Islanding power systems to minimize impact of system disturbances in Southern African Power Pool

Abstract

When power utilities physically connect their transmission equipment they form an interconnection. In an interconnection power utilities trade electricity. In so doing they optimize energy resources for environmental sustainability, regional integration and socio-economic growth. In southern Africa, power systems of nine countries are interconnected. Most of these power systems were originally designed to operate as islands, supplying loads within national boundaries. Most interconnectors were commissioned after 1995 when the Southern African Power Pool (SAPP) was officially formed. The larger the interconnected system grew the more complex it became. Utilities started experiencing disturbances emanating from outside their borders. Due to weak interconnections, most disturbances triggered power oscillations. Often, power oscillations led to cascading equipment failures that threatened the reliability of power supply and could easily culminate into blackouts. Towards the end of 2015, the SAPP commissioned special protection schemes at two key interconnection points. The schemes were designed to intentionally split the interconnection in order to minimize the impact of system disturbances. This paper reviews the application of special protection schemes; gives examples of typical system disturbances in SAPP; and discusses lessons learnt. The paper observes that it is important to endeavor to eliminate the initiating events of system disturbances. However, due to some unforeseen events such as weak connections, hidden failures in protection systems or human error, system disturbances may still occur. In this case, the paper concludes that implementing special protection schemes is one of the cost-effective means of achieving safety and reliability of power supply. To improve their effectiveness, 1) they have to be coordinated among power utilities in an interconnection; 2) power system operators have to be adequately trained; and 3) new technologies such as the use of phasor monitoring units have to be adopted.