Wide area measurements have been widely used in the energy management systems (EMS) of power systems for monitoring, operation, and control. In recent years, the advent of synchronized phasor measurements has added another dimension to the field of wide-area measurements. At the same time, the field of protection systems has been dominated by computer based relays, which make it relatively easy to communicate with relays and thus include them in many innovative protection systems. Special Protection Systems (SPS) or Remedial Action Schemes (RAS) are one example of integration of communication and protection systems. In recent years—due to operational demands imposed on transmission networks by deregulation it has become clearer that wide-area measurements and their integration in protection and control systems is a necessity. This paper documents the state-of-the-art in modern sensor, communication, and protection technologies which show great potential for innovations in protection and control systems for the future.

Wide Area Protection&#8212;Technology and Infrastructures

This thesis deals with various aspects of coordinated voltage control in electric power systems. It consists of an introduction and six papers. The introduction contains a summary of the thesis and tutorials on voltage control and the search methods that have been applied.

The first part deals with the coordination of cascaded tap changers in radial distribution feeders. A tuning rule for the conventional (local) controllers as well as two new (centralized) schemes are proposed: firstly, an optimal controller based on on-line optimization and secondly, a rule based controller based on a fuzzy inference system. A prototype of the fuzzy-rule based controller has been installed and successfully tested in a distribution feeder in the south of Sweden.

The second part of the thesis describes a coordinated system protection scheme (SPS) against voltage collapse based on model predictive control. Using combinatorial optimization and predictions of the system's future behaviour, the SPS optimally coordinates dissimilar and discrete controls such as generator, tap changer and load shedding controls in presence of load dynamics and constraints on controls as well as voltages and currents in the network.

Modelling of power systems has also been addressed. The third part describes a freely available power system library called ObjectStab intended for voltage and transient stability analysis and simulation written in Modelica--a general-purpose object-oriented modelling language. All component models are transparent and can be modified or extended. Power system topology and parameter data are entered in one-line diagram form using a graphical editor. The component library has been validated using comparative simulations with Eurostag.

Coordinated Voltage Control in Electric Power Systems

Synchronized wide area communication has become a mature technology, which makes the real-time interaction between the substations and the wide area protection and control system possible. However, the present protection and control system to handle this real-time data has been recognized to be deficient. This paper begins by reviewing the development history of power system protection, with special attention paid to the recent development in the field of wide-area and integrated protections, in order to look into the future development of protection and control systems. Then the concept of integrated wide area protection and control is introduced, where it can be shown that a hierarchical protection and control system provides the protection and control for wide area or regional power substations/plants and their associated power networks. The system is mainly divided into three levels: the local, the substation/plant, and the wide area/regional. The integrated functions at each level are described in details with an aim to develop an optimal coordination mechanism between each level. The key element in the proposed system is the wide area real-time protection and control information platform, which not only enables the merger of three lines of defence for power system protection and control, but also provides a perfect tool for the application of cloud computing in substations and power networks.

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Developments of power system protection and control

The present invention discloses a system protection scheme comprising of at least two system protection terminals (18), introduced at suitable locations in an electric power system (1). The system protection terminals (18) are interconnected by a communication system (22), preferably using a substantially dedicated communication resource. At least two of the system protection terminals (18) are equipped to collect measurement signals (72) associated with time stamped voltage or current values of the power system at that particular location or quantities derivable therefrom. The measurements are evaluated and related data is spread to other system protection terminals (18). At least one of the system protection terminals (18) is equipped to evaluate the existence of poorly damped power oscillations in the power network and if necessary provide control signals (74) to power system actuating units. The evaluation is preferably based on changes in the node angle differences between the measurement locations.

Power oscillation protection

Adaptive under-voltage load shedding scheme using model predictive control

A wide-area protection system against long-term voltage collapse was designed, implemented, tested, modified, and commissioned. The system is based on the present SCADA system in southern Sweden, which has been complemented with input signals and equipment to execute action orders from the protection system. Bus voltages from the transmission system, reactive power output from generators connected to the transmission system, and current limiter information from main generators were used as input signals to the wide-area protection system. Actions from the protection system are shunt reactor disconnection, shunt capacitor connection, start of gas turbines, emergency power request from the HVDC connection to Germany, low priority load disconnection, and finally shedding of high priority load.

B. Ingelsson

Papers

Wide-area protection against voltage collapse