A New Adaptive High Speed Distance Protection Scheme for Power Transmission Lines

Transcription

1 A New Adaptive High Speed Distance Protection Scheme for Power Transmission Lines M.M. Saha, T. Einarsson, S. Lidström ABB AB, Substation Automation Products, Sweden Keywords: Adaptive distance protection, high speed tripping, numerical simulations, RTDS testing. Abstract This paper describes a new, high-speed protection for EHV transmission lines. The presented scheme also performs well on series compensated lines. The High speed distance protection (ZMFPDIS) function is a six zone full scheme protection with three fault loops for phase-to-phase faults and three fault loops for phase-to-earth faults for each of the independent zones, which makes the function suitable in applications with single-phase auto-reclosing. The zones can operate independently of each other in directional (forward or reverse) or non-directional mode. A new built-in adaptive load compensation algorithm prevents overreaching of the distance zones in the load exporting end during phase-to-earth faults on heavily loaded power lines. It also reduces under-reach in the importing end. The ZMFPDIS function-block itself incorporates a phase-selection element and a directional element, where these elements are represented with separate function-blocks. Implementation of the new distance protection with fast tripping algorithms has been made in a modular line terminal platform. Comprehensive test results on various test systems show the benefits of the scheme. The results with a Real Time Digital Simulator (RTDS) are presented in this paper. The scheme is now in operation under trade name REL Introduction Distance relay is one of the most important components of protection systems available to protection engineers. Distance relays can benefit from ideas in the newly developed field of adaptive protection, and can offer an even more selective and sensitive form of protection, under a variety of system configurations. The benefits of installing series capacitors in the power system include increased power transfer capability, improved power system stability, reduced system losses, improved voltage regulation, and the possibility to regulate power flow. Installation of series capacitors, however, introduces challenges to protection systems with regards for both the series compensated lines and the adjacent lines [1-3]. The protection of systems with series compensated lines is considered one of the most difficult tasks both for relay design and utility engineers. The paper [4] explores such a hybrid solution in order to achieve high speed of operation while maintaining high dependability and security. Direct application of the classic distance protection (designed for traditional lines) to the series-compensated lines results in considerable shortening of the first zone reach and also in poor transient behavior [1]. In order to overcome these difficulties the new distance protection principle for the first zone has been developed [5]. A digital distance relaying algorithm based upon symmetrical components was introduced few decades ago [6]. This protection method utilizes certain relations between voltages and currents in a three-phase post fault power system [6-7], the phase voltages and currents are transformed into symmetrical components by means of digital filtration. Ultra-high speed impedance estimation, in turn, is natural in time domain methods. This group of algorithms based on the resistiveinductive model of the fault loop circuit, usually neglecting shunt capacitances, assumes the appropriate voltages and currents as measured and solves the appropriate differential equation with respect to resistance (R) and reactance (X) of positive sequence. Various approaches to numerical differentiation and solution of relevant equation originate a number of different relaying methods of this kind [8-10]. It is therefore, not possible to use only one algorithm but rather a hybrid solution with parallel and adaptive algorithms have to be implemented. This paper explores such a hybrid solution in order to achieve high speed of operation while maintaining high dependability and security. 2 New Protection Scheme 2.1 Functionality The ZMFPDIS is providing sub-cycle, down towards halfcycle, operate time for basic faults within 60% of the line length and up to around SIR 5. This is a six zone full scheme protection with three fault loops for phase-to-phase faults and three fault loops for phase-to-earth faults for each of the independent zones, which makes the function suitable in applications with single-phase auto-reclosing. The zones can operate independently of each other in directional (forward or reverse) or non-directional mode. This makes them suitable, together with a communication scheme, for protection of power lines and cables in complex network configurations, such as parallel lines, multi-terminal lines, and so on. A new built-in adaptive load compensation algorithm [11] prevents overreaching of the distance zones in the load 1

2 exporting end during phase-to-earth faults on heavily loaded power lines. It also reduces under-reach in the importing end. The ZMFPDIS function-block itself incorporates a phaseselection element and a directional element.the operation of the phase-selection element is primarily based on current change criteria, essentially those of the High-Speed Distance function as in [12]. The directional element utilizes a set of well-established quantities to provide fast and correct directional evaluation during various conditions, including close-in three-phase faults, simultaneous faults and faults with only zero-sequence in-feed. Main Distance Protection The main protection function is a full scheme distance protection with six impedance measuring zones, having a quadrilateral characteristic [13], as shown in figure 1. The setting for each zone are independent for: reactive and resistive reach, resistive reach for single phase to ground and multiphase faults, zero sequence compensation factor and directionally of all zones. This means that the total protection scheme can be seen as a combination of three impedance elements: Phase to phase measuring elements, phase to ground measuring elements and fault detection/phase selection measuring elements for each zone and three phases. can be verified that within the accuracy no operation will occur outside the characteristic. The different measuring criterion can be identified in the characteristic and all of them have to be fulfilled for operation. The characteristic is principally identical for all type of faults. The reactive and resistive reach settings are different for the phase to ground and the phase to phase measuring loops. 2.2 Principles of Operation Filtering Practically all voltage, current and impedance quantities used within ZMFPDIS are derived from fundamental frequency phasors filtered by a half cycle filter. To improve the properties of the filter, it is reset at fault inception. The reset means that just after fault detection the filter size is reduced to sub quarter cycle length and then increased one step for each new sample until the half cycle length is reached. The aim is to only use samples from the fault period in the filter and hereby estimating the properties of the fault signal more quickly. The phasor filter is frequency adaptive in the sense that its coefficients are changed based on the estimated power system frequency. By switching to filter coefficients which are more closely related to the actual power system frequency, the properties of attenuating harmonics are maintained also during off-nominal frequency conditions. Distance Measuring Zones Each quadrilateral distance measuring zone includes six impedance measuring loops; three intended for phase-to-earth faults, and three intended for phase-to-phase as well as threephase faults. It is well-known that transients from CVT s may have a significant impact on the transient overreach of a distance protection. At the same time these transients can be very diverse in nature from one type to the other. Figure 1. Fault detection elements and adaptive expanding characteristic. A filter described in [13] gives initially an underestimation of the current, which increases security of the scheme. The comparison of the currents and voltages gives an impedance circle and the operating time is shorter. The apparent characteristic will increase when the filter factors are adjusted towards a narrower bandwidth and as the estimate of the fault current grows. Basic Characteristic The characteristic can be described by the figure 1. Due to the transient character of the measuring principle static measurement cannot verify the characteristic. Dynamically, it There are basically two types of CVT s from the point of view of ZMFPDIS; the passive and active type, which refers to the type of ferro-resonance suppression device that is employed. The active type requires more rigorous filtering which will inflict negative impact on operate times. However, this will be evident primarily at higher source impedance ratios (SIR s), SIR 5 and above, or close to the reach limit. Circular Characteristic The primary instrument for avoiding overreach and at the same time achieving fast operate times is a supplementary circular characteristic that includes some alternative processing [12]. One such circular characteristic exists for every measuring loop and quadrilateral characteristic. There are no specific reach settings for this circular zone. It uses the normal quadrilateral zone settings to determine a reach that will be appropriate. This implies that the circular characteristic will always have somewhat shorter reach than the quadrilateral zone. 2

3 Phase-Selection Element The operation of the phase-selection element is primarily based on current change criteria [12]. The current change criteria itself can however only be relied on for a short period following the fault inception (during what this will call the current change phase). Subsequent switching in the network may render the change in current invalid. So, naturally, the phase-selection element also operates on continuous criteria. 1 1 k Voltage different between phase L1 and L2 (L2 lagging L1) Memorized voltage difference between phase L1 and L2 (L2 lagging L1) Current difference between phase L1 and L2 (L2 lagging L1) Factor determining the amount of memory voltage The phase-selection element can, owing to the current change criteria, distinguish faults with minimum influence from load and fault impedance. In other words, it is not restricted by a load encroachment characteristic during the current change phase. This significantly improves performance for remote phase-to-earth faults on heavily loaded lines. One exception however is three-phase faults, for which the load encroachment characteristic always has to be applied, in order to distinguish fault from load. The continuous criteria will in the vast majority of cases operate in parallel and carry on the fault indication after the current change phase has ended. Only in some particularly difficult faults on heavily loaded lines the continuous criteria might not be sufficient, e.g. when the estimated fault impedance resides within the load area defined by the load encroachment characteristic. In this case, the indication will be restricted to a pulse lasting for one or two power system cycles. Directional Element Several criteria are employed when making the directional decision. The basis is provided by comparing a sum of positive sequence voltage and memory voltage with phase currents. For extra security, especially in making a very fast decision, this method is complemented with an equivalent comparison where, instead of the phase current, the change in phase current is used. Moreover, a basic negative sequence directional evaluation is taken into account as a reliable reference during high load condition.finally, zero sequence directional evaluation is used whenever there is more or less exclusive zero sequence in-feed. Nominally, the directional sectors that represent forward direction, one per measuring loop, are defined by the following equations. 15 < (1 ) < < (1 ) < 120 Where, 1 Positive sequence phase voltage in phase L1 1 Positive sequence memorized phase voltage in phase L1 Phase current in phase L1 2.3 Measuring Principles Fault loop equations use the complex values of voltage, current, and changes in the current. Apparent impedances are calculated and compared with the set limits. The apparent impedances at phase-to-phase faults as follows: (example for a phase L1 to phase L2 fault) = Here U and I represent the corresponding voltage and current phasors in the respective phase Ln (n = 1, 2, 3) The earth return compensation applies in a conventional manner to phase-to-earth faults (example for a phase L1 to earth fault) as: Where: = +, and are the phase voltage, phase current and residual current present to the IED is defined as: Where: 0 1 = = = 1+ 1 R0 is setting of the resistive zero sequence reach X0 is setting of the reactive zero sequence reach R1 is setting of the resistive positive sequence reach X1 is setting of the reactive positive sequence reach Here is a phasor of the residual current in IED point. This results in the same reach along the line for all types of faults. The apparent impedance is considered as an impedance loop with resistance R and reactance X. Measuring elements receive current and voltage information from the A/D converter. The check sums are calculated and compared, and the information is distributed into memory 3

4 locations. For each of the six supervised fault loops, sampled values of voltage (U), current (I), and changes in current between samples (DI) are brought from the input memory and fed to a recursive Fourier filter. The filter provides two orthogonal values for each input. These values are related to the loop impedance according to: illustrated with the full loop reach while the phase-to-phase characteristic presents the per-phase reach. in complex notation, or: = + ( ) = ( )+ ( ) ( ) = ( )+ ( ) =2 where: Re designates the real component of current and voltage, Im designates the imaginary component of current and voltage and designates the rated system frequency. Figure 2. Characteristic for phase-to-earth measuring, ohm/loop domain. The algorithm calculates measured resistance from the equation for the real value of the voltage and substitute it in the equation for the imaginary part. The equation for the measured reactance can then be solved. The final result is equal to: ( ) ( ) = ( ) ( ) ( ) ( ) = ( ) ( ) The calculated and values are updated each sample and compared with the set zone reach. The adaptive tripping counter counts the number of permissive tripping results. This effectively removes any influence of errors introduced by the capacitive voltage transformers or by other factors. The directional evaluations are performed simultaneously in both forward and reverse directions, and in all six fault loops. Positive sequence voltage and a phase locked positive sequence memory voltage are used as a reference. This ensures unlimited directional sensitivity for faults close to the IED point. 2.3 Measuring Zones Settings All zones operate according to the non-directional impedance characteristics presented in figure 2 (phase-to-earth) and figure 3 (phase-to-phase). The phase-to-earth characteristic is Figure 3. Characteristic for phase-to-phase measuring The fault loop reach in relation to each fault type may also be noted as in particular that the setting RFPP always represents the total fault resistance of the loop, even while the fault resistance (arc) may be divided into parts like for three-phase or phase-to-phase faults. The R1 and jx1 represent the positive sequence impedance from the measuring point to the fault location. 3 Implementation Implementation of the new distance protection with fast adaptive features has been made in a modular line terminal platform that is a part of an automated substation concept. The transformer module, the A/D conversion module, the main processing module, the power supply module, the binary I/O module and the communication module are shown in Figure 4. 4

5 The main processing module has CPU logics and a main logic processor. A 1 ms sampling interval has been used. Figure 4. The protection platform 4 Testing Development and evaluation of the new protection schemes has been done within an interactive software environment. The software testing of the protection schemes is accomplished by using EMTP-MATLAB software tools. EMTP/ATP files and recorded data from a real time power system simulator (with a connected disturbance recorder) have been used as input data. To thoroughly evaluate operation of the proposed protection schemes, several network configurations have been simulated. Additional tests have been done using a RTDS real time power system simulator. The simulator test was carried out in accordance with IEC standard [14]. The responses of the protection in terms of operating times for different types of faults and SIR are shown in figure 5. The parameters for the tests are shown in Table 1 in Appendix. Figure 5. Operating times for different fault types and SIR 5

6 5 Conclusions This paper describes a new, high speed protection for EHV transmission lines. The presented scheme also performs well on series compensated lines. The distance protection is providing a fast operate time for basic faults within 60% of the line length. The protection is as well designed for extra care during difficult conditions in high voltage transmission networks, like faults on long heavily loaded lines and faults generating heavily distorted signals. A new built-in adaptive load compensation algorithm prevents overreaching of the distance zones in the load exporting end during phase-to-earth faults on heavily loaded power lines. It also reduces under-reach in the importing end. Comprehensive test results on various test systems show the benefits of the scheme. The results with a Real Time Digital Simulator (RTDS) are presented in this paper. The scheme is now in operation in several countries around the world under trade name REL Acknowledgements The authors extend their grateful acknowledgement to the colleagues at the Application Function Development department of ABB AB, Substation Automation Products, Sweden, for their substantial contributions during the basic development of this concept, and colleagues at Application department for the evaluation of the algorithms and testing at RTDS. References [1] Application guide on protection of complex transmission network configurations, CIGRE materials, CIGRE SC 34 WG-04, August, (1990). [2] F. Andersson, W. Elmore, Overview of Series- Compensated Line Protection Philosophies, Western Relay Protective Conference, Washington State University, Spokane, Washington, October, (1990). [3] J. Cheetham, A Newbould, G. Stranne, Series- Compensated Line Protection: System Modelling and Test Results, 15th Annual Western Relay Protective Conference, Washington State University, Spokane, Washington, October, (1988). [4] M.M.Saha, S.Ward, Adaptive Distance Protection For Series Compensated Transmission Lines, 29th Annual Western Protective Relay Conference,Spokane,WA,October 22-24,(2002). [5] M.M. Saha, B. Kasztenny, E. Rosolowski, J. Izykowski, First zone algorithm for protection of series compensated lines, IEEE Trans. on Power Delivery, Vol.16, NO.2, pp , (2001). [6] A.G.Phadke, M.Ibrahim,T.Hlibka, Fundamental basis for distance relaying with symmetrical components, IEEE Trans. Power Appar. Syst. PAS-96 (2) (1977) [7] D.I.Walkar, S.Elagovan,A.C.Liew, Fault impedance estimation algorithm for digital distance relaying, IEEE Trans. Power Deliv., 9 (3) (1994) [8] B.Jeyasurya, W.J.Smolinski, Identification of a best algorithm for digital distance protection of transmission lines, IEEE Trans. Power Appar. Syst. PAS-102 (10) (1963) [9] A.M.Ranjbar, B.J.Cory, An improved method for digital protection of high voltage transmission lines, IEEE Trans. Power Appar. Syst. PAS-91 (2) (1975) [10] M.S.Sachdev, M.A.Baribeau, A new algorithm for digital impedance relays, IEEE Trans. Power Appar. Syst. PAS-98, (1979) [11] M.Akke, B.Westman, Load compensation in distance protection of a three-phase power transmission line, European Patent Specification, EP B1,24 March, (2010). [12] M.M.Saha, K.Wikstrom, S.Lindhal, A new approach to fast distance protection with adaptive features, IEE/DPSP, Nottingham, UK, March, (1997). [13] A. Engqvist, L. Eriksson, Numerical Distance Protection for Sub-transmission lines, Cigré, 34-04, Aug./Sept., (1988). [14] IEC Final draft International Standard, Measuring relays and protection equipment-part 121: Functional requirements for distance protection, IEC ,Ed. 1.0,95/319/FDIS, January 3, (2014) Appendix Tests and settings are for 50 Hz and 1 A. ShortLine-20km RLdFw RLdRv XLd ArgLd LongLine-100km ZoneLinkStar Phase Selection Phase Selection t CVTtype Passive type Passive type INReleasePE X R X R RFPP RFPE Table 1: Test settings for RTDS real time power system simulator. 6