Differential Relays Numerical Differential Relays True Unit Protection unit protection: a protection whose operation and section selectivity are dependent on the comparison of electrical quantities at each end of the protected section. CTs which provide these electrical quantities define the boundaries of the protection zone

Differential Relays Kirchhoff's knot Rule Currents in = Positive Currents out = Negative

Differential Relays Numerical Differential relays Basic Principle Calculates the sum of all currents flowing in and out of the protected object. For non faulted protected object this current should be zero Except There is magnetising current Charging currents CT errors Tap changers Currents entering the zone are not detected by the CTs at the end of the zone and a differential current exists.

Differential Relays Numerical Differential relays CT errors To guard against CT errors the pickup threshold is increased in proportion to the total flow current Also called the stabilising current Applications Differential protection is easier for objects where all CTs are located close to each other. Such as: Bus Bars Transformers Motors Generators CTs from all protected terminals are connected directly to the relay. Only one relay required as long as it has the required inputs for each of the protected ends.

Differential Relays Numerical Differential relays Applications for Feeder Protection CTs from each terminals can be 100’s of km apart Protection achieved via communications channels. Eg. Fibre Microwave Feeder protection require the same number of relays as the number of protected terminals.

Differential Relays Numerical Differential relays Advantages of modern relays Disturbance recording Operational Measurement Remote interrogation Test metering Multifunctional (87 and 21, 50, 60, 67 etc) Improved filters for inrush Smart algorithms provide CT saturation stabilisation Self monitoring Monitoring of communications channels

Differential Relays Numerical Differential relays Advantages of modern relays Phase segregated measurement Numerical adaptations of CTs Ratio Correction Vector Correction Transformers and Transformer ended feeders Protection of multi terminal feeders Two ended feeder and three ended are common Four ended rare Five & Six ended – haven’t done one yet SIPROTEC 4 7SD5 ( 2 to 6 ended protection)

Differential Relays Numerical Differential relays Terms Specific to Differential relays Longitudinal Differential Protection Comparison of the magnitude, or; Comparison of the angle Transverse Differential Protection Protection on parallel connected circuits Protection based on the unbalance distribution of currents between them Biased Differential Pickup threshold that increases in proportion to the increasing through current

Differential Relays Numerical Differential relays Terms Specific to Differential relays Phase Comparison Protection Based on the comparison of the phase of the currents The channel conveys only the phase information with respect to the current Sends either the waveform is above or below the axis Same determination is made at the local relay After allowing for the communication channel delay a comparison is made.

Differential Relays Numerical Differential relays Phase Comparison Protection Note that the system, even if implemented digitally, does not call for synchronization of the individual relays. This is because the information exchanged is encoded via timing of the pulses related to the same “analog” or “continuous” time. The core trip decision may be as fast as 6 to 8 ms, and is generally under one cycle, plus channel delays and processing delays in the relays.

Differential Relays Numerical Differential relays Phase Comparison Protection Current Differential uses both magnitude and phase information, and is therefore prone to errors in either of these two components. Phase Comparison, in turn, uses the phase information only in terms of timing a particular current polarity, and therefore is much less sensitive to magnitude errors. As a rule, Phase Comparison is a more secure principle except in cases where low signal magnitude makes the phase information less accurate (such as on series-compensated lines). (GE)

Differential Relays Numerical Differential relays Restricted Earth Fault The residual current from a set of 3 phase CTs is balanced against the residual output from CTs located in the earthing connection of the system

Differential Relays Numerical Differential relays Terms Specific to Differential relays Alpha (α) and Beta (ß) Plane The operating characteristic in the complex plane α = IA/ IB ß = IB/ IA IA is the local terminal IB is the remote terminal Term is generally applied to feeder protection

Differential Relays Numerical Differential relays Protection is: 100% selective Responds to faults within the zone Zones defined by the location of CTs Grading with other protection not required Can be set to trip instantaneously Principle is straight forward Stability for through fault is a concern May be used for Fdrs.“too short” for distance protection Can protect feeders 100’s of km long – (Good Comms)

Differential Relays Numerical Differential relays Basic Principle No current flows in the relay for a through fault I1+I2 flows in the relay for an internal fault From Ziegler – Numerical Differential Protection

Differential Relays Numerical Differential relays Biased Differential Protection False differential currents can arise because of: Transformer Taps Charging currents Magnetising currents CT errors Ziegler – Numerical Differential Protection

Differential Relays Numerical Differential relays Biased Differential Protection Restraint and Operating current IREST = |I1|+ |I2|+ |I3|+ … |In| ( Rectified Mean) IOP = |I1+ I2+ I3 + … In| Pickup can be set to IOP > k * IRES and IOP > B K is the biased factor B is the pickup threshold

Differential Relays Numerical Differential relays Biased Differential Protection Mechanical & Electrical Representation Differential Relay as defined by McCroll

Differential Relays Numerical Differential relays Biased Differential Protection For an external fault IREST = 2 x the through fault current IOP = 0 For an internal fault IREST = Internal fault current IOP = Internal fault current

Differential Relays Numerical Differential relays Biased Differential Protection External fault – calculated example 2000 A entering the zone 2000 A leaving the zone Internal fault – calculated example 2000 A entering the zone from Side 1 2000 A entering the zone from side 2

Differential Relays Numerical Differential relays Biased Differential Protection Fault with fault Resistance 2500 A entering the zone 500 A leaving the zone ( Supplying load) 2000 A into the fault

Differential Relays Numerical Differential relays Biased Differential Protection All manufactures do not use the same equations for their calculations Some Relay manufactures may only use half of the current sum as the restraint quantity Consideration has to be taken when comparing characteristics across different manufacturers Read the manual!

Differential Relays Numerical Differential relays High Impedance Differential Protection CTs connected so that through currents circulate CTs have to be the same CT ratio CTs can’t be used for other relays HZ differential relay is connected as a shunt Sensitive overcurrent relay High impedance in series Becomes a voltage operated relay Voltage is small during load Internal Resistance of CT and CT cable burden on both sides of the bridge are same Plant has to be galvanically connected

Differential Relays Numerical Differential relays High Impedance Differential Protection Stability on through faults Ideal CTs No Saturation Actual CTs Saturate due to the high impedance Assume 1 CT saturated (worst case) Most unbalance All other CTs perform ok Saturated CT is shorted ( no output) Internal resistance of CT remains in circuit.

Differential Relays Numerical Differential relays High Impedance Differential Protection Stability on through faults Voltage across the shunt path is: VShunt = ((ITHRU)/CTRATIO ) * (RLEADS + RCT ) ITHRU = Primary Through Fault Current RLEADS + RCT = resistance of CT and CT leads CTRATIO = CTPRIMARY/CTSECONDARY

Differential Relays Numerical Differential relays High Impedance Differential Protection Stability on through faults Calculated Example

Differential Relays Numerical Differential relays High Impedance Differential Protection Pickup Sensitivity The minimum primary fault current must: Provide the magnetising current of all the connected CTs Provide the pickup current of the shunt connected relay Provide leakage current of voltage limiting varistor Imin = CTRATIO(n * ImR+IR+IV) n = no of feeders in parallel ( 2 in our example) ImR = magnetising current at the relay pickup voltage (30mA) IR = 20 mA IV = 0 ( not connected)

Differential Relays Numerical Differential relays High Impedance Differential Protection Pickup Sensitivity Worked Example

Differential Relays Numerical Differential relays High Impedance Differential Protection Required CT knee point voltage CTs go into saturation for internal faults ( due to High Z) Theory and Practical experiments show that a knee point of 2 x the relay pickup threshold is required. For our case we had worked out a setting of 150V therefore a knee point of at least 300V would be required. As mentioned earlier CTs with same ratios and preferably of the same construction should be used.

Differential Relays Numerical Differential relays High Impedance Differential Protection Application Notes Secondary leakage reactance should be negligibly small Secondary internal resistance should be small This minimises voltage across it during saturation Small magnetising current result in sensitive threshold Large core cross section required This leads to more copper length Leads to more resistance CT dimensions are limited due to switch gear It can be shown that there is an optimum ratio for CTs. This depends on all the application parameters but is generally about 2000/1 – (Alstom NPAG)

Differential Relays Numerical Differential relays High Impedance Differential Protection Response to heavy internal Faults All CTs feed current into the high impedance relay Voltage on all CTs raise sharply CT saturation occurs Large voltages endanger insulation Voltage limitation required Apply Varistors ( Brand Name – Metrosil) Size can be considerable based on the number of CTs

Differential Relays Numerical Differential relays High Impedance Differential Protection Metrosil Single or 3 element

Differential Relays Numerical Differential relays High Impedance Differential Protection Calculation of Maximum Voltage Vpeak = 2 * Sqrt(2 * Vkp * (VF – Vkp)) Vkp = kneepoint voltage of CT VF = Maximum voltage in shunt if no CT saturation occurred Above equation applies if Vkp < VF /2 If Vkp much smaller then Vpeak = 2 * Sqrt(2 * Vkp * VF )

Differential Relays Numerical Differential relays High Impedance Differential Protection Is a Varistor required ? CTRATIO = 1000/1 Vkp = 400 V (Knee point voltage) Relay Setting = 150V Relay pickup Threshold = 20mA Relay internal resistance = 150V/20mA

Differential Relays Numerical Differential relays High Impedance Differential Protection Is Varistor required ? Calculation Example:

Differential Relays Numerical Differential relays High Impedance Differential Protection Summary of Requirement (HZ scheme) All CTs must be the same ratio Can be used with ratio correction CTs (avoid if possible) No other devices can be connected to the CTs (saturation) Varistors required (limit voltage prevent insulation damage) Plant to be galvanically connected Can’t use across magnetically coupled devices Low Impedance Bus Zone will be visited in the relay setting example

Differential Relays Numerical Differential relays Restricted Earth Fault Used as supplementary protection on Transformers High Impedance Type Residual line current is balanced against Neutral CT current Low Impedance Type All currents become inputs into the differential element Used for impedance earthed system Conventional E/F does not provide adequate protection Grading issues Also found in solidly earthed systems

Differential Relays Numerical Differential relays Restricted Earth Fault Although biased differential protection provides excellent protection for phase-to-phase and most phase-to-earth winding faults, this element is less sensitive for single phase to earth faults close to the neutral point in solidly earthed transformers. For these faults, phase currents change very little but large currents flow in the neutral conductor. Labuschagne & Merwe

Differential Relays Numerical Differential relays Restricted Earth Fault Advantages Whole fault current is measured and not only the transformed component in the HV primary winding (assuming secondary winding is star). While fault current decreases as faults get nearer the neutral, it is not affected by the square law which controls the primary current. ( The reactance is inversely proportionate to the square of the short circuited turns) For impedance earthed systems the fault current characteristic is linear Virtually complete cover of earth faults can be obtained.

Differential Relays Numerical Differential relays IP IF Restricted Earth Fault Impedance earthed Fault current is linear Primary current reduces closer to neutral IP IF From NPAG

Differential Relays Numerical Differential relays Restricted Earth Fault Solid Earth Fault high close to neutral Primary current reduces closer to neutral From NPAG

Differential Relays Numerical Differential relays Restricted Earth Fault High Impedance Connection Three phase CTs and Neutral CTs are paralleled Calculations are similar to that shown for the HZ Diff Stability is based on the max through fault current Use short circuit reactance of the transformer HZ can also be used for the delta winding shown System must be earthed Sum of phase currents must be zero (balanced system) In an earth fault current sum > 0  pickup Ziegler

Differential Relays Numerical Differential relays Restricted Earth Fault LZ Connection Siemens (7UT relay) Io* = IN Io** = IA+IB+IC Stabilising Current Is = | Io* - Io**|- | Io* + Io**| Tripping Condition | Io* | > Iset + ko +Is ko controls the tripping range, Faults between the CTs covered CTs brought into the relay individually External fault only stabilising current flows No differential current flows Current angle between Io* and Io** to be between 0 to <90 for tripping Ziegler

Differential Relays Numerical Differential relays Restricted Earth Fault High or Low Impedance? Sensitivity is an issue for impedance earthed system Consider this example: from (Labuschgne & Merwe) CTs in HZ scheme draw 15mA at operating voltage Relay operating current is 20mA Total secondary current = (4 * 15mA + 20 mA) =80mA 4 is the number of CTs Using a CT of 200/1 we get 16A. If our current is limited to 355 A (due to NER) we get: 16/355 = 4.5% This amount of winding that is not covered

Differential Relays Numerical Differential relays Restricted Earth Fault High or Low Impedance In the same example now consider a LZ scheme CTs only draw 2mA of magnetising current due to the low voltage across them Relay Draws 50mA Total secondary current = (4 * 2mA + 50 mA) =58mA Using 200/1 and an NER limiting current to 355A We get a primary current of 11.6A (before 16A for HZ) Clearly the Low Impedance is more sensitive in this case

Differential Relays Numerical Differential relays Restricted Earth Fault High or Low Impedance In the same example now consider a HZ scheme CTs only draw 2mA ( Now using better quality CTs) Relay operating current is 20mA Total secondary current = (4 * 2mA + 20 mA) =28mA Using a CT of 200/1 we get 5.6A. If our current is limited to 355 A(NER) we get: 5.6/355 = 1.6% This amount of winding that is not covered Clearly better than the LZ Use Good CTs – but the AM always wants to buy cheap ! Ok - Use cheap ones – no gold plating.

Differential Relays Numerical Differential relays High Impedance Overall Transformer Protection Only on galvanically connected transformers Auto transformer 7 CTs 3 x HV side 3 x LV side 1 x Neutral CT All CTs same ratio Only responds to E/F From NPAG Note: Can’t use these CTs for inputs to other relays

Differential Relays Numerical Differential relays High Impedance Overall Transformer Protection Only on galvanically connected transformers Auto transformer 9 CTs 3 x HV side 3 x LV side 3 x Neutral CT All CTs same ratio Phase and E/F covered Note: Can’t use these CTs for inputs to other relays From NPAG

Differential Relays Numerical Differential relays High Impedance Overall Transformer Protection Provides high speed sensitive protection Unaffected by ratio changes (Taps) Immune to the effects of magnetising inrush Does not respond to inter-turn faults (rely on other protection) Will not detect faults in delta tertiary windings In Auto transformers delta tertiary is included Limits 3rd harmonic voltages caused by magnetising currents Lowers zero seq. impedance for 5 limb constructions Note: Can’t use these CTs for inputs to other relays

Differential Relays Numerical Differential relays Restricted Earth Fault High or Low Impedance Which to choose Is it solidly earthed? Either will do Which relay do you have Cost Is it impedance earthed Check Availability of CTS Same CT ratios of good quality Use HZ REF ( Less Expensive) Different CT ratios Must use LZ REF (More Expensive) Same CT ratios of less quality Use LZ REF Determine your sensitivity and make your selection

Differential Relays Numerical Differential relays Transformer Differential Requirements Different current values on HV & LV side Cope with different CT ratios Cope with angle shift (Vector group) Handling of Zero Sequence Currents Handle transient Inrush Handle CT saturation

Differential Relays Numerical Differential relays Transformer Differential Requirements Consider 100 MVA Yd5 transformer LV Side = 22kV LV current = 2887 A ( CT 3000/1) LV secondary current = 0.962 HV Side = 110kV HV current = 525 A (750/1) HV secondary current = 0.7 Different secondary current - compensation required Vector Group compensation required

Differential Relays Numerical Differential relays Transformer Differential From Siemens – Lippert & Muller

Differential Relays Numerical Differential relays Transformer Differential

Differential Relays Numerical Differential relays Transformer Differential CT Matching Some relays will calculate the factors Some may need correction factor to be calculated In our example:

Differential Relays Numerical Differential relays Transformer Differential Zero sequence removal required on the Star side as the Stat point is earthed. Discussed when doing the relay setting example. 110kV 20kV It is possible to have a condition when only zero sequence current flows via the CTs. 110kV supply with LV CB open and fault as shown

Differential Relays Numerical Differential relays Transformer Differential Zero Sequence Elimination Consider External Fault as shown A Ph fault (say 2400A) on Star side outside the zone Appears as Ph-Ph on the 20kV side. Which means that it has positive and negative sequence currents and NO Zero Seq. Where has it gone?

Differential Relays Numerical Differential relays Transformer Differential Zero Sequence Elimination

Differential Relays Numerical Differential relays Transformer Differential Zero Sequence Elimination on 110kV side Current leaving the Zone is designated -ve Zero Sequence Current has been removed. (2/3) of 4.57 is left = 3.048

Differential Relays Numerical Differential relays Transformer Differential Vector Correction Matrix (from Manual) No Io Elimination Delta Traps Io Compare results from LV Side Add them and they sum to zero Relay Stable!

Differential Relays Numerical Differential relays Transformer Differential Inrush of Transformers Can falsely trip transformers On energisation Sympathetic Inrush Different inrush currents in each phase Set an inrush blocking function. Inrush detects 2nd harmonic currents present during energisation 2nd Harmonic currents also present for CT saturation Careful selection required for blocking the relay No Cross Blocking All individual phases have to block Cross blocking 1 out of 3 2 out of 3 Time limitations with cross blocking ( Trip after a fixed duration)

Differential Relays Numerical Differential relays Transformer Differential Differential with no Bias Fast Unrestrained Evaluates fundamental wave as well as instantaneous values Not blocked by harmonics, CT Saturation or Inrush Set this above maximum through fault current ! For example if Transformer impedance is 13% Set higher than (1/0.13) * Rated Or determine the through fault current using your fault analysis software. Be sceptical – System analysis team don’t always get it right ! (Most times they do)

Differential Relays Numerical Differential Relays Feeder Protection Protection with Pilot Wires Two main approaches Opposed Voltage Principle Opposed Current Principle Opposed Voltage Principle - Ziegler

Differential Relays Numerical Differential Relays Protection with Pilot Wires Opposed Voltage Principle Current routed via a shunt resistor Voltages U1 & U2 Produced Voltages compared via the pilot wire For through fault or load voltages are in opposition No current flows Internal fault Voltages are in phase Current flows ( few mA) which trip the relays Scheme limited to 25km due to burden Varistor required to limit voltage Twisted pair used to limit voltage rises due to E/F Operating current = I1 + I2 Restraint Current = I1 - I2 Opposed Voltage Principle - Ziegler

Differential Relays Numerical Differential Relays Protection with Pilot Wires Circulating Current Principle Circuit similar to opposing voltage Operation and restrained are swapped The aux txfr. the pilot loop now supplies restraint current The aux txfr. the shunt now supplies operating current CTs are connected in phase opposition Cross over CT shown previously is not done Secondary voltages of the CT are in phase for load Circulating current flows Internal fault Voltages are in opposition With same in feeds Current is zero Opposed Voltage Principle - Ziegler

Differential Relays Numerical Differential Relays Protection with Pilot Wires Comparison of the principles Current Supervision can prevent Pilot failure trips Voltage Comparison Circulating Current No current during normal load Current during normal load No Current for through faults Current for through faults Broken Pilot - No Tripping Broken Pilot - Tripping Shorted Pilot – Trips for Through faults Shorted Pilot – Leads to Blocking of Trip Also called -Tripping Pilot Scheme Also called - Blocking Pilot Scheme Opposed Voltage Principle - Ziegler

Differential Relays Numerical Differential Relays Feeder Protection Digital Communications Matching and processing done numerically Numerical Filtering Advanced algorithms High measuring accuracy Implementation of adaptive techniques High Stability Shorter operating time

Differential Relays Numerical Differential Relays Feeder Protection Digital Communications Manufacturers have different techniques Two main types Continuous online measurement GPS time signal

Differential Relays Numerical Differential Relays Digital Communications Continuous Measurement The salient point is that this method assumes that the communications channel is identical in each direction. If the delays are not same, the error between them is equal to half the transmit – receive time difference . Protection of HV and EHV Systems – B Moor

Differential Relays Numerical Differential Relays Continuous Measurement Relays at A & B sample at TA1 & TA2 & TB1 & TB2 Times are not coincident At Time TA1 Relay A transmits data ((with time tag) to relay B Relay B receives it at TA1 +Tp1 (Tp1 is propagation time) This is recorded as TB* Relay B sends data in identical format at time (TB3) Received by relay A at TB3 + Tp2 ( Record at TA*) The last message from B to A includes time TB3 & the last relay time tag from relay TA1 & the delay time between the arrival of the message from A ( Called Td) The total elapsed time is (TA* -TA1)=(Td+TP1+Tp2)

Differential Relays Numerical Differential Relays Digital Communications GPS For applications where the communications channel is not symmetric The Relays are synchronised to the GPS time signal for accurate time stamping Calculations not required GPS signal to be capable of receipt in all weather conditions At the mercy of the US government Selective availability You hope they don’t go broke! Selective Availability (SA) was an intentional degradation of public GPS signals implemented for national security reasons. In May 2000, at the direction of President Bill Clinton, the U.S government discontinued its use of Selective Availability in order to make GPS more responsive to civil and commercial users worldwide.

Differential Relays Numerical Differential Relays Digital Communications (GE L90)

Differential Relays Numerical Differential Relays Feeder Protection Digital Communications (GE L90)

Differential Relays Numerical Differential Relays Feeder Protection Digital Communications (GE L90) L90 operates in a Master – Master Arrangement All relays make decisions independently Each relay must be able to communicate with each other If there is a failure of communication ( 3 Terminal) then relay operates in master slave. The relay with communication from two other relays is the master. The master Relay makes the calculations and communicates to the slave relays. Local trip also sends a DTT to the slave relays

Differential Relays Numerical Differential Relays Charging Current Long lines with significant capacitance can benefit from charging current compensation. Implement via desensitisation Static Eg. Siemens 7SD512 (No VT) Dynamic One relay introduces as 1.5 cycle delay on the diff if ANY CB is open ( effective for 3 cycles) Has obvious drawbacks

Differential Relays Numerical Differential Relays Charging Current Long lines with significant capacitance can benefit from charging current compensation if terminal voltage measurements are applied to the relay*. The voltage input is also used for some protection and monitoring features such as directional elements, fault locator, metering and distance backup. * Most relays

Differential Relays Numerical Differential Relays Charging Current Self-Compensation Take the average differential of last few cycles – (3 cyc.) Subtract from presently measured differential current Generally zero or close to it Caters for: CT error, Communications channel error; Tapped load Provides good high resistance cover. Not Voltage depended Can be used on multiple terminals Suspended during disturbance detection

Differential Relays Numerical Differential Relays Charging Current Only Consider the positive sequence What about the –ve and zero sequence ? In some 765 kV installations, the positive-sequence charging current is in the range of several hundred to over 1,000 A, and it may be higher than either the load or fault level. -SEL During line energization, the energizing terminal draws the total charging current. If the energizing voltage is balanced and the line well transposed, the charging current is composed predominantly of positive-sequence current and therefore only affects the phase elements. If the line is not well transposed and the total charging current is high, we may have to increase the pickup of the 87LQ and 87LG functions considerably, potentially diminishing their natural protection sensitivity. In general, line-charging current is not a major concern for the 87LQ and 87LG functions, unless the line is not well transposed or is operated under considerable unbalance (e.g., caused by single-phase reactor operation).

Differential Relays Numerical Differential Relays Calculation Example (Areva Relay on 33 kV system)

Differential Relays Numerical Differential Relays Charging Current Compensation for charging current requires the voltage at the terminals be supplied to the relays. The algorithm calculates for each phase, which is then subtracted from the measured currents at both ends of the line. This is a simple C * dv/dt approach that provides adequate compensation of the capacitive current at the fundamental power system frequency.

Differential Relays Numerical Differential Relays Charging Current If the VTs are connected in wye, the compensation is accurate for both balanced conditions (i.e. all positive, negative and zero sequence components of the charging current are compensated). If the VTs are connected in delta, the compensation is accurate for positive and negative sequence components of the charging current. Since the zero sequence voltage is not available, the relay cannot compensate for the zero sequence current. (GE)

Differential Relays Numerical Differential Relays Calculation Example Charging Current In some instances a B1 value will be available for the line data (usually in a pu format) To get the charging current simply : (B1 * Base current) at the Base MVA ( Usually 100 MVA) Using the previous example

Differential Relays Numerical Differential Relays Calculation Example Charging Current If the Capacitance data is available then the charging current can be calculated as:

Differential Relays Numerical Differential Relays Charging Current Different relays have different data requirements Positive and Zero sequence Reactance in kilo ohms is required in some relays Other relays may require the input in mS CT Saturation ( Add on stabilisation) Relays use a number of algorithms to detect CT saturation The relay is dynamically adjusted to provide stability from CT saturation

Differential Relays Numerical Differential Relays Errors due to synchronisation Errors due to conventional and nonconventional CTs Assume a relay synchronising method is based on equal channel delay. If the channel delays differ then HALF this difference appears an and angle error. Using a 50Hz system calculate the phase angle deviation For example a 1ms difference = 18 degrees