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MiCOM P12X

High Impedance Protection

pplication Notes


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Safety Section P12x/EN SS/E11

SAFETY SECTION


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P12x/EN SS/E11 Safety Section


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(SS) - 1

CONTENTS

1. INTRODUCTION 32. HEALTH AND SAFETY 33. SYMBOLS AND EXTERNAL LABELS ON THE EQUIPMENT 43.1 Symbols 43.2 Labels 44. INSTALLING, COMMISSIONING AND SERVICING 45. DE-COMMISSIONING AND DISPOSAL 76. TECHNICAL SPECIFICATIONS FOR SAFETY 76.1 Protective fuse rating 76.2 Protective class 76.3 Installation category 76.4 Environment 7


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STANDARD SAFETY STATEMENTS AND EXTERNAL LABEL INFORMATIONFOR AREVA T&D EQUIPMENT

1. INTRODUCTION

This Safety Section and the relevant equipment documentation provide full information onsafe handling, commissioning and testing of this equipment. This Safety Section alsoincludes reference to typical equipment label markings.

The technical data in this Safety Section is typical only, see the technical data section of therelevant equipment documentation for data specific to a particular equipment.

Before carrying out any work on the equipment the user should be familiar withthe contents of this Safety Section and the ratings on the equipments ratinglabel.

Reference should be made to the external connection diagram before the equipment isinstalled, commissioned or serviced.

Language specific, self-adhesive User Interface labels are provided in a bag for someequipment.

2. HEALTH AND SAFETY

The information in the Safety Section of the equipment documentation is intended to ensurethat equipment is properly installed and handled in order to maintain it in a safe condition.

It is assumed that everyone who will be associated with the equipment will be familiar withthe contents of this Safety Section, or the Safety Guide (SFTY/4L M).

When electrical equipment is in operation, dangerous voltages will be present in certain partsof the equipment. Failure to observe warning notices, incorrect use, or improper use may

endanger personnel and equipment and also cause personal injury or physical damage.

Before working in the terminal strip area, the equipment must be isolated.

Proper and safe operation of the equipment depends on appropriate shipping and handling,proper storage, installation and commissioning, and on careful operation, maintenance andservicing. For this reason only qualified personnel may work on or operate the equipment.

Qualified personnel are individuals who:

Are familiar with the installation, commissioning, and operation of the equipment and ofthe system to which it is being connected;

Are able to safely perform switching operations in accordance with accepted safety

engineering practices and are authorized to energize and de-energize equipment and toisolate, ground, and label it;

Are trained in the care and use of safety apparatus in accordance with safetyengineering practices;

Are trained in emergency procedures (first aid).

The equipment documentation gives instructions for its installation, commissioning, andoperation. However, the manuals cannot cover all conceivable circumstances or includedetailed information on all topics. In the event of questions or specific problems, do not takeany action without proper authorization. Contact the appropriate AREVA technical salesoffice and request the necessary information.


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3. SYMBOLS AND LABELS ON THE EQUIPMENT

For safety reasons the following symbols which may be used on the equipment or referred toin the equipment documentation, should be understood before it is installed orcommissioned.

3.1 Symbols

Caution: refer to equipment documentation Caution: risk of electric shock

Protective Conductor (*Earth) terminal

Functional/Protective Conductor (*Earth) terminal

Note: This symbol may also be used for a Protective Conductor (Earth) terminal if thatterminal is part of a terminal block or sub-assembly e.g. power supply.

*NOTE: THE TERM EARTH USED THROUGHOUT THIS TECHNICALMANUAL IS THE DIRECT EQUIVALENT OF THE NORTHAMERICAN TERM GROUND.

3.2 Labels

See Safety Guide (SFTY/4L M) for typical equipment labeling information.

4. INSTALLING, COMMISSIONING AND SERVICING

Equipment connections

Personnel undertaking installation, commissioning or servicing work for thisequipment should be aware of the correct working procedures to ensure safety.

The equipment documentation should be consulted before installing,commissioning, or servicing the equipment.

Terminals exposed during installation, commissioning and maintenance maypresent a hazardous voltage unless the equipment is electrically isolated.

Any disassembly of the equipment may expose parts at hazardous voltage, alsoelectronic parts may be damaged if suitable electrostatic voltage discharge (ESD)precautions are not taken.

If there is unlocked access to the rear of the equipment, care should be taken byall 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 maintainedfor safety.

Watchdog (self-monitoring) contacts are provided in numerical relays to indicatethe health of the device. AREVA T&D strongly recommends that these contactsare hardwired into the substation's automation system, for alarm purposes.


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(SS) - 5

To ensure that wires are correctly terminated the correct crimp terminal and toolfor the wire size should be used.

The equipment must be connected in accordance with the appropriate connectiondiagram.

Protection Class I Equipment

- Before energizing the equipment it must be earthed using the protectiveconductor terminal, if provided, or the appropriate termination of thesupply plug in the case of plug connected equipment.

- The protective conductor (earth) connection must not be removed sincethe protection against electric shock provided by the equipment would belost.

- When the protective (earth) conductor terminal (PCT) is also used toterminate cable screens, etc., it is essential that the integrity of theprotective (earth) conductor is checked after the addition or removal ofsuch functional earth connections. For M4 stud PCTs the integrity of theprotective (earth) connections should be ensured by use of a locknut or

similar.

The recommended minimum protective conductor (earth) wire size is 2.5 mm(3.3 mm for North America) unless otherwise stated in the technical data sectionof the equipment documentation, or otherwise required by local or country wiringregulations.

The protective conductor (earth) connection must be low-inductance and as shortas possible.

All connections to the equipment must have a defined potential. Connections thatare pre-wired, but not used, should preferably be grounded when binary inputsand output relays are isolated. When binary inputs and output relays areconnected to common potential, the pre-wired but unused connections should be

connected to the common potential of the grouped connections.Before energizing the equipment, the following should be checked:

- Voltage rating/polarity (rating label/equipment documentation);

- CT circuit rating (rating label) and integrity of connections;

- Protective fuse rating;

- Integrity of the protective conductor (earth) connection (whereapplicable);

- Voltage and current rating of external wiring, applicable to the application.

Accidental touching of exposed terminals

If working in an area of restricted space, such as a cubicle, where there is a risk ofelectric shock due to accidental touching of terminals which do not comply withIP20 rating, then a suitable protective barrier should be provided.

Equipment use

If the equipment is used in a manner not specified by the manufacturer, theprotection provided by the equipment may be impaired.

Removal of the equipment front panel/cover

Removal of the equipment front panel/cover may expose hazardous live parts,which must not be touched until the electrical power is removed.

UL and CSA listed or recognized equipment

To maintain UL and CSA approvals the equipment should be installed using ULand/or CSA listed or recognized parts of the following type: connection cables,protective fuses/fuseholders or circuit breakers, insulation crimp terminals, andreplacement internal battery, as specified in the equipment documentation.


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Equipment operating conditions

The equipment should be operated within the specified electrical andenvironmental limits.

Current transformer circuits

Do not open the secondary circuit of a live CT since the high voltage producedmay be lethal to personnel and could damage insulation. Generally, for safety,the secondary of the line CT must be shorted before opening any connections toit.

For most equipment with ring-terminal connections, the threaded terminal blockfor current transformer termination has automatic CT shorting on removal of themodule. Therefore external shorting of the CTs may not be required, theequipment documentation should be checked to see if this applies.

For equipment with pin-terminal connections, the threaded terminal block forcurrent transformer termination does NOT have automatic CT shorting on removalof the module.

External resistors, including voltage dependent resistors (VDRs)Where external resistors, including voltage dependent resistors (VDRs), are fittedto the equipment, 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 recommendedtype and be installed with the correct polarity to avoid possible damage to theequipment, buildings and persons.

Insulation and dielectric strength testing

Insulation testing may leave capacitors charged up to a hazardous voltage. At theend of each part of the test, the voltage should be gradually reduced to zero, todischarge capacitors, before the test leads are disconnected.

Insertion of modules and pcb cards

Modules and PCB cards must not be inserted into or withdrawn from theequipment whilst it is energized, since this may result in damage.

Insertion and withdrawal of extender cards

Extender cards are available for some equipment. If an extender card is used,this should not be inserted or withdrawn from the equipment whilst it is energized.This is to avoid possible shock or damage hazards. Hazardous live voltages maybe accessible on the extender card.

External test blocks and test plugs

Great care should be taken when using external test blocks and test plugs suchas the MMLG, MMLB and MiCOM P990 types, hazardous voltages may beaccessible when using these. *CT shorting links must be in place before theinsertion or removal of MMLB test plugs, to avoid potentially lethal voltages.

*Note: When a MiCOM P992 Test Plug is inserted into the MiCOM P991 TestBlock, the secondaries of the line CTs are automatically shorted, makingthem safe.

Fiber optic communication

Where fiber optic communication devices are fitted, these should not be vieweddirectly. Optical power meters should be used to determine the operation orsignal level of the device.

CleaningThe equipment may be cleaned using a lint free cloth dampened with clean water,when no connections are energized. Contact fingers of test plugs are normallyprotected by petroleum jelly, which should not be removed.


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5. DE-COMMISSIONING AND DISPOSAL

De-commissioning

The supply input (auxiliary) for the equipment may include capacitors across thesupply or to earth. To avoid electric shock or energy hazards, after completely

isolating the supplies to the equipment (both poles of any dc supply), thecapacitors should be safely discharged via the external terminals prior tode-commissioning.

Disposal

It is recommended that incineration and disposal to water courses is avoided.The equipment should be disposed of in a safe manner. Any equipmentcontaining batteries should have them removed before disposal, takingprecautions to avoid short circuits. Particular regulations within the country ofoperation, may apply to the disposal of the equipment.

6. TECHNICAL SPECIFICATIONS FOR SAFETY

Unless otherwise stated in the equipment technical manual, the following data is applicable.

6.1 Protective fuse rating

The recommended maximum rating of the external protective fuse for equipments is 16A,high rupture capacity (HRC) Red Spot type NIT, or TIA, or equivalent. The protective fuseshould be located as close to the unit as possible.

DANGER - CTs must NOT be fused since open circu iting them mayproduce lethal hazardous voltages.

6.2 Protective class

IEC 60255-27: 2005 Class I (unless otherwise specified in the EN 6025EN 60255-27: 2005 equipment documentation). This equipment

requires a protective conductor (earth) connectionto ensure user safety.

6.3 Installation category

IEC 60255-27: 2005 Installation category III (Overvoltage Category III):

EN 60255-27: 2005 Distribution level, fixed installation.

Equipment in this category is qualification tested at

5 kV peak, 1.2/50 s, 500 , 0.5 J, between allsupply circuits and earth and also betweenindependent circuits.

6.4 Environment

The equipment is intended for indoor installation and use only. If it is required for use in anoutdoor environment then it must be mounted in a specific cabinet of housing which willenable it to meet the requirements of IEC 60529 with the classification of degree ofprotection IP54 (dust and splashing water protected).

Pollution Degree - Pollution Degree 2 Compliance is demonstrated by reference to safetyAltitude - Operation up to 2000m standards.

IEC 60255-27:2005

EN 60255-27: 2005


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Application Notes P12x/EN AP/C00

MiCOM P120, P121, P122 & P123

APPLICATION NOTES FOR

MICOM P12X HIGH

IMPEDANCE PROTECTION

Date: May 2007


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P12x/EN AP/C00 Application Notes

MiCOM P120, P121, P122 & P123


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MiCOM P120, P121, P122 & P123 (AP) 1

CONTENTS

1. INTRODUCTION 3

1.1 Theory of high impedance protection 3

1.2 Use of non-linear resistors 4

2. APPLYING THE MICOM P12X RANGE 6

2.1 Restricted earth fault (REF or [87N]) applications 7

2.2 Three phase ([87]) applications 9

2.2.1

Buswire supervision 11

2.3 Typical operating times 11

2.4 Advanced application requirements 12

2.5 Recommended non-linear resistors 12

2.5.1 Metrosils 12

2.5.2 Integrated non-linear resistor and resistors 13

3. SETTING EXAMPLES 16

3.1 Restricted earth fault application 16

3.1.1 Stability voltage 16

3.1.2 Stabilizing resistor 16

3.1.3 Current transformer requirements 17

3.1.4 Non-linear resistor requirements 17

3.2 Busbar applications 18

3.2.1 Stability voltage 18

3.2.2 Discriminating zone effective setting 18

3.2.3 Check zone effective setting 19

3.2.4 Stabilizing resistor 19

3.2.5 Current transformer requirements 19

3.2.6 Non-linear resistor requirements 20

3.2.7 Busbar supervision 20

3.2.8 Integrated non-linear resistor and resistors 20

3.2.9 Advanced application requirements 21

3.3 Motor / generator applications 21


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(AP) 2 MiCOM P120, P121, P122 & P123

FIGURES

Figure 1: Principle of high impedance protection 3

Figure 2: Restricted earth fault protection of a 3 phase, 3 wire system applicable to star connectedgenerators or power transformer windings with neutral earthed at generator/transformer

starpoint. 7

Figure 3: Balanced or restricted earth fault protection for the delta winding of a power transformer withsupply system earthed 8

Figure 4: Restricted earth fault protection of a 3 phase, 4 wire system applicable to star connectedgenerators or power transformer windings with neutral earthed at switchboard 8

Figure 5: Restricted earth fault protection of a 3 phase, 4 wire system applicable to star connectedgenerators or power transformer windings with neutral earthed at generator/transformerstarpoint. 9

Figure 6: Phase and earth fault differential protection for generators, motors or reactors 10

Figure 7: Phase and earth fault differential protection for an auto-transformer with CTs at the neutralstar point 10

Figure 8: A simple, single zone phase and earth fault busbar protection scheme with buswiresupervision 11

Figure 9: X2/DVZ3R connections for 500stabilizing resistance 13



Figure 12: Characteristic of the non-linear resistors used in the X2/DVZ3R 14

Figure 13: Example connection of the X2/DVZ3R with the entire differential path short-circuited following

operation. 15

Figure 14: Example connection of the X2/DVZ3R with the stabilizing resistor short- circuited followingoperation. 15

Figure 15: Restricted earth fault protection on a transformer 16

Figure 16: Three phase and earth fault protection of a double busbar generating station 18


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MiCOM P120, P121, P122 & P123 (AP) 3

1. INTRODUCTION

1.1 Theory of high impedance protection

The application of the P12x numerical overcurrent relays as differential protection formachines, power transformers and busbar installations is based on the high impedancedifferential principle, offering stability for any type of fault occurring outside the protectedzone 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 lessthan the calculated maximum voltage which can appear across its terminals under theassigned maximum through fault current condition.

It can be seen from Figure 1 that during an external fault the through fault current shouldcirculate between the current transformer secondarys. The only current that can flowthrough the relay circuit is that due to any difference in the current transformer outputs forthe same primary current. Magnetic saturation will reduce the output of a currenttransformer and the most extreme case for stability will be if one current transformer iscompletely saturated and the other unaffected.

Figure 1: Principle of high impedance protection

Calculations based on the above extreme case for stability have become accepted in lieu ofconjunctive scheme testing as being a satisfactory basis for application. At one end thecurrent transformer can be considered fully saturated, with its magnetizing impedance ZMBshort circuited while the current transformer at the other end, being unaffected, delivers itsfull current output. This current will then divide between the relay and the saturated current

transformer. This division will be in the inverse ratio of RRELAY CIRCUITto (RCTB+ 2RL) and, ifRRELAY CIRCUITis high compared with RCTB+ 2RL, the relay will be prevented from undesirableoperation, as most of the current will pass through the saturated current transformer.

To achieve stability for external faults, the stability voltage for the protection (V S) must bedetermined in accordance with equation [1]. 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 from the relaying point (RCT + 2RL). The stability of thescheme is also affected by the characteristics of the differential relay and the application(e.g. restricted earth fault, busbar etc). The value of K in the expression takes account ofboth of these considerations. One particular characteristic that affects the stability of thescheme is the operating time of the differential relay. The slower the relay operates thelonger the spill current can exceed its setting before operation occurs and the higher the spill

current that can be tolerated.

VSKIF(RCT+2RL) -----[1]


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(AP) 4 MiCOM P120, P121, P122 & P123

Where:

RCT= current transformer secondary winding resistanceRL= maximum lead resistance from the current transformer to the relaying pointIF= maximum secondary external fault currentK = a constant affected by the dynamic response of the relay and the application

NOTE: When high impedance differential protection is applied to motors orshunt reactors, the external fault current contribution from themotor/reactor is much smaller than the internal fault current. In thesecases, the locked rotor current or starting current of the motor, orreactor inrush current, should be used in place of the external faultcurrent used for stability calculations.

To obtain high speed operation for internal faults, the knee point voltage, VK, of the CTs mustbe significantly higher than the stability voltage, VS. This is essential so that the operatingcurrent through the relay is a sufficient multiple of the applied current setting.

The kneepoint voltage of a current transformer marks the upper limit of the roughly linearportion of the secondary winding excitation characteristic. This is defined exactly in the IEC

standards as that point on the excitation curve where a 10% increase in exciting voltageproduces a 50% increase in exciting current.

The current transformers should be of equal ratio, of similar magnetizing characteristics andof low reactance construction. In cases where low reactance current transformers are notavailable and high reactance ones must be used, it is essential to use the reactance of thecurrent transformer in the calculations for the voltage setting. Thus, the current transformerimpedance is expressed as a complex number in the form RCT+ jXCT. It is also necessary toensure that the exciting impedance of the current transformer is large in comparison with itssecondary ohmic impedance at the relay setting voltage.

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 relayelement (n) and the magnetizing current of each current transformer (Ie) at the stabilityvoltage (VS). This relationship can be expressed as follows:

Iop = CT ratio x (Ir+n Ie -----[2]

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

above. The setting of the stabilizing resistor (RST) must be calculated in the followingmanner, where the setting is a function of the relay ohmic impedance at setting (R r), the

required stability voltage setting (VS) and the relay current setting (Ir).

r

r

SST R-

I

VR = -----[3]

The ohmic impedance of auxiliary powered numerical relays such as the P12x is extremelysmall and so Rrmay be ignored in equation [3]. The standard resistors that can be suppliedfor use in the high impedance application are wire-wound, continuously adjustable and havea continuous rating of 145W based upon AREVA conjunctive testing.

NOTE: The resistors should not be adjusted below 60% of their nominalohmic value otherwise there is a risk that the power dissipation duringfaults will be insufficient.

1.2 Use of non-linear resistors

When the maximum through fault current is limited by the protected circuit impedance, suchas in the case of generator differential and power transformer restricted earth faultprotection, 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 busbarprotection, it is more common to use a non-linear resistor (Metrosil) across the relay circuit(relay and stabilizing resistor). Metrosils are used to limit the peak voltage developed by the


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MiCOM P120, P121, P122 & P123 (AP) 5

current transformers under internal fault conditions, to a value below the insulation level ofthe current transformers, relay and interconnecting leads, which are typically able towithstand approximately 3000V peak.

The following formulae should be used to estimate the peak transient voltage that could beproduced for an internal fault. This voltage is a function of the current transformer kneepoint

voltage and the prospective voltage that would be produced for an internal fault if currenttransformer saturation did not occur.

NOTE: The internal fault level, If, can be significantly higher than the external

fault level, If, on generators where current can be fed from the supply

system and the generator. Similarly the internal fault level on motorsand shunt reactors will be considerably higher than the values used inthe stability calculations (motor start/locked rotor and inrush currentsrespectively).

( )KFKP VV2V2V = -----[4]

( )RSTLCT'FF RRR2RV +++= I -----[5]

Where:

VP= peak voltage developed by the CT under internal fault conditionsVK= current transformer knee-point voltageVF= maximum voltage that would be produced if CT saturation did not occur

If= maximum internal secondary fault current

RCT= current transformer secondary winding resistanceRL= maximum lead burden from CT to relayRST= relay stabilizing resistorRr= relay ohmic impedance at setting

When the value of VP

is greater than 3000V peak, non-linear resistors (Metrosils) should beapplied. They are effectively connected across the relay circuit, or phase to neutral of the acbuswires, and serve the purpose of shunting the secondary current output of the currenttransformer from the relay circuit in order to prevent very high secondary voltages.

Traditionally, Metrosils are externally mounted and take the form of annular discs, of 152mmdiameter and approximately 10mm thickness. Their operating characteristics follow theexpression:

0.25CV I= -----[6]Where:

V = instantaneous voltage applied to the non-linear resistor (Metrosil)C = constant of the non-linear resistor (Metrosil)

I= instantaneous current through the non-linear resistor (Metrosil)

With a sinusoidal voltage applied across the Metrosil, the RMS current would beapproximately 0.52x the peak current. This current value can be calculated as follows:

4

)RMS(S

RMSC

2V52.0

=I -----[7]

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 Metrosil is not sinusoidal butappreciably distorted.

For satisfactory application of a non-linear resistor (Metrosil), the current through the non-linear resistor at the relay voltage setting should be as low as possible but no greater thanapproximately 30mA rms for 1A CTs and approximately 100mA rms for 5A CTs.


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(AP) 6 MiCOM P120, P121, P122 & P123

2. APPLYING THE MICOM P12X RANGE

The P12x range are numerical phase overcurrent and earth fault relays with 3 stages of

phase and/or earth fault protection, I>/ Ie>, I>>/ Ie>> and I>>>/ Ie>>> which can be used in

high impedance applications. The P121, P122 and P123 are three phase relays and can beapplied for use as 3 phase differential protection or restricted earth fault (REF) protectionwhereas the P120 is a single phase relay which can be used for REF protection only.

The protection element used for this specific application depends upon the type of P12xrelay chosen. In the case of the P120 and P121 relays and the first two stages of protectionon the P122 and P123, all stages use an identical Fourier algorithm. However, the third

stage (I>>>/ Ie>>>) on the P122 and P123 may be set to use an element that is peak

measuring (samples) which gives a significant improvement in operating time over theFourier algorithm employed by the other protection elements. In all cases the time delaycharacteristic should be set to definite time (DT) and with a setting of zero seconds to giveno intentional delay.

The Trip Commands menu (AUTMAT.CTRL) should be used to allocate the chosen

protection elements (e.g. tI>>> etc) to the trip relay RL1. Any elements allocated in the trip

commands menu will cause RL1 to pulse for 100ms but for applications where a 100mspulse is insufficient, the pulse time can be extended up to 5 seconds in the tOpen pulse cell(AUTOMAT.CTRL/CB Supervision). This feature is not available in the P120 and P121models but an alternative approach, which is available in all P12x relays, is to latch the tripoutput following a protection operation. This can be done by selecting the appropriate

protection element (e.g. tI> etc) in the Latch Functions menu (AUTOMAT.CTRL).

Any elements not being used should be disabled by selecting No in the appropriatelocation.

The setting ranges of the P12x elements are:

Phase Overcurrent Input Board Rating: 0.1 to 40 In

I> 0.1 25 In

I>> 0.5 40 In

I>>> 0.5 40 In

Input Board RatingEarthFault 0.002 to 1 Ien 0.01 to 8Ien 0.1 to 40 Ien

Ie> 0.002 1 Ien 0.01 8 Ien 0.1 25 Ien

Ie>> 0.002 1 Ien 0.01 8 Ien 0.1 40 Ien

Ie>>> 0.002 1Ien 0.01 8

Ien 0.5 40

Ien

Further details of the general setting capability and use of the MiCOM P12x relay may befound in the Technical Guide (P12x/EN T).

Since the performance of the P12x relay varies depending upon application, considerationmust be given to which relay and protection element to use. The following data should beused as a guide in order to obtain the best performance from the relay.

NOTE: It is recommended that the dual powered MiCOM P124 relay is notused for differential protection because of the start-up time delaywhen powered from the CTs alone. Additionally, the minimum setting

of the phase overcurrent elements, 0.2In, would limit its application for

differential protection.


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MiCOM P120, P121, P122 & P123 (AP) 7

2.1 Restricted earth fault (REF or [87N]) applications

For REF applications it is generally recommended that the 0.01 to 8 Ien input board is usedas this provides a good compromise of sensitivity and CT sizing. For advice on using theother earth fault input boards (0.002 to 1 Ienand 0.1 to 40Ien), please contact AREVA T&DUK Ltd, or your local representative.

On the P120 or P121 any of the three earth fault elements may be used as they all use thesame Fourier based algorithm. However, on the P122 and P123 the third stage element,

Ie>>>, may be used and set to peak measuring (samples) as this will give improved

operating times.

After extensive conjunctive testing of these elements in this application it has been foundthat the value of K used in equation [1] should be 1 and that generally a ratio of VK/VS4 willbe adequate. In other words, for REF applications we can state that;

( )

SK

LCTFS

4VVvoltage,kneepointCTand

R2RVVoltage,StabilityRelay

+= I

The recommended relay current setting for restricted earth fault protection is usuallydetermined by the minimum fault current available for operation of the relay and wheneverpossible it should not be greater than 30% of the minimum fault level.

Figures 2 to 5 show how the P12x earth fault elements can be utilized in REF applications.

Figure 2: Restricted earth fault protection of a 3 phase, 3 wire system applicable tostar connected generators or power transformer windings with neutralearthed at generator/transformer starpoint. (1A connections shown)


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(AP) 8 MiCOM P120, P121, P122 & P123

Figure 3: Balanced or restricted earth fault protection for the delta winding of apower transformer with supply system earthed (1A connections shown)

Figure 4: Restricted earth fault protection of a 3 phase, 4 wire system applicable tostar connected generators or power transformer windings with neutralearthed at switchboard (1A connections shown)


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MiCOM P120, P121, P122 & P123 (AP) 9

Figure 5: Restricted earth fault protection of a 3 phase, 4 wire system applicable tostar connected generators or power transformer windings with neutralearthed at generator/transformer starpoint. (1A connections shown)

2.2 Three phase ([87]) applications

On the P121 any of the three overcurrent elements may be used as they all use the sameFourier based algorithm. These elements are suitable for applications where the system X/Rratio does not exceed 40. Higher system X/R ratios will have a detrimental effect on relayoperating time and hence it is recommended that the third stage element set to peakmeasuring (samples) of a P122 or P123 is used as this will give improved performance.

After extensive conjunctive testing of these elements in this application it has been foundthat the value of K used in equation [1] should be 1.4 and that generally a ratio of VK/VS4will be adequate. In other words, for three phase applications we can state that;

( )

SK

LCTFS

4VVvoltage,kneepointCTand

R2R1.4VVoltage,StabilityRelay

+= I

For busbar protection, it is considered good practice by some utilities to set the minimumprimary operating current in excess of the rated load such that if one of the currenttransformers becomes open circuit, the high impedance relay does not maloperate. Otherutilities prefer to set the relay as sensitive as possible and use a second check relayconnected to a separate set of current transformers to provide security against maloperation.

For generator and motor protection, the relay is generally set as sensitive as possible tomaximize fault coverage. In these applications it is important to recognize that the externalfault level contribution from the generator or motor may be significantly lower than theinternal fault level. The external fault level should be used for the calculation of stabilityvoltage and resistor requirements whereas the internal fault level (that includes systemcontribution) should be used for non-linear resistor (Metrosil) calculations.

Figures 6 to 8 show how the P12x overcurrent elements can be utilized in differentialprotection applications.


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(AP) 10 MiCOM P120, P121, P122 & P123

Figure 6: Phase and earth fault differential protection for generators, motors orreactors (1A connections shown)

Figure 7: Phase and earth fault differential protection for an auto-transformer with

CTs at the neutral star point (1A connections shown)


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MiCOM P120, P121, P122 & P123 (AP) 11

Figure 8: A simple, single zone phase and earth fault busbar protection scheme withbuswire supervision (1A connections shown)

2.2.1 Buswire supervision

The Ie> earth fault element with its low current settings can be used for buswire supervision.

When a CT or the buswires become open circuited the 3 phase currents will becomeunbalanced and residual current will flow. This infers that operation for three phaseproblems in the CTs or buswires is not guaranteed since the currents through the

supervision element may summate to zero. The Ie> earth fault element should give an

alarm for most open circuit conditions but will not stop a maloperation of the differential

element if it is set below rated load. For this reason, a check zone relay is usually used (fedfrom a separate set of CTs mounted on incoming and outgoing feeders only) to ensure thatthe busbar is stable even though there may be a wiring or CT problem.

The supervision element should be time delayed to prevent spurious operation duringthrough fault conditions. The supervision element should be used to energize an auxiliaryrelay with mechanically latched contacts connected to short circuit the buswires and renderthe busbar zone protection inoperative thereby preventing thermal damage to the scheme.Contacts may also be required for busbar supervision alarm purposes.

Whenever possible the supervision primary operating current should not be more than 25A

or 10% of the smallest circuit rating, whichever is the greater. The earth fault element (Ie>)

should be connected at the star point of the stabilizing resistors, as shown in Figure 8. The

time delay setting for the supervision elements (tIe>) is generally 3 seconds or more.

NOTE: It is important that the residual current is checked when the busbar isunder load to ensure that the supervision threshold Ie>, is set aboveany standing unbalance current. The measured residual current maybe viewed in the Measurements of the relay.

The earth fault based supervision element may be supplemented with a spare phase

protection stage (I>) set to the same setting as the Ie> element, or the lowest available

setting of 0.1In, if Ie> is set lower than this.

2.3 Typical operating times

Typical operating times for different VK/VSratios are shown in the following tables:

VK/VSratio 2 4 8 16

Typical operating time (in ms) 48 37 33 18

Table 1: P122 or P123 Third Stage (I>>>) element set to peak measuring (samples)


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(AP) 12 MiCOM P120, P121, P122 & P123

These times are representative of system X/R ratios up to 120 and a fault level of 5Is orgreater.

VK/VSratio 2 4 8 16

Typical operating time (in ms) 49 38 34 32

Table 2: P120 & P121 (all stages), and P122 or P123 First & Second stage (I>, I>>)elements

The operating times shown in table 2 are representative of an X/R ratio up to 40. For X/Rratios greater than 40 it is strongly recommended that the P122/P123 I>>> element is used.

2.4 Advanced application requirements

As stated in section 2.1 and 2.2 a ratio of VK/VS4 will be appropriate for most P12x highimpedance applications. However, in some instances the VScalculated using the standardformula may lead to large kneepoint voltage requirements, especially for busbar applications.In this case, it is possible to reduce the VS setting based upon transient stability limitsderived from the conjunctive testing.

For three phase applications where the system X/R ratio is less than 40, the formula for therequired voltage setting VSmay be modified to:

( )LCTFS R2R05.1R

X007.0VVoltage,StabilityRelay +

+= I ---[8]

If the system X/R ratio is more than 40, the standard formula should be used with the K-factor being 1.4.

This reduced stability voltage only applies to three phase applications the stability voltagecalculation for REF applications cannot be modified for the P12x relays.

2.5 Recommended non-linear resistors

2.5.1 Metrosils

The Metrosil units normally recommended are as follows:

Metrosil Units for 1A CTs

Recommended Metrosil TypeRelay Stabiliy Voltage Setting (VS)

Single Pole Three Pole

Up to 125V rms600A/S1/S256

C = 450

600A/S3/I/S802

C = 450

125 300V rms600A/S1/S1088

C = 900

600A/S3/I/S1195

C = 900

Metrosil Units for 5A CTs (single pole shown only)

Recommended Metrosil Type

Relay Stability Voltage Setting (VS)SecondaryInternal Fault

Current Up to 200V rms 250V rms 275V rms 300V rms

50A rms600A/S1/S1213

C = 540/640

600A/S1/S1214

C = 670/800

600A/S1/S1214

C = 670/800

600A/S1/S1223

C=740/870

100A rms600A/S2/P/S1217

C = 470/540

600A/S2/P/S1215

C = 570/670

600A/S2/P/S1215

C = 570/670

600A/S2/P/S1196

C = 620/740

150A rms600A/S3/P/S1219

C = 430/500

600A/S3/P/S1220

C = 520/620

600A/S3/P/S1221

C = 570/670

600A/S3/P/S1222

C = 620/740

Table 3: Recommended metrosils for use with the MiCOM P12x relays


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MiCOM P120, P121, P122 & P123 (AP) 13

The single pole Metrosil units recommended for use with 5A CTs can also be used with triplepole relays and consist of three single pole units mounted on the same central stud butelectrically insulated from each other. A triple pole Metrosil type and the reference shouldbe specified when ordering. Metrosil units for higher stability voltage settings and faultcurrents can be supplied if required.

2.5.2 Integrated non-linear resistor and resistors

As an alternative to utilizing individual resistors and Metrosils, it is also possible to utilize anintegrated unit of resistors and non-linear resistors (X2/DVZ3R). The X2/DVZ3R unit

contains a number of fixed resistors capable of providing a stabilizing resistance of 500,

1000 or 2000 per phase, dependent upon connection. Since the resistances are fixedvalue it must be considered that limitations in the available stability voltage will occur. Forexample, the minimum stability voltage that can be achieved with a 0.1A setting is 50V, 100V

and 200V for the 500, 1000or 2000 resistances. The connections for the X2/DVZ3Runit are shown in figures 9 to 11.

Figure 9: X2/DVZ3R connections for 500 stabilizing resistance (1A connectionsshown)



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(AP) 14 MiCOM P120, P121, P122 & P123


In addition to the stabilizing resistors, non-linear resistors are included for each phase thatwill limit the voltage across the circuit to approximately 2200V.

Figure 12: Characteristic of the non-linear resistors used in the X2/DVZ3R


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MiCOM P120, P121, P122 & P123 (AP) 15

When using the X2/DVZ3R integrated unit, it is recommended that following operation of thedifferential element, the stabilizing resistor or the entire differential path be short-circuited tohelp reduce the power dissipation requirements of the scheme. This shorting can beperformed using contacts of the P12x relay itself or an external auxiliary device triggeredfrom the P12x high impedance trip function. If the entire differential path is bypassed in thismanner, it may be necessary to ensure that the trip contacts are latched at least until the

fault has been cleared.

Figure 13: Example connection of the X2/DVZ3R with the entire differential pathshort-circuited following operation. (1A connections shown)

Figure 14: Example connection of the X2/DVZ3R with the stabilizing resistor short- circuited following operation. (1A connections shown)


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(AP) 16 MiCOM P120, P121, P122 & P123

3. SETTING EXAMPLES

3.1 Restricted earth fault application

The application of a P12x relay as a high impedance relay can be best illustrated by meansof an example. Figure 13 shows an application for which REF protection is required on theLV winding of a power transformer. In this example the earth fault input is used to providethe restricted earth fault protection and any remaining phase inputs could be utilized toprovide normal overcurrent based protection.

Figure 15: Restricted earth fault protection on a transformer

3.1.1 Stability voltage

The transformer full load current, IFLC, is:

1391.2A

3415

1MVAFLC

=

=I

Maximum through fault level, ignoring source impedance, IF, is:

27824A5%

1391.2A

X

TX

FLCF

==

=I

I

Required relay stability voltage, VS, and assuming one CT saturated is:

( )

( )

35.24V=

+=

+=

08.03.01500

527824x

2RRKV LCTFS I

3.1.2 Stabilizing resistor

Assuming that the relay effective setting for a solidly earthed power transformer isapproximately 30% of the full load current, we can calculate that a setting of less than1.391A is required on the relay. Assuming that a setting of 1A is selected the stabilizingresistor, RST, required is:


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MiCOM P120, P121, P122 & P123 (AP) 17

=>

= 35.24e

VR SST

I

For this application a 47resistor can be supplied on request which can be adjusted to any

value between 28and 47. Thus a setting of 35.2is available.

3.1.3 Current transformer requirements

To ensure that internal faults are cleared in the shortest possible time the kneepoint voltageof the current transformers should be at least 4 times the stability voltage:

141V

4VVVoltage,Kneepoint SK

By re-arranging equation [2], the excitation current for each of the current transformers at therelay stability voltage can be calculated:

0.1A

4

1391.1

nRatioCTvoltage,stabilityatcurrentgMagnetisinCT

RS

e

II

I

In summary, the current transformers used for this application must have a kneepoint voltage

of 141V or higher, with a secondary winding resistance of 0.3or lower and a magnetizingcurrent at 35.2V of less than 0.1A.

3.1.4 Non-linear resistor requirements

If the peak voltage developed across the relay circuit under maximum internal fault

conditions exceeds 3000V peak then a suitable non-linear resistor (Metrosil) should beconnected across the relay and stabilizing resistor, in order to protect the insulation of theCTs, relay and interconnecting leads. Using equation [5] we can calculate the maximumfault voltage assuming no CT saturation:

( )

( )

3303.6V

2)92.75(35.6

24.3508.03.01500

527824x

RRR2RV RSTLCT'FF

=

=

++=

+++= I

Based upon this and assuming that the CT kneepoint voltage is 141V, we can estimate thepeak voltage as:

( )

( )

1888.76V

141-3303.62x1412

VV2V2V KFKP

=

=

=

This value is below the peak voltage of 3000V and therefore a non-linear resistor (Metrosil)is not required.

NOTE: The kneepoint voltage value used in the above formula should be theactual voltage obtained from the CT magnetising characteristic and

not a calculated value.


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(AP) 18 MiCOM P120, P121, P122 & P123

3.2 Busbar applications

Figure 14 shows an application of the P12x for high impedance protection of a typical 132kVdouble bus generating station, consisting of two 100MVA generators and step-uptransformers, two bus-couplers and four overhead transmission lines. Each busbar issectionalized and therefore gives a requirement for four discriminating zone and one overall

check zone.

Figure 16: Three phase and earth fault protection of a double busbar generatingstation

For the purpose of this example it is assumed that all switchgear is rated for 3500MVA, thatthe system is solidly earthed and the system X/R ratio is 20. All circuits have the same CT

ratio of 500/1A with a secondary winding resistance of 0.7 and the largest loop leadresistance is 2.

3.2.1 Stability voltage

The stability level of the busbar protection is governed by the maximum through fault levelwhich is assumed to be the switchgear rating. Using the switchgear rating also allows forany future expansion of the busbar:

15308A

3132kV

3500MVA

3xVoltage

ratingSwitchgearcurrent,faultthroughMaximum F

=

=

=I

Required relay stability voltage, VS, and assuming one CT saturated is:

( )

( )

116V=

+=

+=

20.7500

11.4x15308x

2RRKV LCTFS I

3.2.2 Discriminating zone effective setting

The primary operating current should be made less than 30% of the minimum fault currentand more than the full load current of one of the incomers. Thus, if one of the incomer CTsbecomes open circuit the differential protection will not maloperate. It is assumed that 30%of the minimum fault current is more than the full load current of the largest circuit.


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MiCOM P120, P121, P122 & P123 (AP) 19

437.4A

3132kV

100MVAcurrent,LoadFull FLC

=

=I

Now if we assume that the magnetizing current taken by each CT at 116V is 0.072A and the

relay current setting is 0.8A, we can calculate for the discriminating zone:

( )

( )( )

FLC

OP

of132%

580A

5x0.0720.81

500

nRatioCTcurrent,OperatingEffectivePrimary

I

IIIer

=

=

+=

+=

Since the primary effective setting is greater than the full load current, we can say that thatrelay setting of 0.8A is suitable for the discriminating zone.

3.2.3 Check zone effective setting

Using the same reasoning and assumptions as already used for the discriminating zone, wecan calculate for the check zone:

( )

( )( )

FLC

OP

of141%

616A

6x0.0720.81

500

nRatioCTcurrent,OperatingEffectivePrimary

I

IIIer

=

=

+=

+=

Since the primary effective setting is greater than the full load current, we can say that thatrelay setting of 0.8A is suitable for the check zone.

3.2.4 Stabilizing resistor

The required value of stabilizing resistor, RST, is:

=

=

>=

145

0.8

116

VR SST

I


value between 132and 220. Thus a setting of 145is available.

3.2.5 Current transformer requirements


464V


In summary, the current transformers used for this application must have a kneepoint voltage

of 464V or higher, with a secondary winding resistance of 0.7or lower and a magnetizingcurrent at 116V of less than 0.072A.


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(AP) 20 MiCOM P120, P121, P122 & P123

3.2.6 Non-linear resistor requirements

If the peak voltage developed across the relay circuit under maximum internal faultconditions exceeds 3000V peak then a suitable non-linear resistor (Metrosil) should beconnected across the relay and stabilizing resistor, in order to protect the insulation of theCTs, relay and interconnecting leads. Using equation [5] we can calculate the maximum

fault voltage assuming no CT saturation:

( )

( )

4522V

.7)30.616(147

14527.0500

115308x

RRR2RV RSTLCT'FF

=

=

++=

+++= I

Based upon this and assuming that the CT kneepoint voltage is 464V, we can estimate thepeak voltage as:

( )( )

3881.1V

464-45222x4642VV2V2V KFKP

=

=

=

This value is above the peak voltage of 3000V and therefore a non-linear resistor (Metrosil)is required. The Metrosil is chosen based upon the CT secondary rating, maximum internalfault level and voltage setting, and for this case the most appropriate type is600A/S3/I/S1195. This Metrosil consists of three discs connected independently for thisthree phase application.

NOTE: The kneepoint voltage value used in the above formula should be theactual voltage obtained from the CT magnetising characteristic and

not a calculated value.

3.2.7 Busbar supervision

Assuming that 25A is greater than 10% of the smallest circuit current we can calculate thesupervision setting as:

0.05A

RatioCT

25AsettingnsupervisioBuswire

=

=

Therefore the Ie> element should be set to 0.05A with a time delay setting, tIe>, of 3s.

3.2.8 Integrated non-linear resistor and resistors

Instead of using the external resistors and Metrosils, the X2/DVZ3R integrated unit could be

used in this application. In this case, the stabilizing resistance is fixed as either 500,

1000or 2000which infers that the relay setting current must be modified. (The stabilityvoltage is fixed by the requirements of the scheme). On this basis, and by rearranging thestabilizing resistor calculation we can state:-

( )

( )

( )==

==

==

=>

2000RforA06.0

1000RforA12.0

500RforA24.0

R

VSetting,RelayRequired

ST

ST

ST

ST

SI

Since the minimum setting for the phase overcurrent element, I>, is 0.1A it can be seen that

it is not possible to use the 2000stabilizing resistor, but that either the 500 or 1000


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MiCOM P120, P121, P122 & P123 (AP) 21

resistance connections could be used. In both cases, it would be necessary to use theintegrated non-linear resistor to protect the circuit.

Another consideration of utilizing this integrated box is the effect of the lower relay settingson the effective setting of the scheme. With either lower setting, the effective setting will belower than full load current for all the discriminating and check zones and this may be

perceived as unsatisfactory. In this case, additional shunt resistors may be required toincrease the effective setting please contact your local AREVA T&D representative foradvice in this case.

3.2.9 Advanced application requirements

In order to reduce the kneepoint voltage of the CTs being used, it is possible to decrease thestability voltage for applications where the system X/R ratio is lower than 40. In thisexample, the system X/R ratio is 20 and therefore we can apply formula [8] to reduce thestability voltage:

( )

( ) ( )

98.37V

27.0500

153081.050.14

R2R05.1R

X007.0VVoltage,StabilityRelay LCTFS

=

++=

+

+= I

Based upon this revised stability voltage, it is possible to revise the stabilizing resistor to:

=

=

>=

231

0.8

98.37

VR SST

I


value between 90and 150. Thus a setting of 123is available.


394V


In summary, the current transformers based upon the advanced formula must have a

kneepoint voltage of 394V or higher, with a secondary winding resistance of 0.7or lowerand a magnetizing current at 98.4V of less than 0.072A. This is a reduction of kneepointvoltage of nearly 20% when compared to the calculated requirements based upon thestandard formula.

3.3 Motor / generator applications

The same calculation principles that apply for the three phase protection of busbars can beapplied to motors and generators, although special consideration of the fault current will berequired. For these applications, the machine contribution to an external fault should beused in the stability voltage calculation and this can be significantly lower than the maximuminternal fault current that should be used in the non-linear resistor calculation. For motorsthe starting current or locked rotor current value is usually used in the stability calculationand this will lead to relatively small CT requirements. Often the most sensitive relay settingsare used and buswire supervision is rarely applied.


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(AP) 22 MiCOM P120, P121, P122 & P123


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P

ublication:

P12X/EN

AP/B00

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