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MBC
I
240
Type MBCI Relay: Translay ‘S’ Differential Feeder and Transformer Feeder Protection
Features
MBCI Pilot wire differentialprotection relay
• High stability for through faults
• High speed operation for in-zone faults
• Simultaneous tripping of relays at each line end
• Low current transformer requirements
• Low earth fault settings
• Designed for the unit protection ofoverhead and underground feeders
• Suitable for pilots up to 1kΩ or 2.5kΩ withpilot isolation transformers
• Four electrically separate contacts
• Can be used as definite time overcurrentrelay in the event of pilot failure
Models Available• MBCI 01
• MBCI 02
Optional Extras
MRTP Supervision relay for acpilot circuits
• Alarm and indication of pilot failureand supervision supply failure
• Suitable for pilot circuits insulated for5kV or 15kV with pilot isolationtransformers
MVTW Destabilising andintertripping relay
• For use with pilot wire relays
• Destabilises the feeder protection sothat tripping occurs
• Intertripping: injects ac voltage intothe pilot circuit so that tripping occurs
MCRI Instantaneousovercurrent and start/checkrelay
• High speed operation
• Not slowed by dc transients
• Wide setting range
• Two phase and earth fault relay
MCTH Transformer inrushcurrent detector
• Allows MBCI to be applied totransformer feeders
• Blocks operation of the MBCI relayduring transformer inrush conditions
• Blocking does not occur for zero,normal load, or genuine fault current
Application
The Translay S differential schemes havebeen designed for the unit protection ofoverhead and underground feeders andtransformer feeders.
Plain Feeders
Differential protection
Differential feeder protection requires acomparison of the currents entering andleaving the protected zone. For faultsoccurring within the protected feeder it isdesirable to trip the circuit breakers at eachend to isolate the fault. Two MBCI relays aretherefore required, one for each end of thefeeder. A pair of pilot wires is used totransmit information between the tworelays so that each may be able tocompare the current flowing at itsrespective end with the current at theother.
The relays at both line ends operatesimultaneously, providing rapid faultclearance irrespective of whether thefault current is fed from both line ends oronly one line end.
When applying this protection tooverhead lines the limiting factor isgenerally the length of the pilot circuits:for cable feeders the limiting factors aremore likely to be the level of linecharging current and the method ofsystem earthing.
MBC
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241
Pilot supervision
Correct interchange of information overthe pilot circuit is essential for the properfunctioning of any differential feederprotection. Pilots may be exposed tohazards and some risk of damage andfailure always exists. The most commonpilot failure is to the open circuit state,caused by the accidental excavation ofburied pilots or storm damage tooverhead pilots. With the pilots opencircuited the differential protection will beunstable and will trip the feeder ifsufficient through current is flowing.
For this reason the circulating currentsystem is often preferred as such schemeswill fail safe and trip so that attention isimmediately drawn to the fault.
The addition of pilot supervision will notprevent tripping for pilot faults but willindicate the cause. It will also detectshort circuit and cross-connected pilotconditions which would not otherwise bedetected. Indication is also provided forloss of the supervision supply.
Destabilising/intertripping
When the protected line is connected toa busbar system, a fault on the busbarswill in general be cleared by the busbarprotection by opening some or all of thelocal circuit breakers. Although suchfaults will usually appear to the feederprotection as through faults, withresultant stability of the feederprotection, it may be desirable to openthe remote line circuit breaker also, toclear the line completely. The remote unitof the differential feeder protection canbe caused to operate, provided sufficientline current is flowing, by open circuitingthe pilots. If line current is not flowing,the remote unit can be operated(intertripped) by injecting a current intothe pilots.
Overcurrent check/starting
Although the supervision scheme providesindication of pilot failure it does notprevent the protection operating if primarycurrent above setting is flowing. Wherethis hazard is unacceptable it is necessaryto add an overcurrent check feature.
The current transformer requirements arenot modified by the addition ofovercurrent elements since they present avery low burden.
When the starting feature is used theoverall operation time of the scheme isincreased by 3–5ms. However, there is noincrease in the overall operation time whenthe overcurrent protection performs a checkfunction only.
When overcurrent relays are used theprotection cannot be intertripped by acinjection into the pilots, and destabilisingthe protection will result in tripping only ifan overcurrent condition existssimultaneously.
Emergency use for overcurrentprotection
In the event of a pilot failure whichcannot quickly be rectified, the TranslayS scheme may be adapted for use as adefinite time overcurrent relay as follows:
If overcurrent relay is fitted
• disconnect pilot wires and leaveterminals open circuited
• set Kt to 3(300ms)
• check that overcurrent elements are onthe required setting above maximumanticipated load current.
If overcurrent relay is notfitted
• disconnect pilot wires and connect a1kΩ resistor across pilot terminals ofrelay
• set padding resistors Rpp to maximum(600Ω)
• set Kt to 3 (300ms)
• set Ks to required open circuit setting:Ks = 1 gives three phase equal torated load In.
Note: Other fault settings will dependupon the summation ratio.
A
B
C
Rs
Ts
Tt
RVD
Trip
øcT1
T2
To Pilot wires
Ro
Rpp
Tr
Ro
Rpp
To
Tr
RVD
øc
V
T2Trip
RsT1
ToTrTrRo
- Operating winding- Restraining winding- Tertiary winding- Linear resistor
Rppøc
- Pilots padding resistor- Phase comparator
T1T2RVDTs
- Summation transformer- Auxiliary transformer- Non linear resistor- Secondary winding
V
Figure 1:
Basic circuit arrangement
MBC
I
242
RL1–1
PilotWires
Trip/Alarm
Outputs
MBCI
IA
IB
ICIN
RS
SeeNote 5
SeeNote 2
VX–
+PowerSupplyCircuits
Enable
Enable
RL12
RL22
RL31
Protected Zone
RES
OP
LevelDetector
Squarer
Squarer
KS
&
&
IA
IB
IN
MBCI
PhaseRotation
RS
Protected Zone
SeeNote 4
ABC
RO
Case EarthSee Note 3
A
BC
Moduleterminal blockviewed fromrear
(a) C.T. shorting links make before (b) & (c) disconnect.
(b) Short terminals break before (c)(c) Long terminal.
RPP
Kt
3. Earthing connections are typical only.
4. C.T. connections are typical only.
5. For overcurrent start schemes, terminal 12 must be connected directly to D.C. +VE to provide a supply for the L.E.D. and reset circuits.
1.
2. Link terminals 11 and 13 except when used with overcurrent check replay type MCRI.
23
RL1–2
RL2–1
RL2–2
IC
Notes
19
18
1710
8
97
642
531
11
14
13
12
282726252423
P2 P1S2 S1
2425
2627
28
P1 P2S1 S2
1 23 45 67 89 1011 1213 1415 16
17 18
19 20
21 22
23 24
25 26
27 28
Case earth
RL4–11
3
5
2
Output Contacts Change state for pilot fail1 2
3 45 67 89 1011 1213 1415 1617 18
19 20
21 22
23 2425 2627 28
Case earth
Module terminal blockviewed from rear
Note 1. (a) C.T. shorting links make before (b) & (c) disconnect.(b) Short terminals break before (c).(c) Long terminal.
RL42
RL4–2 4
6
7
RL5–1 9
11
8
RL5–2 10
12
Output Contacts Change state for supply fail
RL52
RL71
RL61
RL81
SupplyFail
PilotS/C
PilotO/C
RL22
RL32
OPReset
OPReset
OPReset
tRL3–1
RL2–1
RL1–1
+VE
StartRL1–2
RL2–2
RL3–2
RL12
A.C.AuxiliarySupply
A.C.PowerSupply
D.C.PowerSupply
Case Earth
20
19
27
28
13
14Vx
18
17
17 1818
17
17 18
MBCI MBCIPilots
MRTP02Case Earth
19
20
MRTP01
Figure 3 Application diagram: pilot supervision relay and injection filter type MRTP 01
Figure 2 Application diagram: differential feeder protection relay type MBCI
MBC
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243
RL4–11
3
5Output Contacts Change state for pilot fail
See Note 7
Module terminal blockviewed from rear
Note 1.
(a) C.T. shorting links make before (b) & (c) disconnect.(b) Short terminals break before (c).(c) Long terminal.
RL42
RL4–2 4
6
7
RL5–1 9
11
8
RL5–2 10
12
Output Contacts Change state for supply fail
RL52
RL71
RL61
RL81
SupplyFail
PilotS/C
PilotO/C
RL22
RL32
OPReset
OPReset
OPReset
tRL3–1
RL2–1
RL1–1
+VE
StartRL1–2
RL2–2
RL3–2
RL12
A.C.AuxiliarySupply
A.C.PowerSupply
D.C.PowerSupply
Case Earth
20
19
27
28
13
14Vx
18
17
18
17
MBCI MBCIPilots
MRTP 03
S1
P1
S2
P2
P6P5P4P3P2
P1
S2
X2
X1S1
Supervisionisolation transformer
P6P5P4P3P2
P1
Pilot isolationtransformer
S1
X1
X2
S2
1 23 45 67 89 1011 1213 1415 1617 18
19 20
21 2223 24
25 26
27 28
Case earth
2
Pilot isolationtransformer
Figure 4 Application diagram: pilot supervision relay 15kV isolation type MRTP 03
Powersupplycircuits
PhaseRotationRs
P2P1S2
SeeNote 4
ABC
Case earthSee Note 3
A
BC
Module terminal blockviewed from rear
(a) C.T. shorting links make before (b) & (c) disconnect.(b) Short terminals break before (c).(c) Long terminals.
S1
1.
MCRI
MBCI
Case earthSee Note 3
D.C. +_
23
24
IS
RL12
14
25
26
( )
27
28
IS
IS
2.
3.4.
Connection for overcurrent startConnection for overcurrent checkEarthing connections are typical onlyC.T. connections are typical only
Notes
RL1–2624
RL1–1513
13232425262728
11
1314
Case earth1 2
3 4
5 6
7 8
9 10
11 12
13 14
15 16
17 18
19 20
21 22
23 24
25 26
27 28
(C)
(A)
(A)(B)
See Note 2(B)
See Note2(A)
Figure 5 Application diagram: overcurrent relay type MCRI 01
MBC
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244
Transformer Feeders (useof transformer inrushcurrent detector)
In the case of transformer feeders wherethere is no circuit breaker separating thetransformer from the feeder, thephenomenon of transformer magnetisinginrush must be considered. This is atransient condition which may occur atthe instant of transformer energisation, orimmediately following a system voltagedrop due to a nearby heavy faultcondition. Magnetising inrush current isnot a fault condition and therefore doesnot necessitate the operation ofprotection equipment which, on thecontrary, must remain stable during theinrush transient. The inclusion of a typeMCTH relay, designed to provide ablocking signal in the presence oftransformer inrush currents, enables apilot wire differential protection schemeto be applied to a transformer feeder.Where line and therefore transformerenergisation can occur at one end onlyof the transformer feeder, then a MCTHunit would be required on that side only.
When the feeder transformer isenergised any resulting inrush currentwill be detected by the MCTH relay, theoutput blocking unit of which will pick-upcausing the pilot wires of the Translay Sto be short-circuited. This will stabilisethe differential relay and prevent it fromresponding to what would otherwiseappear to be an in-zone fault. Theimmunity to operation due to inrushcurrent is coupled with fast faultclearance times and the built-inovercurrent detectors of the MCTH relayensure that the blocking feature isoverridden if a fault is detected in onephase whilst inrush is present in another.
Symbols:
15kV Isolating transformer
15kV Isolating transformerwith injection filter
Scheme Pilot Supervision O/CInsulation Start/Check Arrangement of EquipmentLevel (kV) (Viewed from front)
A 5kV — —
B 15kV — —
C 5kV • —
D 15kV • —
E 5kV — •
F 15kV — •
G 5kV • •
H 15kV • •
Table 1. Typical scheme arragements for plain feeders. See key below.
Scheme Pilot Supervision TransformerInsulation Arrangement Arrangement of EquipmentLevel (kV) (Viewed from front)
I 5kV —
J 15kV —
K 5kV •
L 15kV •
M 5kV —
N 15kV —
O 5kV •
P 15kV •
Table 2. Typical scheme arragements for transformer feeders. See key below.
No. Type of relay
1 MBCI 01/02 Differential2 MRTP 01 Pilot supervision and injection filter3 MRTP 02 Injection filter4 MRTP 03 Pilot supervision5 MCRI 01 Overcurrent start/check6 MVTW 01 Destabilising7 MVTW 03 Destabilising and Intertripping
Schemes A to D can be fitted with relay types 6 or 7.Schemes E to H can be fitted with type 6 which will providedestabilising if the overcurrent start/check relays (MCRI 01) haveoperated. Schemes I to L must use type 7 or 8.
8 MCTH 01 Transformer inrush current detector9 MFAC 14 High impedance earth fault relay10 MMLG Test plug/block
It is advisable on all schemes to include the test unit to facilitatecommissioning and routine testing. The unit will be situated on theright–hand side of the scheme.
1 1
1 1
2 1 3 1
4 1
1 5 1 5
1 5 1 5
4 1 5 1 5
1
2 1 5 3 51
1 8 9 7 1 8
1 8 9 7 1 8
1 8 2 9 7 1 8 3
1 8 4 9 7 1 8
1 8 1 8
1 8 1 8
1 8 2 1 8 3
1 8 4 1 8
Pilots
MBC
I
245
Description
Differential protection
The differential feeder protection circuitis derived from the well known Merz-Price circulating current system butemploys phase comparators as themeasuring elements. This novelcombination provides high stabilityperformance for external faults with theminimum of bias (restraint) quantitythereby ensuring that the low earth faultsettings are effectively retained evenwhen load current is flowing. Figure 1shows the basic circuit arrangement. Asummation current transformer T1 ateach line end produces a single phasecurrent proportional to the summatedthree phase currents in the protectedline. The neutral section of thesummation winding is tapped to providealternative sensitivities for earth faults.
The secondary winding supplies currentto the relay and the pilot circuit inparallel with a non-linear resistor (RVD).The non-linear resistor can beconsidered to be nonconducting at loadcurrent levels. Under heavy faultconditions it conducts an increasingcurrent and thereby limits the maximumsecondary voltage. At normal currentlevels the secondary current flowsthrough the operating winding To ontransformer T2 and then divides into twoseparate paths, one through Ro and theother through the restraint winding Tr ofT2, the pilot circuit and resistor Ro of theremote relay.
The resultant of the currents flowing in Trand To is delivered by the third windingon T2 to the phase comparator and iscompared with the voltage across Tt ofTransformer T1. The emf developedacross Tt is in phase with that across thesecondary winding Ts which is in turnsubstantially the voltage across Ro.
Taking into account the relative values ofwinding ratios and circuit resistancevalues, it can be shown that thequantities delivered for comparison inphase are:
(IX + 2IY) and (2IX + IY)
where IX and IY are the currents fed intothe line at each end (for through faultsIX=IY). The expressions are of oppositesign for values of IY which are negativerelative to IX and are between 0.5IX and2IX in value. The system is stable with thisrelative polarity and operates for allvalues of IY outside the limits.
The phase comparator has angular limitsof ± 90° giving a circular biascharacteristic in the complex plane. If thepilots are open circuited, current input willtend to operate the relay. Conversely,short-circuited pilots will cause the relay torestrain, holding its contacts open.
Transformers T1 and T2 also provide thenecessary insulation barriers for staticcircuitry. The input circuits of the phasecomparator are tuned to the powerfrequency so that the threshold ofoperation increases with frequency. Thisde-sensitises the relay to the transienthigh frequency charging current thatflows into the line when it is energised.A further advantage provided by thetuned input is that the waveform of thederived signal, which may be severelydistorted by current transformersaturation, is improved, ensuring highspeed operation under adverseconditions. In order to maintain the biascharacteristic at the designed value it isnecessary to pad the pilot loopresistance to 1kΩ.
A padding resistor Rpp is provided inthe relay for this purpose.
Pilot isolation transformers
When pilot isolation transformers areused, the range of primary taps enablespilots of loop resistance up to 2.5kΩ tobe matched to the relay. The pilotinsulation level is also raised to 15kV bythese transformers.
Telephone type pilots
When the pilots to be used are of thetelephone type, an alternative limiterbased on a zener diode is available toensure that the maximum voltage whichcan appear on the pilot system is withinprescribed limits. Pilot isolatingtransformers can be used in thisarrangement also, both to provideinsulation to 15kV and also indirectly toenable pilots of relatively high resistanceto be used.
Pilot supervision
Figure 3 shows the arrangement for pilotsupervision in a pilot circuit insulated for5kV. In this instance the injection filtersand the supervision unit are assembledwith the relay case. (Types MRTP01/02).
Figure 4 shows the similar arrangementfor pilot circuits insulated for 15kV (TypeMRTP 03).
The injection filters are then assembledas part of the isolation transformer andhave to be isolated from the supervisionrelay. The supervision isolationtransformer provides the necessary 15kVisolation barrier. For further technicalinformation see Publication R6026.
Destabilise and IntertripFacilities
MVTW01
Refer to Figure 6.
Operation of the destabilising relay (UN)results in the summation currenttransformer in the differential relaybeing short circuited and the local relayprevented from tripping. The remoterelay then sees a single end feedcondition and trips, provided the throughcurrent exceeds the no-load fault settingof the protection (see Table 5 page 18).Typical operating times are shown inFigure 7.
Terminals 17 and 20 should normally belinked together on the destabilising relay.However, the operating level of theremote equipment can be reduced toone half of the normal fault setting(under destabilising operation only) ifthis link is omitted.
It should be noted that, with this linkomitted, if the destabilising relay UN isoperated for longer than the supervisiontime delay (6-10 seconds) an indicationof pilot failure will be given. This doesnot apply if pilot isolating transformersare used.
When overcurrent elements are used toprovide a starting or check function thereis no advantage in removing this linksince, for operation, the through currentmust exceed the overcurrent setting.
MVTW 03
A circuit diagram for the MVTW 03 typerelay which depicts the destabilising,intertrip and inverter function is shown inFigure 8. On energising the relay, agreen LED illuminates and the normallyclosed contacts of RL1 open to indicatethe supply is healthy and the inverter isoperating.
MBC
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246
The MVTW 03 incorporates a full bridgeinverter, which receives complementarysquare wave signals from the oscillatorcircuit at a frequency of 80Hz. Thisfrequency was chosen because it liessufficiently far from the pilot frequency of50 or 60Hz and cancellation of theintertripping signal will not be caused bythe beat frequency that may be produced.
The inverter is continually energised andsupplies a transformer which isolates thepilots to 5kV from the input circuits. Thetransformer supplies the intertrippingcurrent and the power supply for theoutput relay (RL2).
Intertripping is initiated by applying atrip signal to terminal 11 whichenergises the output relay.
The signal is isolated from the pilots to5kV by the opto isolator. When theoutput relay (RL2) is operated, the localMBCI is made stable by shortingterminal 18 to terminal 19. This actiondestabilises the remote MBCI. If the loadcurrent level is greater than thedifferential current setting the remoteMBCI trips; however if the current level islower than the setting the remote MBCIdoes not trip. To ensure intertrippingoccurs the output relay (RL2) injects a20mA intertrip current into the pilots; theremote MBCI sees the intertrip current asa differential current which causes it totrip.
For further technical information seePublication R6027.
Transformer Inrush CurrentBlocking
Transformer inrush currentdetector feature
Refer to Figure 9.
The principle of operation of thetransformer inrush current detector(MCTH) is based upon a unique featureof substantially zero for significantperiods in each cycle. During load orfault conditions, the current waveformremains at zero for negligible periods ineach cycle. The relay detects these zeroperiods in the inrush waveform andinitiates a blocking relay, which causesthe pilot wires of the Translay S relay tobe short-circuited, preventing tripping fortransformer inrush current.
A typical scheme for a delta–star powertransformer is shown in Figure 9. Theline current transformers are connectedin star on the delta side of thetransformer. Appropriate choice of CTratios ensures that for normal load andthrough fault conditions, equal currentsflow into the differential tripping units(MBCI) at each end.
A high impedance differential relay(type MFAC 14) is included in theneutral lead of the star-connected linetransformers to give lower earth faultsettings on the delta side of the powertransformer. The MFAC 14 highimpedance differential relay may beused to initiate an intertrip unit (type MVTW 02).
1 2 3 4 5 6 7 8 9 10 20 30 40 50 800
40
80
120.
160
200
Current - Multiple of setting
Milli
seco
nds
Kt = 6
Kt = 14Kt = 20Kt = 40
Figure 7 Time characteristic for destabilised operation
1 23 45 67 89 1011 1213 1415 1617 18
19 2021 2223 2425 2627 28
Pilot isolation transformer
Pilots
P6 S2
P1 S1MBCI
17
18
19
Case earth
MVTW 01
UN-1
UN-2
UN-3
UN3
17
18
19
20
11
12
14
Vx(1)+Vx(2)+Vx(3)+ 13
Case earthSee note 2
Module terminal blockviewed from rear
(a)
(b)(c)
CT shorting links makebefore (b) & (c) disconnectShort terminals break before (c)Long terminals
2. Earthing connections are typical only
Notes1.
Figure 6 Application diagram: destabilising relay type MVTW 01
MBC
I
247
1 23 45 67 89 1011 1213 1415 1617 18
19 2021 2223 2425 2627 28
Module terminal blockviewed from rear
(a)
(b)(c)
CT shorting links makebefore (b) & (c) disconnectShort terminals break before (c)Long terminals
Earthing connections as shown is typical only1. 2.
Case earth
Pilot isolation transformer
Pilots
S2
P1MBCI
17
18
19
MVTW 03
17
RL11
Case earth See note 2
S1
P6
Trip send
Vx Supply fail12
RL1-1RL2-1
RL2-2
Supplyhealthy
Powersupply
RL22
181911+VE
13+VE
Notes
14–VE
Figure 8 Destabilising and intertripping relay type MVTW 03
A
B
C
S1 S2
P2I
II
III
i
ii
iiiyn
A
B
C
P2 P1
S2 S1
N.E.R.
MCTH MCTH
MBCI MBCI
Pilots
MFAC14
RVD3
232425262728
232425262728
232425262728
232425262728
17
18
17
1819
1819
17
1
3
27
28
17
19
17
19
Note1: It is essential that the current transformer connections are earthed at one point only.
11
MVT
W03
Figure 9 Typical application diagram: overall protection of transformer feeders
MBC
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248
Figure 13
Minimum earth fault current foroperation with through load
%In
Through Load (xIn)
100
80
60
40
20
Earth Fault SettingKs = 2.0
N = 3 N = 6
0 0.5 1.0 1.5 2.0
%In
Through Load (xIn)
50
40
30
20
10
Earth Fault SettingKs = 1.0
N = 3 N = 6
0 0.5 1.0 1.5 2.0
60
70
A-N
C-NB-N
C-NB-N
A-N
C-N
A-N
B-N
C-NB-N
A-N
Figure 12 Effect of pilot capacitance and pilot isolating transformers on setting
Facto
r by w
hich
settin
g is
incre
ased
2.0
Pilot intercore capacitance -mF
Without 15 kV pilot isolating trans.With 15 kV pilot isolating trans. (Ks = 1.0 or 2.0)With 15 kV pilot isolating trans. (Ks = 0.5)(Note: Pilot isolating transformer ratio = 1:1)
1.8
1.6
1.4
1.2
1.00 1 2 3 4 5
Pilot
Volta
ge (V
Peak
)
200
160
120
80
40
0 1 0 20 30
A-N fault current (x In)
MetrosilLimiter
ZenerLimiter
Figure 10: Current circuit burden
15
10
5
0
Volta
ge a
cross
summ
atio
n cu
rrent
trans
form
er p
rimar
y – vo
lts 1
A re
lay
Current in summation current transformer primary – Amps
N = 6 A–N
N = 6 C–N
N = 3 A–N
N = 3 B–N
A–C
A–B
Volta
ge a
cross
summ
atio
n cu
rrent
trans
form
er p
rimar
y – vo
lts
3
2
1
210
1A relay5A relay
15
Figure 11
Pilot voltage characteristics
MBC
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249
Figure 16
Response to spurious induced loop voltage in pilots
60
40
20
0 0.5 1.0 1.5 2.0
Setting multiplier – Ks
Induc
ed p
ilot lo
op vo
ltage
to o
pera
te
Figure 15 Time characteristics for internal faults
240
200
160
120
80
40
01 2 3 4 5 6 10 20 30 40 50 60 80 100
Kt = 6Kt = 14Kt = 20Kt = 40
Current (multiples of setting)
Ope
ratio
n tim
e (M
illise
cond
s)
Figure 14 Mesh type switchgear arrangements
CTs
CTs
RL RL RL
CTs
PilotsEnd ARelay
End BRelay
MFAC
MBCI
MCTHinrush
detector
23 25 27 17
24 26 28 19
REFA B C
232527
1.251
6242628
Feeder
MCTHinrush
detector
17 23 25 27
19 24 26 28
Delta StarN.E.R.
MBCI
232527
242628
1.251
6
17
18
17
18Pilots
Transformer (Dy 11)
A B C
Figure 17
Overall protection of transformer feedersshowing connections to MBCI relay
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250
Technical Data (MBCI Relay)
Current rating (IIn)
1A, 2A or 5A
Frequency rating
50Hz or 60Hz
Current withstand ratings
Duration (s) DifferentialContinuous 2In3 45In2 55In1 80In 0.5 100In Table 2
Current circuit burden
Highest phase burden(with three phase rated current)6VA N = 6 3.5VA N = 3 At setting current 0.5VA
Auxiliary supply
Rated Operative Current drain (mA)voltage (Vx) range (V) Quiescent Operated24/27 19.2–32.4 30 17.530/34 24–37.5 15 17548/54 37.6–72 15 175110/125 87.5–150 15 90
Contacts
Contact arrangements2 make and 2 change-over (See Figure 2)
Contact ratings
• Make and carry for 0.2s 7500VAsubject to maxima of 30A and 300Vac or dc
• Carry continuously5A ac or dc
• Breakac 1250VA dc 50W resistive
25W inductive L/R = 0.045s
subject to maxima of 5A and 300V
Durability
• Loaded contact10,000 operations minimum
• Unloaded contact100,000 operations minimum
Reset time
Less than 100ms
Indication
A non-volatile LED trip indicator is used.If the auxiliary supply is lost the LED willreturn to the same state when the supplyis restored.
Stability level
The stability of the protection for throughfaults is greater than 50In
High voltage withstand
• Dielectric withstand
IEC 60255-5: 19772.0kV rms for 1 minute between allterminals and case earth.
2.0kV rms for 1 minute between allterminals of independent circuits, withterminals on each independent circuitconnected together.
5.0kV rms for 1 minute between pilotterminals and all other terminals andcase earth.
1.0kV rms for 1 minute across normallyopen contacts.
• High voltage impulse
IEC 60255-5: 1977Three positive and three negativeimpulses of 5.0kV peak, 1.2/50µs, 0.5Jbetween all terminals and all terminalsand case earth.
Electrical environment
• DC supply interruption
IEC 60255-11: 1979The unit will withstand a 10msinterruption in the auxiliary supply,under normal operating conditions,without de-energising.
• AC ripple on dc supply
IEC 60255-11: 1979The unit will withstand 12% ac ripple onthe dc supply.
• High frequency disturbance
IEC 60255-22-1: 1988 Class III2.5kV peak between independentcircuits and case.
1.0kV peak across terminals of the samecircuit.
• Fast transient disturbance
IEC 60255-22-4: 1992 Class IV4.0kV, 2.5kHz applied directly toauxiliary supply.
IEC 60801-4: 1988 Level 4 4.0kV,2.5kHz applied to all inputs.
• Surge immunity
IEC 61000-4-5: 1995 Level 32.0kV peak, 1.2/50ms between allgroups and case earth.
2.0kV peak, 1.2/5-ms betweenterminals of each group.
• EMC compliance
89/336/EECCompliance to the EuropeanCommission Directive on EMC is claimedvia the Technical Construction File route.
EN 50082-2: 1994EN 50082-2: 1995Generic Standards were used to establish conformity.
• Product safety
73/23/EECCompliance with the EuropeanCommission Low Voltage Directive.
EN 61010-1: 1993/A2: 1995EN 60950: 1992/A11: 1997Compliance is demonstrated by reference to generic safety standards.
Atmospheric environment
• Temperature
IEC 60255-6: 1988Storage and transit –25°C to +70°COperating –25°C to +55°C
IEC 60068-2-1: 1990 Cold
IEC 60068-2-2: 1974 Dry heat
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• Humidity
IEC 60068-2-3: 196956 days at 93% RH and 40°C
• Enclosure protection
IEC 60529: 1989IP50 (dust protected)
• Mechanical environment
Vibration IEC 60255-21-1: 1988Response Class 1
Pilots
Pilots isolation
Pilot isolation transformers are requiredwhen any longitudinally induced voltagein the pilot circuit is likely to exceed 5kV:in effect this means when protectingfeeders operating at voltages in excessof 33kV, unless these are short in length.
The use of pilot isolation transformersalso extends the acceptable range ofpilots. This is achieved by the matchingratios available as shown in Table 4.
Pilots
Pilot voltage
The voltage applied across the pilotsvaries with fault current as shown inFigure 12. For normal through loadconditions the peak pilot voltage will bein the order of 50V rising to a maximumof: 200V for MBCI 01, 80V for MBCI 02under fault conditions.
When pilot isolation transformers areused this value of voltage is multiplied byKM .
Note: Types MBCI 01 and 02 are notcompatible. Relays should be ofthe same type at either end.
Pilot current
The pilot current is typically 30mA fornormal through load conditions andrises to a maximum of 300mA underthrough fault conditions.
Line charging current
In applications pertaining to cables, withor without in zone shunt reactors, andoverhead lines, it is necessary for themost sensitive fault setting to beincreased to:
• 1.1 times the steady state linecharging current for solidly earthedsystems
• 3.2 times the steady state linecharging current for resistance earthedsystems
• 1.9 times the steady state linecharging current for resistance earthedsystems with one relay per phase
In all cases, allowance should be madefor some system overvoltage. This requirement ensures stability duringexternal ground faults which will causethe three phase capacitance currents tobe unequal, resulting in an increasedoutput from the summation transformer.
The high frequency line in-rush currentscan be neglected because the setting ofthe relay automatically increases at highfrequencies by a factor:
Inrush frequency Rated system frequency
KM 0.8 1.0 1.2 1.5 2.5 Matching ratio
Loop resistance 800 1000 1200 1500 2500 Ω
Capacitance 6.25 5 4.2 3.3 2 µF
Terminals P1-P6 P1-P5 P1-P4 P1-P3 P1-P2
Where KM = (turns ratio)2 for respective tap of pilot isolation transformers.
When pilot isolation transformers are not used KM = 1.
The optimum value for KM is the nearest value Rp/1000 in Table 4, where Rp is themeasured pilot loop resistance.
There are two types of pilot isolation transformers: ZC0244-002 for schemes withoutpilot supervision: HN0068-001 for schemes with pilot supervision. The latter includesthe injection filter for the pilot supervision circuit.
The pilot padding resistor (Rpp) at each end should be set to:
1/2(1000 – Rp/KM)
Table 4
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Overcurrent starter/checkelement settings
The maximum resetting value of theovercurrent elements is not less than 90%of the operating value. Thus, to ensurethat they will reset when the current isrestored to the full load level, the settingshould be at least 1.2 times themaximum anticipated through loadcurrent.
The setting of the earth fault elementsshould be at least 1.2 times anystanding zero sequence current and nothigher than 80% of the minimum earthfault current.
The value chosen as an assessment ofminimum earth fault current must beconservative, due allowance being madefor all aspects of minimum systemoperating conditions and faultimpedance; alternatively, a largertolerance below the nominal minimumfault current value is advisable.
Unit Protection of PlainFeeders
Fault settings for plain feeders
The input transformer has a summationratio of 1.25:1:N where N = 3 fornormal use. N = 6 is used where lowearth fault settings are needed. Theminimum operating current will thereforebe dependent on the phase or phasesinvolved in the fault. The minimum earthfault current (If) should be greater thantwice the least sensitive earth fault settingto ensure rapid fault clearance.
The range setting of fault settings isshown in Table 5.
Minimum operation for earthfaults with through load
Bias being a direct function of throughcurrent, increases minimum operatingcurrent with through load. Figure 14shows the minimum earth fault currentrequired for various levels of throughload.
The curves shown are for the first relay totrip. The second relay will trip sequentiallyprovided the resulting in-feed at that endis above the setting value. To ensure thatsimultaneous tripping will occur theminimum fault current should be greaterthan twice the minimum operating currentgiven in Figure 14.
Line current transformerrequirements for plain feeders
Class X BS 3938
• VK ≥ 0.5N Kt In (RCT + XRL)
Where
VK = knee point voltage of currenttransformers for through faultstability
RCT = resistance of current transformersecondary circuit (Ω)
RL = lead resistance of single leadfrom relay to currenttransformers (Ω)
X = 1 for 4 wire connectionsbetween the main currenttransformers and the relay: 2for 6 wire connections
N = relative neutral turns onsummation transformer winding(3 or 6, as shown underheading Current Settings)
Kt = the selected time-dependentconstant (40, 20, 14, 6 or 3)
*Note; For all applications at or above220kV where X/R ratios arelarge use:
VK ≥ N Kt In (RCT + XRL)
Differential Fault Settings(Summation ratio = 1.25/1/N) N=3 N=6
Ks is a setting multiplier A – N 0.19Ks.In 0.12Ks.In and may be varied B – N 0.25Ks.In 0.14Ks.In from 0.5-2.0 C – N 0.33Ks.In 0.17Ks.InIn is the rated relay current A – B 0.8Ks.In
B – C 1.0Ks.In C – A 0.44Ks.In A – B – C 0.5Ks.In
Table 5
The minimum operating current of therelay will be increased by any shuntimpedance connected across the pilotwires, for example, pilot capacitanceand the magnetising path of the pilotisolation transformers. The effect of thepilot capacitance is shown in Figure 12.
Values of Ks from 0.5 to 1.0 areprovided to achieve effective faultsetting equal to the nominal valueindicated in Table 5. This is achieved bysingle end injection tests duringcommissioning. Values of Ks from 1.0 to2.0 are used to increase the settingwhen the application demands. Refer tosection: Line charging current.
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It is not necessary for the line currenttransformers at the two ends of theprotected system to have the same kneepoint voltage. Differences of up to 4:1can be tolerated provided both areabove the minimum value. However, thethree line current transformers at anyone end should have similar magnetisingcharacteristics.
• Magnetising current:
In order that the minimum effectiveoperating current of the relay remainslow, it is necessary to apply a limit to thevalue of the magnetising currentdemanded by the line currenttransformers:
IM ≤ 0.05 In at 10 VIn
• Mesh type switchgear arrangements
The relay may be fed by parallelconnected current transformers as shownin Figure 15. It is essential to balancethe lead resistance in the circulatingsecondary current path to ensurestability for a through busbar fault.Connecting the current transformers asshown in Figure 15 will result in therequired balance being obtained. It isessential that the current transformers atthe same end should have similarmagnetising characteristics. The value ofRCT to be used in calculations should bethe resistance of one current transformerplus the resistance of one lead betweenthe two parallel connected currenttransformers. The value of RL should bethe resistance of a single lead from thecommon connection of the currenttransformers to the relay.
• Methods of reducing the currenttransformer requirements:
In general the larger the currenttransformer the better the overallperformance. However, when currenttransformer size is critical the followingnotes should prove helpful.
• The operation time varies with faultcurrent as shown in Figure 16. Stabilityis maintained with smaller currenttransformers if the value of Kt is reduced.
This will of course result in the operationtime increasing, typical operation timesbeing:
Kt 40 20 14 6 3 Time at 5x setting 30 50 65 90 300 (ms)
• Kt= 20 will suit most currenttransformers on distribution systemsKt = 40 is the preferred setting for EHVsystems where high speed operation isrequired
• It should be noted that the knee pointrequirement increases with the nominalcurrent rating In. Advantage is thereforeobtained by using a low value of ratedcurrent eg. 1A or even 0.5A.
• Wires may be connected in parallel toreduce the lead resistant (RL) and hencethe current transformer requirements.
• If the relay is fed from delta connectedline current transformer then N = 1.
• If one relay is used per phase thenassume N = 1
Stabilising resistance
Rs = VK Ω but not greater than 12 Ω40In In2
Note: This resistor is not required forsingle phase protection or whenTranslay S is fed from deltaconnected current transformers.
Additional requirements:
• It is a stability requirement that therelays at both ends have the same valueof N & Kt selected.
• It is preferred, although not absolutelyessential, that the equipment at the twoline ends have the same rated current In.
Small in-zone teed loads
Small three phase loads may beconnected to the feeder within theprotected zone; normally these will besupplied by a delta/star transformerconnected to the line through HRC fuses.Substantial faults on this circuit will causethe fuses to blow very quickly before thedifferential relay can operate.
The limiting condition is a value of faultcurrent which will just produce an A–Cphase setting current in the relay: thiscurrent must cause the fuse to operatequickly enough to discriminate. Mostfusing time curves show pre-arcing timeand some allowance must be made for thearcing period.
To accommodate the largest teed load, usemay be made of the Ks (setting multiplier)adjustment, and/or selection of a value ofKt corresponding to a lower operatingspeed.
The particulars tabulated below for teedloads connected to a fused 11kV feedermay be helpful as a general example.
Feeder 11kV 300A rating
Tee Transformer Fuse Ks Ktrating(kVA) rating (A) 300 20 1 40400 25 1.7 40400 25 1.5 6500 30 1.75 3
Table 6
The table above refers to individual teedloads. When smaller loads are connectedat separate locations, on the basis thatonly one will be subject to a fault at anyinstant, the aggregate load may begreater. For example twelve 100kVAtransformers each protected by 10A fusescould be connected to the above line, withmain protection settings Ks = 1 and Kt =40. Similarly for ten 300kVA transformerseach protected by 20A fuses, relaysettings Ks = 2.0 and Kt = 20 would besuitable. In general the aggregate tee-offload should not exceed 0.25Ks x currenttransformer rating.
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Maximum induced pilot loopvoltage
Ideally the pilot cores should be wormed(twisted together) so that the inducedloop voltage is kept to a minimum. Therequired level of this voltage to causeoperation varies with the settingmultiplier Ks as shown in Figure 17.
Unit Protection ofTransformer feeders
Fault Setting
The relay internal summation is identicalto that used for plain feeders but theturns ratio used is 2.25:6. This isconnected as shown in Figure 18 andwill result in secondary settings as givenin the table below:
Relay setting in amps = Ks x In constantin table below A to N 0.44B to N ∝C to N 0.17A to B 0.44B to C 0.17C to A 0.123 Phase 0.14
Table 7
Where Ks = setting multiplier which may be
adjusted between 0.5 and 2.0
In = relay rated current.
NB. The Figures quoted in this table arethose to be expected underconditions of secondary injectiontesting
Note 1: As shown in Figure 18 there is arestricted earth fault relay in theneutral of the star connected CTs onthe delta side of the powertransformer. This provides protectionagainst earth faults on the delta sideof the power transformer when theinfeed is into the delta. It will providesettings lower than any of the phaseto neutral settings given above.
Note 2: The MBCI relay, when used in thetransformer feed application, doesnot require a stabilising resistor.
Line Current TransformerRequirements for TransformerFeeders:
Operating times less than 80ms will beachieved and through fault stabilityassured provided the following CTrequirements are satisfied (Kt = 14):
For star connected CTs
Vk ≥ 50 In (2.2 + RCT + RL)In2
For delta connected CTs
Vk > 50In (9.7 + RCT + RL)√3 In2
If longer operating times can be toleratedCT requirements to the following formulaewill give operating times less than 160msand assured through fault stability (Kt =14):
For star connected CTs
Vk ≥ 35 In (2.2 + RCT + RL)In2
For delta connected CTs
Vk > 35In (9.7 + RCT + RL)√3 In2
Where Vk = kneepoint voltage (V) In = rated current of relay (A)
RCT = resistance of CT secondarywinding (Ω)
RL = resistance of a single lead fromthe CTs to the relay (Ω)
Note 1: Operating times are quoted at 5xrated current.
Note 2: The above equations for through faultstability are applicable for up to 20%CT mismatch.
Note 3: In normal applications, to ensure thefast operation of the MFAC, the kneepoint voltage Vk must be greater thantwice the voltage setting Vs of theMFAC relay. However, when usedwith the MBCI/MCTH relaycombination, lower knee pointvoltage, down to Vs, may be usedprovided operating times up to thescheme operating time of 80ms areaccepted.
Vs = If ( 3 + RCT + RL)In2
Where
Vs = setting of MFAC (V)
If = maximum through fault currentreferred to CT secondary forwhich stability is required (A-rms)
In = rated currency of relay (A) RCT= resistance of secondary
winding (Ω)
RL = resistance of a single lead fromthe CTs to the relay (Ω)
The effective primary operating circuit(Iop) of the MFAC 14 is given by:
Iop = n(IR + NIIu)
Where
IR = relay operating current andmetrosil current at settingvoltage (see MFAC publication)
Iu = current transformer magnetisingcurrent at setting voltage (A)
Nl = number of connected currenttransformers
n = current transformer turns ratio
The following notes on this applicationare also important:
• A setting of 14 is recommended for Ktto ensure sufficient time for inrushblocking. Tripping for internal faults willthen occur (typically) within 60 – 80 ms.
• The N = 6 setting on the MBCI relaymust be used to achieve increasedsensitivity.
• Where the CT lead resistance is apredominant part of the CT burden atone, or both, line ends then the use of1A line CTs is recommended. Theselected rating of current transformersmust be the same as the relays (MBCIand MCTH) which they supply.
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• Additional conductors may beconnected in parallel in order to reducethe lead resistance (RL) and, hence, thecurrent transformer requirements.
• The pilot resistance should not exceed700Ω.
• With 15kV pilots, the MCTH outputcontacts should be connected on therelay side of the isolating transformer toterminate numbers 17 and 18 of theMBCI relay.
• The MCTH overcurrent settings foreach phase, set by 3 front-mountedpotentiometers (one per phase), shouldbe set at least 50% above the maximumpossible load current.
• The steady state magnetising currentmust not exceed the three phase faultsetting of the MCBI relay. For a Ks = 1setting, the three phase fault setting is14% of rated current. If the transformeris likely to be subjected to overfluxing,with the corresponding increase insteady state magnetising current, thenthe three phase setting must bepermanently set above this highermagnetising current by increasing Ks.
Cases
Relay type MBCI is provided in case size6 as shown in Figure 18.
Auxiliary Equipment
For outline drawings of pilot isolationtransformers, and stabilising resistor, seeFigures 20, 21, 22 and 23.
Associated Publications
R6001 Midos system
R6026 MRTP supervision for ac pilotcircuits
R6027 MVTW destabilising andintertripping relay
R6028 MCRI instantaneous overcurrentrelay
R6004 MMLG/B test block
R6006 MSTZ power supply
R6007 MFAC high impedancedifferential relay
R6066 MCTH transformer inrush currentdetector relay
Information Required withOrder
Basic scheme reference (refer to Table 1)
Type(s) of relay
Type of pilots (private or telephone)
Pilot loop resistance and intercorecapacitance values.(This information isrequired to determine whether pilotisolating transformers are required formatching purposes.)
Pilot insulation level (5kV or 15kV). Is pilotsupervision equipment required?
Is the overcurrent relay required?
Is the destabilising facility ordestabilising/intertrip facility required?
Pilot voltage: Metrosil (MBCI 01) or Zenerlimiting (MBCI 02).See Figure 11.
Current rating
Frequency rating
Auxiliary dc supply rating
Auxiliary ac supervision supply rating
AC intertrip supply rating
Push buttonprojection 10 max
All dimensions in mm
Panel cut-out:Flush mounting fixing details
4 holesø 4.4
Flush mounting
149
155
21232 25 min.
11
Reset
157 max.
151
159168
103.624
177
Figure 18
Case outline size 6
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116 mm
64.5 mm
9 mm
4 Fixing holes, M6 clearance
30 mm
171.5 mm
244 mm
190 mm
45 mm
8-M6 Terminals
176 mm
P1 2.5 1.51.2 1.0 0.9 S1 S2
X1 X2
InjectionsInputs
172 mm
340 mm
6 fixing holes, M6 clearance
64.5 mm
154 mm
9 mm116 mm176 mm
30 mm
Figure 20
Pilot isolation transformer without filter.With insulation for 15kV
Figure 19 Pilot isolation transformer with filter.With insulation for 15kV
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48 mm
48mm
2BA connectionscrews
354 mm
310 mm
342 mm
2 holes 6.5 mm
S2
S2
69 mm
98 mm
19 mm
2 off M5 studs
2 off fixing holes M5
134 mm
2 off M5 studs
19 mm
52 mm50 mm
69 mm
8.5 mm
52 mm
Figure 21 Pilot supervision isolation transformer.With insulation for 15kV
Figure 22 Stabilising resistor