Alstom MBCH User manual

Service Manual
Type MBCH
Biased Differential Relay

HANDLING OF ELECTRONIC EQUIPMENT
A person's normal movements can easily generate electrostatic potentials of several thousand volts.
Discharge of these voltages into semiconductor devices when handling electronic circuits can cause
serious damage, which often may not be immediately apparent but the reliability of the circuit will have
been reduced.
The electronic circuits of ALSTOM T&D Protection & Control Ltd products are immune to the relevant levels
of electrostatic discharge when housed in their cases. Do not expose them to the risk of damage by
withdrawing modules unnecessarily.
Each module incorporates the highest practicable protection for its semiconductor devices. However, if it
becomes necessary to withdraw a module, the following precautions should be taken to preserve the high
reliability and long life for which the equipment has been designed and manufactured.
1. Before removing a module, ensure that you are at the same electrostatic potential as the equipment
by touching the case.
2. Handle the module by its front-plate, frame, or edges of the printed circuit board.
Avoid touching the electronic components, printed circuit track or connectors.
3. Do not pass the module to any person without first ensuring that you are both at the same
electrostatic potential. Shaking hands achieves equipotential.
4. Place the module on an antistatic surface, or on a conducting surface which is at the same
potential as yourself.
5. Store or transport the module in a conductive bag.
More information on safe working procedures for all electronic equipment can be found in BS5783 and
IEC 60147-0F.
If you are making measurements on the internal electronic circuitry of an equipment in service, it is
preferable that you are earthed to the case with a conductive wrist strap.
Wrist straps should have a resistance to ground between 500k – 10M ohms. If a wrist strap is not
available, you should maintain regular contact with the case to prevent the build up of static.
Instrumentation which may be used for making measurements should be earthed to the case whenever
possible.
ALSTOM T&D Protection & Control Ltd strongly recommends that detailed investigations on the electronic
circuitry, or modification work, should be carried out in a Special Handling Area such as described in
BS5783 or IEC 60147-0F.
Service Manual
Type MBCH
Biased Differential Relay

4
CONTENTS
SAFETY SECTION 7
1 DESCRIPTION 11
2 INSTALLATION 11
2.1 General 11
2.2 Unpacking 12
2.3 Storage 12
2.4 Site 12
3 COMMISSIONING 12
3.1 Commissioning preliminaries 12
3.2 Commissioning tests 13
4 APPLICATION NOTES 17
4.1 General 17
4.2 Matched line current transformers 17
4.3 Ratio and phase matching interposing transformers 18
4.4 Application of matching transformer 19
5 SETTINGS 23
DIAGRAMS
Flowchart 1 24
Flowchart 2 25
Flowchart 3 26
Flowchart 4 27
Flowchart 5 28
Flowchart 6 29
Figure 1 Connections for checking relay settings 30
Figure 2 Connections for checking relay operating time 30
Figure 3 Connections for checking the bias curve 31
Figure 4 MBCH 12/13/16 bias curve 32
Figure 5 Connections to the relay to simulate magnetizing inrush current
waveform 33
Figure 6 Mesh busbar arrangement requiring three bias inputs to the
differential relay 34
Figure 7 Three winding transformer – one power source 35
Figure 8 Switchgear arrangement where six bias inputs may be required 36
Figure 9 Example of a 30 MVA transformer with current flow indicated 37
Figure 10 Disposition of windings on matching transformer 38
Figure 11 Two winding transformer with unmatched line current transformers 39

5
Figure 12 Three winding transformer showing interposing CTs 40
Figure 13 Block diagram: biased differential protection relay Type MBCH12
with two biased inputs 41
Figure 14 Block diagram: biased differential protection relay Type MBCH13
with three biased inputs 42
Figure 15 Block diagram: biased differential protection relay Type MBCH16
with six biased inputs 43
Figure 16 Connection for six change-over tripping contacts for three phase
tripping of up to six circuit breakers 44
6 COMMISSIONING TEST RECORD 45
REPAIR FORM 47

6

7
SAFETY SECTION
This Safety Section should be read before commencing any work on
the equipment.
Health and safety
The information in the Safety Section of the product documentation is intended to
ensure that products are properly installed and handled in order to maintain them in
a safe condition. It is assumed that everyone who will be associated with the
equipment will be familiar with the contents of the Safety Section.
Explanation of symbols and labels
The meaning of symbols and labels which may be used on the equipment or in the
product documentation, is given below.
Caution: refer to product documentation Caution: risk of electric shock
Protective/safety *earth terminal
Functional *earth terminal.
Note: this symbol may also be used for a protective/
safety earth terminal if that terminal is part of a
terminal block or sub-assembly eg. power supply.
*Note: The term earth used throughout the product documentation is the direct
equivalent of the North American term ground.
Installing, Commissioning and Servicing
Equipment connections
Personnel undertaking installation, commissioning or servicing work on this
equipment should be aware of the correct working procedures to ensure safety.
The product documentation should be consulted before installing, commissioning or
servicing the equipment.
Terminals exposed during installation, commissioning and maintenance may present
a hazardous voltage unless the equipment is electrically isolated.
If there is unlocked access to the rear of the equipment, care should be taken by all
personnel to avoid electric shock or energy hazards.
Voltage and current connections should be made using insulated crimp terminations
to ensure that terminal block insulation requirements are maintained for safety.
To ensure that wires are correctly terminated, the correct crimp terminal and tool for
the wire size should be used.

8
Before energising the equipment it must be earthed using the protective earth
terminal, or the appropriate termination of the supply plug in the case of plug
connected equipment. Omitting or disconnecting the equipment earth may cause a
safety hazard.
The recommended minimum earth wire size is 2.5 mm2, unless otherwise stated in
the technical data section of the product documentation.
Before energising the equipment, the following should be checked:
Voltage rating and polarity;
CT circuit rating and integrity of connections;
Protective fuse rating;
Integrity of earth connection (
where applicable )
Equipment operating conditions
The equipment should be operated within the specified electrical and environmental
limits.
Current transformer circuits
Do not open the secondary circuit of a live CT since the high voltage produced
may be lethal to personnel and could damage insulation.
External resistors
Where external resistors are fitted to relays, these may present a risk of electric shock
or burns, if touched.
Battery replacement
Where internal batteries are fitted they should be replaced with the recommended
type and be installed with the correct polarity, to avoid possible damage to the
equipment.
Insulation and dielectric strength testing
Insulation testing may leave capacitors charged up to a hazardous voltage. At the
end of each part of the test, the voltage should be gradually reduced to zero, to
discharge capacitors, before the test leads are disconnected.
Insertion of modules and pcb cards
These must not be inserted into or withdrawn from equipment whilst it is energised,
since this may result in damage.
Fibre optic communication
Where fibre optic communication devices are fitted, these should not be viewed
directly. Optical power meters should be used to determine the operation or signal
level of the device.

9
Older Products
Electrical adjustments
Equipments which require direct physical adjustments to their operating mechanism to
change current or voltage settings, should have the electrical power removed before
making the change, to avoid any risk of electric shock.
Mechanical adjustments
The electrical power to the relay contacts should be removed before checking any
mechanical settings, to avoid any risk of electric shock.
Draw out case relays
Removal of the cover on equipment incorporating electromechanical operating
elements, may expose hazardous live parts such as relay contacts.
Insertion and withdrawal of extender cards
When using an extender card, this should not be inserted or withdrawn from the
equipment whilst it is energised. This is to avoid possible shock or damage hazards.
Hazardous live voltages may be accessible on the extender card.
Insertion and withdrawal of heavy current test plugs
When using a heavy current test plug, CT shorting links must be in place before
insertion or removal, to avoid potentially lethal voltages.
Decommissioning and Disposal
Decommissioning: The auxiliary supply circuit in the relay may include capacitors
across the supply or to earth. To avoid electric shock or energy
hazards, after completely isolating the supplies to the relay
(both poles of any dc supply), the capacitors should be safely
discharged via the external terminals prior to decommissioning.
Disposal: It is recommended that incineration and disposal to water
courses is avoided. The product should be disposed of in a safe
manner. Any products containing batteries should have them
removed before disposal, taking precautions to avoid short
circuits. Particular regulations within the country of operation,
may apply to the disposal of lithium batteries.

10
Technical Specifications
Protective fuse rating
The recommended maximum rating of the external protective fuse for this equipment
is 16A, Red Spot type or equivalent, unless otherwise stated in the technical data
section of the product documentation.
Insulation class: IEC 61010-1: 1990/A2: 1995 This equipment requires a
Class I protective (safety) earth
EN 61010-1: 1993/A2: 1995 connection to ensure user
Class I safety.
Installation IEC 61010-1: 1990/A2: 1995 Distribution level, fixed
Category Category III installation. Equipment in
(Overvoltage): EN 61010-1: 1993/A2: 1995 this category is qualification
Category III tested at 5kV peak, 1.2/50µs,
500Ω, 0.5J, between all
supply circuits and earth and
also between independent
circuits.
Environment: IEC 61010-1: 1990/A2: 1995 Compliance is demonstrated by
Pollution degree 2 reference to generic safety
EN 61010-1: 1993/A2: 1995 standards.
Pollution degree 2
Product safety: 73/23/EEC Compliance with the European
Commission Low Voltage
Directive.
EN 61010-1: 1993/A2: 1995 Compliance is demonstrated
EN 60950: 1992/A11:1997 by reference to generic safety
standards.

11
Section 1. DESCRIPTION
The MBCH is a range of high-speed biased differential relays suitable for protection
of two or three winding power transformers, auto-transformers or generator
transformer units.
The MBCH may also be regarded as an alternative to the high impedance relays for
the protection of reactors, motors and generators.
The relay is extremely stable during through faults and provides high speed operation
on internal faults, even when energized via line current transformers of only moderate
output. Immunity to false tripping due to large inrush currents on energization of the
power transformer, and during overfluxing conditions, is guaranteed without the use
of harmonic filter circuits, therefore eliminating their associated delay.
A tapped interposing transformer for ratio matching of the line current transformers is
available where required. The transformer taps are spaced at intervals of 4% and
better, allowing matching to well within 2% in most cases.
The relay models available are as follows:
Type No of bias Publication
designation circuits Application ref no
MBCH12 2 Two winding power transformer R6070
MBCH13 3 Generally 3 winding power
transformer, where bias is required R6070
for each of the 3 groups of CTs
MBCH16 6 For all applications requiring R6070
4, 5 or 6 bias circuits
Section 2. INSTALLATION
2.1 General
Protective relays, although generally of robust construction, require careful treatment
prior to installation and a wise selection of site. By observing a few simple rules the
possibility of premature failure is eliminated and a high degree of performance can
be expected.
The relays are either despatched individually or as part of a panel/rack mounted
assembly in cartons specifically designed to protect them from damage.
Relays should be examined immediately they are received to ensure that no
damage has been sustained in transit. If damage due to rough handling is evident,
a claim should be made to the transport company concerned immediately, and
ALSTOM T&D Protection & Control Ltd should be promptly notified.
Relays which are supplied unmounted and not intended for immediate installation
should be returned to their protective polythene bags.

12
2.2 Unpacking
Care must be taken when unpacking and installing the relays so that none of the
parts are damaged or their settings altered, and relays must only be handled by
skilled persons.
Relays should be examined for any wedges, clamps, or rubber bands necessary to
secure moving parts to prevent damage during transit and these should be removed
after installation and before commissioning.
Relays which have been removed from their cases should not be left in situations
where they are exposed to dust or damp. This particularly applies to installations
which are being carried out at the same time as constructional work.
2.3 Storage
If relays are not installed immediately upon receipt they should be stored in a place
free from dust and moisture in their original cartons and where de-humidifier bags
have been included in the packing they should be retained. The action of the de-
humidifier crystals will be impaired if the bag has been exposed to ambient
conditions and may be restored by gently heating the bag for about an hour, prior to
replacing it in the carton.
Dust which collects on a carton may, on subsequent unpacking, find its way into the
relay; in damp conditions the carton and packing may become impregnated with
moisture and the de-humidifying agent will lose its efficiency.
The storage temperature range is –25°C to +70°C.
2.4 Site
The installation should be clean, dry and reasonably free from dust and excessive
vibration. The site should preferably be well illuminated to facilitate inspection.
An outline diagram is normally supplied showing panel cut-outs and hole centres.
For individually mounted relays these dimensions will also be found in Publication
R6017.
Publication R7012, Parts Catalogue and Assembly Instructions, will be useful when
individual relays are to be assembled as a composite rack or panel mounted
assembly.
Publication R6001 is a leaflet on the modular integrated drawout system of protective
relay.
Publication R6014 is a list of recommended suppliers for the pre-insulated
connectors.
Section 3. COMMISSIONING
3.1 Commissioning preliminaries
3.1.1 Electrostatic discharges
The relay uses components which could be affected by electrostatic discharges.
When handling the withdrawn module, care should be taken to avoid contact with
components and connections. When removed for the case for storage, the module
should be placed in an electrically conducting anti-static bag.

13
3.1.2 Inspection
Remove the polycarbonate front cover by undoing the two knurled plastic nuts with a
small screwdriver. The module can now be withdrawn by the handles provided.
Carefully examine the module and case to see that no damage has occurred during
transit. Check that the relay serial number on the module, case and cover are
identical and that the model number and rating information are correct.
3.1.3 Wiring
Check that the external wiring is correct to the relevant relay diagram and scheme
diagram. The relay diagram number appears inside the case. Note the shorting
switches shown on the relay diagram are fitted internally across the relevant case
terminals and close when the module is withdrawn. It is essential that such switches
are fitted across all CT circuits.
If a test block type MMLG is provided, the connections should be checked to the
scheme diagram, particularly that the supply connections are to the ‘live’ side of the
test block (coloured orange) and with terminals allocated with odd numbers (1, 3, 5,
7 etc). The auxiliary supply voltage to the scheme should be routed via test block
terminals 13 and 15.
3.1.4 Earthing
Ensure that the case earthing connection above the rear terminal block, is used to
connect the relay to a local earth bar.
3.1.5 Insulation
The relay and its associated wiring, may be insulation tested between:
– all electrically isolated circuits
– all circuits and earth
An electronic or brushless insulation tester should be used, having a dc voltage not
exceeding 1000V. Accessible terminals of the same circuit should first be strapped
together. Deliberate circuit earthing links, removed for the tests, subsequently must be
replaced.
3.1.6 WARNING
DO NOT OPEN THE SECONDARY CIRCUIT OF A CURRENT
TRANSFORMER SINCE THE HIGH VOLTAGE PRODUCED
MAY BE LETHAL AND COULD DAMAGE INSULATION.
When the type MMLG test block facilities are installed, it is important that the sockets
in the type MMLB01 test plug, which correspond to the CT secondary windings, are
LINKED BEFORE THE TEST PLUG IS INSERTED INTO THE TEST BLOCK. Similarly, a
MMLB02 single finger test plug must be terminated with an ammeter BEFORE IT IS
INSERTED to monitor CT secondary currents.
3.2 Commissioning tests
3.2.1 Test equipment
For relays with a rated current In = 1A, the variable auto-transformer and resistor
listed below can be used as an alternative to the overcurrent test set.
Overcurrent test set (with timing facilities or separate timer).
DC power supply (to suit relay auxiliary voltage Vx).

14
2 multimeters
Double pole switch
Single pole switch
MMLB01 test plug
MMLB02 single finger test plug
8A variable auto-transformer
2 Variable resistors 0 – 100 Ohms, suitably rated
Diode rated 7A for magnetising inrush test, if required.
Note: The following test instructions are based on injecting current directly into the
relay terminals, however if a MMLG test block is incorporated in the scheme,
then it is more convenient to inject current into the MMLG test block. Refer to
the relevant scheme diagram for connections.
3.2.2 DC auxiliary supply
Check the rated auxiliary voltage Vx on the front plate and connect a suitable
smoothed dc supply or station battery supply to relay terminals 13(+ve) and 14(–ve).
3.2.3 Relay settings
Connect the overcurrent test set to the relay as shown in Figure 1. Adjust the relay
front panel switches to give a relay setting Is = 0.1 x In (10% setting, In = relay
rated current).
Slowly increase the current until the relay operates, indicated by a light emitting
diode (led) on the front plate. Note the operate (differential) current and check that
this is within ±10% of the expected current (ie. 0.09 to 0.11A for a 1A relay, or
0.45 to 0.55A for a 5A relay, with a 10% relay setting).
Check that the relay trip contacts (terminals 1,3 and 2,4 ) are closed with the current
above the setting, and that these contacts open as the current is removed.
Check also that the relay alarm contacts (terminal 9,11) are closed with the current
above the setting and remain closed as the current is removed.
Press the reset button on the relay front plate and check that the LED indicator resets
and that the alarm contacts open.
Repeat the test with the relay adjusted to settings of 0.2 x In, 0.3 x In, 0.4 x In and
0.5 x In in turn. Check that the settings are within ±10% of the nominal value.
Notes: 1. The setting may also be checked using a variable auto-transformer,
0 – 100 Ohm resistor and ammeter, as an alternative to using an
overcurrent test set.
2. During commissioning do not disconnect the dc auxiliary supply
without first removing the ac operating current, otherwise the trip
contacts on terminals 1,3 and 2,4 may remain operated.
If this does occur the contacts may be reset by removing the ac
operating current, and then switching on the dc auxiliary supply at rated
voltage.
3. It is prudent to switch off the dc supply before inserting or removing
modules.
If MMLG Test Block
is supplied
}

15
3.2.4 Operating time
Connect the test circuit as shown in Figure 2. Set the relay to Is = 0.2 x In (20%
setting).
Inject 3.5 x In and record the relay operating time. For 50Hz, this should be within
the range 24ms ±5ms (60Hz relays, within the range 20ms ±4ms). To check
operation of the instantaneous circuit (high set), inject 4.5 x In and record the mean
relay operating time. For 50Hz relays, this should be less than 20ms (60Hz relays,
less than 17ms).
Note: For relays with a rated current (In) of 1A the operating time may be checked
using a variable auto-transformer and 0 – 100 Ohm (non inductive) resistor
(suitably rated), as an alternative to using the overcurrent test set.
3.2.5 Bias check
3.2.5.1 Connect the test circuit as shown in Figure 3. Ensure that both variable resistors are
non-inductive.
With the relay set to Is = 0.2 x In (20% setting), adjust resistor R1 to about 40 Ohms
(8 Ohms if In = 5A) and resistor R2 to about 100 Ohms (20 Ohms if In = 5A).
Switch on the supply and increase the applied voltage until ammeter A1 indicates
0.6 x In for MBCH 12, 13, 16. Slowly increase the differential current by decreasing
resistor R2 until the relay operates as indicated by the front plate LED. Record the
values of current A1 and A2.
Calculate the mean bias using the formula:
A2
2
Use the bias curve Figure 4 for MBCH 12, 13, 16 to determine the theoretical
differential current and check that the measured current A2 is within ±20% of this
theoretical value. Note that for a 5 amp relay (In = 5A) the values of the calculated
mean bias have to be divided by 5 before applying the bias curve and the
theoretical differential current multiplied by 5 before comparing with the measured
current A2.
3.2.5.2 MBCH 13 only
Repeat the above test with the third bias coil (terminal 21).
3.2.5.3 MBCH 16 only
Repeat the above test with the third to sixth bias coils (terminals 21, 19, 17 and 15
respectively).
3.2.5.4 Reconnect the 2nd bias coil as shown in Figure 3 and adjust the current shown on
ammeter A1 to be 1.7 x In for MBCH 12, 13, 16. (Note that for a 5A relay this
current may exceed the continuous rating of the variable auto transformer and should
therefore be switched on for short durations only).
Increase the differential current until the relay operates and check that this value is
within ±20% of the theoretical value by calculating the mean bias as described in
3.2.5.1 above.
3.2.5.5 Repeat tests 3.2.2, 3.2.3, 3.2.4, and 3.2.5 for the two relays associated with the
other phases.
Mean bias = A1 + amps

16
3.2.6 Bias interconnection
Check that the terminals no 12 on all three phase relays are interconnected using
screened leads, the screen connection being made to the dc negative supply
(terminal no 14).
A suitable screened lead should be provided with each relay. Only two will be
required for the interconnection.
3.2.7 Circuit breaker tripping
By interconnecting terminal no 10 of all three phase relays, up to six self-resetting
changeover contacts can be provided for the three phase tripping of up to six circuit
breakers.
If this is required, check terminals no 10 are connected together, and check that the
relay trip contacts (terminals 1,3 and 2,4) on all three phase relays close as the
current injected into a single phase relay (as shown in Figure 1) exceeds the relay
setting.
3.2.8 On load tests
The object of the on-load tests is to check that the relay is connected correctly to the
system.
If the relay is protecting a transformer with no tap changer then the differential
current could be less than 1% of the load current. However, if the transformer has a
tap changer and the CTs are not matched to the transformer, then the normal
differential current ,with the tap changer away from the nominal position, could be as
much as 20% of the load current.
Check that the load current in each bias coil is close to the value which is expected
for the particular application. For the MBCH 16 relay particularly, it may be
preferable to energize the transformer in different ways to ensure that all connections
are satisfactory. Check that the differential current under any of these conditions is
within 1–20% of the load current. The actual figure of differential current depends
upon the particular application as stated above.
Since the magnetizing current may exceed 5% of rated current for small transformers,
and bearing in mind the comments of the above paragraph, it is recommended that
the standard setting of the relay should be Is = 0.2 x In.
Check that the currents measured in the same bias or differential coils of each phase
relay are similar.
3.2.9 Magnetizing inrush test
The relay may be tested with a simulated waveform representing magnetizing inrush,
by connecting a diode in series with the relay to produce a half wave rectified
waveform.
With reference to Figure 5, close switches S1 and S2 and set the current to 1 x In
(rated current). Check that the relay operates.
Open switch S2, close switch S1 and check that the relay does not operate.
If it is preferred to test the relay with the magnetizing inrush current of the
transformer, it is suggested that the transformer is energized ten times at full rated
voltage on no load and checked that the relay does not maloperate.

17
Section 4 APPLICATION NOTES
4.1 General
The type MBCH relay is a high speed biased differential relay suitable for the
protection of two or three winding power transformers, autotransformers, or
generator transformer units.
Three versions of the relay are available as follows:
Designation Number of bias inputs Application
MBCH 12 2 Two winding transformers or three
winding transformers where
significant fault infeed can pass
through the transformer from one
winding only. See Figure 7.
MBCH 13 3 Generally three-winding transformers
where bias is required from each of
the three groups of CTs or;
Where a two-winding transformer has
one or other of the windings
controlled by two circuit breakers as
in mesh or one-and-a-half breaker
arrangements. See Figure 6.
MBCH 16 6 For applications requiring 4, 5, or
6 bias circuits. See Figure 8.
4.2 Matched line current transformers
For optimum performance, the differential scheme should be arranged so that the
relay will see rated current when the full load current flows in the protected circuit.
Where line current transformers are matched, but secondary current with full load
current flowing is less than the relay rated current (as illustrated in Figure 9), the
effective sensitivity of the relay will be reduced.
The transformer current is 262.4A at 66kV, giving a secondary current of 4.37A
from the 300/5A current transformer. For a 20% relay setting, the relay will operate
when the differential exceeds 0.2 x 5 = 1A.
1
4.37
Thus the effective setting is 22.9% for a relay setting of 20%.
1A = x 100% = 22.9% of transformer full load current.

18
4.3 Ratio and phase matching interposing transformers
Matching transformers are available for use in cases where the current transformers
on one side of the protected transformer do not match, in current ratio or phase
angle, with the current transformers on the other side of the protected transformer.
The following versions of matching transformer are available:
Description Reference No
Single phase transformer 1/1A GJ0104 010
Single phase transformer 5/5A GJ0104 020
Single phase transformer 5/1 GJ0104 030
Three phase transformer 1/1A GJ0104 050
Three phase transformer 5/5A GJ0104 060
Three phase transformer 5/1A GJ0104 070
4.3.1 Details of matching transformers
The winding details of the three current ratings of the matching transformers are given
in the table below and in Figure 10.
Number of turns
Transformer rating
Primary
tap terminals 1/1A 5/1A 5/5A
1 – 2 5 1 1
2 – 3 5 1 1
3 – 4 5 1 1
4 – 5 5 1 1
5 – 6 125 25 25
X 7 25 5 5
7 – 8 25 5 5
8 – 9 25 5 5
S1 – S2 125 125 25
S3 – S4 90 90 18
Table 1.
Notes on combinations of windings.
For star-output windings:
It is permissible to use either S1-S2 or S1-S4 (with S2-S3 linked). Where S1-S2 alone
is used, the secondary winding S3-S4 is available for formation of an isolated delta
connection to prevent zero sequence currents due to external earth faults being seen
by the relay. This is for applications where phase correction is not required, but
where a zero sequence trap is needed.
For delta output windings:
S1-S4 (with S1-S3 linked) must be used to obtain optimum protection performance.

19
4.4 Application of matching transformer
Where the line current transformer ratios on the two sides of the protected
transformer are mutually incompatible, the matching transformer may be used as in
the following examples:
4.4.1 Single phase transformer
Matching transformer ratio required = 3.9/4.875. Using secondary windings S1-S2
gives 25 turns. The number of turns required on the input (primary) winding is given
by:
31 turns are available between input winding terminals 4—7 with terminals 6—X
linked.
4.4.2 Three phase transformer with unmatched current transformers
See Figure 11.
30MVA transformer 11/66kV Delta star
11kV winding:
30 x 106
√3 x 11 x 103
Because the 11kV winding is delta connected, the associated current transformers
will be star connected and under rated load conditions will give the following current
per pilot phase:
1574.6 x 1A
1600
This current is sufficiently close to the relay rated current (1A) and furthermore
requires no phase correction.
66kV winding:
30 x 106
√3 x 66 x 103
Normally the current transformers associated with the star winding of the main
transformer should be connected in delta to provide appropriate phase shift
correction. However, since the latter in this case are connected in star the necessary
phase correction may be carried out by means of a star delta connected matching
transformer.
Tp = x Ts = 25 x = 31.25 = 31 turns
4.875A
3.9A
Normal current at 11kV = = 1574.6A
Is = = 0.984A
Normal current at 66kV = = 262.43A
200/5 A 1000/5 A
195 A 780 A
4.875 A 3.9 A
S1 4
S2 7
Is
Ip

20
The output current, per phase pilot, of the 300/1A current transformers is given by:
262.43 x 1
300
This should be adjusted by the interposing transformer so that 0.984A flows into the
relay.
The windings S1–S2, and S3–S4 must be used in series as output windings giving
(125 + 90) = 215 turns.
Primary turns (Tp) required, therefore are given by:
Is/√3
Ip
0.984 x 215
√3 x 0.875
say Tp = 140 turns
ie. connect each phase pilot from the 300/1A current transformers to primary
terminal nos. 2 and 6 (see Figure 11 and Table 1). Complete connections to the
interposing transformer as given below:
4.4.3 Three winding transformer
An example of the three winding transformer is shown in Figure 13. The voltage and
power rating of each winding is indicated. The current transformer ratios are chosen
as a function of the winding voltage and power rating of the particular windings with
which they are associated.
For transformer differential protection matching of current transformers is correct
when the CT ratios are determined on a basis of associated winding voltage only.
500kV Winding:
Based on 400 MVA the rated current is given:
400 x 106
√3 x 500 x 103
Secondary current from 500/5 current transformers:
462 x 5
500
The 500/5 star connected CTs are associated with the 500kV star winding, and thus
the transition to delta connected secondaries must be made by means of an
In = = 462A
Is = = 4.62A
Is = = 0.875A
∴Primary turns (Tp) = = 139.6
Tp = x Ts
A
B
C
To
Relay
To 66kV
line
current
transformer
P1
P1
P1
A
B
C
N
S1
S1
S1 S2
S2
S2
S3
S3
S3
S4
S4
S4
P2
P2
P2
6
6
6
2
2
2