Pamphlet RK 63-200 E Edition 1
Basic theory of
bus differential protectiontype RADSS
Broszura RK 63-200 wydanieo
Podstawy teoretycznedzialaniaZABEZPIECZENIA R6zNICOWEGO
SZYN
ZBIORCZYCH,
typ RADSS
Outstanding
I
A simplified schematic dia-
gram of the RADSS protectionisshown in Fig.1, which rep re-sents one phase of a single
The sensitivity is basically occurs the magnitude of The operating and restraint
unaffected by the the fault current and its d.c. voltages can thereforenumber of circuits included in component måy be so be regarded as being
the differential scheme large thaI the line C.T.'s developed instantaneous-
(current transformers) ly, or at basically the sameLine C. T.'s may be of standard saturate within 2 -3 ms. In speed as the primary currentdesign with relatively poor such cases it is essentiai that variation in the case of a fau It.
characteristics and with the bus differential protec-different turn ratios tion operates and seals in The combined operating and
with in 2 ms, i.e. prior to the restraint circuit of theStandard C.T. pilot wires with saturation of the line RADSS may be denoted the
a large loop resistance may C.T.'s. This high speed is differential relaycomparatorbe used necessary because when a line circuit, because a com-
C.T. saturates its output parison is here madeOther protective relays may e.m.f. tends to drop to zero. between two voltages with
be included within the respect to both amplitudesame C.T. circuits Intheeventofanexternalfault, and phase relation. The
just outside the line output relay (dR) of theC.T.'s of a relatively small comparator circuit is of the
feeder, the fault current high speed (1 ms) dry-reedmay in an extreme case be type, which ensures thatas large as 500 times the rating decisive operation will
of the feeder. The line always, be achieved under
C.T.'softhe faultyfeeder are internai fault conditions.
then likely to saturate at an
even higher speed, particularly
so if the remanence left
in the core from a previous
fault has an unfavourablepolarity. The response of
the restraint circuit of the
differential relay must there-
2
features Introduction
Percentage restraint bus Internai bus faults occur tore be of at least the same The RADSS protection is
differential protection for less frequently than line high speed as that of the based on the following twc
line to line and earth faults. faults. On the other hand, a operating circuit, if mal- fundamental princip les:
bus fault tends to be operation is to be avoided.
High speed operation about appreciably more severe! both 1. For externa I faults. the5-10 ms with respect to the safety The RADSS bus differential secondary circuit of a fully
of personnel, system protection has been designed saturatedlineC.T.canbeFull stability in the event of stability and the damage at to cope with the above represented by ils total
through faults, with the point of fault.The fact that mentioned requirements. d.c. loop resistance only,infinite fault MVA and bus faults occur relat- Ils restraint and operating i.e., with negligible
complete saturation of ively seldom is therefore circuits consist basically of reactance. Fline C. T.'s. of little comfort to two resistors, across which are 2. For interna I faults, the '-.;:
the engineer-in-charge sub- developed a restraint volt- secondary circuit of an
Low differential relay pick- sequent to a major system age and an operating voltage unloaded line C. T. canupsetting, fixed within shut-down caused by respectively. The actual time be represented by a relat-20 -70 per cent of the current the lack of adequate bus constants (L/R) of these two ively large magnetising I"
rating of the heaviest loaded protection. circuits are for all practical impedance, mainly '-feeder purposes zero. reactive with a large
When an internai bus fault (L/R) time constant.
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bus-zone arrangement with A restraint voltage Vs is ob- The differential current willthe feeder circuits LA' LB ...tained across the terminals still remain zero and the
Lx. The current distribution K and L and this drives a restraint voltage Vs will beis shown for an assumed certain current III:! through identical to that obtained
positive reference half-cycle. the diade D:! and the resistor during the positive referenceThe RAl?SS protection is Rd3 towards the output half-cycle.
normalJy arranged as terminal L. The differentialthree separate single-phase relay (dR) is then blocked and It should be noticed that the
units and a common trip unit. cannot operate. diade O2 puts Rd3 in parailei
with Rs. The effective The mechanism of C.T.Normal service During the subsequent nega- burden (R..,) of the restraint saturation and relay response
The total incoming current live half-cycle all the line cur- circuitduring normal service may briefly be explained
I,."enterstherelayatterminal rents will be reversed, i.e. can therefore be reduced as foliows:K, and the totaloutgoing Ix:, will enter the relay at by making RcJ03 as small ascurrent leaves at terminal L. terminal K and the total possible.
During normal service these outgoing current IA3 + IB3currents are equal and the will leave the relay at
differential current Id! is terminal L.therefore zero.
External faultsThe positive reference di-
rections of the currentsshown in Fig. 1 may still be
regarded as applying. The
outgoing current lxI may beassu med to increase owing toan external fault on the
feeder Lx.
SymbolsA Alarm relay, RXIG 2SR Staning relaydR Differential relayVs Restraint voltageV da Operating voltage
IM Blocking current throughdiodeD.
TMA AuxiliaryC.T.(ratiocorrection, n = lA.1I A3)
no = Overall C.T. rationo = lA,IIA3 =lx,lIX3no = nA nMA = nx nMX
Rs. Rd3 Restraint and differential circuit resistancesRdl = Ud l/Jd l = nd2Rd3 resistance referred to T Md
primary sideRdll Variable differential circuit resistornd = UdlIUd%. ratioof TMd (n. = 40rS)UdT Total voltage of differential circuitIdl Differential currentRdT = Rdl + Rdll = UdT/Jdl" total resistance of diffe-
rential circuitIn Total incoming relay currentI.. Current leaving at terminal LJRl Current through dR-relay
LA Lo
11
Lx
Ouring the initial rise of thefault current no saturationwill occur for the first few
milliseconds (see Fig. 9 on
page 9).The relay current IT3 will be
proportional to the total
incoming primary current(/TI = IAI + IBI) and the
restraint voltage Us will
increase causing alarger cur-rent I R2 to pass throughdiode O2. Also, the volt-
ages Ud3 and Us will bebasically equal. The differ-ential current Id! remains
practically zero as longas no saturation occurs.This condition isshown in a
simplified way in Figs. 2aand 2b.
Finally, saturation starts in themost exposed line C. T. (T x)and VI. is reduced whereas
U T:\ still increases without any
saturation. This represents thesituation most detrimen-tal to the stabil ity of any dif-
ferential relay. In Fig. 2 c it isseen thatU d1' becomes larger
as the unbalance between
UT3and UL increases.ltwouldappear that the currentIdl is free to flowas soon as
UdT exceeds zero. This,
however, is not the case.
I II II I TxI I nx
I IH~
I I~ I I
J IT MX I
I I nMX
rB ~nBI
IBlfT/B2
rTA nA
IAI1
T MA nMA T MO
1-- nMR Id' tRI
~
II I
I I ')[31I I-"
7 I I
Id3 iRd3 [Jj Ud3IA3
--
A TMd--:;-w:-;- n
N Udlll~tRdl
SR
"Rdll R.I, [ II dR~
&02
J~U,I
.-I
K 'Ta f RO
Fig. 1. Schematic diagram for one phase of a single-zone bus differential protection with feeders.If feeder LA has the largest primary rating, a secondary rating with lAo = 1A is normaliv selected.
3
Fig. 2 a Basic relay circuit duringthe initial rise of externa I faultcurrent, prior to C.T. saturation,i.e. UT3 = UL and IT3 = IL.
In order that ldlshall flow
a secondary currentl d2 must
alsoflow (see Fig.1), i.e. the
driving e.m.f. (Vd2) of theT Md secondary winding must
exceed the voltage Vd3. TheT Md primåry voltage V dl,
obtained owing to the
unbalance between the
incoming and outgoing lineC.T. 's, must therefore exceed
a certain voltage vali,Je before
the differential current canflow. This voltage isgiven by:
11'1 Vd3 or Ild V,,;; where the
ratio of T Md may be selectednd = 5. Hence, in the cage of
a through fault, if Vs =.30 Vit is necessary that the total
differential yoltage exceeds5x30 = 150 V in order to pro-
duce a differential current.
This restraint or blocking
action, imposed on the flowof differential spill-
currents is of minor impor-
tance for the stabil ity of the
RADSS protection. On theother hand, itenables a very
sensitive earth fault relay
(RXIG 2) to be inserted in thedifferential circuit in the cageof resistance earthed net-
works, where the earth faultcurrent may be limited to 10-
20 per cent of the largest
line C.T. rating.
In the case of Fig. 2 d he In a co-ordinate system with In this example therefore,
e.m.f. of UL has been f Ily Idl vs. IT3' equation 3 re- the protection will, remain
reduced to zero owing t satu- presents a straight line stable if the totalloop resis-ration and the effects f the through origin with the tance seen atthe L terminal to-
total loop resistance R x. as slopeS. This line is denoted wards the Lx circuit varies
seen at the L terminal the stability line, and the between 0-1000 O.lf, in the
wards the line Lx, mus be value of S may be varied case of an actual installation,taken into account. The alue between 0.5 -0.8 depending R LX should exceed 1000 O,of R LX has here been on the stabil ity r,equirements of the protection can still be
increased to such an e tent the installation. Hence, with made stable by further increas-
thatldlwilljuststarttofl w. the maximum restraint ing the slope or by increasingIt can be shown that th setting the differential the resistance of the
currentIRl flowing tow rds spill-current must exceed differential circuit. However,
the output relay (dR) is j st 80 per cent of the total in- increasing the slope de-zero when coming current in order to creases the sensitivity of the
cause maloperation. protection..3]Idl = S IT3
The maximum permissible
loop resistances for the lineC. T. secondary circuits canthen be found from:
where S = a constant, d t en- ding on the selected
setting of the compara rcircuit.
1000 n andR A2 = (n;:;-:J2
1000 nR X2 = (n:;;;}2
This is the most import ntstability equation (seeFig. 4 b on page 6).It indicates that the oper tingand restraint voltages iIIjust balance when Idl iequal to a certain fixedpercentage of the total in-coming currentIT3. Al .itcan be deduced that increasing Idl above thispercentage makes V d3exceed Vs. Similariy.. r uc-ing.Jdl makes Vd3 small rthan Vs.
Referring to Fig. 2 d it is seenthat the percentage current
distribution in the Id! andIL circuits is dependent
only on the RdT and RLx re-
sistance values. The influenceof Rs/2 is in this respect so
small that it may be ignoredFor the conditions corres-
ponding to the stability line,we have: where RA2 and RX2 represent
the total secondary burden
RLX =~ RdT [Eq. 5] o~ th~ TA and T x line C.T. s.Clrcults.
Example: If RdT = 250.0. and This burden includes: theS = 0.8 secondarywinding resistance
then RLX = 4x250 = 1000 il the pilot-wire loop resistance
and the resistance of any
additional apparatus. Theburden of the auxiliary C. T's
D can normally be disregarded.la. 2~ ~
jUs
IT3 K
UT3'R' D,o. ~ -AUs
1T3 K
.Ud.
UT3
R./2fUd3I Rs/2 UdT
Id'C::J-,...
UL IN RdT
Rs/21
~dT1. RdT
Idl I R./2IN
RI!3Rd3
UL
IL L~=~
IL L
Fig. 2 b. Voltage distribution forconditions shown in Fig. 2 a.UdT =Oand Idl = O. Also UX3 =UB3 = UTI and UXI = UL. Fig. 2 c. Voltage distribution
when line C. T. (T xl has started tosaturate. UdT and hence Udl areesch much less than nd Us.(where nd = 5). Hence. Idl re-mains at zero.
4
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Fig. 3. Basic circuit under inter-nal fault conditions ZL = ~JI L = ZLM. total magnetisin im-
pedance.
In practice, the differential
circuit of the RADSS is
designed to include two
separate resistors, Rd3 andRdll' The Rd3 is used primar-ily for setting the slopeS, andthe Rdll 'IS used simply for
increasing the total resistance
of the differential circuit.
~ closer examination of Fig.
2 d and Equations 3 and 5
shows that the stabilityof the protection is in-
dependent of the actual-nagnitude of IT3' i.e.evenif UT3 increases to an infinitely
large transient voltage theprotection remains stableprovided the value of RLXdoes not exceed that given by
Equation 5. The selectivityof the RADSS protection is,
The minimum operating cur-rent of the differential re-lay is given by:
Kldlmin = -A,1-S
where K and S areconstants depending on theselected settings of thecomparator circuit.The following approximatevalues apply to standard
settings:
Slope (S) 0.5 0.66 0.8Idlmin.(Amp.) 0.2 0.33 0.65
It is seen tha:t increasing theslope from 0.5 to 0.8.'-oincreases the minimumpick-up GYffent frorr0.2 to 0.65 Amp
Fig. 2 d. Approximate voltagedistribution required to make'dl>O. Line C.T. (TxJ is fully,aturated.
D.fT3 K 'ROUT3
Rs/2) I
UdT
RAll ofthesefeatureswill assist
to ensure decisive operation
in the event of internai bus
faults, irrespective of thenumber of circuits connected
to the RADSS protection.
rULAs an example consider:
Total incoming current [Ta =10A.Effective restraint circ it re-sistance
RdaRsRSe= -_=30
Rd3 + Rs
U. = Ud3 = l~.. = 30 V
NIdl = o and JUdlc UdT = ndUd3 = 150 ~I
150-15 !RLX= ~~~ ~=13.5n
10
5
I
therefore, independent of themagnitude of the systemfau" lever and the system d.c.time constant.
It should also be notic dthat no requirement w atso-
eve r has been establish d with
respect to the charact r-
istics ofthevarious line .T's,for example their indivi ual
matching, accuracy, wi dingdistribution, saturationlevel, remanence, timeconstant, etc. The reas nis that these factors do
rnot in any way affect th
through fault stability o theRADSS. I
In the above discussion the
positive reference halt ycleonly has been conside ed. It
can, however, be shown thatthe differential relay co -
parator circuit works in xactlythe same way during t e
negative half-cycle.
Ud3
Internai faults
In the case of an internai bus
fault, the fault current may
be considered to enter the
bus via the feeders L\ and Lo.All the other feeders, Lc...
Lx, may be assumed to be
disconnected or carrying noprimary current (idle C.T.'s).
The schematic diagram
Fig. 3 may be used to rep re-sent this situation. At the L
terminal of the relay the
impedance:
ZL = UL/IL = ZLM
is obtained, which corres-
ponds to the total magnet-
ising impedance of all the un-loaded line C. T .'5. This imped-anGe however, is normally
quite large and highly reactive,with a relatively large timeconstant, about 200 ms.
In the ca se of an internai fault,
the differential current
is therefore larger than the
magnetising current. Also, thetime constant of differentialcircuit is practical ly zero
and the rate-of-rise of the
operating voltage will
therefore exceed the rate-of-rise of the restraint voltage.
Finally, the inductive nature ofthe magnetising circuit causesa certain phase displacementbetween the operating and
restraint voltages.
_..J-_Ji
,-,.
The speed of operation ofthe differential relay isabout 1-3 ms for fault cur-rents of more than 2 times the
rating of the largest line C. T.
To ensure relay operation evenwhen the line C. T.'s saturate
quickly, the dR-relay together
with the SR-relay are arrangedto energise an impulse
storing (capacitor plusresistor) unit and a self-
sealing high speed, 3 ms,
auxiliary relay (RXMS 1).
Id2 IRIN
UT3~
SdR
102-'-0,~
Characteristicsof RADSSThe operating and restraint
characteristics of the RADSScan easily be determined
by using the test circuitin Fig. 4 a. The injection
test volt~e UT3 and the exter-
nal test circuit impedanceZL are varied to give the
required [-dl and [T3 cur-
rent distribution values.
IT3 JR2
Fig. 4 a. Test circuit used to determine the restraint and operatingareas on external ancj internai faults.
UT3 = Test supply voltage; ZL = Test circuit impedance.
Differential currentld,(A) Operating area5 x 3 + 10= u.tS II
0.13
The test circuit impedance
ZL is varied to represent a
pure resistance in the case ofan external fault, and to
represent the total magnet-ising impedance of all Jdlmln = = 0.6$ Athe idle line C.T.'s in the 1-0.8 i
caseofaninternal fault. When Thestability line in Fig. ~bcan
ZL is infinite, i.e. open.. be made to correspond tb a
circuited. all the incoming test circuit where:Ic~rrent ~ust .pas~ thro.u~h the S I
differential clrcurt. This IS ZL = RLx = -'~represented by the straight line 1 S
Id! = IT3. It should be noted -.Q:L100 = 400 nthat the area above this line 1-0.8 08
is of no significance, because or ZL = RLX = ~ 2
I dl can never exceed IT3"
--RdT
=
0.5
= 1000 il
The small narrow area be-
tween the stability line andthe operating line il) Fig. 4 bis a dispersion area caused bythe operating VA require-ment of the dR-relay. The relaycurrentJRI is here largerthan
zero, but less than the steady-state pick-up value. Complete
stabil ity, without any relay
operation, for all types of
system transient conditionscan not, therefore, be guaran-
teed in this narrow area.
'TaTotal incoming currentIn(A)
Fig. 4 b. Restraint characteristic upon external fault; Relay slopeS = 0.8ZL = RLX = linear resistance.Id' = S In = stability lineId' = S In + K = operating line
Differential current I d,(A)
1.0'
Idlmin.
0.5.
0.5 1.0 1.5 'T3ATotal incoming current IT3(A)
Fig. 4 c. Operating characteristic upon internai fault; Retaystope S = 0.8.ZL = ZLM = magnetising impedance.
6
The characteristics sho n inFig. 4 b and 4 c apply t arelay with the following pical
setting: iRd3 = 3 fl; Rs/2 = 10 ;nd = 5whichgivesthec ns-
tantK=0.13andtheslo e:
Rss- --nd Rd3 + Rs/2 -
2 x 10 ~ ~
and a minimum operatirgcurrent:
simplybyvarying thevatueofthe total differential cir-cuit resistance (RdT) between100-250,0.. This is mosteasilycarried out by varying IRdll between 0-150 n.lltshduld be noted that varyingR"'I does not affect theslope (5) nor the minimumoperating current (/dlmi~).
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Auxiliary C. T .'sratios
Auxiliary C.T.'s are requiredwhell the line C. T .'s have a se-
condary rating of 5 A., andalsowhen the line C. T.'s havedifferent turns ratios. The
over,all line and auxiliaryC.T. ratio no. is given by:
Max. bus transfer current
I Tln =4000 A, and that
for the line LA:
nA = 1000/5 A, andnMA = 5/1 A.
Check features
The principles and methods of
applying additional relaysfor increasing the reliabi-lity of bus differential
schemes vary appreciablyfrom one country to an-other .The inadvertent open ing
of a C.T. secondary cir-
cl,iit has been of particular
concern, because this maylead to maloperation of
a bus protection during normalservice conditions.
Some power companies
permit tripping of the buszone protection if a C. T. secon-dary is open-circuited,whereas other companies
require an alarm only, withouttripping. The method which
is adopted depends oftenon past experience. re-
Iiability of C.T. secon-dary wiring and whethertripping of the protection canbe accepted from the
system stability pointofview.
10005
.e.
no =no = nA nMA = nXnMX =
Ixl-'-Ix2
Ix!
Ix3x =
IA3 lx3
The total incoming, nominal
prim ary current to the busmay be denoted.
Also, it may be assumed
thatforthis station the oper-ating characteristic of Fig.4 c will apply. The primary
fault setting (IFS) the n
becomes:IFS = noxldlmin =1000 x 0.65 = 650 A.
The percentage primary faul
setting, based on the
maximum bus transfercurrent is therefore:
6404000. x 100 = 16 per cent.
This is an acceptable sensi-tivity even in comparison with
a generator or transformerdifferential protection.
=JTln = JAln + JBln + ...
Max. bus transfer current
This normally corresponds tothe rated current carrying
capacity of the busconductors.
In order to prevent u nnecess-
ary heating of the compar-stor circuit resistors, themaximum circulating relay IfseparatelineC.T. coresare
current during normal fullload available for the bus pro-conditions should be limited to tection, it is recommended4 A. It should be checked that the auxiliary C.T.'s be ir
therefore, that: stalled near the line C. T.'s. InlT1D thiswaythe auxiliary C. T. pri-
-mary winding can be re-
-" : -.garded as being securely
connected to the line C.T.The .auxiliary C.T.'s may be secondary winding. If an
omitted altogether if all open-circuit should occurthe line C.T,'s have the same within the wiring up to the
turns-ratio, if their second- differential protection theary rating is 1 A and the auxiliary C.T. becomes
maximum permissible damaged but itwill protect thEcirculating relay current line C.T. from being open-
is not exceeded. circuited.
IT3n=no
does not exceed 4 A
There is al~o one relativelyweil C. T. open-circultkn~wn philosophy, which alarm
clalms that the very impor-tant bus zone protection A static alarm relay (typeschemes should ~ot be RXIG 2) is included in the
allowedtotripbytheclosing differential circuit, set toof one relay contact only. operate at about25 mA. The
Two separately actuated actual sensitivity of thisrelays, with their contacts alarm relay will depeAd
connected in series, arethen on the total magnetis~
required to operate si- ingcurrentofallthelineandmultaneously in order to auxiliary C.T.'s. A primary
achieve tripping. setting of about 5 per cent
of the largest lineC. T.ratingFor the purpose of satis- is normally obtained..
fying most requirements
a simple overcurrent start-
ing relay SR has therefore
been included as a standard
check feature in the RADS Sdesign. This relay is of the
same high speed (I ms) dry-
Si nce maloperation .of a bus
differential protection maylead to a complete system
shut-down. the alarm relay is
normal ly arranged to discon-
nect the main tripping relayafter a time delay of about
5 secorids.
As an example, consider thatfor a particular bus in-
stallation:
reed type as the com~rator
circuit output relay dR. The
SR-relay has a fixed (non-
variable) setting, normallyarranged to coincide with
the largest line C.T. primarycurrent rating. When a sensi-
tive bus protection is parti-
cularly requested, theistartingrelay setting will be re-
duced to coincide with
the dR-relay setting. If con-sidered unnecessary, the
starting relay contact maybe short-circuited. I
When V.T.'s (voltage trans-
formers) are installed on the
bus, within the bus zone pro-tection, the high-speed 1-8
ms, three-phase undervoltagerelay RXOTB 2 can be used
as a check feature, i.e. its con-
tact may be connected in ser-ies with the differential
relay contact so that bothrelays must operate to give
tripping.
Full scale heavy 1\current testing
The basic formulae for stabil-
ity and operatiorl of the
RADSS protection have beenthoroughly tested under the
most severe fault conditions.Such tests have been carried
out both at the Central
Development Department ofASEA and at the independ-
ent, heavy current testing
laboratories of KEMA, Ar~-
hem, Holland. i
The ability of the protectionto operate prior to satu ration
i.e. on the initial rise of theinternai fault current, was
checked by using fault cur-rents equal to 375 times the
the rating of the incoming
feeder. The line and auxiliary
C.T.'s were premagnetised inthe worst direction. In these
tests the incoming line C.T.saturated in less than 2 ms.
The starting and differential
relays, however, operated inlessthan 1 ms, causing animmediate seal-in action bymeans of an impulsestoring capacitor unit and a
self-sealing auxiliary relayRRMS 1 (the same as
RXMS 1).
Altogether. some hundred
heavy current test shotshave been carried out rep re-
senting both internai andexternal faults of varying
magnitudes.ln none ofthesetests did the protection
maloperate. or refuse tooperate when required. Thestability of the protection,u nder the most u nfavourable
through fault conditions,was checked with faultcurrents equal to 600 times
the rating of the fau Ity feeder
circuit.
Line C.T.req u irementsThelineC.T.'s neednotbe
matched nor must theybe of the same type, forexample wound or bar type.
In the majority of installa-tions it is found that standard. C '"lIne .T.'s have a knee-
point (saturation) voltageand a secondary windingresistance Iying weil within
the requirements of the
protection.In the ca se of internaI busfaults it is normally required
that a differential relay
current of about 2 Amp. shallbe produced before the lineC.T.'s start to saturate. It isthen assumed that thestarting relay is set to operate
at a differential current of
about 1 Amp.
The total resistance (RdT)of the differential circuitmay be varied between 50-
250 il. The required relay
input voltage (UT3) maytherefore vary between
100- 500 V depending on
the stabil ity requirement of
the installation. In the ca se of
line C.T.'s with a seco!1daryrating of 1 Amp., their requir-
ed knee-point voltages are
of ten found to lie in the regionof 100-500 V. For 5 Amp. line
C. T.' s these requ irements are
normally reduced to 20-100 V.
The test circuitof Fig. 7wasused to check the stability ofthe protection during externalfaults. Varying the valueof the resistanceRx2' in the
secondary circuit of the line
current transformerT x. madeit possible to determine the
limiting condition for
tt should be noted that these
minimum knee-point voltage
requirements are re-
lated only to the operating
ability of the protection during
internai faults. The stability
of the protection duringexternal faults is not in
any way aftected by thesaturation levets of line
or auxiliary C.T.'s.
L
'A,TA
400/2A
CJ'RUTWA Twc Twx
2/1Ar=I 2/1A~ 2/1A~ 2/0.1A-lu...1 / /01..'!' --N ~ T MO
UOIII~~ R..nt U.. Ro. / ~SR TI,.. fL L
1J-O- -b l1>t'R3R./21
L
,, J.. R./211~u~
oK
---;;;--y---"-~ Osci Ilog rap h
Fig.5. Internai fault with max ITI = 24 kA r~ms (first peak 62 kAj.The oscillograph loops were arranged to record the current at relayterminal K and the current I KIl/RO
8
Briel report testNo. 6868 AA brief review of the main test
circuits and of two typical
oscillograms will be givenin the following:
The test circuit of Fig. 5 was
used to check the ope-
rating characteristic of
the protection during internaifaults. The primary currentITI is directed towards thebus, and at the same time thesecondary current I T3 enters
the relay at terminal K. The
outgoing relay currentILis relatively small andlimited by the value ofRlIl" The differential cur-
rent I dl is thereforelarge and causes a current
Id2 to pass through the relay
SR and acurrentIRI through
the relay dR.
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stability.ltwasfoundthatRx2could be increased byabout10 per cent above the cal-
culated value without anyrisk of maloperation.
Operating time of diffe-
rential relay dR.Id
RADSS three-phase
designsThe RADSS components aredesigned to suit the
COMBIFLEX modular
mounting and wiring system.A standard protection for
3 phases and 1 zoneis described in CatalogueRK 63-10 E.
Operating time of start-
ing relay SR-
ts
Totaloperating time(measured on auxiliary
self-sealing relayRRMS 1 (RXMS 1), see
Fig. 6).
represents the bIock-
ing current IR2 through
the diode O2 (blocks
operation of dR).
IT3' IL
This trace represents
IT3 when ITt flows inthe positive reference
direction. When ITt is
negative, the cur-
rent IL from the auxilia-
ry current transformer
T MX enters the relayat terminal K.
The measuring loops of a high-
speed"multi-channel os-cillograph were arrangedto provide traces of:
tT
JTl Primary fault current.
Th is always starts
in a positive referencedirection.
JRl/JR2
The positive trace ,ab ove
the zero line, rep-resents JR! through the
differential relay dR.The negative trace.below the zero line,
LA
lA"
TA
4O0/2A-
L.L=~-<&)1X,~ ""' tIr.-
Tx=40/2A -
'r-
It1..1
rR",
i:-TMA
2/1A~
T T'~~ MC2/1A~ 2/1A I='
TMx
2/0.1Ar=tN
lA' I 183
~In
Fig. 7. External fault with/T! = 24 kA r.m.s.
1S 1.6 MS I ~L-",, A---Å- --A-
Fig.9. Oscillogram No. 48, ITl = 24 kA r.m.s. External fault. Nooperation of differential relay (trace td) and the protection remainsful ly stable.
AppendixDerivation of comparator circuit operating and
restraint characteristicsMaximurn permissible loop reslstance R,.x
Referring to Fig. 2 d, the maximum value of R,.x required to
bring the protection to its stability limit, isdetermined by:The dR-relay just operates when (see Fig. 1):
[Eq. 1JUIi:1 -Us= JRI ZR = JR()ZR
where IRQ 'o; Operating current of dR-relay andZR = Impedance of dR-relay + diode 01. Idl = SIT3. and Il = (1-S) IT3
Also: U,l:! = {/'12 -/llu)RII3 = {"III,;. -/III,)RII:1
Rsand Us = IT3 Rs + IRo Rs -Idl 2
Also, neglecting the influence of Rs/2 and assuming UdT =
UL, gives:
5JT3 RdT = (1-5) JT3 RLx
5Hence RLX = T=s RdT [Eq.5)
Substituting these values in Equation 1 gives the minimum
operating line for the case when U da and U s are in-phase
i.e. corresponding to an external faultwhereZL in Fig. 4 ais purely resistive.
where, the total differential circuit resistance:
RdT = nd2Rd3 + ZMd + ZRXIG + Rdll [Eq.6]
Rs21'11 Rd:1 + Id! = J'I'3 Rs + JRO (RIt:, + Rs + ZII) and, where the differential circuit components are given by:
nd2Rd3 = Rdl = Resistance referred to T Md primary side.
ZMd = Primary short-circuit impedance of T MdZRXIG = Impedance of alarm relay RXIG 2
Rdll = Variable resistor max. range: 0-150 flR":I + Rs + ZH
Il" R,,:, + Rs/2
R"I", = 1"':1 R +"" R /2 + IRII1/" ":1 S
Hence, the minimum operating line becomes:
Jdl=SJT3+K [Eq. 2]
Typical settings:R<lT = 52x3 + 12 + 12 + R<l11 = 100 + R'III
i.e. R<lT is normally set within the range 100-250 .fl, byadjustlngRdl1 between 0-150 n, as required.
It is seen that S and K are constants, depending on the
comparator circuit setting and the operating sensitivity of
the output relay.
Full stability can be guaranteed when the output currentlRo = O. i.e. K = O, giving the stability line:
Id! =S/T3
The minimum operating current is obtained when all the in-
coming current goes through the differential circuit, i.e.
Idl = IT3 = Idlmin inserted in Equation 2 gives:
Idlmin = S Idlmin + K
KHence, Idlmin =[Eq.4]1-.\'
10
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Line C. T. knee-point voltage requirementThe line and auxiliary C. T .'s must have knee-point voltagescapable of driving the current 2 I A3n through the differentialcircuit, i.e. the relay input voltage should be equal toor exceed:
Calc:ulations for a typical station
Example 1. Assume that:/I A = Largest line C. T. ratio = 2000/5 A
flx = Smallest line C.T. ratio = 200/5 A
ITtn = Max. bus transfer current = 4000 A
Minimum primary fault current ~ 2000 AOn the basis of these data the following relay settings may
be selected:Overall C.T. ratio:
U T3K ~ 2/ A3n RdT [Eq. 7]where, / A3n = Normal full-load current
It is here assumed that the overall ratio for the largest feeder
is given by no = / Aln / / A3n' It is also assumed that the
normal rate d current IA3n = 1A and that the starting relay is
set to operate at about/dl =/A3n = 1 A.
The following values may be substituted in Equation 7.
The max. circulating relay current then becomesIT3n = 2 A,Selecting the slope, S = 0.8 gives:Primary fault setting, IFS =2000xO.65 = 1300 ASelecting Rdll = 150 n gives RdT = 250 il
RLx 1000andRx2 =(n .-
1000
~ (nMX)2Rx2s
which gives theknee-pointvoltage requirementforthe line
C.T. (TA):MXJZ 2500 = 0.4 il
= 40 fl (iA 1n2 l~rnAr tn~n R___\
-=-
RA2= 52 ,, '_.~-".'-""X2'
Hence, the total burden of the T xsecondarycircuit shouldbe lim ited to 0.4il. The secondary winding resistance of a200/5 A C.T. is normally about 0.2 iland by installing theauxiliary C.T. close to the line C.T. the permissibleresistance value will not be exceeded.Assuming that the starting relay will be set to operate at/ A3n =I.dl = 1 A. the relay inputvoltage should be equal to,
or larger than:UT3K = 2/A3n RdT = 2X250 = 500 V
hence, the line C. T. secondary voltages should be equal toor exceed:
r, UT3K 500 ~
-;;-:-::--}U,\2K = 2 1-5~ IA2g -s-(nMX)2 RX2
1
1
1-Ss- ~)2U,\:!K = 2 I A2n RX2 nMX2 ( nX [Eq. 8]
[Eq.9]
and for (T x):
liXU X:!I; = U ,\:!I; -;;-:-[Eq. 10]
These knee-pointvoltages are required only in thosecircuits
which can feed fault currents towards an internai bus
fault. In load circuits, Which are not called upon to
initiate tripping of the bus protection, the knee-point
voltages may be reduced to halfthe above values.
UT3K 500UX2K = -= --so- = 10 V
nMX
For the line C.T. (Tx) the knee"'point voltage requirement
of only 10 V is so small that even a measuring C.T. core(e.g.10 VA, accuracy lim it factor = 5) may be used.lf this
core can be made free from all other burdens, then the
auxiliary C.T.'s (T MX) can be installed close to the lineC.T.'s and the effects of the pilotwire resistance become
negligible.
1
Typical settingsTable 2 Typical settings of Rd3 and Rs/2
Ratio of T Md" nd = 5 (fixed)
Example 2. Assume that:nA = 2000/1 A, nx = 200/1 A
and with bus ratings as in above example.
Slope(.'i) 0.5 0.66 0.8 0.8 0.8
Rd3(n) 3 3 3 2 1.5
R,;/2(n) 5 7.5 10 6.66 5
A/so, with S = 0.8 gives:JFs = 1300 A and RdT = 250 il
which will give
K 0.1 0.11 0.13 0.16 0.2
Rs...(il) 2.3 2.5 2.6 1.74 1.3
Pn(W) 38 40 41 28 20
JCllmin(A) 0.2 0.33 0.65 0.8 1.0Compared with example 1 these permissible resistance
values are 52 larger. The voltage requirements are:
Nate:
Increasing the slope S and reducing the value of Rd3. increases theminimum operating current (ldlmln).
The effective resistance Rse refers to the resultant resistance of
Rd:1 inparallel with Rs, as seen by the circulating relay current(lT3) during normal service.
The calculated valuePn = (lT3n)2RSe isbased on the max. heatdeveloped withlT3n = 4 A.lf alarger heating effect is required the
physical size of the resistors must be increased and, probably, alsothe ventilation of the relay cubicle.
500=50VUX2K = 10
The knee-point voltage requirement of 500 V for a 2000/1 A
C. T. and the total secondary burden limitation of 10.fl for a200/1 A C.T. is in practice quite moderate and will easilybe satisfied in the majority of install~tions.
Reference publications:Test report 6868 A: Full scale heavy current testing
Test report AKU 40- 30 E: Aesult from tests carried out at
KEMA, Arnhem, Holland.Pamphlet AK 63-201E: Bus differential protection type
AADSS, Introduction.
Catalogue AK 63-1 O E: Bus differential protection type AADSS
standard design for 3 phases and 1 zone.
ASEARelay DivisionS-721 83 VÄSTERAs SWEDENTel. 021-100000
Pamphlel RK 63-200E Reg.7451 Printed in Sweden Västerås 9.1976 Tryckcentra AB 5000+30
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