MREF
Restricted Earth Fault Relay
P&B Engineering
Belle Vue Works
Boundary Street
Manchester
M12 5NG
Tel: 0161 230 6363
Fax: 0161 230 6464
15/05/96
0
MREF 01/96G
Contents
Contents
1. Introduction ............................................................................................................. 1
2. Applications ............................................................................................................. 2
3. Features and Characteristics...................................................................................... 2
4. Design...................................................................................................................... 3
4.1. Application Diagrams ................................................................................. 3
4.1.1 Analogue input circuits............................................................................. 3
4.1.2 Output relay ............................................................................................. 4
4.1.3 Power supply............................................................................................ 4
4.2 Front panel.................................................................................................. 4
4.3 Code Jumpers.............................................................................................. 5
5. Working Principles ................................................................................................... 5
5.1 Analogue Circuits........................................................................................ 5
5.2 Digital Circuits ............................................................................................ 5
5.3 Operation of High Impedance Differential Protection................................... 6
5.4 Requirements for Current Transformers....................................................... 6
6. Operation and Setting............................................................................................... 7
6.1. Relay Setting.............................................................................................. 7
6.2. Indication ................................................................................................... 7
6.3. Setting Value Calculation ........................................................................... 7
7. Relay case ................................................................................................................ 8
7.1 Individual case............................................................................................. 8
7.2 Rack mounting ............................................................................................ 8
7.3 Terminal connections................................................................................... 8
8. Test and maintenance ............................................................................................... 8
9. Technical Specification ............................................................................................. 9
9.1 Measuring Input Circuits ............................................................................. 9
9.2 Auxiliary power supply................................................................................ 9
9.3 Common data.............................................................................................. 9
9.4 Output contact ratings ................................................................................. 9
9.5 System data................................................................................................. 10
9.6 Housing ...................................................................................................... 11
9.7 Terminal Connection Details........................................................................ 12
10. Order Form ............................................................................................................ 13
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MREF 01/96G
1. Introduction
The application of powerful microprocessors opens a new chapter for power system protective
relaying. The digital processing of measured values and the ability to perform complex arithmetic
and logic operations, give digital protection relays significant performance and flexibility
improvements over their traditional analogue counterparts. Additional advantages - very small
power consumption, adaptability, self-supervision, fault diagnosis through fault data recording,
smaller physical construction and selectable relay characteristics - all combine to allow the
implementation of accurate and highly reliable protection schemes at a significantly reduced
financial burden.
The development of microprocessor based protective relays and their introduction into the market
has been stimulated by the recent trend to replace analogue with digital equipment. This modern
trend has prompted the development of two new P&B protective relay families - the MI & MR
relay series. The superiority of digital protective relaying over traditional analogue devices, as
embodied by the MREF and MI series relays, is summarised by the following features:
•
•
•
•
Integration of many protective functions in a single compact case
High accuracy owing to digital processing
Wide setting ranges with fine interim steps.
User friendly setting procedure by means of DIP-switches.
The MI series family includes the Digital Overcurrent and Earth Fault Relay, MIRI, a universal
protection device for medium voltage networks, and, for protection against undervoltage,
overvoltage and neutral voltage displacement, the MIRV.
The MI relay family was designed as a low cost range of protection relays for application in
medium voltage networks. Similar protection relays with extended functions, the MR series, are
also available. They provide an additional alpha-numeric display for the indication of measured
values and faults as well as allowing data exchange via a serial interface and increased operational
reliability through self-supervision. This comprehensive family of protection relays can satisfy the
demands of even the most complex protection schemes:
MRI
MRI-V
MRAR
MRMF
MRVT
MRFT
MROS
MRNS
MRRP
MRCS
MRFF
MRDG
- Overcurrent Relay (Independent time/I.D.M.T + earth + directional facilities)
- Voltage Dependent Overcurrent Relay
- Auto-Reclosing Relay
- Mains Failure Relay
- Voltage Protection
- Frequency Protection
- Vector Surge or Rate of Change of Frequency
- Negative Sequence Relay
- Power Relay
- Check Synchronising Relay
- Field Failure Relay
- Differential Relay
To complement the MI & MR series, a range of Auxiliary, Timing and Tripping devices are also
available.
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MREF 01/96G
2. Applications
The MREF digital overcurrent relay, is a universal protection device for high, medium and low
voltage networks. It is aimed primarily at high impedance differential schemes such as restricted
earth fault and generator differential protection.
In this type of protection scheme, based on circulating current measurement, heavy through fault
currents during a fault condition can cause current transformers to approach or exceed their
saturation level and allow sufficient unbalance current to operate the relay. This is due to the
asymmetrical current of extreme magnitude which has dissimilar effects on seemingly identical
CT's.
In order to ensure stability under these circumstances, it is usual practice to employ a high
impedance differential scheme, the stability of which is achieved through careful selection of CT's,
stabilising resistors, settings and non-linear resistors.
For further details of the application of the MREF relay, please refer to technical note MR901.
3. Features and Characteristics
• Single or three phase current measurement
• Stabilisation by means of a resistor in series with the current relay in differential circuit
• The harmonics produced by current transformer saturation should be rejected by a series
resonant circuit.
• The total impedance of the relay should be low enough to prevent the current
transformers developing high voltages during maximum internal faults
• The current setting range of 5 - 82.5% x In (in steps of 2.5% and 5% x In)
• The time delay is not selectable, but for currents of 1 to 2 x Is there should be a small
•
•
•
•
time delay of about 100 mS so that no maloperation occurs due to a heavy through fault.
Frequency Range 50/60 Hz
Rated Current 1A/5A
Single output relay with two change over contacts
Code jumpers behind front panel for manual or auto-reset
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MREF 01/96G
4. Design
4.1. Application Diagrams
Supply
1
2
CASE
POWER
SUPPLY
5
27
PROTECTED
OUTPUT
TRIP SIGNAL
OBJECT
RS
I> D
28
3
7
6
CIRCUIT
4
8
RESET
MREF-1
Supply
1
2
CASE
POWER
SUPPLY
21
L1
22
5
23
PROTECTED
OUTPUT
L2
OBJECT
RS
I> D
24
CIRCUIT
TRIP SIGNAL
3
7
6
4
8
25
RESET
L3
26
L2 ONLY SHOWN
MREF-3
4.1.1 Analogue input circuits
The constantly monitored measuring values are galvanically decoupled, filtered and finally fed to
the analogue/digital converter.
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MREF 01/96G
4.1.2 Output relay
A single output relay with two changeover contacts; the relay contacts can be self or manual reset
depending upon the position of the code jumpers.
4.1.3 Power supply
Two auxiliary power supply versions are available:
Vaux = 24V
or
Vaux = 110V
or
in a range from 16V to 60V AC
in a range from 16V to 80V DC
in a range from 50V to 270V AC
in a range from 70V to 360V DC
4.2 Front panel
The front panel of the MREF comprises the following operation and indication elements:
• 1 Reset Button
• 2 LED's for indication
• 1 6 Way DIP Switch for setting
Front panel
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MREF 01/96G
4.3 Code Jumpers
Behind the front panel of the MREF are two code jumpers used to pre-set the following functions:
Position
OFF
ON
Jumper 1
Jumper 2
LED: Manual Reset
LED: Auto Reset
Trip Relay: Manual Reset
Trip Relay: Auto Reset
Note: LED and/or Trip Relay can only be reset via RESET push-button when jumpers in position "OFF".
The following figure shows the position and designation of the code jumpers
Front Board
Code Jumper
J1
J2
Code Jumper ON
Code Jumper OFF
5. Working Principles
5.1 Analogue Circuits
The incoming currents from the external current transformers are converted to internal signals in
proportion to the currents, via the internal input transducers and shunt resistors. The noise signals
caused by inductive and capacitive coupling are suppressed by an analogue RC filter circuit. The
analogue signals are fed to the A/D converter of the micro-processor and transformed to digital
signals through sample-hold circuits. The analogue signals are sampled with a sampling frequency
of 800 Hz, namely a sampling rate of 1.25 mS for every measured quantity.
5.2 Digital Circuits
The essential component of the MREF relay is a powerful micro-controller. All of the operations,
from the analogue digital conversion to the relay trip decision, are carried out by the microcontroller digitally. The relay program, located in EPROM, allows the CPU of the micro-controller
to calculate currents in order to detect a possible fault.
The calculated current values are continuously compared with the set value, which is adjusted
using the DIP switches. When initiated, the output relay is operated and the front panel LED
illuminated.
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MREF 01/96G
5.3 Operation of High Impedance Differential Protection
The relay is connected in the differential circuit across the CT's as shown below.
In the case of a through fault condition, both CT's will reflect the fault current into the secondary.
If the CT magnetising curves, internal impedance’s (Rs) and lead resistances (RL) are similar for
both the CT's, the voltage Vs will be approximately zero.
Equivalent Circuit to describe the working principles of the MREF
IF
IF
Protected Object
Rs
RL
RL
Rsr
Rs
Vs
Z
Z
During saturation
is short circuit
R
However, if the fault current passes only through one CT (in zone fault) then the second CT will
appear as an open circuit (magnetising current only), and Vs will be of sufficient magnitude to
operate the relay through Rsr.
During severe fault conditions, this voltage may rise to an extremely high value. Thus, a voltage
dependent non-linear resistor may be required to limit this voltage.
In practice if one CT becomes saturated first, during a through fault, inadvertent operation may
occur. In order to prevent this a stabilising resistor Rsr is fitted.
The relay input circuit is tuned to the network supply frequency (50/60 Hz) by means of a resonant
circuit and harmonics produced by the CT's can be effectively rejected.
5.4 Requirements for Current Transformers
In order to ensure the correct operation of the MREF protection relay, the characteristic of the CT
must be selected to suit its application.
In general the CT's must be Class X, all of the same ratio and similar secondary resistance. They
should have a knee point of at least twice the setting voltage.
or
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Vkn = 2 × Vset
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MREF 01/96G
6. Operation and Setting
6.1. Relay Setting
The relay is set using the front panel DIP switches and examples of this are shown below:
2
5
0
3
10
0
4
20
0
5
40
ID
%
In
5
1
0
2
7.5
5
0
3
10
0
4
20
0
5
40
6
ID
%
In
6
ID=7.5+5=12.5%In
5
1
0
2
5
0
3
10
0
4
20
0
5
40
ON
0
7.5
ON
1
ON
ID
%
In
5
7.5
6
ID=5+5+20=30%In
ID=5+5+10+20+40=80%In
6.2. Indication
• Power On Indication, green ON LED
• Trip Indication, red ID LED
6.3. Setting Value Calculation
The setting current Is, and the stabilising resistor Rsr must be selected as;
Is =
Vs
R sr
where;
Vs ≈
IF
× (Rct + 2RL + Ra )
n
Where;
Vs
IF
n
Rct
RL
Ra
=
=
=
=
=
=
Setting Voltage
Fault Level in CT Primary
CT Ratio
Resistance of CT secondary
Load Resistance
Additional Resistance due to contacts, connections etc.
With the CT knee point in excess of 2 × Vs
The value of the stabilising resistance is selected so that the relay setting current will fall within the
setting range (5% - 82.5% In)
If the relay measuring current is higher than the setting value, the relay trips instantaneously with
the operating time approximately 30 mS (this includes the time taken for the output relay to
operate).
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MREF 01/96G
Careful selection of Rsr and Is must be made such that excessive voltage is not present for high
fault currents.
For further details of the setting of the MREF relay, please refer to technical note MR901 for
restricted earth fault, or MR904 for Bus-Zone protection.
7. Relay case
The MREF is delivered in an individual case for flush mounting.
7.1 Individual case
The MREF is supplied in a UK manufactured industry standard drawout case suitable for flush
mounting. For case dimension and cut-out, refer to Technical Data.
7.2 Rack mounting
MREF relays may be supplied mounted in 19" racks if specified by the user.
7.3 Terminal connections
The MREF plug in module is supplied in a case which has a very compact plug and socket
connector. The current terminals are equipped with self closing short circuit contacts. Thus the
MREF module can be unplugged even with current flowing without endangering personnel.
8. Test and maintenance
Currents may be supplied to the input transformers to test the behaviour of the relay. By switching
on test currents and measuring the tripping time, the whole system can be accurately tested. A
portable test case can be supplied which is suitable for testing the MREF.
All measuring input circuits of the MREF are of static design and the relay functions are fully
digitised. Thus, the MREF has no particular demand on maintenance.
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MREF 01/96G
9. Technical Specification
9.1 Measuring Input Circuits
Rated Data
Rated current, IN
Rated frequency, fN
1A or 5A
50 - 60Hz
Power consumption
Power Consumption
<0.12VA @ In
Thermal withstand
Thermal Withstand
4 x In continuous
9.2 Auxiliary power supply
Supply Voltage
Vaux = 24V
or
Vaux = 110V
or
in a range from 16V to 60V AC
in a range from 16V to 80V DC
in a range from 50V to 270V AC
in a range from 70V to 360V DC
Power Consumption
1W @ 24V Quiescent
2W @ 24V Operating
1W @ 110V Quiescent
2W @ 110V Operating
Automatic Restart Time 50mS
9.3 Common data
Drop Off/Pick Up ratio
Drop Off time
Minimum Operating Time
>97%
50mS
20mS
9.4 Output contact ratings
Maximum rated voltage = 380V AC / 125V DC
Maximum breaking capacity
Resistive
Inductive
= 1250VA / 150W
= 500VA / 90W
Maximum rated current
Making current (16mS)
= 5A
= 20A
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MREF 01/96G
9.5 System data
Design standard
= IEC 255-4, BS 142
Operating temperature range
Storage temperature range
Relative humidity
= -20°C to 70°C
= -40°C to 85°C
= 95% @ 40°C for 56 days
Test Voltages to EN50081-1, EN50082-2
Isolation Test
= 2.5kV / 50Hz / 1 min.
Impulse Test
= 5kV, 1.2 / 50mS, 0.5J
High frequency interference Test
= 2.5kV / 1MHz
Burst transient Test
= 4kV / 2.5KHz, 15mS
ESD Test
= 8kV
RFI Suppression Test
= 10V/m, 27 - 500MHz, 1 Octave/ 3 min.
EMI Suppression Test
= 10V/m
Mechanical Tests:
Shock
Vibration
= IEC 41B (CO) 38, Class 1
= IEC 41B (CO) 35, Class 1
Degree of Protection
= Front - IP52
Rear - IP00
Weight
= Approx. 2kg.
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MREF 01/96G
9.6 Housing
Throughout the MR series range a modular housing system has been employed, utilising the latest
high quality UK manufactured industry standard case components. This approach affords maximum
flexibility for both the relay scheme designer and the maintenance engineer. The relay modules are
fully withdrawable for ease of maintenance and where applicable incorporate automatic shortcircuiting CT connections to avoid dangerous open circuit CT overvoltages. A clear plastic front
cover is provided for inspection purposes.
MREF units are supplied in standard height (179mm≅7in.) cases, complying with IEC 297 size 4U.
The rigid case wall is manufactured from a single sheet of hot dipped galvanised steel coated
externally with Plastisol PVC and internally with a low gloss alkyd paint finish. This construction
technique provides improved thermal transfer characteristics over plastic walled cases and
combines exceptional corrosion and flame resilience with good electromagnetic and electrostatic
screening properties allowing many relays to be freely situated in close proximity and hazardous
environments. When the relay is inserted a leaf spring along the top edge of the module makes
contact with a solidly bonded nickel plated steel strip on the interior of the case, providing
excellent earth continuity. This strip is brought out at the rear of the case, above the terminal block,
where it forms a separate earthing terminal. A rigid front mounting flange is provided allowing the
entire range of standard cases to be flush mounted without alteration. These flanges are also used
to mount the relay inspection cover which is secured by thumbscrews. Securely bonded channels
can be provided on the top and bottom surfaces toward the rear of the case allowing large rigid
assemblies to be created by the use of joining strips located in these channels.
This uniform but highly flexible housing system integrates excellent mechanical strength with good
electrical practice in industry standard sizes.
97
52
23.5
45
4 HOLES 4.4mm DIAMETER
10
OPTIONAL
168
159
PANEL CUT OUT FLUSH
MOUNTING FIXING DETAILS
Min
28
99
NOTE Minimum gap between vertical
PUSH BUTTON
spacing is required in order to
PROJECTION 10mm
withdraw relay from the case above.
OPTIONAL
32
177
103
157
178
Required to open case
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Clearance
25 min
212
NOT SHOWN TO SCALE
11
OPTIONAL
SIZE 100 CASE
MREF 01/96G
9.7 Terminal Connection Details
The rear terminal block accepts both pre-insulated screw and push-on blade type connectors which
may be used singly or in combination. Each terminal has 1 screw type and 2 blade type connectors.
Screw:
Each connection uses a 4mm (M4) screw outlet and accepts standard
L-shaped ring type connectors designed for 4mm screws.
Blade:
Each connection facilitates 2 pre-insulated push-on blades 4.8mm wide
0.8mm thick complying with BS5057.
Combinations:
Each terminal will accept either;
2 ring type connectors
or
2 push-on blade type connectors
or
1 ring type connector & 1 push-on blade type connector
Earth
1
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
Each terminal
1 screw &
2 spade
MREF
Rear terminal block connections.
All information subject to change without notice
Publication number MREF-01/96G
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MREF 01/96G
10. Order Form
Restricted Earth Fault Relay
QUANTITY
MREF
1
3
Single Phase
Three Phase
1
5
Rated current 1A
5A
* Aux Voltage
L
H
24 V
110 V
* Range
Housing
19" Rack
Flush mounting
A
D
16-60V ac, 16-80Vdc for 24V device
50-270V ac, 70-360Vdc for 110V device
Non-standard voltages
available on request
PBSI Ltd Trading as
P&B ENGINEERING
Belle Vue Works,
Boundary Street,
Manchester.
M12 5NG.
Tel: 0161- 230-6363
Fax: 0161-230-6464
15/05/96
13
MREF 01/96G