Research Objectives and Methodology

With these motivations, an exhaustive research work is carried out to investigate the impact of process bus communication network on the performance and the reliability of substation protection functions. The highlight of research objectives are listed below:

1. Detailed dynamic modeling and performance analysis of process bus

communication networks for different protection functions of a typical substation.

2. Studying the number of sampled values lost/delayed at various substation

protection IEDs over the process bus, and developing a sampled value estimation algorithm which can counteract adverse effects of multiple loss/delay of sampled values.

3. Development of hardware facility based on IEC61850-9-2 process bus network to

demonstrate the implementation and testing of the proposed sampled value estimation algorithm as a part of some of the typical protection functions (e.g. biased-differential protection of substation busbar, and distance protection of transmission line) within a laboratory environment.

4. Evaluation of the reliability and availability of the process bus based substation

protection systems, and also the comparison of the reliability and availability of various practical process bus communication networks for a typical substation layout.

5. Extension of Markov model for protection system reliability evaluation by

considering process bus communication network, and studying the impact of process bus on protection reliability indices.

The specific research methodology to carry out the above listed research objectives at different stages is described below:

Stage-1. Performance of substation protection over IEC 61850-9-2 based process bus communication network

According to IEC 61850, the maximum acceptable delay for the time critical messages of substation protection, such as raw data SVs and GOOSE should be within 3 to 4 msec. Therefore, to reduce additional time delay caused by TCP/IP (Transmission Control Protocol/Internet Protocol) layers, IEC 61850 has proposed direct mapping of these time critical messages over data link layer by eliminating the TCP/IP layers. However, this elimination affects reliability of message transmission, i.e. reception of time critical messages is not guaranteed. Thus, IEC 61850 proposes repetition of the same GOOSE message several times over the communication networks. On the other hand, repetition of SV time critical messages cannot be proposed in order to limit the huge traffic over the network. Hence, there is no assurance for the SV message communication over the IEC 61850-9-2 based process bus. Thus, the detailed dynamic performance analysis of the process bus for time critical SV messages is carried out using industrial simulation tool, OPNET. The models for IEC 61850-9-2 process bus devices, such as protection IED and merging unit are developed in the OPNET. Moreover, as suggested in IEC 61850 part-9, various communication protocols such as VLAN (IEEE 802.1Q) and QoS (priority tagging based on IEEE 802.1p) are implemented in to the process bus model of the OPNET. Finally, the sample value packet loss and packet delay are studied for different process bus scenarios.

Stage-2. Sample value estimation algorithm for IEC 61850-9-2 process bus

To improve the performance of digital substation protection functions in case of SV loss/delay, a generic SV estimation algorithm is proposed which can be applicable to any digital protection system. To test the performance of the proposed algorithm for various scenarios, a typical substation is simulated in PSCAD/EMTDC software to obtain raw data SVs, and different scenarios of multiple SVs loss are created with the help of MATLAB. Finally, the error analysis is carried out, before applying to digital relaying

algorithms, to study the accuracy of proposed SV estimation algorithm by considering the effects of Source Impedance Ratio (SIR) and Point-On Wave (POW) of the fault, noise level into power signal, sampling frequency, and instance of SV loss.

Stage-3. Hardware implementation of sampled value estimation algorithm in the laboratory environment

The proposed sampled value estimation algorithm should be implemented and tested before applying to any commercial protective relaying. Therefore, hardware laboratory is setup for a typical IEC 61850-9-2 based process bus network with substation protection systems. The developed testing facility includes the development of various digital protection devices, e.g. protection IEDs, and MU real-time playback simulator over industrial embedded systems with a hard-real-time platform; implementation of IEEE 1588 based time synchronization over the process bus using the GPS signals; and configuration of IEC 61850-9-2 based Ethernet networks using the commercial Ethernet

switches. The performance improvement of the busbar differential and the transmission

line distance protections using the proposed sampled value estimation algorithm is analyzed with a series of experiments.

Stage-4. Reliability analysis of process bus communication architectures

To study the impacts of process bus communication network on the reliability of substation protection functions, the analysis is started with the development of Reliability Block Diagrams (RBD) for various practical process bus architectures, considering Ethernet configurations (e.g. cascaded, ring, star-ring and redundant-ring), as well as, time synchronization techniques (e.g. IRIG-B and IEEE 1588). Thereafter, the combinatorial reliability of protection system is analyzed using RBD method. Also, the failure rates of these process bus architectures obtained from RBD method can be used for the further Markov modeling.

Stage-5. Impact of process bus communication on reliability of substation protection

After obtaining the failure rates of practically possible process bus communication architectures with different time synchronization techniques using the RBD, the existing Markov models of protection system (from literature) is extended to accommodate the IEC 61850-9-2 based substation protection functions. With this proposed Markov model, the reliability indices of protection systems, such as, protection unavailability, abnormal unavailability, and loss of security are obtained to compare the traditional protection functions with the next generation IEC 61850-9-2 based protection functions. Moreover, the impact of SV loss/delay on the reliability indices of a protection system, and possibility of improvements with the help of the proposed SV estimation technique is studied using the proposed model. In addition to that, the sensitivity analysis of time synchronization techniques, process bus network reconfiguration, monitoring effectiveness, and failure rate of protection IED on protection reliability indices is carried out in detail.