New Chapter 6 Areva Transformer Differential Protection

CHAPTER 6 AREVA TRANSFORMER DIFFERENTIAL PROTECTION

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AREVA MICOM P63X RELAY

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The MiCOM P63x series differential protection devices are intended for the fast and selective short-circuit protection of transformers, motors, generators and other installations with two, three or four windings, respectively. The MiCOM P63x series provides high-speed three-system differential protection using a triple-slope characteristic and two high-set differential elements in combination with transformer inrush restraint, over fluxing restraint and through-stabilization. Amplitude and vector group matching is done just by entering the nominal values of transformer windings and associated CTs. In addition many supplementary protective functions are incorporated in the devices. Protective functions which are available several times are freely assignable to the windings. For ring bus and breaker-and- a-half applications a virtual winding can be defined for which the current measuring inputs are based on the vector sum of currents from two freely selectable windings. P63x provide four setting groups for easy adaptation to varying system operation conditions. The intuitive user interface as well as the various communication interfaces allow easy and entire device settings and readings from extensive recordings. Numerous integrated communication protocols, including IEC 61850, allow easy interfacing to almost any kind of substation control or SCADA system. Furthermore the integrated protection interface InterMiCOM provides direct end-end communication between two protection devices. The especially flat compact case of P630C as well as the standard 19'' modular cases of P631, P632, P633 and P634 with variable number of plug-in modules provide a flexible solution for easy integration of the devices into the substation. Both case variants are available for flush mounting and wall-surface mounting and provide the option of detachable HMI.

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APPLICATION MiCOM P63x series differential protection devices provide a wide range of protection functions. The device selection depends on the protected object and the required protection scheme:  P630C: Two-end/winding schemes (two 3 pole CT inputs)  P631: Two-end/winding schemes (two 3 pole CT inputs)  P632: Two-end/winding schemes (two 4 pole CT inputs, one VT input)  P633: Three-end/winding schemes (three 4 pole CT inputs, one VT input)  P634: Four-end/winding schemes (three 4 pole CT inputs, one 3 pole CT input, one VT input)

GLOBAL FUNCTIONS The following functions are generally available in all devices:  Parameter subset selection (4 alternative setting groups)  Metering  Operating data recording  Overload recording incl. overload data acquisition  Fault recording of all CT/VT inputs and binary events incl. fault measurands

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MAIN FUNCTIONS Main functions are autonomous function groups and can be individually configured or disabled to suit a particular application. Function groups that are not required and have been disabled by the user are masked completely (except for the configuration parameter) and functional support is withdrawn from such groups. This concept permits an extensive scope of functions and universal application of the device in a single design version, while at the same time providing for a clear and straight-forward setting procedure and adaptation to the protection and control task under consideration.

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DIFFERENTIAL PROTECTION On the basis of the primary transformer currents, the differential protection devices can be flexibly adapted to the reference currents of the protected object. Amplitude matching is by means of a straight-forward input of the reference power common to all windings plus the nominal voltages and the nominal transformer currents for each winding. The resulting reference currents and matching factors are automatically deduced by the device and checked for compatibility with the internally allowed value ranges. Matching to the vector group of the protected object is via a straight-forward input of the relevant vector group identification number. The mathematical formula to be applied to the measured values is automatically selected internally according to the relevant vector group.

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Zero-sequence filtering may be deactivated separately for each winding in case of an operational grounding within the protected zone. The tripping characteristic of the differential protection device has two knees. The first knee is dependent on the setting of the basic threshold value Id> and is on the load line for single-side feed. The second knee of the tripping characteristic is defined by a setting. Above the user-selected differential current level Id>>>, the restraining current is no longer taken into account. Up to a certain limit, stability in the event of external faults is ensured by means of the bias. Due to the triple-slope tripping characteristic, the stabilization is particularly pronounced for high currents. However, as an additional safeguard for through currents with transformer saturation, the MiCOM P63x series differential protection devices are provided with a saturation discriminator. Particularly the start-up of directly switched asynchronous motors represents a problem in differential protection due to transient transformer saturation caused by a displacement of the start-up current for relatively high primary time constants. Even under such unfavorable measurement conditions, the MiCOM P63x series differential protection devices perform with excellent stability. Stabilization under inrush conditions is based on the presence of second harmonic components in the differential currents. The ratio of the second harmonic component to the fundamental wave for the differential current of the measuring systems serves as the criterion. Optionally, tripping is blocked either across all three measuring systems or selectively for one measuring system. However, from a user-selected differential current level Id>>, the blocking criterion is no longer taken into account. For application as differential protection device for motors or generators, the harmonic restraint can be deactivated. For stabilization under over fluxing conditions, the ratio of the fifth harmonic to the fundamental wave for the differential current of the measuring systems serves as criterion. Tripping is blocked selectively for each measuring system. For differential current levels of 4·Iref or higher, the blocking criterion is no longer taken into account. The over fluxing restraint function may be deactivated.

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RESTRICTED EARTH FAULT PROTECTION Restricted earth fault protection is based on the principle of comparison of measured variables by comparing residual currents and is applied on transformers in order to detect ground faults on a given winding more sensitively than overall transformer differential protection is able to do. The required through-fault stabilization is provided by two different measuring principles:  Biased restricted earth fault protection  High impedance restricted earth fault protection Compared to the biased restricted earth fault protection high impedance restricted earth fault protection can also be applied to non-grounded objects, especially to delta windings. With biased restricted earth fault protection one of the following operating modes can be chosen:  Biasing by residual current  Biasing by maximum phase current The advantage of ground differential protection resides in the linear dependence of the sensitivity on the distance between the fault and the neutral point. Chapter 6

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DEFINITE TIME AND INVERSE TIME OVERCURRENT PROTECTION Both the definite-time and the inverse-time overcurrent protection operate with separate measuring systems for the evaluation of the three phase currents, the negativesequence current and the residual current. Three stages each are provided for the three measuring systems of the definite-time overcurrent protection. The inverse-time overcurrent protection offers a multitude of tripping characteristics for the individual measuring systems.

THERMAL OVERLOAD PROTECTION The highest of the three phase currents serves to track a first-order thermal replica according to IEC 60255-8. The temperature of the cooling medium can be taken into account in the thermal replica using the optional PT-100 input or the 0 to 20 mA input. The user has a choice of using a thermal replica on the basis of either absolute or relative temperature. A warning signal can be issued in accordance with the set warning level an alternative method of generating a warning, the cyclically updated measured operating value of the predicted time remaining before tripping is monitored to check whether it is falling below a set threshold.

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OVER-/UNDERVOLTAGE PROTECTION The over/undervoltage protection function evaluates the fundamental component of the voltage by way of two definite-time overvoltage and undervoltage stages each.

OVER-/UNDERFREQUENCY PROTECTION The four-stage frequency protection can be operated as pure over- and under frequency monitoring as well as combined with differential frequency gradient monitoring (df/dt) for system decoupling applications or with medium frequency gradient monitoring ( for load shedding applications.

OVEREXCITATION PROTECTION Over excitation protection detects impermissible high magnetic flux density in the iron core of power transformers which can occur in case of increase in voltage and/or decrease in frequency. Flux density above the rated value saturates the iron core which may result in power transformer overheating due to large iron losses. Over excitation protection processes the voltage to frequency ratio (V/f) in relation to their nominal values. The inverse time characteristic may be set via 12 value pairs and therefore enables accurate adaptation to power transformer data. In addition a definite-time alarm stage and a definite-time tripping stage are available.

CIRCUIT BREAKER FAILURE PROTECTION The new Px3x platform Circuit Breaker Failure protection is now also available in modular P63x devices. Individual protection elements are provided for each end. CB failure operates if the current does not fall below a low set threshold within permitted time. In case of trip conditions with no fault current the CB auxiliary contacts open condition can be monitored additionally. A second (re-)trip command and an upstream CB trip command can be raised. This CBF provides also trip function in case of downstream CB failure, stub bus protection and pole discrepancy monitoring.

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MEASURED DATA INPUT AND OUTPUT For the acquisition of an externally measured variable or the output of measured data P63x provides optionally one 0 to 20 mA input and two 0 to 20 Ma outputs. Settable scaling allows simple adaptation of the input and output ranges resp. (e.g. 0 to 10 mA, 4 to 20 mA). Direct temperature acquisition can be served by the optional PT-100 input.

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Transformers are high capital cost assets in electrical power systems. Elimination of all electrical and mechanical stresses, although desirable to preserve transformer life, is impractical. Adaptive techniques to measure and alarm (or trip) in such instances, and advise on cumulative service duty, can help to schedule preventive maintenance – before a costly failure occurs. Internal faults are a risk for all transformers, with short-circuits dissipating the highest localized energy. Unless cleared quickly, the possibility to rewind windings reduces, and core damage may become irreparable. Customer benefits • Universal IED for all transformer (or reactor) configurations • Protection, control, monitoring, measurements and recording in-one device • Backup and logging of through faults • Simple to specify, set, and commission • Programmable function keysA

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Figure 1: Front view of P645 The MiCOM P642, P643 and P645 address all these issues - preserving service life, and offering fast protection for transformer faults. Hosted on an advanced IED platform, the P64x incorporates differential, REF, thermal, and over fluxing protection, plus backup protection for uncleared external faults. Model variants cover two and three winding transformers (including autotransformers), with up to 5 sets of three-phase CT inputs. Large CT counts are common in ring bus/mesh corner applications, where the P64x summates currents to create each total winding current, easing application of backup protection. Backup overcurrent can be directionalized, where the user includes the optional 3phase VT input in their chosen model.

KEY FEATURES  High-speed transformer differential protection  Simple setting – wizard requires only nameplate data  Restricted earth fault (REF) boosts trip sensitivity  Voltage, frequency, thermal and over fluxing elements  CT, VT, trip circuit and self-supervision:  Patented CT supervision ensures no spurious trip for CT or wiring failures  Integrated backup overcurrent per winding  Readily interfaces to multiple automation protocols, including IEC 61850

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APPLICATION The MiCOM P642 is intended for two-winding transformer applications, with one set of three phase CTs per winding. The P643 covers up to 3 bias inputs (three CT sets) either a three winding application, or two-winding with dual CTs on one side. Where 4 or 5 feeding connections to the protected transformer exist, the P645 offers five bias input sets. All models have a single-phase VT input, mainly for over fluxing application, and the P643 and P645 allow an additional 3-phase VT input to be connected. This allows overcurrent backup to be directionalised, and expands the measurement and recording analog channels available. As well as transformer protection, the P64x range may be applied to other unit applications, such as reactor and motor. The MiCOM P64x series is supplied with a full suite of protection and control functions as standard. The configuration column of the menu is used to control which functions the user requires in the intended application, and which may be disabled. Disabled functions are completely removed from the menu, to simplify setting. Differential elements have an inbuilt configuration wizard, to avoid settings errors.

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MAIN PROTECTION 87T Transformer Differential The algorithm has a triple slope percentage bias restraint, as shown in Figure 3. An internal fault will generate differential current. The bias current is that which merely flows through the protected unit, as a load or through-fed external fault. The initial characteristic is flat, for ease of commissioning, rising then to bias slope (k1). K1 is a low slope for sensitivity to faults whilst allowing for mismatch when the power transformer is at the limits of its tap changer range, in addition to any current transformer ratio errors. At currents above rated, extra errors may be gradually introduced as a result of CT saturation, hence the bias slope is increased to k2. The P64x incorporates transient bias and this combined with the k2 bias ensures that CT knee point voltage requirements are minimized. Energization of a transformer causes magnetizing inrush current to flow in one winding only, and the differential elements may need stabilizing whilst the inrush persists. A proven second harmonic current ratio scheme is used. The differential protection may also be restrained when the transformer is overfluxed so that an instantaneous trip is not issued for transient overfluxing. Over fluxing restraint is conditioned by the percentage of fifth harmonic current present. A high set instantaneous differential element, not subject to harmonic restraint, is provided to ensure rapid clearance of high current faults. The differential protection setting configuration utility requires only known data – that which resides on the transformer rating plate, the CT rating plate, and information on any in-zone earthing transformer.

REF: Restricted Earth (Ground) Fault Restricted earth fault protection is included to cover a larger percentage of the transformer windings than might be possible with the main differential elements. A separate element per winding is provided (P642: HV and LV. P643/P645: HV, LV, and if required, the TV tertiary too). Figure 5 shows a typical restricted earth fault application. Biased REF is used, to avoid the need for any stabilizing resistor or varistor/Metrosil. REF elements operate independently of inrush detection, potentially offering faster tripping for low or moderate fault currents, in addition to enhanced sensitivity.

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Thermal Overload All models offer thermal overload protection, with the extent of protection being the choice of the customer. The most simple application employs I²t characteristic. Time constants are set, such that the thermal model can follow the correct exponential heating and cooling profile, replicating the winding hotspot temperature. Alarm and trip thresholds are available as outputs. To enhance the thermal replica, ambient and/or top-oil temperature compensation may be applied. This is achieved by fitting the RTD board option, and positioning the PT100 probes appropriately (outdoors, or within the transformer tank). Additionally, alarm and trip setpoints can be applied for any probe input, should an absolute measured temperature at the probe location be of interest. Ten independent probe inputs are available, making radiator pump and fan control an additional possibility using the relay’s programmable scheme logic (PSL). Thermal overload protection is a closelyrelated companion function to the Loss of Life monitoring feature described later.

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> V/Hz Overfluxing Protection The single phase voltage input may be connected ph-ph or ph-neutral and is provided to enable over fluxing detection. Alarm and tripping characteristics, which are based on a measurement of the voltage/frequency ratio, are provided. The alarm is definite time delayed whilst the trip characteristic may be applied with up to four definite time (DT) elements, or an IDMT curve plus up to three DT elements. The optional additional 3phase VT input available in P643 and P645 allows over fluxing to be applied on both HV and LV sides of the transformer, to ensure optimum protection, irrespective of the load flow direction. Both thermal overload and over fluxing elements are essentially thermal based, modeling winding and oil heating, or heating of core bolts and laminations. Due to time constants being in minutes (rather than seconds), heating and cooling of both replicas can be relatively slow. A pre-trip countdown is provided, displaying the time remaining to trip if the present level of load, or flux were to be maintained. A pre-trip alarm can be applied, notifying the dispatcher that he/she has a certain number of minutes for remedial action, before a trip is likely. After any injection testing, all replicas can be forced to reset via a user command.

Circuit Breaker Failure The breaker failure protection may be initiated from internal protection within the P64x, and also from external devices. In the case of Buchholz (sudden pressure) relays, the CBF elements for all breakers must be initiated in parallel. Where external feeder or busbar protection is applied to trip only one (or more) breaker(s), the P64x has the ability to initiate the CBF scheme on a per breaker basis. Re-tripping and back tripping schemes are supported, all with a fast acting undercurrent check.

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SUPERVISORY FUNCTIONS Voltage transformer supervision is provided to detect loss of one, two or three VT signals (P643 and P645 models fitted with a 3-phase VT). Current transformer supervision is provided to detect loss of phase CT input signals. Using the “differential CTS” feature (patented), the relay performs an intelligent comparison of the negative sequence current imbalance at all CT terminals, to determine which, if any, CTs have failed. This comparison detects all CT shorts, open circuits, and wiring disconnections without an inherent time delay. Operation of the differential protection can be blocked during the failure, or alternatively temporarily desensitized to avoid an unwanted trip. The CTS thus assures real-time stability of the differential elements, and any applicable REF protection.

BACK-UP PROTECTION The MiCOM P642, P643, and P645 are delivered with comprehensive back-up protection. Typically this will be used in time-delayed mode to improve fault detection dependability for system (out-of-zone) faults. System integrity can also be improved, utilizing internal elements for load-shedding, interlocking, alarm, or other purposes.

Current-Based Protection Each winding, whether the current is directly measured from one CT input, or is a virtual summation from two CTs, has the following elements available: •

Phase fault overcurrent

•

Negative sequence overcurrent

•

Earth (ground) fault.

Simple Up to four stages of each element, per winding, are available – with a choice of standard IEC and ANSI/IEEE IDMT curves, instantaneous, and definite-time operation. Where a P643/P645 has the 3-phase VT option fitted, any of the current protection applied on the same winding as the VT location may be directionalized. Overcurrent elements, directionalized if necessary, can be useful to clear reverse-fed upstream faults, or for protection of adjacent busbars. At distribution and industrial voltage levels, Chapter 6

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low-cost bus protection schemes can be configured using the “reverse interlocking” principle. This is a logic-based scheme, which will trip should a fault current flow onto the busbar not be accompanied by an external fault start on an outgoing circuit. The earth fault protection is configurable to operate either in measured, or derived mode. “Measured” denotes that the winding (or external earthing transformer) has a star-point single phase CT available in the Y-ground connection, and the user wishes this current to be used to implement standby earth fault (SBEF). “Derived” is set for delta windings, or other cases where the user prefers to use the calculated residual current from the three phase CTs.

Voltage Protection * Two stages each are available for phase overvoltage, phase undervoltage, and residual overvoltage (neutral displacement). Such elements are particularly useful to detect voltage regulation errors. (* - Available when optional 3-phase VT input is ordered in P643 or P645).

Frequency Protection Four stages of under frequency and two stages of over frequency are provided, permitting load shedding and restoration schemes to be implemented.

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The Application of High Impedance Relays

Introduction The application of the MCAG 14/ 34, M FAC 14/ 34 and M CTI14/ 34 relays to the protection of machines, power transformers and busbar installations is based on the high impedance differential principle, offering stabiliy for any type of fault occurring outside the protected zone and satisfactory operation for faults within the zone A high impedance relay is defined as a relay or relay circuit whose voltage setting is not less than the calculated maximum voltage which can appear across its terminals under the assigned maximum through fault current condition. It can be seen from Figure 1, that during an external fault the through fault current should circulate between the current transformer secondaries. The only current that can flow through the relay circuit is that due to any difference in the current transfomer outputs for the same primary current. Magnetic saturation will reduce the output of a current transformer and the most extreme case for stability will be if one current transformer is completely saturated and the other unaffected. This condition can be approached in busbar installations due to the multiplicity of infeeds and extremely high fault level. It is less likely with machines or power transformers due to the limitation of through fault level by the protected unit’s impedance, and the fact that the comparison is made between a limited number of current transformers. Differences in current transformer Chapter 6

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remanent flux can, however, result in asymmetric current transformer saturation with all applications. Calculations based on the above extreme case for stability have become accepted in lieu of conjunctive scheme testing as being a satisfactory basis for application. At one end the current transformer can be considered fully saturated with its magnetising impedance ZM B short circuited while the current transformer at the other end, being unaffected, delivers its full current output which will then divide between the relay and the saturated current transformer. This division will be in the inverse ratio of RRELAY CIRCUIT and RCTB +2RL and obviously, if RRELAY CIRCUIT is high compared with RCTB + 2 RL, the relay will be prevented from undesirable operation, as most of the current will pass through the saturated current transformer. To achieve stability for external faults, the stability voltage for the protection (VS) must be determined in accordance with formula 1 for the MCAG/MFAC. The setting will be dependent upon the maximum current transformer secondary current for an external fault (If) and also on the highest loop resistance value between the current transformer common point and any of the current transformers (RCT + 2 RL). ≥

Ir (RCT + 2RL) ……………………………………………….. 1

RCT

=

current transformer secondary winding resistance

RL

=

maximum lead resistance from the current transformer to the

Vs

Where

common point To ensure satisfactory operation of the relay under internal fault conditions the current transformer kneepoint voltage should not be less than twice the relay voltage setting i.e. VK >_ 2VS for the MCAG/ MFAC The kneepoint voltage of a current transformer marks the upper limit of the roughly linear portion of the secondary winding excitation characteristic and is defined exactly in British practice as that point on the excitation curve where a 10% increase in exciting voltage produces a 50% increase in exciting current. The current transformers should be of equal ratio, of similar magnetising characteristics and of low reactance construction. In cases where low reactance current transformers are not available and high reactance ones must be used. It is essential to use in the calculations for the voltage setting, the reactance of the current transformer and express the current transformer impedance as a complex number in the form RCT + jXCT. It is also necessary to ensure that the exciting impedance of the current transformer is large in comparison with its secondary ohmic impedance at Chapter 6

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the relay setting voltage. The MCAG 14/34 The M CAG 14/ 34 is an electromechanical current calibrated relay with setting ranges of: 0.025 – 0.100 A 0.050 – 0.200 A 0.100 – 0.400 A 0.200 – 0.800 A 0.250 – 1.000 A 0.500 – 2.000 A 1.000 – 4.000 A

The relay has a fixed burden of approximately 1VA at setting current and its impedance varies with the setting current used. To comply with the definition for a high impedance relay, it is necessary, in most applications, to utilise an externally mounted stabilising resistor in series with the relay coil. The standard ratings of the stabilising resistors normally supplied with the relay are 4700, 220CI and 47CI for 0.5A, 1A and 5A current transformer secondary respectively. In applications such as busbar protection, where higher values of stabilising resistor are often required to obtain the desired relay voltage setting, non-standard resistor values can be supplied. The standard resistors are wire wound, continuously adjustable and have a continuous rating of 145W.

Applying the MCAG 14/34 The recommended relay current setting for restricted earth fault protection is usually determined by the minimum fault current available for operation of the relay and whenever possible it should not be greater than 30% of the minimum fault level. For busbar protection, it is considered good practice by some utilities to set the minimum primary operating current in excess of the rated load. Thus, if one of the current transformers becomes open circuit the high impedance relay does not maloperate. In the case of the high impedance relay, the operating current is adjustable in discrete steps. The primary operating current (Iop) will be a function of the current transformer ratio, the relay operating current (Ir), the number of current transformers in parallel with a relay element (n) and the magnetising current of each current transformer (Ie) at the stability voltage (Vs). This relationship can be expressed in three ways: To determine the maximum current transformer magnetising current to achieve a specific primary operating current with a particular relay operating current.

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To determine the maximum relay current setting to achieve a specific primary operating current with a given current transformer magnetising current

To express the protection primary operating current for a particular relay operating current and with a particular level of magnetising current.

Iop = (CT ratio) x (Ir + nIe) In order to achieve the required primary operating current with the current transformers that are used, a current setting (Ir) must be selected for the high impedance relay, as detailed in the second expression above. The setting of the stabilising resistor (RST) must be calculated in the following manner, where the setting is a function of the relay ohmic impedance at setting (Rr), the required stability voltage setting (VS) and the relay current setting (Ir).

Note: the MCAG 14/ 34 is a fixed burden relay, therefore the ohmic impedance of the relay will vary with setting. The ohmic impedance (Rr) of the MCAG 14/ 34 can be calculated using the relay VA burden at current setting (B) and the relay current setting (Ir);

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The stabilising resistor supplied is continuously adjustable up to its maximum declared resistance. In some applications, such as generator winding differential protection, the through fault current is low which results in a low stability voltage setting. In many such cases, a negative stabilising resistor value can be obtained from the above formula. This negative result indicates that the relay will be more than stable without a stabilising resistor. When astabilising resistor is not required, the setting voltage(VSA) can be calculated using the following formula and the current transformer kneepoint voltage should be at least twice this value.

Use of Metrosil non-linear resistors - M CAG 14/34 When the maximum through fault current is limited by the protected circuit impedance, such as in the case of generator differential and power transformer restricted earth fault protection, it is generally found unnecessary to use non-linear voltage limiting resistors (metrosils). However, when the maximum through fault current is high, such as in busbar protection, it is always advisable to use a non-linear resistor (metrosil) across the relay circuit (relay and stabilising resistor). Metrosils are used to limit the peak voltage developed by the current transformers under internal fault conditions, to a value below the insulation level of the current transformers, relay and interconnecting leads, which are normally able to withstand 3000V peak. The following formulae should be used to estimate the peak transient voltage that could be produced for an internal fault. The peak voltage produced during an internal fault will be a function of the current transformer kneepoint voltage and the prospective voltage that would be produced for an internal fault if current transformer saturation did not occur. This prospective voltage will be a function of maximum internal fault secondary current, the current transformer ratio, the current transformer secondary winding resistance, the current transformer lead resistance to the common point, the relay lead resistance, the stabilising resistor value and the relay VA burden at relay operating current.

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where Vp = peak voltage developed by the CT under internal fault conditions. Vk = current transformer knee-point voltage. Vf = maximum voltage that would be produced if CT saturation did not occur. If = maximum internal secondary fault current. Rct = current transformer secondary winding resistance. RL = maximum lead burden from current transformer to relay. RST = relay stabilising resistor. Rr = Relay ohmic impedance at setting. When the value given by the formulae is greater than 3000V peak, non-linear resistors (metrosils) should be applied. These non-linear resistors (metrosils) are effectively connected across the relay circuit, or phase to neutral of the ac buswires, and serve the purpose of shunting the secondary current output of the current transformer from the relay in order to prevent very high secondary voltages. These non-linear resistors (metrosils) are externally mounted and take the form of annular discs, of 152mm diameter and approximately 10mm thickness. Their operating characteristics follow the expression:

Where

V =Instantaneous voltage applied to the non-linear resistor (metrosil) C = constant of the non-linear resistor (metrosi) I =instantaneous current through the non-linear resistor (metrosil).

With a sinusoidal voltage applied across the metrosil, the RM S current would be approximately 0.52x the peak current. This current value can be calculated as follows;

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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 nonlinear resistor (metrosil) is not sinusoidal but appreciably distorted. For satisfactory application of a non-linear resistor (metrosil), it’s characteristic should be such that it complies with the following requirements: At the relay voltage setting, the nonlinear resistor (metrosil) current should be as low as possible, but no greater than approximately 30mA rms for 1A current transformers and approximately 100mA rms for 5A current transformers. At the maximum secondary current, the non-linear resistor (metrosil) should limit the voltage to 1500V rms or 2120V peak for 0.25 second. At higher relay voltage settings, it is not always possible to limit the fault voltage to 1500V rms, so higher fault voltages may have to be tolerated.

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Applying the M FAC14/34 As the M FAC 14/ 34 is a voltage calibrated relay with setting ranges of 25 -175V in 25V steps, 25 -325V in 50V steps and 15 -185V in 5V steps, it is inherently a high impedance relay requiring no external resistors. Due to the relay circuit impedance always being relatively high, significant voltages can be produced across the current transformers and secondary wiring during an internal fault. To limit the voltage to a value below the insulation level of the current transformers, relay and interconnecting leads, a non-linear resistor (metrosil) is always required and should always be used by connecting in parallel with the relay. Refer to metrosil publication for selection chart). The operating current is virtually fixed at around 20mA, but there is some slight variation with relay voltage setting as a result of variation in the current drawn by the nonlinear resistor (metrosil). The operating current, including the nonlinear resistor (metrosil) current, for the various voltage settings is stated to the right: The relay effective current setting can be calculated in the same manner as described for the MCAG 14/ 34. For busbar protection, it is considered good practice by some utilities to set the minimum primary operating current in excess of the rated load. Thus, if one of the current transformers becomes open circuit the MFAC 14/ 34 does not maloperate.

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TYPICAL SETTING EXAMPLES Restricted earth fault protection using M FAC14 The correct application of the M FAC 14 high impedance relay can best be illustrated by taking the case of the 11000/415V 1000kVA power transformer shown in figure 9, for which restricted earth fault protection is required on the L.V. winding. It is assumed that the relay effective setting for a solidly earthed power transformer is approximately 30% of full load current.

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Areva Transformer Differential Protection

Chapter 6

Page 6-52

Areva Transformer Differential Protection

Chapter 6

Page 6-53

Areva Transformer Differential Protection

Chapter 6

Page 6-54

Areva Transformer Differential Protection