Development of an Improved Forced Island and Load Shedding Scheme to Prevent System Collapse

Development of an Improved Forced Island and Load Shedding Scheme to Prevent System Collapse.

Abstract

This research work presents a scheme that identifies line outages and generator outages using contingency analysis and effectively splits the network into a set of predefined islands, with a load shedding strategy to minimize the adverse effect of each outage and to ensure system security.

In this research work, power flow was used for the power flow analysis of the network.

Also, Active Power Loading Performance Index was used during the contingency analysis to rank the transmission line and generator outage based on the severity of each outage.

For each outage causing a line overload and voltage violation, the network splits into predefined islands and power flow analysis is performed on the new island to check the stability of the network.

For each island found to be unstable, a power mismatch, and under-voltage load shedding scheme is used to ensure the system stability.

The developed algorithm was implemented on the IEEE 6 and 14 test bus network.

From the IEEE 6 bus network, ten outages resulted in the split of the network into two islands. Also, the developed load shedding scheme was applied on each island that was found to be unstable after power flow analysis.

The average voltage profile improvement of the island network over the base case was found to be 13.02% after load shed o f42.3MW.

Also, from the IEEE 14 bus network, the contingency analysis considered twenty-four outages, with twenty-one outages causing a split of the network into two islands.

Load shedding scheme was also applied on each newly formed island found to be unstable. The average voltage profile improvement of the islands over the base case was 2.89% after a load shed of 80.92 MW.

The validation of this research work was performed by simulation, and comparing with the work of Soman et al,.(2015a), using load shed speed and voltage profile as performance metrics.

The developed method obtained an average load shed speed improvement of 63.3% and an average voltage profile improvement of 1.01%.

Also, from the results obtained, it is quite evident that the developed scheme has a better performance than Soman et al,.(2015a).

Introduction

Background of study

Power system networks mostly operate close to their stability limits as a consequence of the deregulated electricity market, growth in energy consumption, and lack of expansion of transmission networks due to economic and environmental constraints.

Under such operating conditions, a severe disturbance such as a loss of generating units or faults along transmission lines may lead to cascading events.

Thus, the risk of collapse and blackout of the overall power system is increased (tang et al., 2013). Security of a power system refers to the degree of risk in its ability to survive imminent disturbances (contingencies) without interruption of customer service.

It relates to the robustness of the system to imminent disturbances and hence depends on the system operating condition as well as the contingent probability of disturbances.

Electric power system security analysis encompasses three functions namely system monitoring, contingency analysis, and corrective control in which safe island formation and load shedding are some of the corrective control (ezhilarasi & swarup, 2009).

Power system blackout is the state when partial or complete areas of the system collapse due to cascading of failure events which causes mass scale tripping of transmission lines and generating units (soman et al., 2015a).

Islanding (also known as loss of grid or loss-of-mains – lom) represents “a condition in that a portion of the power system that contains both load and generation remains energized while isolated from the remainder of the power system”.

An island represents a condition where a portion of an area electric power system (eps) is energized solely by one or more local power sources while that portion of the area eps is electrically separated from the rest of the area eps. Islanding appears when some part of the utility grid loses connection with the rest of the system (banu & istrate, 2014).

References

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