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The importance of the wide-area monitoring system is rapidly increasing and many transmission system operators have already made several steps toward implementing wide-area monitoring, protection, and control. Underfrequency load shedding (UFLS) is undoubtedly one of areas that can be significantly improved within such an upgrade. However, in order to pursue finding a concept for practical implementation, as little measurements as possible should be used for the purpose. In this paper, an approach for obtaining a few-seconds-in-advance frequency prediction is described. It enables making decisions about the amount of load to be shed and, at the same time, being aware that inaccuracies in the procedure are inevitable. An extensive testing of the approach was performed on the 110 kV part of the Slovenian power-system model and the predictive UFLS appears to be highly superior to traditional UFLS on evoked frequency conditions as well as the total amount of disconnected loads. Consideration of practical limitations, such as all major time delays, unveiled a limitation for using the procedure at extreme values of system-wide frequency gradients. Consequently, the proposed algorithm is to be considered a “smart” supplement to already operational traditional UFLS. [ABSTRACT FROM PUBLISHER]

Underfrequency load shedding is one of the most important protection mechanisms in a power system. However, in the majority of power systems it has remained unchanged for decades, despite the advances in computer and communication technologies. Wide Area Monitoring System proved to be very useful and reliable in the last few years and this is why much effort is given into development of Wide Area Monitoring, Protection and Control concepts. Adaptive underfrequency load shedding is certainly a very suitable candidate for the task. In order to accelerate changes in this area, a proposal for an adaptive scheme has been developed, which is a good option from both the technical and economic points of view. In contrast to majority of adaptive schemes, it handles problems of active-power deficit estimation and its variations during the transient due to voltage dependent loads differently, i.e., by on-line forecasting the operating point trajectory in a phase space. The scheme has been tested on previously proven (by comparison to WAMS measurements) dynamic model of a part of a Slovenian power system and the results indicate a large improvement compared to the traditional scheme. [ABSTRACT FROM AUTHOR]

The importance of a secure supply with electrical energy in the world is increasing. With every problem concerning a security of supply with electrical energy, industry is faced with huge financial consequences. This is why even partial power system blackouts should be always prevented. This is possible by implementing sophisticated control and protection mechanisms. One of the most important power system protection mechanisms, which are able to prevent blackout during underfrequency conditions, is underfrequency load shedding. However, in the majority of power systems it has remained unchanged for decades, despite the advances in computer and communication technologies. To support changes in this field, a proposal for an adaptive load-shedding scheme has been developed, which is suitable from both the technical and economic points of view. By using this scheme any unnecessary load disconnections and power system blackouts are prevented. This is achieved by forecasting the response of the regulatory mechanisms that influence the system frequency by applying a locus diagram of the frequency versus the frequency's first time derivative. The scheme has been tested on models of the IEEE 9-bus system and a part of Slovenian power system. Using both models, the presented approach shows remarkable results. [ABSTRACT FROM AUTHOR]

The actual activation of underfrequency load shedding is rarely needed in a power system. However, this does not diminish its significance, as it is the last barrier before a system blackout if underfrequency conditions occur. Like in all other areas of application, a tendency also exists to maximize the efficiency of the underfrequency load-shedding scheme and to bring its operation to the limits. By default, the scheme should be able to stop a frequency fall under any circumstances. However, trying to lower the quantity of the disconnected load in the same process is the purpose of this paper. After the theoretical background is summarized, in terms of the reaction of individual generators in the islanded system to a certain power imbalance, a procedure is described showing how it is possible to adjust any of the adaptive load-shedding schemes to utilize as much primary frequency control as possible. As a reference, a theoretically optimal underfrequency load-shedding scheme is presented. [ABSTRACT FROM PUBLISHER]

Underfrequency load shedding is one of the most important protection systems, as in many cases it represents the last chance to prevent a system blackout after a serious disturbance occurs in a power system. In order to improve traditional schemes, many efforts have been concentrated on attempts to use the frequency gradient as an indicator for determining the lack of active power in a system. This paper concentrates on analyzing the factors that influence the gradient. Analyses have shown that the gradient can give misleading information about the active-power deficit, as long as certain factors are ignored or assumed to be constant. A draft of a gradient-based underfrequency load-shedding scheme is presented, which also highlights the problems associated with the use of gradient. On the one hand, a frequency gradient alone does not appear to be sufficient for the active-power deficit estimation. Nevertheless, for the actual load-shedding procedure it is found to be very useful. [ABSTRACT FROM AUTHOR]