IEEE ISGT Asia 2013 1569815463
Sampled Values Packet Loss Impact on IEC 61850 Distance Relay performance Ikbal Ali1, Senior Member, IEEE, Mini S. Thomas2, Senior Member, IEEE, and Sunil Gupta3, Student Member, IEEE Department of Electrical Engineering FET, Jamia Millia Islamia New Delhi, India 1
[email protected],
[email protected],
[email protected] non-conventional instrument transformers for collecting voltage, current and other sampled value and status data from merging units (MU’s) and communicated it to bay level IEDs on a multicast basis. This data exchange is possible via IEC 61850-92 that defines a “configurable” dataset that can be transmitted on a multi-cast basis from one publisher to multiple subscribers [6], [7]. This integration of substation data through MUs enables to replace a number of Instruments transformers at the process level by a fewer ones to achieve lower installation, maintenance and transducer costs [8], [9].
Abstract—Modern substations are implementing Switched Ethernet based IEC 61850-9-2 Process bus communication networks for realizing their protection functions. Packet loss is a common phenomenon and plays a crucial role in the implementation of communication network based time-critical real time protection applications of power system. This paper presents the modeling and simulation of communication based digital distance relay protection scheme using MATLAB. In the first phase, the scheme is tested under various fault types, locations and fault resistance conditions to verify the operating performance of simulated distance relay model. In second phase, it is tested under sampled values packets loss condition to analyze the impact of lost data on the trip time performance of relay model. The result shows that the lost sampled values data in input signals adversely affects the tripping of distance relay.
A successful configuration, implementation & testing of IEC 61850 based time critical real time protection functions in substation is required to enhance and meet the stringent performance criterion defined in IEC 61850 communication standard. Unlike conventional hardwired protection applications their performance is influenced by the variation in communication network parameters such as network configuration, data rate, sampling frequency, packet size, network load condition, network background traffic etc. The timing performance of GOOSE & Sampled Values (SVs) messages as defined in IEC 61850 standards is essential to realize their use in real time applications. The message transmission time depends not only on communication network parameters but also on network situations and the processing capabilities of devices used. For this, the message transmission time requirements as per IEC 61850-5 must be fulfilled under all network load operating conditions [10]. Thus, appropriate substation communication network architecture must be designed to assure diverse levels of performance issues in IEC 61850 substation automation system. The dynamic performance of substation communication network architecture in different topologies and under different network configuration parameters are analyzed and discussed in literature [11]-[14].
Index Terms--Communication Schemes; Distance Protection; IEC 61850 Substation Automation System (SAS); IEC 61850-9-2 Process Bus; Switched Ethernet.
I.
INTRODUCTION
The advancement in the state-of-the-art microprocessor and digital signal processing techniques in protective relaying has led to the evolution of numerical protection relays commonly known as Intelligent Electronic Devices (IEDs) that integrates a number of protection, control and other protective relay functions in a single unit [1],[2].The communication standard IEC 61850, Communication Networks and Systems in Substations, was introduced primarily to allow interoperability between the IEDs in Substation Automation System (SAS). In IEC 61850 based modern digital substations, the strategy is to replace a multiple network of heavy copper cables with a fewer and lighter switched Ethernet technology based shared communication links between the primary and secondary equipments[3].
Switched Ethernet technology based substation communication network plays an important role in the design and operating performance of IEC 61850 based digital substation applications. The Quality of Service (QoS) features like full duplex, priority tagging (IEEE 802.1p), VLAN (IEEE 802.1q), Rapid Spanning Tree Protocol (IEEE 802.1w) etc. offered by Switched Ethernet technology satisfy the real time performance requirements in substation automation applications [15]-[18]. These features allow the efficient use of available
The advanced technological features like GOOSE & Process Bus inherited in IEC 61850 permits the design of new and innovative power system applications that achieves high reliability, availability and optimized performance at low cost [4],[5]. IEC 61850-8-1 GOOSE replaces a complex network of hardwired connections with an Ethernet network at the station level for inter-IED communication. IEC 61850-9-2 Process bus connects process level switchyard equipments to conventional or
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network bandwidth and minimize the several delays by segregating and prioritizing the network traffic. The communication mechanism inherited in client-server communication provides the guarantee to the delivery of information. However, it makes the transmission too slow to be used for time critical communications. To reduce the additional overhead introduced by TCP/IP layers during transmission, time critical messages such as SVs from MUs and GOOSE messages from IEDs are mapped directly to the link layer of the Ethernet [19],[20]. This feature accelerates the transmission of time critical IEC 61850-9-2 SVs messages but adversely affects the reliability of transmission. Thus owing to the reduced transmission reliability, there might be a loss or delay in the transmitted time critical IEC 61850-9-2 SVs messages across a process bus communication network caused by network congestion, poorly selected queuing mechanism or poor configuration of system etc. [21],[22]. Hence it is crucial to analyze the performance of Ethernet communication based timecritical applications under various sampled value loss scenarios before these can be realized in real time applications.
fault enters into the protection zone operating characteristic i.e. when the ratio V/I fall inside a circle. The characteristic of Mho type distance relay which is described on the R-X diagram is shown in Fig. 1. TABLE I. S. No.
Fault Type
Impedance Estimation Formula
1.
AG
VA/(IA+3KI0)
2.
BG
VB/(IB+3KI0)
3.
CG
VC/(IC+3K0)
4.
AB or ABG
(VA-VB)/(IA-IB)
5.
BC or BCG
(VB-VC)/(IB-IC)
6.
CA or CAG
(VC-VA)/(IC-IA)
Where, K = (Z0 – Z1)/(3Z1) Z0 = Zero sequence line impedance Z1 = Positive sequence line impedance I0 = Zero sequence component of current
The paper presents the modeling and simulation of communication based digital distance relay protection scheme using MATLAB. In the first phase, the scheme is tested under various fault types, locations and fault resistance conditions to verify the operating performance of simulated distance relay model. In second phase, the paper discussed the impact of sampled values packet loss data on the trip time performance of the distance relay model. For this a program is developed in a MATLAB that creates different sampled values loss scenario in fault voltage and current waveforms before they are presented to distance relay algorithm. The rest of the paper is organized as follows. Section II provides a brief overview of distance protection scheme. Section III discusses the detailed dynamic modeling of distance protection scheme in MATLAB SIMULATION software. Section IV discussed the simulation and testing results. Finally, Section V concludes this paper. II.
FAULT IMPEDANCE CALCULATION FORMULAE
Figure 1. Mho distance relay characteristic
DISTANCE RELAY BASICS
The distance protection scheme is normally a multi-zone arrangement in which three zones of protection are usually included forming the stepped time-distance characteristic. The distance relay zone reach setting for zone 1 is generally set up to 80% of the protected line impedance for instantaneous zone1 protection. This is done to avoid the effect of overreaching in relay operation that might be due to any measurement errors in instrument transformers, errors in relay settings or due to the presence of dc offsets. This provides high speed and reliable protective relaying with accurate discrimination between the internal and external faults. Zone 2 reach settings usually include 120% of the protected line impedance which may vary depending on the application. Zone 3 includes 150% of the protected line impedance which is normally applied as a backup protection for zone 2. The Zone 2 and Zone 3 elements are generally set with some delay in their operations [28].
Distance relaying is generally used for the protection of medium and long transmission lines in power system where high speed operation is required for fault rectification. They operate by using both voltage and current phasors for impedance estimation that determines whether the fault impedance is within a relay’s zone of protection or not. In simulated distance relay model, the impedance estimation is performed by incorporating six operation elements that includes three elements for phase faults and another three for earth faults. Based on the particular fault type, the relay computes the fault impedance from the relay location to the fault point using a set of equations given in Table I. The distance relay algorithm implemented in distance relay then compares the estimated impedance with its pre-set value which is determined by the relay’s protection zone reach. The relay element operates when the impedance trajectory of the
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III.
MODELING OF DISTANCE PROTECTION SCHEME
Fig. 2 shows the SIMULINK model developed in MATLAB for a 132 KV 50 Hz power system comprising of a 100 km long three phase transmission line with two three phase sources connected one at each end. The Sim-Power-Systems SIMULINK tool in MATLAB provides the necessary components to model the designed power system [29].The parameters in each block can be varied accordingly to obtain suitable results.
Figure 3. Sample fault signals from LG fault on a transmission line
Fig. 4 shows the shape of a typical faulty phase current waveform for a single line to ground fault on a simulated power system that contains decaying dc components, subsystem frequency transients, and high frequency oscillation quantities. The efficient and reliable operation of distance relay algorithm depends upon the accuracy of instantaneous voltage and current samples presented to it for impedance estimation up to the fault point. Therefore, it is important to reduce the effects of these unwanted noisy components from the generated fault voltage and current signal waveforms for accurate phasor estimation by the relay algorithm [23]-[25].
Figure 2. SIMULATED power system model
The parameters of the designed single circuit transmission line power system are given in Table II [26]. TABLE II.
PARAMETERS OF THE DESIGNED POWER SYSTEM MODEL Figure 4. Distorted fault current waveform from a single line to ground fault
Three phase sources parameters Parameters
Value
Power system voltage 132 Frequency 50 3 phase short circuit level at bus 300 voltage Source X/R ratio 6 Transmission line parameters Transmission Line length 100 Positive sequence resistance 0.01239 Zero sequence resistance 0.1239 Positive sequence inductance 0.00043386 Zero sequence inductance 0.00130157 Positive sequence capacitance 1e-9 Zero sequence capacitance 1e-9
Unit
Fig. 5 shows the snapshot of the signal processing subsystem model simulated in MATLAB which is used to carry out various signal processing tasks such as analog filtering, A/D conversion, digital filtering etc. on the fault signal waveforms.
KV Hz MVA km Ω/km Ω/km H/km H/km F/km F/km
The simulated three phase voltage and current waveforms for a numerous number of faults under different power system conditions are extracted from the voltage transformer (VT) and current transformer (CT) respectively and scaled down to a low magnitude levels for use in numerical distance relays. Fig. 3 shows the sample of pre-fault, fault and post-fault voltage and current waveforms for a LG fault appear on a transmission line in a simulated power system model.
Figure 5. Signal processing model simulated in MATLAB
The processed output signals are then used to obtain the phasors of the fundamental frequency components of voltage and current signals which are further utilized to plot the impedance trajectory of the fault on R-X diagram. In the signal processing SIMULINK model, there is a scope & sink after every block to observe the output, and the finally obtained
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magnitude and phase of the signals are sent to the workspace for further use in relays.
frequency components from the distorted fault signals by eliminating decaying dc components. Fig. 8 shows the magnitude and phase waveform obtained for current signal, where it is then transformed to rectangular form. The expressions for calculating the DFT for the input voltage signal at nth sample v (n) and current signal at nth sample i (n) are given in (1) & (2) [27]
Figure 6. Current signal as appeared after analog filtering
XR
(1)
XL (k) =
(2)
Where, N is the number of samples per cycle.
To avoid the phenomenon of aliasing in which the high frequency components of the inputs appear to be parts of the fundamental frequency components, low pass anti-aliasing analog filters with appropriate cut-off frequency are used. These anti-aliasing filters eliminate high frequency components from the fault signal but cannot remove decaying dc components and reject low frequency components. Fig.6 shows one of the output signals of an analog filter.
Distance relay algorithm calculates the resistance and the reactance values using the estimated voltage and current phasors at relay point. The efficient and reliable operation of distance relay depends on the accuracy of fundamental frequency phasors presented to it for estimating impedance from the relay point to the fault point. Mho circle relay compares the impedance estimated with its pre-set impedance setting as per the protection zone characteristics and issues a trip command to the corresponding circuit breaker when the fault impedance lies within the operating characteristics zone of relay.
Since the microprocessor based digital numerical relays can process only digital signals therefore the voltage and current signals are sampled at discrete times. For this, these signals are passed through a sample and hold module, and conveyed, one at a time, to an Analog to Digital converter (ADC). The Sample & Hold converts the input signals from the analog filter into samples, which are then input into the quantizer. The basic function of quantizer is to quantize a smooth signal into a stairstep output using the round-to-nearest method. The quantizer produces a digital output signals which is then sent to the digital filter. It is noted that the amount of dc offsets present in the faulted waveforms depends upon the parameters such as fault location, fault inception angle, fault resistance etc. The effect of dc offsets on the operating performance of a relay can be directly observed. The presence of dc offsets in fault waveforms can cause relay to overreach or may produce saturation in current instrument transformers. Thus the accurate estimation of fundamental frequency components requires the removal of dc offsets from the faulted signals. The output of a digital high pass filter which is used to eliminate sub-harmonics and the effect of the dc offsets is shown in Fig.7
(a) Current signal magnitude from DFT
(b) Current signal phase from DFT Figure 8. DFT output waveforms
IV.
SIMULATION RESULTS
The relay parameters are set as per the designed distance protection scheme for a single circuit transmission line in power system. It includes protection zones reach settings, the time delays provided for zone 2 & zone 3 protections etc. A MATLAB program is developed to plot the characteristics of
Figure 7. Signal as appeared after digital filtering
The Discrete Fourier transform (DFT) algorithm is a digital filtering algorithm used to extract the fundamental
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mho distance relay i.e. the impedance characteristics variation based on the input voltage and current samples. The apparent impedance seen by the relay prior to the fault is large whereas during fault it is small. Thus the impedance will move into protection zone characteristic impedance during fault condition. The relay response evaluates whether the impedance trajectory of the fault enters the operating zone characteristics of the relay or not. The relay operating time is time elapsed between the fault initiation and the instant when the impedance trajectory of fault intersects the relay operating characteristics. In the first phase of relay testing, the faults at different locations with varying fault inception angle and resistance are simulated. The results so obtained are presented in graphical form using an R-X diagram. The effects of these parameters on the impedance trajectory of the fault are addressed. It is noted that the impedance trajectory of a fault and hence the tripping decision and trip time depends on fault parameters and system configuration. The effect of varying fault resistance on the reach and tripping decision of the distance relay for a LG fault are discussed in the presented simulation results in Fig. 9.
(c) Rf = 25ohm Figure 9. R-X plot for A-G fault at different fault resistance (Rf)
It is seen that the relay shows consistent behavior under different fault scenario conditions. However, the impedance trajectory of the fault i.e. the performance of relay is adversely affected at high fault resistance values. These results are generally as expected noting that at high fault resistance, relay fails to operate in its operating zone fault and the majority of the faults are detected in Zone 2 time. The trip time results under varying fault resistance conditions are presented in Table III. TABLE III.
VARIATION OF TRIP TIME WITH FAULT RESISTANCE (RF)
Fault Type
Rf=5ohm
Rf =15ohm
Trip Time (ms) Rf = 25ohm (No Operation State)
A-G
16.6
22.2
169.5
B-G
15.3
20.5
175.3
C-G
15.7
24.1
164.4
In the second phase of relay testing, the distance relay is tested from the performance of process bus communication network point of view and its impact on the tripping time performance of the relay is investigated. A program developed in MATLAB generates various suitable sampled values packet loss scenario in the input voltage and current waveforms for the distance relay impedance estimation algorithm. From the results obtained as shown in Fig. 10, it has been found that for a consecutive five sampled value packets loss in input fault waveforms, immediately after the fault occurs, the tripping decision and hence the trip time performance of distance protection IED is adversely affected. The trip time of distance relay without sampled values loss is within a specified range which means that the relay detects fault within its protection zone 1. However, the operation of relay is delayed with sampled values loss in input signals under different scenarios. The additional delay introduced due to the loss of SV packets is add to the various deterministic and non-deterministic communication delays in switched Ethernet network like store
(a) Rf = 5ohm
(b) Rf = 15ohm
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and forward delay, queuing delay, processing delay & propagation delay etc. Due to this, the relay misjudges the fault location and does not operate for zone 1 protection. These results are generally as expected noting that a significant trip time difference is observed in between the healthy and corrupted state of the IEC 61850-9-2 process bus communication channel.
[7] IEC 6850-9-2 LE: Implementation guideline for digital interface to instrument transformers using IEC 61850-9-2, UCA International Users Group. [8] R. Hunt, “Process Bus: A practical approach”, PAC World, Spring 2009Issue. [9] A. Apostolov, B. Vandiver, “Understating the IEC 61850 9-2 process bus and its benefits”, in Proc. 2009 Georgia Tech Protective Relay Conference. [10] IEC 61850-5: Communication requirements for functions and device models, IEC INTERNATIONAL STANDARD, July 2003. [11] T.S. Sidhu and Yujie Yin, “Modelling and simulation for performance evaluation of IEC61850-based subsation communication systems”, IEEE Transactions on Power Delivery, vol.22, no. 3, pp. 1482–1489, July 2007. [12] L. Andersson, K.P. Brand, and D. Fuechsle, “Optimized architectures for process bus with IEC 61850-9-2, “presented at the CIGRE Session Paris, France, Aug. 2008, paper B5-105. [13] I. Ali and M. S. Thomas, “Substation communication networks architecture”, POWERCON & IEEE Power India Conference, New Delhi, India, October 2008, pp. 12–15. [14] M.S. Thomas and I. Ali, “Reliable, Fast, and Deterministic Substation Communication Network Architecture and its Performance Simulation”, IEEE Transactions on Power Delivery, vol.25, pp. 2364–2370, Oct. 2010. [15] T. Skeie, S. Johannessen, and C. Brunner, “ETHERNET in substation automation”, IEEE Control Systems Magazine, vol. 22, no. 3, June 2002, pp. 43–51. [16] K. C. Lee and S. Lee, “Performance evaluation of switched Ethernet for real-time industrial communications”, Elsevier Computer Standards & Interfaces, vol. 24, no. 5, pp. 411–423, Nov. 2002. [17] J.D.Decotignie,“Ethernet-based real-time and industrial communications” , Proceedings of the IEEE, vol. 93, no. 6, pp. 1102–1117, June 2005. [18] C. Hoga, “New Ethernet technologies for substation automation”, IEEE Power Tech, Lausanne, 1-5 July 2007, pp. 707–712. [19] K.P. Brand, “The Standard IEC 61850 as prerequisite for intelligent applications in substations”, IEEE Power Engineering Society General Meeting, 6-10 June 2004, pp. 714–718. [20] T.S. Sidhu and P.K. Gangadharan, “Control and automation of power system substation using IEC61850 communication”, Proceedings IEEE Conference on Control Applications , Toronto, Canada, 28-31 August 2005, pp. 1331–1336. [21] T.S. Sidhu, M.G.Kanabar, and P.P.Parikh, “Implemetation issues with IEC61850 based subsation automation systems”, presented at the National Power System Conf. Mumbai, India, Dec. 2008. [22] M.G.Kannabar and T.S.Sidhu,“Performance of IEC 61850-9-2 process bus and corrective measures for digital relaying”, IEEE Transactions on Power Delivery, vol.26, no. 2, pp. 725–735, 2011. [23] S.G.A. Perez, M.S. Sachdev, and T.S. Sidhu, “Modeling relays for use in power system protection studies,” Proceedings, 2005 Canadian Conference on Electrical and Computer Engineering, pp. 566-569. [24] Hamid Sherwali and Abdlmnam Abdlrahem, “Simulation of numerical distance relays”, Matlab-Modeling, Programming and Simulations, Emilson Pereira Leite (Ed.), ISBN: 978-953-307-125-1, InTech. [25] Li-Cheng Wu, Chih-Wen Liu, and Ching-Shan Chen “Modeling and testing of a digital distance relay using MATLAB/SIMULINK”, Power Symposium, Proceeding of the 37th Annual, North America, pp.253259,23-25 Oct. 2005. [26] M. H. Idris, S. Hardi, and M. Z. Hasan, “Teaching distance relay using Matlab/Simulink Graphical User Interface”, Procedia Engineering, Elsevier, pp. 264-270, 2013 [27] Vidyarani K.R., R. Nagaraja, G.K. Purushothama, and Somnath Guha, “Implementation of advanced DSP techniques in distance protection scheme”, International conference on Advances in Power Conversion and Energy Technologies (APCET), Andhra Pradesh, India, 2012. [28] H. Saadat, Power System Analysis, WCB/McGraw-Hill, 1999. [29] The Math Works, Inc., “Sim-Power-Systems user’s guide”, version 4.6, 2008.
Figure 10. Variation in Trip Time with and without missing samples for L-G fault
V.
CONCLUSION
Switched Ethernet based IEC 61850-9-2 Process bus communication networks play an important role in the design and operating performance of fully digital protection applications in modern substations. This paper analyzed the performance of MATLAB simulated communication based digital distance relay protection scheme under sampled values packets loss condition. Simulation results showed that the lost data in input signals had an adverse impact on the trip time performance of distance relay. The paper recommends the deployment of optimized process bus communication network architecture to realize time critical protection applications in IEC 61850 digital substation that can handle substation data in diversified network load conditions without any loss or delay in real time SVs data. REFERENCES [1]
J.D. McDonald, “Substation automation, IED integration and availability of information”, IEEE POWER Energy Mag., vol. 1, no. 2, pp. 22-31, Mar./Apr.2003. [2] A. G. Phadke and J. S. Thorp, “Computer relaying for power system”, Taunton, Somerset, England, Research Studies Press, and Wiley, New York, 1988. [3] IEC 61850: “Communication networks and systems in substations”, 2002–2005 (www.iec.ch). [4] R.E. Mackiewicz, “Overview of IEC 61850 and benefits”, IEEE Power Systems Conference and Exposition, PSCE’06, Atlanta, Oct.29-Nov.1 2006, pp. 623–630. [5] M.C. Janssen and A. Apostolov, “IEC 61850 impact on substaion design”, IEEE/PES Transmisssion and Distribution Conference and Exposition, Chicago, 2008, pp. 1-7. [6] INTERNATIONAL STANDARD IEC 61850-9-2, Communication networks and systems in substations-Part 9-2: Specific Communication System Mapping (SCSM) – Sampled Values over ISO/IEC 8802-3, First edition 2003-05.
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