MiCOM P721 & P723. Numerical High Impedance Differential Relay. Protection Relays. Customer Benefits

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1 01 MiCOM P721 & P723 Numerical High Impedance Differential Relay The new P72x high impedance differential protection series provides an independent and simple high impedance differential protection solution for generator, reactor, motor and busbar applications where fast clearance of faults is required. Combined with the P79x, a standalone metrosil and resistor unit, it provides simplified scheme engineering for single or three-phase differential applications. Customer Benefits Simple scheme engineering Reliable algorithm Supervision functions included in the same protection box As well as offering the same application benefits as traditional high impedance electromechanical protection schemes, it combines the added benefits of numerical technology to provide advanced communications, event records, fault records, disturbance records and ancillary protection features.

2 02 APPLICATION When circulating current protection schemes are subjected to through faults, the sudden and often asymmetrical growth in the system current can cause the line current transformers to reach saturation. Under these conditions the variation in current transformer magnetising characteristics can cause large ratio errors with a consequent circuit imbalance and maloperation of the protective relays. To ensure stability, it is common practice to employ high impedance relays set to operate at a slightly higher voltage than that developed in the worst theoretical case of this condition for a given through fault current. On a balanced earth fault system for example, this is when one current transformer of a group is saturated whilst the others remain unaffected. The saturated transformer presents a low impedance path in parallel with the relay and limits the voltage applied. For internal faults this limitation does not exist and voltages of twice the setting are easily reached resulting in fast operation of the differential relay. The P72x relay together with the P79x unit, is designed for applications where sensitive settings with stability on heavy through faults are required. It offers a reliable protection for busbars, transformers windings, generators, reactors and motors. FUNCTIONAL OVERVIEW OVERVIEW Two models are available: MiCOM P721 is a single-phase high impedance differential protection. It is suitable for restricted earth fault or balanced earth fault applications. MiCOM P723 can be configured either as high impedance differential three-phase protection (87) or as high impedance differential earth protection (87N). The connection setting determines whether 87 or 87N is being used. MAIN functions Phase segregated high impedance differential protection (87A, 87B, 87C) The application of P723 numerical relay as differential protection for machines and busbar installations is based on the high impedance differential principle. This principle requires stabilising resistors in each phase and a metrosil is generally required across the relay circuit. These are included in the P79x accessory as shown in Figure 1. Stability for any type of fault occurring outside the protected zone and satisfactory operation for faults within the zone are achieved. Figure 1 shows the typical application diagram where P723 is configured as a phase segregated high impedance differential protection. Protected Plant I diff R ST Metrosil P79x 87P 87N Remote Comm. Port Local Communication Self Monitoring Fault Records Measurements Disturbance Record 95P LEDs 95N 50BF Binary Input / Output P72x P72x and P79x together offer a simple and reliable high impedance differential protection

3 03 PROTECTION functions OVERVIEW ANSI Features P721 P723 87P Phase segregated high impedance current differential protection 95P Phase bus-wire supervision 87N Restricted earth fault protection 95N Earth bus-wire supervision 87CZ Check zone input 50BF Breaker failure detection GENERAL FEATURES Features P721 P723 Number of opto-isolated inputs 2 5 Number of relay output contacts 4 8 Event recording Fault recording Disturbance recording 5 5 Instantaneous records 5 5 Setting groups 2 2 Auxiliary timers 4 4 Communication protocols Modbus RTU (rear port) DNP3 (rear port) IEC (rear port) Modbus RTU (front port) Setting software MiCOM S1 Studio Logic equations AND, OR and NOT gates 8 Boolean equations Measurements Neutral supervision/differential currents (INdiff) Phase supervision/differential currents (IAdiff, IBdiff and ICdiff) Maximum differential currents (ImaxAdiff, ImaxBdiff and ImaxCdiff) Maximum differential neutral currents (ImaxNdiff) Figure 1: P723 configured as phase segregated high impedance differential busbar protection

4 04 High impedance differential earth fault protection (87N) The P721 has a single differential element and may only function as an 87N protection. The P723 can be configured as 87N when only one analogue current input is required. Restricted earth fault protection is included to cover a larger percentage of the transformer windings than might be possible with the main biased differential transformer protection. 87N is commonly referred as restricted earth fault (REF) protection for star windings and balanced earth fault (BEF) protection for delta windings. Figures 2 and 3 show the P72x configured for these applications. GENERAL SCHEME LOGIC The schemes in figures 4 and 5 describe the logic of the three-phase high impedance differential protection and high impedance differential earth fault protection relays. External blocking inputs are available to allow the user to configure an input to enable or disable the differential element. A selectable logic input is also available as a check zone input from another P72x or differential relay. [87] DIFF Blocking Logic 1 / 2 & DT [87CZ] Check Zone & Start [87] DIFF Temporized [87] tdiff [95] SUP DT Start [95] SUP Temporized [95] tsup Figure 4: General scheme of the three-phase high impedance differential protection [87N] DIFF Start [87N] DIFF Blocking Logic 1 / 2 & DT Temporized [87N] tdiff [95N] SUP Start [95N] SUP DT Temporized [95N] tsup Figure 2: BEF protection for the delta winding of a power transformer with supply system earthed Figure 5: General scheme of high impedance differential earth fault protection CONTROL AND INDICATION Control Two blocking logics are available where for example opto inputs can be independently configured to block the differential element or any of the auxiliary timer functions. Logic equations The MiCOM P72X relays integrate complete logic equations to allow customization of the product based on customer application. Up to 8 independent Boolean equations can be used. Each equation offers the possibility to use AND, OR & NOT logical gates. Up to 16 parameters can be used for each equation. Every result of equation can be time delayed and assigned to any output relays, trip, trip latching and/or HMI LEDs. Figure 3: REF protection of a 3 phase, 4 wire system applicable to star connected generators or power transformer windings with neutral earthed at generator/transformer star point

5 05 Optos Protection elements Automatic Control Gate Logic Timers Relay contacts LED's Phase segregated outputs are available from the buswire supervision such that each phase can be stabilised by short circuiting the bus-wires as required. The differential elements are internally blocked due to bus-wire supervision (although this will not negate the requirement for external shorting of bus-wires). LEDs Four programmable LEDs are available. Alarms Protection and control functions generate alarms on the screen and LEDs as configured. Certain alarms can be inhibited or automatically reset by preliminary parameter setting. Alarms can be stored in non-volatile memory. SUPERVISORY functions Open circuit failures of the CTs or bus-wires will cause a spill current to flow through the differential element and operation will occur if this current is greater than the setting. Two methods are used to prevent nuisance tripping due to open circuit conditions: 1. Bus-wire supervision elements are provided to monitor for a continuous low level of spill current. After a time delay, this relay would usually short circuit the AC bus-wires to ensure stability and also give an alarm. The disadvantage of this scheme is that the differential elements must be set higher than the bus-wire supervision elements (and usually above maximum load conditions) if nuisance tripping is to be avoided. This may create sensitivity issues. 2. Duplicate differential elements (main and check zones) fed from separate CTs using segregated (phase by phase) bus-wires. Tripping is only permitted when both differential elements operate. The advantage of this scheme is that the differential settings can remain very sensitive (below full load current values), but this will require a total duplication of the scheme. The selectable logic input from another P72x deployed as a check zone is available for this function. The above two methods are usually used in conjunction with each other, although to reduce costs either scheme could be used independently. Bus-wire Shorting To limit the power dissipated in the resistance, after the differential trip protection [87] or bus-wire supervision [95] are activated, the bus-wires must be short-circuited. This function is latched in the internal logic. The bus-wire shorting signal can be connected as shown in figures 1, 2 and 3. CB Failure function The CB failure output from the P72x is an input to the breaker failure scheme. The general scheme for the breaker failure detection function is shown in figure 6. Trip [87] tdiff Idiff< Pick up timer CB Fail time tbf Figure 6 CBF detection logic & CB Fail OPERATION AND CONfIGuRATION Setting groups External conditions may request the need for different settings. provide two independent setting groups. The active setting group can be switched to the other from the local HMI or from external conditions (opto input change of state or DCS control). Multi-language user interface (HMI) All functions, including protection, automation, communication, LEDs, inputs and outputs, can be programmed and modified using the front panel user interface (Human Machine Interface, HMI). The backlit LCD informs the user about settings, measurements & faults thanks to the pull-down structure menu allowing easy and quick access to any data. Moreover, even if the relay is delivered with the language specified at the order, the user can change the language used within the relay from the HMI.

6 06 Clear Key Enter Key The P721 and P723 measure the neutral supervision/differential currents (I Ndiff ). Trip Led Alarm Led Read Key The P723 measures the maximum differential currents (Imax Adiff, Imax Bdiff and Imax Cdiff ). The P721 and P723 measure the maximum differential neutral currents (Imax Ndiff ). Fault records Device fail Power supply [ Cursor Keys 25 time-tagged fault records that indicate the faulted phase, protection operation, active setting group, pre-fault and differential fault currents magnitudes are available. Disturbance records Freely Programmable Leds RS 232 A disturbance record is generated for every trip. In addition, the disturbance recorder can be triggered by an opto-isolated input, or a remote control signal. Battery is not used anymore MEASuREMENTS AND RECORDING Measurements The following measurements are available for display and remote measurements: The P723 measures Phase supervision/ differential currents (I Adiff, I Bdiff and I Cdiff ). Up to five records of 3 seconds each one can be stored. The disturbance records are 32 samples per cycle. Event records 250 time-tagged event records can be stored. Events include operation and reset of inputs/ outputs, alarms, internal signals, and contacts. All events are time stamped to 1ms, and can be accessed either locally or remotely. Figure 7: Disturbance Record

7 07 COMMUNICATION & SYNCHRONIZATION The offers a wide range of communication protocols, allowing its utilisation in most of the network control and data acquisition systems (via Modbus, IEC , and DNP3.0). It has been designed for permanent multi-drop connection through the rear RS485 communication port. The incorporates an internal clock used for the time tagging of alarms, events, fault and disturbance record. The relay s internal clock is synchronise to the timing information to an accuracy of 1ms.To avoid any drifting of the time tagging clock, it s necessary to periodically synchronize the relays. To do this P72x offers two solutions: 1. Synchronization from the substation control system via the rear communication port 2. Synchronization from an external GPS clock via a dedicated digital input STABILISING RESISTOR AND VARISTOR ACCESSORY The P79x accessory contains a number of fixed resistors that provide stability during external faults. In addition to the stabilising resistors, a varistor is included for each phase. Two models are available: the P791 used in single phase applications and the P793 used in three phase applications. Stabilising resistor Three options of internal stabilising resistors are offered. In each option is possible to get 4 combinations of serial and parallel connections to give various values of resistance as required by the application. Varistor Varistors 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 wiring, which are typically able to withstand approximately 3000V peak. Two options of varistors are available. The half power option may withstand up to 10kJ of transient energy and the full power up to 20kJ. Figures 8 and 9 show P721/P791 and P723/P793 connection diagram. See P79x brochure for more information. A B C P791 Auxiliary supply WD RL1 RL2 RL3 RL Watch dog RL1 to RL4 are programmable outputs P791 L1 to 2 are programmable inputs L L2 MiCOM P721 Link terminals 30 and 32 if the relay is connected at the end of the RS485 bus * RS485 communication port Figure 8: MiCOM P721/P791 diagram connection Simple and reliable high impedance differential scheme protection

8 08 A B C PROTECTED ZONE Three-Phase High Impedance Differential TC1 CT1 TC2 CT2 TCN CTN Auxiliary supply P793 P793 L1 to 5 are programmable inputs L L L L L5 WD RL1 RL2 RL3 RL4 RL5 RL6 RL7 RL8 MiCOM P723 (*) Link terminals 30 and 32 if the relay is connected at the end of the RS485 bus * Watch dog RL1 to RL8 are programmable outputs RS485 communication port Figure 9: MiCOM P723/P793 diagram connection Device Track Record - High Impedance Differential Protection MiCOM P12x series relays used in high-impedance applications since 1998 MiCOM P14X series relays used in high-impedance applications since 1999 Schneider Electric 35, rue Joseph Monier CS Rueil-Malmaison Cedex, France Tel: 33 (0) RCS Nanterre Capital social As standards, specifications and designs change from time to time, please ask for confirmation of the information given in this publication. This document has been printed on ecological paper Publishing: Schneider Electric Design: Schneider Electric Printing: 2010 Schneider Electric - All rights reserved EnergyAutomation-DS-P72x-2200-EN