commissioning of multi function digital relays.pdf

7/27/2019 COMMISSIONING OF MULTI FUNCTION DIGITAL RELAYS.pdf

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www.netaworld.org Winter 2009-2010 NETA WORLD

Special Feature

Phase

Fault

Ground

Fault

Breaker

Failure

Phase

Fault

Ground

Fault

Breaker

Failure

Overexcitation

PRIMARY ZONE

Underfrequency

BACKUP

ZONE

Protection and Commissioning

of MultifunctionDigital Transformer Relays

Charles J. Mozina, Consultant

Beckwith Electric Co., Inc.

T

he application o multiunction digital relays to

protect power transormers has become a com-mon utility practice. Tis paper discusses the basics

o transormer protection including phasing standards,

through-ault withstand capability, dierential protection

with slope, C requirements, and harmonic restraint, using,

overcurrent protection, and communicating these properly

to new digital relays. Te rationale or providing transormer

overexcitation protection on all major transormers is also

addressed. Hopeully, this inormation will be helpul to less

experienced engineers.

Advancements in digital technology have allowed relaymanuacturers to include more and more relay unctions

within a single hardware platorm as well as address increas-ingly more transormer winding congurations. Tis hasresulted in digital transormer relays requiring an Einsteinto set and an Edison to commission. Since there are ewEinsteins or Edisons among us, the next generation o trans-ormer relays needs to concentrate on this complexity issuein addition to technical improvements. Tis paper addressesthese issues that the author believes are the major shortcom-ings o existing digital transormer protective relays.

I. Introductionransormer Protective Zones -raditionally, the protectiono power transormers has been relegated to the applicationo transormer phase dierential and backup overcurrentrelays to provide short-circuit protection. With the advento modern multiunction transormer relay packages, phasedierential and overcurrent are only two o the many pro-tection unctions that are incorporated into these packages.Fig. 1 indicates both the primary and backup zone protec-tion areas typically provided by todays digital transormerprotection packages.

Figure 1 Transormer Zone Protection

At many utility acilities, many o these additional pro-tective unctions are provided by discrete electromechani-cal (E-M) relays or not applied at all because o economicconsiderations.

Tis exibility and added unctionality has resulted incomplicated relays that are difcult to set and commission.

Also, the ailure o the hardware platorm typically resultsin the loss o all protective unctions within the transormer

package and must be considered in the design o the pro-tection system to maintain separate primary and backupprotection. Tis paper addresses these issues and providespractical solutions. Tese shortcomings, however, are aroutweighed by the many advantages o digital relays. Usersalso have seen the many benets o digital relays with almostall new installations using this technology.

Causes o ransormer Failure - Contrary to popular belie,transormers do experience short circuits and abnormal elec-trical conditions that result in their ailure. As transormers


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NETA WORLD Winter 2009-2010 www.netaworld.org2

Improper Maintenance

% of FailuresCause

Insulation Failure 26%

Manufacturing Problems 24%

Unknown 16%

Loose Connections 7%

Through Faults 5%

5%

Overloading 4%

Oil Contamination 4%

Fire/Explosions 3%

Lightning 3%

Floods 2%

Moisture 1%

become older, the likelihood o ailure increases as insulationbegins to deteriorate. An example o one such abnormalcondition is overexcitation, which is discussed in this paper.Many industry experts have concluded that overexcitationand through-aults are more detrimental to transormer liethan load-associated aging. [1] Trough-ault ailures werea major industry concern in the U.S. during the late 1970sand 1980s when the industry experienced an unusually largenumber o through-ault ailures due to design deciencies.

As a result, the IEEE ransormer Committee developedguidelines (C57.109-1985) or duration and requency otransormer through-aults. Figs. 2a and 2b depict theserequirements or Category III (5-30 MVA) and Category IV(above 30 MVA) transormers. Te through-ault standardsor smaller Category III transormers are dened by twosets o curvesone or requent aults and one or inre-quent aults. wo sets o curves are used since this size otransormer is oten used in utility distribution substationsand are subjected to requent through-aults and multipleautomatic reclosing attempts. Te multiples o normal cur-rent in Figs. 2a and 2b are based on the OA rating o thetransormer being 1.0 base current. Tese curves should beused when developing transormer time overcurrent relaysettings.

Figure 2b IEEE Category IV Transormer above 30 MVA

A detailed analysis o transormer ailures [2] conductedby the Hartord Steam Boiler (HSB) Inspection and Insur-ance Company (HSB is a major electrical equipment insurer) breaks down the causes o transormer ailures based onthe transormers they insure. able 1 shows the breakdowno the causes o ailure. One o HSBs conclusions is tha

whatever the cause o ailure, age compounds the problemTereore, the proper protection o aging transormers war-rants careul attention rom utility protection engineers.

Figure 2a IEEE Category III Transormer 5 to 30 MVA

Table 1 Causes o Transormer Failures


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1.0

1.0

1.0

1.0

1.0

1.0

A) Three Phase Fault B) Phase to Phase Fault in pu of Three Phase Fault

0.577

0.577

0.577

C) Line to Ground Fault

0.866

0.866

0.5

O.5

1.0

0.5

0.5

1.0

0.577

0.577

0.5770

00

0

TransformerH1

H2

H3

X1

X2

X3

Wye-Wye H1 and X1 at zero degreesDelta-Delta H1 and X1 at zero degreesDelta-Wye H1 leads X1 by 30 degrees

Wye-Delta H1 leads X1 by 30 degrees

II. Transformer Basicsransormer Grounding and Winding Conguration - ransormer groundingand winding conguration (wye or delta) play a large role in the developmento the protection or a specic transormer.Te use o a delta-wye transormerintroduces a rearrangement o ault current on the primary o the transormeror secondary aults that aect protection when overcurrent devices such asovercurrent relays or uses are used

Figure 3 DeltaWye Transormer Fault Current Distribution

Fig. 3 illustrates how secondary ault currents are redistributed or varioustypes o aults when viewed rom the primary side o the transormer. Fault cur-rents are shown in per unit (pu). Fuses which are commonly used by utilities toprotect solidly-grounded transormers o 10 MVA and smaller cannot be used toprotect transormers whose wye winding is grounded through grounding resistors.Section III o this paper discusses protection o transormers that are groundedthrough resistors in the wye neutral. Tis type o transormer grounding is com-monly used at industrial acilities and at power plants or auxiliary and start-uptransormers. As an example, i a secondary wye ground ault current is limitedto 400 A on a 10 MVA 138/13.8 kV transormer, a bolted secondary ground

ault would only produce 23 A (see Fig. 3C) o ault current on the primary sideo the transormer. Tis is less than the 42 A o ull load current. Tus, it is notpossible to use such a transormer because the uses are not sensitive enough todetect secondary ground aults. Tese transormers must be protected by relays.

ransormer Phasing Standards - Tere are two major phasing standards usedworldwide or transormers.

Te ANSI/IEEE standardwas developed by North American transormer manu-acturers and is used in the U.S., Canada, and many other countries. Te IECphasing standard was developed by European transormer manuacturers andis used in Europe and countries worldwide with electric systems inuenced by

European manuacturers. ransormer protective relays that are sold worldwidemust be able to handle both phasing standards thereby adding to the complex-ity o the digital transormer relays. Properly communicating the phase shitintroduced by delta-wye transormers to a digital relay is the biggest source osetting errors in digital transormer relays. Te level o complexity required tocommunicate transormer phase shit to a digital relay is one o the design eaturesthat dierentiates one relay manuacturers product rom another. IEEE/ANSIphasing standards are shown in Fig. 4. For delta-wye or wye-delta transormers,the primary (H) current leads the secondary current (X) by 30 degrees.

Primary Secondary

Figure 4 IEEE/ANSI Phase Shits

How the 30 degree-phase shit isaccomplished within the transormeris a mystery to some engineers, but itis accomplished by the congurationo the delta winding within the trans-ormer. Figs. 5 and 6 show how thisis done or delta-wye and wye-deltatransormers wound to meet IEEE/

ANSI standards. For IEEE/ANSIstandard transormers, a simple wayto communicate the phase shit is todetermine how the delta winding iscongured. One can look at a phas-ing diagram or the delta winding andeasily determine i the A-phase deltais a delta AB (IaIb) or AC (IaIc)congured. Te delta AB shits thephase angle 30 degrees in the posi-tive direction with the delta current

leading the wye current. Te delta ACshits the phase angle 30 degrees inthe negative direction with the deltacurrent lagging the wye current. Usingthis simple convention, it is possibleto communicate the phase shit to adigital relay or IEEE/ANSI standarddelta-wye transormers.


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A

H1

B

H2

C

H3

X1

a

X2

b

X3

c

a

b

c A

B

C

H1 (A) leads X1 by 30

degrees. IA = Ia - Ib

Delta AB ( AB) can be

used to describe how the

delta winding is made-up.

HV LV

a

b

c A

B

C

IA

IB

IC

Ia =IA - IC

Ib =IB - IA

Ic = IC - IB

A

H1

B

H2

C

H3

X1

a

X2

b

X3

c

H1(A) leads X1(a) by 30

degrees. Ia =IA - IC.

Delta AC ( AC ) can be

used to describe how the

delta winding is made-up.

HV LV

0

6

39

8

7

10

11 1

2

5

4

For Delta Primary Transformers:

1 = Dy1 = X lags H by 300

3 = Dy3 = X lags H by 900

7 = Dy 7 = X lags H by 2100

For Wye Primary Transformers: 1= Yd1 = X lags H by 300

3 = Yd3 = X lags H by 900

7 = Yd7 = X lags H by 2100

300 CLOCK EXAMPLES

IEEE/ANSI Phasing Standard IEC Phasing Standard

letter Y a wye winding. Te letter that is capitalized is theprimary or H winding o the transormer. Fig. 7 illustratesthe clock concept and the standard IEC phasing examples

Figure 6 IEEE/ANSI WyeDelta Transormer Phasing

IEC phasing standardscannot use the method describedabove to communicate the phasing to a digital transormerstandard. Te Euro-designation uses a clock system witheach hour designated as a 30-degree increment o laggingphase angle rom the X1 bushing to the H1 bushing. Teclock is divided into 12 segments. Each segment numberindicates the number o 30 degree increments that the phaseangle is shited (Example: 1=30and 11= 11X 30 = 330).

Te letter D is used to designate a delta winding and the

Figure 7 IEC Transormer Phasing

o handle both o these phasing standards, relay manuacturers have used various techniques. Some have chosento diagram every possible transormer phasing connection

which results in over 250 three-line phasing diagramsTe number o specic cases is increased because o theact that delta-connected Cs must be considered. Otherhave chosen to adopt the Euro clock, which can be usedto describe IEEE/ANSI standard transormers as well asIEC transormers. One manuacturer has chosen to use thesotware to allow users to select either the IEEE/ANSI oIEC standard.

Figure 5 IEEE/ANSI Delta-Wye Transormer Phasing

Figure 8 Programmable Phasing Standard Selection

Tis method allows North American users to dene

phase shit by the simple method o how the delta is congured on A-phase (Delta AB or Delta AC). I the relayis being used to protect an IEC standard transormer, acustom mode can be selected that uses the 30 clock de-scribed above. Tis results in a less complicated methodto communicate transormer phase shit to a digital relayor standard IEEE/ANSI phasing applications while stilproviding the exibility to address IEC transormers. Fig8 illustrates this sotware.


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0.5

1.0

1.5

2.0

0.5 1.0 1.5 2.0

87T Pick Up

Slope 1

Slope 2

Slope 2

Breakpoint

TRIP

RESTRAIN

IR

= Iw1

+ IW2

2

RESTRAINT (IR) - pu

OPERATE

(I0)-pu

UNITIw1

Iw2

Iw1

+ Iw2

= 0

Where:

IO

= IW1

+ IW2

In percentage restraint dierential relays, the higher thethrough-ault current, the greater the value o dierentialcurrent it takes to operate the relay. Fig. 9 illustrates thisconcept or a digital relay. Te operating current (I

O) is the

vector sum o the primary and secondary per unit currents.Per unit current (pu) in dierential relays is the C cur-rent on the primary and secondary divided by the relay tapsetting or that winding. Te dierential relay pickup mustbe set above steady-state transormer magnetizing current

and generally is set in the 0.2 0.3 pu range.

III. Transformer Differential Protectionransormer dierential protection is typically installed

on transormers that are 10 MVA or larger [5]. ransormerdierential protection is a challenge to apply because o ac-tors such as current magnitude and phase angle balancing,inrush and overexcitation restraint, and C perormance.Digital relays have allowed manuacturers to improve manyo the design elements that comprise transormer dieren-

tial protection. ransormer dierential protection can bedivided into two categories: phase and ground.

Current Magnitude Balancing- Current magnitude balanc-ing within a dierential relay is accomplished through theselection o the appropriate transormer tap settings withinthe relay. Te older technology o E-M relays used ve orsix discrete tap settings to balance current magnitudes onthe primary and secondary o the transormer. Tey couldbalance a current mismatch o approximately 3 to 1. Digitalrelays have continuously settable tap settings and can balancea 10 to 1 current mismatch making them more accurate

and providing the exibility to handle larger mismatches.ypically, the tap settings on the primary and secondaryare selected by determining the ull load current at the OArating o the transormer and then checking to make surethe relay current coil ampere rating is not exceeded underemergency loading conditions.

Phase Angle Compansation - Phase angle compensationis discussed ully in Section II o the paper. E-M trans-ormer relays balance phase angle externally through theconnection o the input Cs. Wye transormer windingC inputs are connected in delta and delta winding Cinputs are connected in wye to balance the 30 phase shit.Frequently, when older transormers are upgraded withdigital protection it is easier to retain the delta C wiringconnections. Te magnitude o the delta C currents un-der normal balanced load conditions is 1.73 times higherthan the individual line current or each o the Cs phasesbeing subtracted to orm the delta that is, Ia-Ic = 1.73 Ia=1.73 Ic. Many digital relays do not compensate or theincrease in current magnitude so that overcurrent relaying

within the digital relay package being supplied rom thesesame Cs must be compensated by setting the relays 1.73times higher. Relay metering is also incorrectly displayedby the same value. Tis has been a source o conusion insome applications. Tere are, however, manuacturers thatdo compensate and provide overcurrent and metering withcorrect magnitude line currents.

Percentage Restraint Slope - Percentage restraint slope is aconcept that is universally used in both E-M and digitalrelays to provide security against alse operation duringthrough-aults. It is recognized that the higher the through-ault current, the greater the possibility that mismatch inC perormance will cause a alse dierential error current.

Figure 9 Digital Relay Dual-Slope Characteristic

As shown in Fig. 9, the higher the through-ault current,the higher the value o the restraint current, which is the sumo the primary and secondary pu current magnitudes dividedby 2. Some relay designers use the larger o the two windingcurrents rather than the average o the two windings as the

restraint current. Te higher the restraint current, the moreoperating current it takes to cause the dierential unit totrip. Almost all digital transormer dierential relays use thedual slope approach. At a settable breakpoint (usually at 2.0pu restraint current), the slope is increased rom slope 1 toslope 2. Slope 1 is set based on expected C error (typi-cally 10% or C class Cs), LC tap range (usually 10%),magnetizing losses (about 1%) and a saety margin (about5%). Tus or a transormer without LC, the slope 1 set-ting is typically 15-20%. For LC transormers, the slope 1setting is set higher to accommodate the ratio change withtypical settings o 25-30%. Te slope 2 setting is usuallydouble the slope 1 setting. Te quality o the Cs used to

supply transormer dierential relays generally require thatthey operate in their linear range or worst-case symmetricalthrough-aults. A C burden calculation can be done to

veriy linear operation. In addition, manuacturers generallyprovide specic guidance on minimum C quality basedon through-ault current levels.

Harmonic Inrush Restraint - Harmonic restraint is usedwithin transormer dierential relays to provide both inrushand overexcitation restraint. Inrush restraint is required

when a transormer is energized. Te transient magnetiz-


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2nd and 4th

Harmonics

During

Inrush

Figure 10 Typical Transormer Inrush and Harmonics

C C C C

OPEN

OPEN

OPEN

SHUNT REACTOR DISCONNECTSWITCH INADVERTENTLY LEFT OPE

FAILEDAUTOTRANSFORMER

C = DISTRIBUTED LINE

CAPACITANCE

CLOSED

CLOSED

Figure 11 Overexcitation Failure o an EHV Autotransormer

ing current to energize the transormer can be as high as8-12 times the transormer rating. With high current in theprimary winding and no current in the secondary windings,a high dierential current will result. Te transormer di-erential relay sees this unbalance as a trip condition.

Te magnitude o inrush current depends on the residualmagnetizing ux in the transormer core, the source imped-ance, and the point on the voltage wave when the circuitbreaker contact closes. When the circuit breaker closes, all

three phase contacts close at approximately the same time.Te three phase voltages, however, are displaced rom eachother by 120. Tus, two o the phase voltages will be neara maximum while one is near zero degrees. Tis imbalancein voltage results in inrush currents being unsymmetricalin each o the three phases. Inrush current is not entirely60 Hz sinusoidal current but is comprised o a signicantlevel o even harmonics with the most dominant being the2nd harmonic. For over 50 years, relay designers have used2nd harmonic restraint to prevent alse dierential opera-tion on transormer energizing. Most digital relay designersalso use 2nd harmonic or inrush restraint. Digital relays arealso designed so that the 2nd harmonic in all three phasesare combined in some manner to restrain the dierential.

oday, newer transormers are being built with low loss steelcores, which result in much less 2nd harmonic current onenergizing. Tis has caused dierential relay tripping duringenergizing. At least one digital relay designer has reinorcedthe 2nd harmonic restraint by also adding the 4th harmonic

which is typically around 40% o the 2nd harmonic.

levels o even harmonics. ransormer relays are equippedwith a nonharmonically restrained high set dierentiaelement (87H) that provides protection or high currenmagnitude internal aults where C saturation can occur

Tis element is set above expected inrush current.

HarmonicOverexcitation Restraint- Harmonic restraint ialso used to prevent the transormer dierential relay romoperating during overexcitation events. Overexcitation oc-

curs when the volts per hertz (V/Hz) level rises signicantlyresulting in transormer saturation. ransormer core uxis proportional to voltage and inversely proportional to re-quency. Overexcitation events can occur on utility systemsduring major system disturbances. During the 1996 Cali-ornia disturbances a number o distribution transormerssupplied rom the 230 kV system ailed when an island wasormed in the northern part o the state where the voltageremained at 120% o normal or a signicant period result-ing in the ailure o six transormers due to excessive V/HzV/Hz ailures have also occurred on EHV systems due toswitching error during the restoration o EHV lines whenthe shunt reactors at one end o the line were inadvertentlylet out-o-service when the line was restored resulting ina high voltage at an autotransormer that was connectednear the end o the line. Fig. 11 illustrates this situation

Te voltage rise on the unloaded line is due to the distrib-uted capacitance o the line. Tis event occurred requentlyenough at a Midwest utility that they nally installedV/Hz protection.

Tis design improved inrush restraint or low loss steelcore transormers. Fig. 10 shows inrush current or a typicaltransormer and the relative level o 2nd and 4th harmoniccontent. Programming a digital relay with too low an inrushrestraint setting risks the relay restraining or an internalault because C saturation can also produce signicant

When a transormer is saturated, excitation current

which is normally very low, increases and unbalances thedierential causing it to operate. Protection is required oroverexcitation events, but the dierential relay operationoccurs so quickly that power system voltage control devicesuch as generator automatic voltage regulators (AVRs) andthe switching o o capacitor banks are not given the timeto operate to correct the problem. When a transormer isoverexcited, a signicant amount o 5th harmonic current igenerated. Tis harmonic is used to restrain the dierentiarelay rom operating. Most digital relay manuacturers blockrelay operation when the 5th harmonic exceeds a specic

value generally around 30%. Because the transormer is un


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0.5

1.0

1.5

2.0

0.5 1.0 1.5 2.0

87T Pick Up

Slope 1

Slope 2

Slope 2Breakpoint

TRIP

RESTRAIN

IR

= IW1

+ IW2

2

RESTRAINT (IR) - pu

OPERATE

(I0)-pu

UNITIw1

Iw2

Iw1

+ Iw2

= 0

Where:

IO

= IW1

+ IW2

87T Pick Up With 5thHarmonic Restraint

der stress during overexcitation, one manuacturer thoughtit more prudent to desensitize the relay rather than blockits operation. Tis is done by raising the pickup when the5th harmonic exceeds a specic value, which is settable inthe relay. Fig. 12 illustrates how the relays slope is modiedduring overexcitation conditions.

capability varies based on transormer design. Protectionor overexcitation is provided by relay unction (24) withina digital relay. Tis unction measures the ratio o V/Hz.It is important that this unction be implemented whenprotecting transormers with a digital package since digitalrelay dierentials, unlike earlier E-M relays, are designednot to operate or an overexcitation event, thereby leavingthe transormer unprotected. WhereasE-M relays providedonly discrete time element V/Hz protection, digital relays

oer an inverse time curve that closely matches most trans-ormer overexcitation capability curves. Tis is illustratedin Fig 14.

Figure 12 Example o How 87T Pickup is Increased whenSignicant 5th Harmonic is Present

Overexcitation Protection -Overexcitation o a transormercan damage the transormer i the event is allowed to persist.Overexcitation results in excessive core ux resulting in ahigh interlamination core voltage, which, in turn, resultsin iron burning. Also, at this high ux level, the normalmagnetic iron path designed to carry ux saturates andux begins to ow in leakage paths not designed to carry

it, again causing damage. Fig. 13 illustrates this ux path.

Figure 13 Overexcitation Transormer Core F lux

Te continuous overexcitation V/Hz transormer capa-bility is specied in IEEE C-57.12 [3] standard developedby the IEEE ransormer Committee. It species a 1.05pu (on the transormer secondary base) at rated load and0.8 PF or greater or loaded transormers. Te short time

Figure 14 Typical Transormer V/Hz Short-Time Capability

Ground Diferential (87GD) Protection Industrial and

power plant auxiliary and start-up transormers are gener-ally grounded through a resistor in the transormer wyeneutral. Many o these installations rely solely on the phasedierential (87) to provide ground ault protection. Someless experienced protection engineers do not understandthat phase dierential protection alone does not provide thelevel o sensitivity to detect a ground ault over the entire

wye winding. A signicant portion o the wye winding nearthe neutral will not be protected i only phase dierentialis applied. Even or ground ault on the transormer wyeterminal, additional sensitivity is required where groundault current is limited to the 200-400 A level. Considerthe ollowing example shown in Fig. 15.


8/13


Open

R

87T

2000/5

200/5

400A

13.8KV

138KV

I=0

400A

40A/1.73

40A/1.73*40/1=0.58A

45 MVA 87T

Figure 16 Transormer Digital Ground Diferential Relay

High-speed protection can be provided by use o aproduct-type ground dierential relay described above. Teconcept was available in E-M technology and is now avail-able in digital transormer protection packages. For aultsexternal to the protective zone, the net operating quantityis negative and the relay will restrain rom operating. Forlow values o 3Io, the relay automatically switches rom aproduct to a balancing algorithm (3Io Rct In). Tis allowsit to detect internal aults when the low-side transormer

breaker is open as in Fig 15. Rct is a ratio matching auxiliaryC, which is provided as part o the sotware algorithm indigital relays as opposed to being an actual C as it was inE-M technology. Tis scheme provides excellent securityagainst misoperations or external, high-magnitude aultseven or cases where the phase Cs saturate. Tere arehowever, some digital relay manuacturers that try to employonly a simple dierential (In 3Io). Tis method is prone tomisoperate during high-magnitude through-aults. ransormer ground dierential relaying substantially improvetransormer ground ault sensitivity and is recommendedin the IEEE Guide or Protective Relay Applications toPower ransormers [5]. Te example described above ac-

tually occurred at an industrial installation where the aulwas caused by human contact. High-speed protection orthis event was extremely important and was provided bythe digital ground dierential that was recently installedon the transormer.

Figure15 Example o Secondary Ground Fault

Example: A 45 MVA, 138/13.8 kV transormer with

200/5 138 kV Cs and a typical 87 relay pickup o 0.3pu. Fig.3 illustrates the current distribution or a delta-wyetransormer ground ault. Te relay tap is set at the 45 MVAtransormer rating (ap = 4.71A secondary amperes) andcan only respond to a aults greater than 1.41A (4.71 A X0.3 =1.41 A). Te maximum ground ault current (0.58A)is below the threshold o operation o the 87 phase di-erential. Sensitive detection o secondary ground aultscan be substantially improved through the addition o an87GD ground dierential relay, which uses a product ap-proach, utilizing the ollowing equation. Te relay-operatingcharacteristic is:

IOP

= (-3IO)I

Ncos Where: IOP = relay operating current

-3IO= residual current rom the bus side CsI

N= transormer neutral current

= phase angle between the currents

Te ground dierential sensitivity is very low and willoperate or an In current o 0.2A. Fig. 16 illustrates thezone o operation where the 87 relay cannot detect groundaults. In many cases, the 51G neutral time overcurrent relayprovides time delay protection or aults in this zone.


9/13


Feeder Back-up Protection Logic Te sel-test ailure out-put contacts on digital eeder protective relays can be used inconjunction with logic and programmable multiple settinggroups within the transormer protection package to provideback-up protection or the ailure o a eeder digital relay.

Te logic or such a scheme and use o an alternate settinggroup are shown in Fig. 18 and 19. A scheme such as thiscan eliminate the need or separate independent back-uprelays on each eeder panel.

Figure 18 Feeder Digital Relay Failure Functional Diagram

IV. Use Of Logic Within TransformerRelays

Logic capabilities within multiunction digital trans-ormer relays can be used to enhance the beneits odigital protection. Tese schemes can integrate the logic otransormer and eeder digital relays to provide bus aultand eeder relay ailure protection and can also be used toincrease the utilization at two bank distribution substations.

Distribution Substation Bus Fault Protection Relativelyhighspeed distribution bus ault protection can be accom-plished by using instantaneous overcurrent ault detectors inthe eeder and transormer relay packages. Such a scheme isshown in Fig.17. A transormer instantaneous overcurrentrelay (50) is used as a ault detector. It is set to overreachthe bus, and its operation is blocked by eeder instantaneousrelay elements. A slight time delay o 5 to 8 cycles is usu-ally added to ensure that the blocking has taken place. Tescheme provides relatively high-speed bus ault protection

without the addition o separate bus dierential relaying. Tescheme is primarily used in distribution substations where

the eeders supply radial load. However, the scheme has beenapplied using directional (67) eeder ault detection relaysat locations where the eeders are a source o ault current.

Figure 17 Bus Fault Protection Logic Diagram

Figure19 Feeder Digital Relay Failure Back-up Logic Diagram

woBank Substation Load Shedding Increasing the uti-lization o two-bank distribution substations can providea substantial economic benet to a utility. Fig. 20 shows atypical two-bank distribution substation.


10/13

10 NETA WORLD Winter 2009-2010 www.netaworld.org

N-1 Rating 30 MVA Bank = 40 MVA

Full Capacity (30MVA x 2) = 60 MVA

Loading Increase = 20 MVA

Figure 20 Two-Bank Distribution Substation Overload Shedding

It is common practice to operate under normal conditions with the bus tiebreaker open to reduce duty or eeder aults. On the loss o a transormer, or insome cases also the supply line, the aected bank breaker (A or B ) is opened andthe bus tie breaker (B) is closed to automatically transer the aected bus sectionto the companion bank. o accommodate this type o automatic restoration, theloading o the two-bank substation is limited to the N-1 rating o one trans-

ormer. Tis is typically a value above the nameplate rating o one transormerand is a short-time rating. In the example above, this rating is 40 MVA. Te timeinvolved in establishing this rating is usually based on the utilitys estimate ohow quickly (typically one day) load can be relieved through load shits to othersubstations or through the installation o a mobile substation transormer. Asshown in Fig. 20, the load level o the substation can be substantially increased ithe substation is loaded to the ull capacity rating o the sum o both banks - 60MVA. Tis provides a signicant load capacity increase.

ypically, distribution peak loads occur only during a small percentage o timeeach year. Tus, concurrent loss o a transormer or supply line at peak load is arare event. Reerence 4 provides an analytical method o evaluating the impacton reliability o increasing the load to the rating o the sum o both banks (60

MVA) and shedding load i the loss o a bank occurs at a load level above theN-1 rating o 40 MVA.

Figure 21 Two Bank Substation Load Shedding Logic

A logic scheme shown in Fig. 21can be implemented within a digitatransormer relay package that cantrip eeders to shed load to protect theremaining transormer rom being ex-posed to load above its N-1 short-timerating. he increase in distributioncapacity by adopting such a plannedprotection philosophy can be signi-

cant. Tis philosophy has been adoptedby a number o utilities.

V. Application andCommissioining of DigitalTransformer Relays

Multiunction transormer digitarelays have eatures that were noavailable on electromechanical or staticrelays. hese include oscillographyand event recording, multiple settinggroups, multiple output and inpu

contacts, metering, monitoring oexternal inputs/outputs,communications, sel-monitoring and diagnosticsand programmable logic. Tese are theeatures that make digital relays thetechnology o choice or the protectiono transormers. Many o these eaturesalso add to the complexity o settingand commissioning o these relays.

Te design o modern digital relayis such that all voltage and curreninputs are multiplexed through com-mon components. I a componenails, generally all protective unctions

within the multiunction relay areinoperative. Te relay engineer musbe aware o this act in deciding thelevel o redundancy or a particularapplication. For the protection oimportant generators or transormersthe eect on the system o removingthese components rom service or arelay ailure may be unacceptable. Inthose cases, dual digital relays are used

A typical dual protection scheme or a

transormer is shown in Fig. 22. Fulinput redundancy can be achieved byusing separate C and V inputs oprimary and backup relays. Because opractical limitations, many users supplyboth primary and backup relays romthe same C and V circuits. Usingthe multiple digital output contacts totrip the high- and low-side breakersdirectly and also trip the lockout relaycan provide output redundancy. Tisprovides tripping even i the lockou


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relay ails. Also, some users reduce the unctionality o thebackup relay. An example o this is the use o overcurrentrelaying as backup or transormer protection rather thanully redundant dierential relaying as illustrated in Fig. 22.

Figure 22 Dual-Relay Transormer Protection

esting o Digital Relays - esting multiunction digitaltransormer relays oers some unique challenges to theuser. Multiunction relays have protective unctions thatinteract with each other, making testing more complicated.

Tey can also be programmed to do control logic, whichmust be veried. In addition, digital relays can have multiplesetting groups that may be switched to address varyingsystem conditions. Tis exibility increases the commis-sioning complexity. Tese relays also have signicant input

monitoring capability that can greatly assist the user indetermining whether these relays are properly connectedto their C and V inputs, helping to veriy that the relayis unctioning properly.

Digital relays also have sel-diagnostics that check thehealth o the relay and can immediately detect internalailures. Tis is perhaps the most important single eaturein digital relays. Te ability to detect a ailure beore theprotection system has to operate contrasts with traditionalprotection where a ailed or deective relay remains unde-tected until it does not operate correctly during a ault oruntil the next maintenance test. It is important that the thor-oughness o sel-diagnostics be considered in developing a

maintenance testing program or multiunction digital relaysand that relay ailure be alarmed to a manned location sothat mill personnel can immediately take appropriate action.

Commission esting o Digital Relays - Commission testingo digital transormer relays still requires the test engineerto veriy the proper setting, internal logic and operation ora new installation or veriy a setting/logic/control change atan existing installation. Tis typically requires:

1. Injection o current and voltage into the relay to veriyrelay setting and timing

2. Veriying proper relay inputs and outputs3. Veriying proper relay logic.4. Veriying tripping and targets.

Clearly, communicating settings to the relay is therst step in the eld commissioning process. Simple andstraightorward setting screens are important, and not allrelay manuacturers provide setting screens that clearlyshow how the desired setting should be communicated to

the relay. Section II o this paper discusses communicationo the phase shit. Fig. 23 shows an example o a simplesetting screen or the phase dierential element. Problemsin properly communicating the desired setting to the relayare a requent source o errors.

Figure 23 Example o Relay Setting Screen

Relay screens can also be used to provide valuable inor-mation to the eld test engineer to conrm that the relayis connected to trip the proper outputs and all protectiveunctions that have been specied to be in service are, inact, programmed to be in service. Tese summary screensor both I/O assignments and setting unctions are shownin Figs. 24a and 24b below. Tese screens are particularlyimportant because during the injection testing processes,it is necessary to temporarily disable interering unctionsto test the desired unction. Tese screens provide positiveeedback that all desired unctions have been returned toservice ater testing.


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Figure 25 Relay Phase Angle Measurements

Tese meters provide positive indication that the relay isproperly set and wired. I there is a problem, it will point thecommissioning engineer to the root cause: wiring externato the relay or improper setting within the relay. Tis is apowerul commissioning tool that has proven its value innumerous installations.

Te commissioning tools cited above are one o the major advantages o digital relays. Tere are major dierencebetween manuacturers in the graphics used to communi-

cate to the user, with some doing a better job than othersGood, well thought-out graphics can greatly reduce relaycomplexity, commissioning, and setting errors.

Figure 24a I/O Assignment Summary

Figure 24b Setting Summary

Te blank space or the 24 #2 setpoint in Fig. 24b in-dicates this unction is out o service. Determining thatthe phasing and current balancing o the phase dierential87 is correct is a major part o commissioning a digitaltransormer relay. Again, graphics in the relay can providemajor help to determine i the relay is wired correctly andthat the settings result in proper balancing o input andoutput currents. Fig. 25 illustrates built in digital phaseangle meters within the relay that looks at both o the in-put currents as well as the internally compensated currents.

VI. ConclusionsSelecting, setting, and commissioning o new multiunc-

tion transormer digital relays oer unique challenges tothe user. Te advantages o numerous relay unctions beingavailable in a single hardware platorm are oset to someextent by the need to provide or the ailure o that platorm

Also, it makes testing more difcult.Digital relays reduce external control wiring required by

electromechanical and static relay technologies by incor-porating control logic within the relay itsel. Tis, howeverresults in more complex relay testing to veriy proper relaycontrol logic. Tese shortcomings, however, are ar out-

weighed by the many advantages o digital relays cited inthis paper. Users also have seen the many benets o digitarelays with almost all new installations using this technology


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VII. References

[1] Working Group 09 of CIGRE Study Committee 12,Lifetime Evaluation of Transformers, Electra No. 150,39-51,Oct. 1993.

[2] Bartley, William H. Analysis of Transformer Failures,Presented at the International Association o Engi-neering Insurers 36H Annual Conerence, Stockholm,

2003,IMIA WG 33.[3] IEEE Standard General Requirements for Liquid-Im-

mersed Distribution, Power and Regulating TransformersANSI/IEEE Standard C57.12

[4] Mozina, C.J.Increasing the Utilization of Two-BankDistribution Substations. PEA Relay Committee, Sept.1980.

[5] IEEE Guide for Protective Relay Applications to Power Trans-formers, ANSI/IEEE C37.91.

Charles (Chuck) Mozina (M65) received a B.S. degree in electrical

engineering rom Purdue University, West Laayette, in 1965. He is aConsultant, Protection and Protection Systems or Beckwith Electric Co.Inc., specializing in power plant and generator protection. His consult-ing practice involves projects relating to protective relaying applications,protection system design, and coordination. Chuck is an active 20-yearmember o the IEEE Power System Relay Committee and was the pastchairman o the Rotating Machinery Subcommittee. He is active in theIEEE IAS I&CPS Committee, which addresses industrial protectionsystems. He is the past U.S. representative to CIGRE Study Commit-tee 34 on System Protection. He has over 25 years o experience as aprotective engineer at Centerior Energy, a major utility in Ohio, wherehe was Manager o System Protection. During the past 10 years, hewas employed by Beckwith Electric as the Manager o ApplicationEngineering or Protection and Protection Systems. He is a RegisteredProessional Engineer in the State o Ohio.

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