A andB, with the same type and percentage of polarisation for blocking and tripping

characteristics, the areas where the co-ordinationrequirements are not met (shaded on Figure 4.4) aremuch smaller but they are still there. Such areas areexposing the protection scheme to overtripping.

The co-ordination of tripping and blockingcharacteristics becomes more complicated whererelays with different polarisation methods and/ordepths are used. Protection engineers have to

examine the whole range of operating conditions for a particular feeder in order to evaluate possible scenariosthat could lead to incorrect operations. Factors such as loading of the line (magnitude and direction), possibleconfigurations of the surrounding network and generation patterns that influence source impedances as well as

expected fault resistance have to be included in order to predict possible shapes of polarised characteristics.Such calculations are very complicated and require intimate knowledge of relay algorithms and filteringtechniques. Relay manufacturers normally do not provide such information in their manuals.

BA

ReverseBlocking

Zone

Forward Zone 2

Figure 4.4 Example of lack of co-ordination between

cross polarised relays.

Some impedance relays use negative sequence voltages and currents to enhance their operation. Co-ordinationof such relays with relays that measure positive sequence only creates another challenge. One of such

application was examined on the ESKOM Transmission network and due to different profiles of positive andnegative sequences along the network for a variety of faults and system configurations, co-ordination was not possible.

Quadrilateral characteristics with well-defined borders of resistive reach are much easier to checkfor co-ordination of weak infeed functionality.

Reverse Blocking

Zone

A B

Forward Zone 2

Figure 4.5. Simple example of proper co-ordination of

quadrilateral characteristics.

Figure 4.5 illustrates a simple example of requiredco-ordination where the entire forward zone 2 polygon of the relay at substation B that can operatefor faults beyond substation A is covered withsufficient safety margin by the reverse blocking zone

of the relay at substation A.

Such co-ordination is possible, however, only onrelays that are equipped with independent resistivereach settings for blocking and trippingcharacteristics, which is not always available.

The real characteristics are often carved to avoid load encroachment, which introduces another complication and

may expose certain areas on the X/R impedance plain for maloperations. One such example is shown in Figure4.6, where characteristics of the relays installed at both ends of the line are plotted together to highlight requiredoverlapping areas.

Relay characteristics at end A of the line are indicated by continuous line and characteristics of the relay

installed at end B are rotated 180 deg. and indicated by dotted line. On this particular application, digital relaysare installed which use the reverse reach of fault detector (FDR) characteristics for blocking in the weak infeedlogic.

318 - 8



Only zone 2 and fault detector characteristics are plotted to make the picture more readable. The following

abbreviations were used:FDA - Forward reach of fault detector at end A;FDR A - Reverse reach of fault detector at end A;FDB - Forward reach of fault detector at end B;

FDR B - Reverse reach of fault detector at end B;Z2A - Zone 2 at end A;

Z2B - Zone 2 at end B;

The four shaded squares indicate areas where co-ordination has not been achieved.

The above picture, although complex and difficult to

read, is still a simplification of real values ofimpedances that the relays measure during faults, particularly high resistance faults, when load playssignificant role. For precise evaluation, theimpedances measured at end A and B should be

plotted on separate X/R planes. The Figure 4.6 is justfor illustration of required overlapping in no loadcondition on "homogenous" system.

During this incident, a healthy line tripped for a highresistance fault on a parallel line in heavy loadedcorridor of the network, almost resulting in system

islanding. Investigation based on plots of measuredimpedances recorded during the incident together withrelay characteristics on the X/R plain revealed areas oflack of co-ordination between blocking and trippingcharacteristics on this line as approximately illustratedin Figure 4.6. During the investigation separate

impedance plots were done for end A and end B.

Max load B

Z2 B

FD B

FDR B

Max loadA

Z2A

FDA

20

40

60

80

100

-60 -40 0 20 40 60

-20

-40

-60

FDR A

-20

X [sec ohms]

R

B

A

Figure 4.6. Example of incorrect co-ordination areas

identified on ESKOM 400kV network.- Relay characteristics at end A of the line

- Relay characteristics at end B of the line

- Incorrect co-ordination areas

- Movement of impedance locus during incident

In this particular application two adjustments to the relay settings could be considered in order to avoid incorrecttripping:

• increase the resistive reach of fault detectors at both ends of the line to +/- 36 ohms to maintain required20% safety margins of overlapping, which would result in possible load encroachment or

• reduce resistive reach of zone 2 at both ends of the line to +/- 15 ohms, which would result in poor resistive

coverage of the relay.

Figure 4.7. Illustration of increasedreactive reach of zone 2 d

ue

to series compensation.

A B

Reverse Blocking

ZoneForward Zone 2

A B

Reverse Blocking

ZoneForward Zone 2 Both solutions or any compromise between the two would result

in increased risk to reliability and/or security of the protectionscheme, considering ESKOM fault environment and possibilityof short time line overloading as a result of disturbance

development.

On series compensated lines, care has to be taken to ensure that proper overlapping by the blocking zone is provided in the worstcase scenario when the series capacitor does not by-pass. Insuch cases, which are common for faults in reverse direction, the

reactive reach of forward zone 2 is much higher due tocompensation of the capacitor as illustrated on Figure 4.7.

This situation can become much more complicated for adequateanalysis where series capacitors are protected by MOV withoutspark-gap and bypass breaker. The MOV during fault

conditions will introduce nonlinearity to this circuit, which may

impact shapes of relay characteristics.

318 - 9



During ARC cycle when the breaker at substation B

(Figure 4.10) closes onto a fault while the breaker atsubstation A is still open (dead line charging), thesubstation A becomes a weak infeed end due to open breaker. In this case undercurrent supervision would

enable weak infeed tripping but undervoltage relaycould measure any value between 0 and 1p.u. depending

on fault resistance.

~

If A=0 If B>

B

ZsBZsA

~

Vf B>>0Vf A>0

Rf>>

Vf>0

A

The undercurrent and undervoltage supervision with lowsettings could provide desired security against incorrectoperations for high resistance faults. Very complicatedstudies would have to be provided, however, to evaluate

areas of possible lack of co-ordination and to calculate values of corresponding voltages and currents for whichweak infeed should not be activated. Values of such currents depend on actual system configuration andgeneration patterns that can be predicted only with limited confidence level. During the development of adisturbance the network may be severed beyond N-2 condition which is very difficult to study due to multiple possible scenarios.

Figure 4.10. Current and voltage distributionduring high resistance fault and open

breaker at substation A

The undervoltage supervision with low setting would disable weak infeed operation in ARC cycle for highresistance faults where most of the benefits could be expected.

5. CONCLUSIONS

The weak infeed function was developed to improve sensitivity of protection relays at the substations where thefault current could be insufficient to reliably operate protection relays. Some of the older electromechanicalrelays require as much as 250mA secondary current for operation. With the introduction of more sensitiveelectronic and digital relays and growth of ESKOM Transmission network the necessity for weak infeed trippingdiminished to a degree that the benefits of this functionality are very limited. Possible faster fault clearance forend zone faults during the ARC cycle when one breaker of the line is open are reliably covered by the use of

SOTF function in the ARC cycle.

At the same time increasing load of Transmission network moved focus of protection co-ordination towardsimproved security of protection schemes. Sympathy trips can result in cascading tripping of parallel connectionsleading to major disturbances. Over the past few years, lack of appropriate co-ordination of weak infeedfunction was a major contributor to overtripping of Transmission feeders in ESKOM.Very complex protection co-ordination analysis, as indicated in this paper, is required to ensure acceptable

security level of weak infeed tripping. In many cases required data is not available for adequate studies and insome cases relays' limitations do not allow for appropriate co-ordination.High resistance faults are major contributors to incorrect operations and Transmission system in ESKOM isexposed to high percentage of such faults.Small deterioration of drop off delays of the impedance relays may also result in a lack of weak infeed trippingco-ordination and overtripping.

The high level of potential insecurity and very limited benefits of application of weak infeed tripping ledESKOM to a decision to use this functionality only on selected and very carefully studied applications where benefits are apparent and justifiable and uncertainties can be quantified and minimised.

6. RECOMMENDATIONS

• For secure operation of weak infeed facility overlapping of the zone 2 characteristic has to be provided by

reverse blocking zone on the entire X/R plain and for all possible system configurations that influenceshapes of the dynamic characteristics of the relays. Co-ordination of the reactive components alone is

insufficient to avoid overtripping during high resistance faults.

• On relays where resistive reach of blocking zone can not be set independently from tripping zones used in

weak infeed logic, very careful studies have to be conducted to ensure acceptable security via undervoltageand undercurrent guards.

318 - 11



• Weak infeed should not be used on relays that are not compatible in terms of measuring principals unless performance of such relays is well known and co-ordination well studied. In some cases it may not be possible to co-ordinate positive sequence measuring relays with relays that are equipped with zero and/ornegative sequence directional overcurrent measuring elements.

• To ensure proper time co-ordination of weak infeed function, the drop off time of impedance relays at bothends of the line should be known, either from tripping records or tests, for adequate time delay settings.

Wherever deterioration of drop off time is detected, the co-ordination time setting should be increased orweak infeed function disabled.

• Undercurrent and undervoltage guards provide only limited security improvements during high resistancefaults.

• In well-interconnected areas of Transmission networks where genuine weak infeed does not occur, benefitsof weak infeed tripping / echo carrier transmission are very limited once switch-onto-fault function isinserted during ARC dead time.

7. BIBLIOGRAPHY

[1] M. J. Mackey "Optimisation of Protection Performance during System Disturbances" -CIGRE WG B5.09 Final Report

[3] IEEE Special Publication, “Application of Fault and Disturbance Recording Devices

for Protective System Analysis,” IEEE Publication No. 87TH-0195-8 PWR,Operations Center, Piscataway, NJ, 1987.

[2] Adam Bartylak “Application of Disturbance Recorders as near real time information

support for National Control in ESKOM” - IEE Conference – Developments in PowerSystem Protection, (Amsterdam, April 2001).

[4] Protective Relays Application Guide, GEC Alsthom, Stafford 1990.

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evaluation of weak in-feed tripping technique on the eskom transmission network

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