Differential current relays
4189340154B
Application notes
DEIF A/S Frisenborgvej 33 Dk-7800 Skive Denmark
Tel.: +45 9614 9614 Fax: +45 9752 4720 E-mail: deif@deif.com
APPLICATION NOTES
Differential current relays 4189340154B
List of contents
1. Differential current protection ....................................................................................................3
1.1. Uni-Line RMC-131D/DELOMATIC CRM-1 diff. .............................................................................3
1.1.1. RMC-131D/CRM-1 diff. wiring .......................................................................................................4
1.1.2. RMC-131D/CRM-1 diff. hints .........................................................................................................4
1.2. Multi-Line MDR-1 relay ..................................................................................................................4
1.2.1. MDR-1 wiring .................................................................................................................................5
1.2.2. MDR-1 programmable trip curve ...................................................................................................5
2. Conclusion ...................................................................................................................................6
DEIF A/S Tel.: +45 9614 9614 Fax: +45 9752 4720 E-mail: deif@deif.com Page 2 of 6
APPLICATION NOTES
Differential current relays 4189340154B
1. Differential current protection
Differential current protection is protecting a generator and (typically) the cable between the generator
and the main switchboard against internal short circuits.
Symbol: I'>
DEIF has 3 differential current protection relays:
- Uni-Line RMC-131D
- Multi-Line MDR-1
- DELOMATIC module CRM-1 diff.
generator
star
point
swbd busbar
cable
generator
breaker
star point current produced current
protected equipment
The differential current is the difference between the current measured in a phase in the generator
terminal box (star point) and the current measured in the same phase in the switchboard. This value
should be zero but due to imbalance in the measuring circuits (current transformer deviations,
measuring input deviations etc.) a deviation occurs.
This deviation can be compensated for externally or internally in the protection relay.
- RMC-131D requires external compensation.
- MDR-1 has an internal compensation system.
- DELOMATIC CRM-1 diff. requires external compensation.
1.1. Uni-Line RMC-131D/DELOMATIC CRM-1 diff.
RMC-131D is a simple DIN-rail mounted differential current relay. The relay accepts 3 current inputs. As
the measurement of I' requires 6 current transformers (2 per phase), it can be seen that the I' value
must be achieved before connecting the relay. This means that RMC-131D does not have internal
compensation. Therefore take very much care during the installation to avoid false trips.
DELOMATIC CRM-1 diff. is a 19" double-height plug-in module for the DELOMATIC multi-function
system. The working manner is similar to RMC-131D, i.e. a simple differential current relay.
DEIF A/S Tel.: +45 9614 9614 Fax: +45 9752 4720 E-mail: deif@deif.com Page 3 of 6
APPLICATION NOTES
Differential current relays 4189340154B
1.1.1. RMC-131D/CRM-1 diff. wiring
Only phase L1 is shown:
terminals
CRM-1 diff: 7 8
RMC-131D: 23 24
s1 s2 s1 s2
CT1 CT2
generator
The differential current (measured on the terminals) occurs as a result of the connection between CT1
and CT2.
1.1.2. RMC-131D/CRM-1 diff. hints
In order to maintain the balance in the system all the time (even in external short circuit or large
consumer start situations) it is essential that:
- The current transformers CT1 and CT2 are of the same make and of the same type.
- The saturation level of the CTs is 8 times better than the nominal generator current.
- Impedance is the same in the circuits CT1 to RMC-131D/CRM-1 diff. and CT2 to RMC-
131D/CRM-1 diff. This can be achieved by using the same cable lengths or as this is not always
possible by using cables of different sizes as the circuit CT1 to RMC-131D/CRM-1 diff. normally
is much longer than the other one.
Finally it may be necessary to set the time delay higher than originally planned.
As it can be seen from the above RMC-131D/CRM-1 diff. has a number of technical disadvantages. By
choosing the Multi-Line MDR-1 relay these disadvantages can be avoided.
1.2. Multi-Line MDR-1 relay
MDR-1 is an advanced differential current relay mounted on the panel front. The difference between
RMC-131D and MDR-1 is that RMC-131D has 3 current inputs and MDR-1 has 6 current inputs. Also
MDR-1 is a programmable unit with a display showing either the actual current or the differential current,
while RMC-131D is a "blind" unit, adjusted via potentiometers in the front.
This means that a compensation curve related to the actual running current could be programmed, i.e.
MDR-1 is a more advanced unit and can easier avoid false trips.
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APPLICATION NOTES
Differential current relays 4189340154B
1.2.1. MDR-1 wiring
Only phase L1 is shown
36 35 terminals 30 29
s1 s2 s1 s2
CT1 CT2
generator
It is essential that:
- The current transformers CT1 and CT2 are of the same make and of the same type.
- The saturation point of the CTs is 8 times better than the nominal generator current.
1.2.2. MDR-1 programmable trip curve
l’
In
TRIP 1 FIXED VALUE 100%
100%
TRIP 3 FIXED VALUE 85%
Upper adjustable value
Trip 2 adjustable
Pre-warning adjustable PREWARNING
50%
100% 200% 300% 400% 500% Ig
In
The trip curve shows that if the generator current (Ig) exceeds the nominal value (In) MDR-1 will start
compensating for the differential current (I') simply by moving the trip and warning setpoint values
upwards as the generator current increases. This will compensate for the previously mentioned
measuring imbalances.
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APPLICATION NOTES
Differential current relays 4189340154B
2. Conclusion
The most obvious conclusion is of course always to use MDR-1, as this is the best unit. However the
economical aspect have to be considered, as MDR-1 is more expensive than RMC-131D and CRM-1.
Generally:
Use RMC-131D if:
- The generator is fairly small.
- There are no heavy consumers whose start current makes the generator current exceed the
nominal value even for a short time (island operation).
Use CRM-1 diff. if:
- A DELOMATIC system is part of the delivery.
- The generator is fairly small.
- There are no heavy consumers whose start current makes the generator current exceed the
nominal value even for a short time (island operation)
Use MDR-1 if:
- There are heavy consumers whose start current makes the generator current exceed the
nominal value, even for a short time.
- The generator is fairly big.
The above is meant only as a guidance. Other aspects may be considered regarding the use of a
differential current protection unit.
Errors and changes excepted.
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