Alstom MCAG User manual

MCAG/MFAC
User Guide
Application of High Impedance Relays
Publication Reference: R6136C
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GRID

2
The Application of
High Impedance Relays
Figure 1:
Principle of high impedance
protection
Introduction
The application of the MCAG14/
34, MFAC14/34 and MCTI14/34
relays to the protection of machines,
power transformers and busbar
installations is based on the high
impedance differential principle,
offering stabiliy for any type of fault
occurring outside the protected zone
and satisfactory operation for faults
within the zone
A high impedance relay is defined
as a relay or relay circuit whose
voltage setting is not less than the
calculated maximum voltage which
can appear across its terminals
under the assigned maximum
through fault current condition.
It can be seen from Figure 1, that
during an external fault the through
fault current should circulate
between the current transformer
secondaries. The only current that
can flow through the relay circuit is
that due to any difference in the
current transfomer outputs for the
same primary current. Magnetic
saturation will reduce the output of
a current transformer and the most
extreme case for stability will be if
one current transformer is
completely saturated and the other
unaffected. This condition can be
approached in busbar installations
due to the multiplicity of infeeds and
extremely high fault level. It is less
likely with machines or power
transformers due to the limitation of
through fault level by the protected
unit’s impedance, and the fact that
the comparison is made between a
limited number of current
transformers. Differences in current
transformer remanent flux can,
however, result in asymmetric
current transformer saturation with
all applications.
Calculations based on the above
extreme case for stability have
become accepted in lieu of
conjunctive scheme testing as being
a satisfactory basis for application.
At one end the current transformer
can be considered fully saturated
with its magnetising impedance
ZMB short circuited while the current
transformer at the other end, being
unaffected, delivers its full current
output which will then divide
between the relay and the saturated
current transformer. This division will
be in the inverse ratio of RRELAY CIRCUIT
and RCTB +2RLand obviously, if
RRELAY CIRCUIT is high compared with
RCTB + 2RL, the relay will be
prevented from undesirable
operation, as most of the current will
pass through the saturated current
transformer.
To achieve stability for external
faults, the stability voltage for the
protection (VS) must be determined
in accordance with formula 1 and
formula 2 for the MCAG/MFAC
and MCTI respectively. The setting
will be dependent upon the
maximum current transformer
secondary current for an external
fault (If) and also on the highest loop
resistance value between the current
transformer common point and any
of the current transformers
(RCT + 2RL).

3
I
op
= (CT ratio) x (
I
r
+ n
I
e
)
Ir<(Iop - nIe)
CT ratio
I
e<1x (
I
op -
I
r)
CT ration
protection, it is considered good
practice by some utilities to set the
minimum primary operating current
in excess of the rated load. Thus, if
one of the current transformers
becomes open circuit the high
impedance relay does not
maloperate. The MVTP11/31
(busbar supervision relay) should
give an alarm for open circuit
conditions but will not stop a
maloperation if the relay is set below
rated load.
In the case of the high impedance
relay, the operating current is
adjustable in discrete steps. The
primary operating current (Iop) will be
a function of the current transformer
ratio, the relay operating current (Ir),
the number of current transformers in
parallel with a relay element (n) and
the magnetising current of each
current transformer (Ie) at the stability
voltage (Vs). This relationship can be
expressed in three ways:
To determine the maximum current
transformer magnetising current to
achieve a specific primary operating
current with a particular relay
operating current.
Vs > If(RCT + 2RL) 1
Vs > 1.25If(RCT + 2RL) 2
where RCT = current transformer
secondary winding
resistance
RL= maximum lead
resistance from the
current transformer to
the common point
Note:
When high impedance differential
protection is applied to motors or
reactors, the external fault current
will be low. Therefore, the locked
rotor current or starting current of
the motor, or reactor inrush current,
should be used in place of the
external fault current.
To ensure satisfactory operation of
the relay under internal fault
conditions the current transformer
kneepoint voltage should not be less
than twice the relay voltage setting
i.e. VK≥2VSfor the MCAG/MFAC
and 1.6VSfor the MCTI.
The kneepoint voltage of a current
transformer marks the upper limit of
the roughly linear portion of the
secondary winding excitation
characteristic and is defined exactly
in British practice as that point on
the excitation curve where a 10%
increase in exciting voltage
produces a 50% increase in
exciting current.
The current transformers should be
of equal ratio, of similar
magnetising characteristics and of
low reactance construction. In cases
where low reactance current
transformers are not available and
high reactance ones must be used.
It is essential to use in the
calculations for the voltage setting,
the reactance of the current
transformer and express the current
transformer impedance as a
complex number in the form
RCT + jXCT. It is also necessary to
ensure that the exciting impedance
of the current transformer is large in
comparison with its secondary
ohmic impedance at the relay
setting voltage.
The MCAG14/34
The MCAG14/34 is an
electromechanical current
calibrated relay with setting ranges
of:
0.025 - 0.100A
0.050 - 0.200A
0.100 - 0.400A
0.200 - 0.800A
0.250 - 1.00A
0.500 - 2.00A
1.00 - 4.00A
The relay has a fixed burden of
approximately 1VA at setting current
and its impedance varies with the
setting current used. To comply with
the definition for a high impedance
relay, it is necessary, in most
applications, to utilise an externally
mounted stabilising resistor in series
with the relay coil.
The standard ratings of the stabilising
resistors normally supplied with the
relay are 470Ω, 220Ωand 47Ωfor
0.5A, 1A and 5A current transformer
secondary respectively. In
applications such as busbar
protection, where higher values of
stabilising resistor are often required
to obtain the desired relay voltage
setting, non-standard resistor values
can be supplied. The standard
resistors are wire wound, continuously
adjustable and have a continuous
rating of 145W.
The MCTI14/34
The MCTI14/34 is an electronic
instantaneous overcurrent relay
suitable for high impedance
circulating current protection. The
MCTI14 single phase earth fault relay
or MCTI34 three phase relay, when
used with a stabilising resistor is
designed for applications where
sensitive settings with stability on
heavy through faults are required. The
setting ranges available are as
follows;
5% to 322.5%In in 2.5% steps (K=1)
50% to 3225%In in 25% steps (K=10)
Applying the MCAG14/34 &
MCTI14/34
The recommended relay current setting
for restricted earth fault protection is
usually determined by the minimum
fault current available for operation of
the relay and whenever possible it
should not be greater than 30% of the
minimum fault level. For busbar
To determine the maximum relay
current setting to achieve a specific
primary operating current with a
given current transformer
magnetising current
To express the protection primary
operating current for a particular
relay operating current and with a
particular level of magnetising
current.
In order to achieve the required
primary operating current with the
current transformers that are used, a
current setting (Ir) must be selected
for the high impedance relay, as
detailed in the second expression
above. The setting of the stabilising
resistor (RST) must be calculated in
the following manner, where the
setting is a function of the relay
ohmic impedance at setting (Rr), the

4
V
p
= 2 2 V
K
(V
r
- V
K
)
I
(rms) = 0.52 (
Vs(rms) x 2
)
4
C
VSA =B
Ir
R
r
=B
Ir
2
RST =VS- Rr
Ir
required stability voltage setting (VS)
and the relay current setting (Ir).
Note: the MCAG14/34 is a fixed
burden relay, therefore the ohmic
impedance of the relay will vary
with setting. The ohmic impedance
(Rr) of the MCAG14/34 can be
calculated using the relay VA
burden at current setting (B) and the
relay current setting (Ir);
The ohmic impedance (Rr) of the
MCTI14/34 over the whole setting
range is less than 0.25Ωfor 1A
relays and less than 0.02Ωfor 5A
relays i.e. independent of current.
The stabilising resistor supplied is
continuously adjustable up to its
maximum declared resistance. In
some applications, such as
generator winding differential
protection, the through fault current
is low which results in a low
stability voltage setting. In many
such cases, a negative stabilising
resistor value can be obtained from
the above formula. This negative
result indicates that the relay will be
more than stable without a
stabilising resistor. When a
stabilising resistor is not required,
the setting voltage(VSA) can be
calculated using the following
formula and the current transformer
kneepoint voltage should be at least
twice this value.
Use of Metrosil non-
linear resistors -
MCAG14/34 & MCTI14/
34
When the maximum through fault
current is limited by the protected
circuit impedance, such as in the
case of generator differential and
power transformer restricted earth
fault protection, it is generally found
unnecessary to use non-linear
voltage limiting resistors (metrosils).
However, when the maximum
through fault current is high, such
as in busbar protection, it is always
advisable to use a non-linear resistor
(metrosil) across the relay circuit
(relay and stabilising resistor).
Metrosils are used to limit the peak
voltage developed by the current
transformers under internal fault
conditions, to a value below the
insulation level of the current
transformers, relay and
interconnecting leads, which are
normally able to withstand 3000V
peak.
The following formulae should be
used to estimate the peak transient
voltage that could be produced for
an internal fault. The peak voltage
produced during an internal fault
will be a function of the current
transformer kneepoint voltage and
the prospective voltage that would
be produced for an internal fault if
current transformer saturation did
not occur. This prospective voltage
will be a function of maximum
internal fault secondary current, the
current transformer ratio, the current
transformer secondary winding
resistance, the current transformer
lead resistance to the common
point, the relay lead resistance, the
stabilising resistor value and the
relay VA burden at relay operating
current.
where Vp= peak voltage
developed by the CT
under internal fault
conditions.
Vk= current transformer
knee-point voltage.
Vf= maximum voltage that
wouldbe produced if
CT saturation did not
occur.
If= maximum internal
secondary fault
current.
Rct = current transformer
secondary winding
resistance.
RL = maximum lead burden
from current
transformer to relay.
RST = relay stabilising resistor.
Rr= Relay ohmic impedance
at setting.
When the value given by the
formulae is greater than 3000V
peak, non-linear resistors (metrosils)
should be applied. These non-linear
resistors (metrosils) are effectively
connected across the relay circuit,
or phase to neutral of the ac
buswires, and serve the purpose of
shunting the secondary current
output of the current transformer
from the relay in order to prevent
very high secondary voltages.
These non-linear resistors (metrosils)
are externally mounted and take the
form of annular discs, of 152mm
diameter and approximately 10mm
thickness. Their operating
characteristics follow the expression:
V = CI0.25
where V = Instantaneous voltage
applied to the non-linear
resistor (metrosil)
C= constant of the non-
linear resistor (metrosi)
I= instantaneous current
through the non-linear
resistor (metrosil).
With a sinusoidal voltage applied
across the metrosil, the RMS current
would be approximately 0.52x the
peak current. This current value can
be calculated as follows;
where Vs(rms) = rms value of the
sinusoidal voltage applied
across the metrosil.
This is due to the fact that the
current waveform through the non-
linear resistor (metrosil) is not
sinusoidal but appreciably
distorted.
For satisfactory application of a
non-linear resistor (metrosil), it’s
characteristic should be such that it
complies with the following
requirements:
At the relay voltage setting, the non-
linear resistor (metrosil) current
should be as low as possible, but
no greater than approximately
30mA rms for 1A current
transformers and approximately
100mA rms for 5A current
transformers.
Vf= If(RCT + 2RL+ RST + Rr)

5
I
op
= (CT ratio) x ( I
r
+ n I
e
+ I
SH
)
ISH = VS
RSH
At the maximum secondary current,
the non-linear resistor (metrosil)
should limit the voltage to 1500V
rms or 2120V peak for 0.25
second. At higher relay voltage
settings, it is not always possible to
limit the fault voltage to 1500V rms,
so higher fault voltages may have to
be tolerated.
Applying the MFAC14/34
As the MFAC14/34 is a voltage
calibrated relay with setting ranges
of 25-175V in 25V steps, 25-325V
in 50V steps and 15-185V in 5V
steps, it is inherently a high
impedance relay requiring no
external resistors. Due to the relay
circuit impedance always being
relatively high, significant voltages
can be produced across the current
transformers and secondary wiring
during an internal fault. To limit the
voltage to a value below the
insulation level of the current
transformers, relay and
interconnecting leads, a non-linear
resistor (metrosil) is always required
and should always be used by
connecting in parallel with the relay.
Refer to metrosil publication for
selection chart).
The operating current is virtually
fixed at around 20mA, but there is
some slight variation with relay
voltage setting as a result of
variation in the current drawn by the
non-linear resistor (metrosil). The
operating current, including the non-
linear resistor (metrosil) current, for
the various voltage settings is stated
to the right:
The relay effective current setting
can be calculated in the same
manner as described for the
MCAG14/34. For busbar
protection, it is considered good
practice by some utilities to set the
minimum primary operating current
in excess of the rated load. Thus, if
one of the current transformers
becomes open circuit the MFAC14/
34 does not maloperate. The
MVTP11/31 (busbar supervision
relay) should give an alarm for open
circuit conditions but will not stop a
maloperation if the relay is set
below rated load. Thus, if the
resultant value of Iop is too low, it
may be increased by the addition of
a shunt resistor (RSH) to give a
current of:
The increased primary operating
current with the shunt resistor
connected is:
Figure 2:
Phase and earth fault differential protection for generators,motors or reactors
Figures 2 to 8 show how high
impedance relays can be applied
in a number of different situations.
Figure 3:
Restricted earth fault protection for a 3 phase,3 wire system-applicable to star
connected generators or power transformer windings
Setting range: 25-175V
Setting Voltage 25 50 75 100 125 150 175
I
r(mA) 19 19 20 23 27 36 53
Setting range: 25-325V
Setting Voltage 25 75 125 175 225 275 325
I
r(mA) 19 19 20 22 24 31 44

6
Figure 4:
Balanced or restricted earth fault protection for delta winding of a power
transformer with supply system earthed
Figure 6:
Restricted earth fault protection for 3 phase 4 wire system applicable to star connected
generators or power transformer windings earthed directly at the star point
Figure 5:
Restricted earth fault protection for 3 phase,4 wire system-applicable to star connected
generators or power transformer windings with neutral earthed at switchgear

7
Figure 7:
Phase and earth fault differential protection for an auto-transformer with CT’s at the neutral star point
Figure 8:
Busbar protection – simple single zone phase and earth fault scheme

8
=1000 x 10
3
3 x 415
= 1391A
.
I
e
@ 50V <1.4 - 0.02
.. 4
< 0.345A
= I
f
(R
CT
+ 2R
L
)
= 27820 x 5
(0.3 + 0.08)
1500
= 35.2V
VK= 2VS
= 2 x 50
= 100V
The next highest setting should be
selected on the MFAC14, this being
50V.
Current transformer
requirements
The minimum current transformer
kneepoint voltage
The exciting current to be drawn by
the current transformers at the relay
voltage setting, VS, will be:
= 100MVA x 1391
5%
= 27820A
Typical setting examples
Restricted earth fault
protection using MFAC14
The correct application of the
MFAC14 high impedance relay can
best be illustrated by taking the case
of the 11000/415V 1000kVA
power transformer shown in figure
9, for which restricted earth fault
protection is required on the L.V.
winding.
It is assumed that the relay effective
setting for a solidly earthed power
transformer is approximately 30% of
full load current.
Voltage setting
The power transformer full load
current
Maximum through fault level
(ignoring source impedance)
Required relay stability voltage
(assuming one CT saturated)
As previously stated metrosils are
always required with an MFAC. The
metrosil type is chosen in
accordance with the maximum
secondary current of 27820 x 5/
1500 = 93A.
Therefore the metrosil reference is
600A/S2/P with a constant (C) of
620/740. (Please refer to Metrosil
publication for selection chart).
Restricted earth fault
protection using MCAG14
The correct application of the
MCAG14 high impedance relay
can best be illustrated by taking the
case of the 11000/415V 1000kVA
power transformer shown in figure
9, for which restricted earth fault
protection is required on the L.V.
winding.
Figure 9:
Restricted earth fault protection on a power transformer L.V.winding
I
e
< I
S
- I
r
n
where I
S
= relay effective setting
= 30 x 1391 x 5
100 1500
=1.4A
Ir= relay setting
= 1A
n = number of current
transformers in parallel
with the relay
= 4

9
=1000 x 103
3 x 415
= 1391A
= 100MVA x 1391
5%
= 27820A
=1000 x 10
3
3 x 415
= 1391A
= I
f
(R
CT
+ 2R
L
)
= 27820 x 5
(0.3 + 0.08)
1500
= 35.2V
Vp= 2 2 VK(Vr- VK)
= 100MVA x 1391
5%
= 27820A
Vp= 2 2 VK(Vf- VK)
= 2 2 x 70.4 x (3298 - 70.4)
= 1348V
V
f
= I
f
(R
CT
+ 2R
L
+ R
ST
+ R
r
)
= 27820x 5 x
(0.3+0.08+34.2+1)
1500
= 92.7 x 35.58
= 3298V
. Ie@ 35.2V <1.4 - 1
.. 4
< 0.1A
VK= 2VS
= 2 x 35.2
= 70.4V
Stability voltage
The power transformer full load
current
Maximum through fault level
(ignoring source impedance)
Required relay stability voltage
(assuming one CT saturated)
Stabilising resistor
Assuming that the relay effective
setting for a solidly earthed power
transformer is approximately 30% of
full load current, we can therefore,
choose a relay current setting of
20% of 5A i.e. 1A. On this basis
the required value of stabilising
resistor is:
5A rated MCTI14 relays are
supplied with stabilising resistors
that are continuously adjustable
between 0 and 47Ω. Thus, a
stabilising resistance of 34.2Ωcan
be set using the standard resistor.
Current transformer
requirements
The minimum current transformer
kneepoint voltage
The exciting current to be drawn by
the current transformers at the relay
stability voltage, VS, will be:
Metrosil non-linear resistor
requirements
If the peak voltage appearing
across the relay circuit under
maximum internal fault conditions
exceeds 3000V peak then a
suitable non-linear resistor (metrosil),
externally mounted, should be
connected across the relay and
stabilising resistor, in order to
protect the insulation of the current
transformers, relay and
interconnecting leads. In the present
case the peak voltage can be
estimated by the formula:
where VK= 70.4V (In practice this
should be the actual current
transformer kneepoint voltage,
obtained from the current
transformer magnetisation curve).
Therefore substituting these values
for VKand Vfinto the main formula,
it can be seen that the peak voltage
developed by the current
transformer is:
This value is well below the
maximum of 3000V peak and
therefore no metrosils are required
with the relay. If, on the other hand,
the peak voltage VPgiven by the
formula had been greater than
3000V peak, a non-linear resistor
(metrosil) would have to be
connected across the relay and the
stabilising resistor. The
recommended non-linear resistor
type would have to be chosen in
accordance with the maximum
secondary current at the relay.
Restricted earth fault
protection using MCTI14
The correct application of the
MCTI14 high impedance relay can
be again illustrated by taking the
case of the 11000/415V 1000kVA
power transformer shown in Figure
9, for which restricted earth fault
protection is required on the L.V.
winding.
Stability voltage
The power transformer full load
current
Maximum through fault level
(ignoring source impedance)
Required relay stability voltage
I
e
<I
S
- I
r
n
where
I
S
=relay effective setting
=30 x 1391x 5
100 1500
=1.4A
I
r=relay setting
=1A
n =number of current
transformers in parallel
with the relay
=4
= 1.25 If(RCT + 2RL)
= 1.25 x 27820 x 5 (0.3 + 0.08)
1500
= 44V
R
ST
=V
S
- B
I
r
I
r
=35.2 -1
1 1
2
= 34.2 Ω
2

10
RST =VS- Rr
Ir
=44 - 0.02
1
= 43.98 Ω
V
f
= If (R
CT
+ 2R
L
+ R
r
+ R
ST
)
= 27820x 5 x
(0.3+0.08+0.02+44)
1500
= 27820 x 0.148
= 4117.36V
= If(RCT + 2RL)
= 7.98 (0.3 + 0.02)
= 2.55V
V
p
= 2 2 V
K
(V
f
- V
K
)
= 2 2 x 70.4 x (4117.36 - 70.4)
= 1509.7V
Vp= 2 2 VK(Vr- VK)
.
I
e@ 44V <1.4 - 1
.. 4
< 0.1A
Stabilising resistor
Assuming that the relay effective
setting for a solidly earthed power
transformer is approximately 30% of
full load current, we can therefore,
choose a relay current setting of
20% of 5A i.e. 1A. On this basis
the required value of stabilising
resistor is:
5A rated MCTI14 relays are
supplied with stabilising resistors
that are continuously adjustable
between 0 and 47Ω. Thus, a
stabilising resistance of 44Ωcan be
set using the standard resistor.
Current transformer
requirements
The minimum current transformer
kneepoint voltage
VK= 1.6VS
= 1.6 x 44
= 70.4V
The exciting current to be drawn by
the current transformers at the relay
stability voltage, VS, will be:
Metrosil non-linear resistor
requirements
If the peak voltage appearing
across the relay circuit under
maximum internal fault conditions
exceeds 3000V peak then a
suitable non-linear resistor (metrosil),
externally mounted,
should be connected across the
relay and stabilising resistor, in
order to protect the insulation of the
current transformers, relay and
interconnecting pilots. In the present
case the peak voltage can be
estimated by the formula:
where VK= 70.4V (In practice this
should be the actual current
transformer kneepoint voltage,
obtained from the current
transformer magnetisation curve).
Therefore substituting these values
for VKand Vfin the above formula it
can be seen that the peak voltage
developed by the current
transformer is:
This value is well below the
maximum of 3000V peak and
therefore no metrosils are required
with the relay. If, on the other hand,
the peak voltage VPgiven by the
formula had been greater than
3000V peak, a non-linear resistor
(metrosil) would have to be
connected across the relay and the
stabilising resistor. The
recommended non-linear resistor
type would have to be chosen in
accordance with the maximum
secondary current at the relay.
Generator winding
differential protection using
MCAG34
For the vast majority of MCAG14/
34 and MCTI14/34 applications
(restricted earth fault protection and
busbar protection), the
aforementioned formulae may be
followed without any additional
consideration. For MCAG34/
MCTI34 generator winding
differential applications, which are
less common applications, some
caution may be necessary in using
these formulae alone.
The relatively low level of current
that is delivered by a generator to
an external fault means that stability
of high impedance generator
differential protection can easily be
achieved with a relatively low
protection stability voltage. In many
applications, there is no need to
utilise stabilising resistors in series
with the high impedance relay
(indicated by negative stabilising
resistor value); the impedance of the
relay elements alone will offer more
than adequate stability. Where the
protection is over stable, literal
application of the published CT
kneepoint voltage formula by a CT
designer would result in the
provision of CT`s with extremely low
kneepoint voltage, which could
result in lack of protection scheme
sensitivity and slow relay operation
for an internal fault.
The correct application of the
MCAG34 when applied as
generator winding differential
protection is illustrated in Figure 10
which shows a typical application of
the MCAG34 relay where the use of
the procedure detailed in example 2
would lead to an extremely low
kneepoint voltage requirement. This
can be avoided by calculation of
the kneepoint voltage using the
actual value for VSA (setting voltage)
rather than the value of Vsrequired
for stability in the event of an
external fault.
Stability voltage
The required relay stability voltage
(assuming one CT saturated)
Using the standard formula, this
would lead to a CT VK requirement
of 2 x 2.55 = 5.1V
I
e
<I
S
- I
r
n
where
I
S
=relay effective setting
=30 x 1391x 5
100 1500
=1.4A
I
r=relay setting
=1A
n =number of current
transformers in parallel
with the relay
=4

11
Iop = (CT ratio) x (Ir+ nIe)
VK= 2VS
= 2 x 10
= 20V
Figure 10:
Generator winding differential protection
Metrosil non-linear resistor
requirements
If the peak voltage appearing
across the relay circuit under
maximum internal fault conditions
exceeds 3000V peak then a
suitable non-linear resistor
(metrosil), externally mounted,
should be connected across the
relay and stabilising resistor, in
order to protect the insulation of
the current transformers, relay and
interconnecting leads.
In this case however the peak
voltage will be substantially less
than 3000V and so metrosils are
not required.
Current transformer
requirements
The minimum current transformer
kneepoint voltage
The exciting current to be drawn by
the current transformers at the relay
stability voltage, VS, will be
determined by the maximum tolerable
protection primary operating current
and is defined by:
Stabilising resistor
The negative result indicates that the
relay is more than stable ithout any
stabilising resistor. With the relay
alone, the actual voltage setting can
be calculated as follows;
Consequently, if the formulae had
been literally applied, a CT with a
Vkof 5.1V would not have been
sufficient to provide correct
operation of the relay under fault
conditions.
If= 7.98A
Ir= 0.1A
V
SA
=B
I r
=1
0.1
= 10V
R
ST
=V
S
- B
Ir Ir
=2.55 -1
0.1 0.1
2
= 74.5 Ω
2

PXXX
Product Description
GRID
A
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