IEC 61850-9-2 Based Process Bus

Authors: Tarlochan S. Sidhu, Mitalkumar G. Kanabar, and Mohammad R. Dadash Zadeh, Department of Electrical and Computer Engineering, University of Western Ontario, Canada

The merging unit (MU) is a key element of the IEC 61850 process bus. It gathers information, such as phase voltages and currents from instrument transformers, and status information from transducers using proprietary links.

All these analog values are converted to digital, and merged into a standard data packet format. This data packet is sent to corresponding bay level   protection and control IEDs using standardized Ethernet based communication links. As Figure 1 shows, MU has a time synchronization source (GPS clock), which provides a time stamp on each data packet. Time synchronization of each packet is required for protection and control IEDs to estimate accurate phasor.

Benefits from IEC 61850-9 Process Bus are:

  • Interoperability:  All devices connected to IEC 61850-9 process bus are able to share any piece of information with any IED
  • Simple connection: Point -to-point copper wire connectors are reduced to just few connections of communication links
  • Overall cost reduction: Deployment of IEC 61850-9 process bus reduces the installation and labor cost, even though material cost may be higher

Challenges with the performance of IEC 61850-9-2 process bus network: According to IEC 61850, the acceptable maximum communication delay for the highest class is as few as 3 msec. This has to be achieved independent from the traffic flows on the process bus communication network. The time critical messages on process bus are of two types:

  • GOOSE
  • Raw data message or sampled values (SV)

Process bus time critical messages are directly mapped to the Ethernet link layer. That means, these time critical messages are communicated with no transmission reliability due to elimination of TCP/IP layer (TCP acknowledges the receipt of the packet, and re-transmit if packet is lost). Therefore, to enhance the transmission reliability of GOOSE, IEC 61850-8-1 has proposed repetition of the same GOOSE messages multiple times. However, IEC 61850-9-2 does not suggest same message repetition for SV packets, which is justified because SV packets are continuously injected from several MUs at the rate of 80 samples per cycle. Repetition of the same SV packet several times would increase the network load tremendously. This means, there is no assurance or reliability measures for the SV packet communication over the process bus.

All the time critical messages will have priority tagging along with VLAN (IEEE 802.1Q) to enhance the performance over the process bus. However, IEEE 802.1Q may not be able to improve the performance especially in the worst case scenarios. For example, if higher priority packets arrive during the transmission of the lower priority packets with large size, higher priority packets will have  to wait until the transmission of the lower priority packets ends. This worst case scenario also depends upon the size of the packet and traffic on the network. That is why implementation of QoS using IEEE 802.1Q in Ethernet switch networks does not provide deterministic ETE  (End-To-End) delay on the network during worst case conditions. Detailed  analysis and performance evaluation of SV packets over the IEC 61850-9-2 based process bus has already  demonstrated the SV packet loss and delay in certain worst case scenarios.

Set-up for IEC 61850-9-2 based distance protection

Figure 1 shows the laboratory test set-up based on IEC 61850-9-2 process bus for distance protection devices. High speed (3.25 GHz) computers with real-time operating platform are used as an IEC 61850 IED and MU to implement and test the proposed sampled value estimation algorithm for transmission line distance protection. Commercial Ethernet switches have been used to set-up a single (distance protection) bay of an IEC 61850-9-2 process bus. Priority tagging based on IEEE 802.1Q to achieve the quality of service (QoS) in IEC 61850-9-2 process bus has been implemented for time critical messages. Distance protection IED, traffic generator IED, and merging unit are connected in star configuration to the process bus Ethernet switch.

Merging unit is implemented as real time data playback using high speed computer with real time operating system. A typical power system has been simulated using PSCAD/EMTDC simulation tool. The signal of 3-phase to neutral currents and voltages corresponding to distance protection are recorded using COMTRADE recorder in PSCAD at 20 kHz frequency. Special file conversion code is developed using C/C++ programming to convert 20 kHz COMTRADE data to IEC 61850-9-2 compliance sampled value Ethernet data packets at 4800 Hz, and saved as a Data.dat file. With the help of timer functions of real time operating system, the IEC 61850-9-2 sampled value packets (in sequence from Data.dat file) are sent to distance protection IED over process bus network by scheduling at every 0.20833 (1/4800) msec. Moreover, this implemented MU has capability to create various scenarios over the IEC 61850-9-2 process bus by simulating number of sampled value loss/delayed in order to study the impact of SV loss/delay over the protection function.

A protection and control IED with IEC 61850-9-2 features has been implemented using the same real time platform. Ethernet NIC of a P&C IED receives all the traffic over the process bus network and filters sampled value packet from the NIC buffer. Further, the received sampled value packets are dissected and the sampled values of 3-phase-to-neutral voltages and currents are stored in the sampled values buffer of the IED. A real-time thread for protection function is implemented in order to perform protection simultaneously with SV packet buffering. Commercial phasor-based DFT algorithm with phase comparator concept for Mho relay characteristic has been implemented as distance protection with memory polarization.  Mimic filter has been applied to the current signals to attenuate decaying DC prior to DFT calculations. Adaptive counting technique has been used to make a fast and secure tripping decision. Standard algorithms for fault detection and trip strategy have also been programmed using this real time platform. Secondary voltages and currents have been recorded in PSCAD/EMTDC after using anti-aliasing filter. Distance protection IED also has capability to send an IEC 61850-8-1 GOOSE message to breaker control unit according to the final trip logic.

Traffic generator is implemented same as P&C IED, except it can send several different types of traffic over the network. In this work, the traffic streams used over the process bus are GOOSE, GSSE, and IP packets. The packet length as well as inter-arrival time of the traffic stream can be configured as per the Ethernet NIC buffering capability. The GOOSE messages create a total of 5.18 Mbps traffic; GSSE creates 4.49 Mbps; and IP packets create 23.29 Mbps traffic. Therefore, total traffic injected from traffic generator is 32.9 Mbps.

Impact of sampled values loss on distance protection

Simulated power system model:  An appropriate model of 230 kV power system, as shown  in Figure 2, has been modeled in PSCAD/EMTDC simulation tool. COMTRADE recorder records the secondary values of 3-phase and neutral voltages and currents sampled at 20 kHz after applying analog (anti-aliasing) filter for all signals. Distance protection relay at Bus-1 is considered for the analysis. The distance relay zone settings are tabulated in Table-1.

Distance relay settings also incorporate the fault resistance coverage, almost 20 Ω for single-phase-to-ground faults, and almost 10 Ω for phase-to-phase faults.

Results: Extensive testing is important in order to understand the impact of sampled value loss on a particular protection algorithm. In order to investigate the implemented distance protection relay based on IEC 61850, various scenarios such as  different Source Impedance Rations (SIRs), Point-On-Wave (POW), and fault types have been examined. Full-cycle DFT based distance relay is set to protect 80% of the transmission line in Zone-1 using phase comparator based Mho characteristic. Table-2 shows the operating times of distance protection IED, considering round-trip communication delays over IEC 61850-9-2 process bus. Line-to-ground (LG) and line-to-line (LL) faults are applied at 75% of the Line-1, and the time of operation has been observed for low and medium SIRs {0.2, 1}; different fault inception {zero, mid, peak} over the A-phase voltage. Consecutive ten SV lost has been simulated using real time data playback after a half cycle (8.33 msec) of fault inception with 4800 Hz sampling frequency.

It can be observed from the table that without SV loss, the operating of the distance relay (including round-trip communication delay) is between 19 to 28 msec. This implies that the fault is detected in Zone-1. However, with ten consecutive sampled value loss, the Zone-1 element of the distance relay delayed, and operated in 39 to 49 msec.

The impact of SV loss on digital protection relaying depends upon various parameters, such as, number of SV loss per cycle, sampling frequency, position of SV loss occurrence, and system condition (i.e. SIRs, POW, fault types, etc.).

Estimation of lost samples and its performance in a distance relay

There are several numerical methods available for the estimation of lost samples e.g. polynomial approximation, spline techniques, curve fitting, etc. However, implementations of any of these complex numerical methods require additional computational capability in the IEDs. In our research we used an estimation techniques based on the Lagrange polynomial method, which is easy to implement as compared to other available methods, and also applicable to any digital relaying algorithm. According to Lagrange polynomial method, a unique polynomial, pn(t) of degree ≤ n, can be obtained from the given n+ 1 distinct sampled values. SV estimation techniques utilize this polynomial function to estimate the lost or delayed sampled values at a given time tk, as shown in Figure 4.

Quadratic SV estimation: To obtain the quadratic polynomial, there will be a need for three known SVs (t0,f0), (t1,f1) and (t2,f2). The lost SV at tk can be calculated using the polynomial equation.

Implementation: As shown in Figure 4, there are a total of three possible scenarios for the SV lost:

1) Next samples have not arrived

2) Next sample at tk+1 has arrived

3) Next sample at tk+2 has also arrived

The polynomial coefficients can be obtained according to these three scenarios.

The sampled value estimation algorithm has been implemented in sampled value buffer, which is used by protection algorithm at every 0.4166 msec (40 times in a cycle). When sampled value packet is received, it is dissected to obtain the sample count value in the data field of the SV packet, as well as, three-phase-to-neutral voltages and currents. Sampled count is the IEC 61850-9-2 based packet format field used for time synchronization purpose. This sample count is used in this proposed sampled value estimation algorithm in order to check the exact sequence of the randomly received sampled values from the IEC 61850-9-2. With the help of sample count, it can be determined how many sampled values are lost or delayed, as well as, whether those next to the lost sampled values are available for the estimation or not. With this information, set-1, set-2 or set-3 coefficients can be selected for the estimation. Finally, the lost sampled values are estimated using polynomial equation, and stored in the sampled value buffer for the protection algorithm. Further details of sampled value estimation algorithm have been explained in the article titled “Performance of IEC 61850-9-2 Process Bus and Corrective Measure for Digital Relaying”, available in Early Access of IEEE Power Delivery Transactions.

As the order of polynomial (n) increases, accuracy of the estimated sampled value increases, however, the computational complexity increases too. The implementation of quadratic SV estimation algorithm in conjunction with already existing traditional digital relaying algorithm has been explained in the above referred article.

Test results: The same power system model and scenarios explained earlier is considered for the testing of the proposed SV estimation algorithm in this section. It has been shown that due to the SV loss in IEC 61850-9-2 process bus network, the Zone-1 element of distance protection gets delayed by more than a cycle (16-19 msec). For the same real-time data playback with exactly the same SV loss scenario, Table-3 shows the comparison of operating time (including round-trip communication delays) for a distance protection IED equipped with and without the proposed SV estimation algorithm. It can be observed that with the help of estimation of sampled values, the distance relay Zone-1 can detect the fault within the normal operating time of  Zone-1 element. Moreover, it can be observed that there are some additional delays incurred due to waiting and processing of lost sampled values.

Conclusions: Time-critical IEC 61850-9-2 sampled value messages are mapped directly over the Ethernet data link layer. This reduces the transmission reliability due to elimination of TCP/IP layer, and may result in several sampled value losses or delays. To study the impact of sampled value loss and analyze the performance of the countermeasure, IEC 61850-9-2 process bus based on Ethernet switched network has been set-up in the laboratory environment for transmission line distance protection. The simulation of SV loss event using real-time playback has been explained in this paper.

Also, impact of sampled value loss on line distance protection has been analyzed for different power system cases simulated in PSCAD/EMTDC for 230 kV power system model. It has been shown with the help of various scenarios, such as, different SIRs, POWs, fault types, etc. that the Zone-1 element of a distance protection relay may be delayed due to 10 consecutive sampled value losses after a half cycle of fault inception.  Further, implementation of sampled value estimation algorithm for distance protection has been presented in order to counteract the impact of sampled value loss on substation protection. With testing in laboratory set-up, it has been shown that with the quadratic sampled value estimation algorithm, distance protection IED is able to detect Zone-1 fault correctly with some delay, even for 10 consecutive sampled value loss.  

Biographies

Tarlochan S. Sidhu (M'90-SM'94-F'04) received the B.E. (Hons) degree from the Punjabi University, Patiala, India, in 1979 and the M.Sc. and Ph.D. degrees from the University of Saskatchewan, Saskatoon, SK, Canada, in 1985 and 1989, respectively. He was with the Regional Computer Center, Chandigarh, Punjab State Electricity Board, India, and Bell-Northern Research Ltd., Ottawa, Canada. From 1990 to 2002, he was with the Department of Electrical Engineering, University of Saskatchewan. Currently, he is Professor and Chair of the Electrical and Computer Engineering Department at the University of Western Ontario, London. He is also the Hydro one Chair in Power Systems Engineering. Dr. Sidhu is a Fellow of the IEEE, the Institution of Engineers (India) and of the Institution of Electrical Engineer (U.K.). He is also a Registered Professional Engineer in the Province of Ontario and a Chartered Engineer in the U.K.

Mitalkumar G. Kanabar is currently pursuing his Ph.D. degree at Electrical and Computer Engineering Department, the University of Western Ontario, Canada. He received his B.E. degree from Sardar Patel University, Gujarat, India, in 2003, and M.Tech from Indian Institute of Technology (IIT) Bombay, India in 2007. His research areas include Power system protection, control and automation; Implementation of IEC 61850 standard; Applications for Smart Grid; Grid Integration issues with DERs.

Mohammad R. Dadash Zadeh was born in Iran. He received a B.S. and M.S. degrees in Electrical engineering from the University of Tehran in 2002 and 2005, respectively and received a Ph. D. degree in 2009. He was a post doctoral fellow at the University of Western Ontario from May 2009 to January 2010. Currently, he is working with GE Digital Energy as an application engineer. Mohammad joined the University of Western Ontario, London, in 2005. His areas of interest include power system protection, control, and analysis.

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