A control system for an electrical substation for transmission and distribution of electrical energy and equipment, comprising equipment for control, measurement, monitoring and protection of the substation. The control system comprises a substantially common bus means for communicating measurements and signals and between transmission and distribution equipment and functions for control, measurement, monitoring and protection run in computing means. Real time access for all measurement signals is enabled by the invention. More effective operation control is achieved with reduced cabling and duplication of hardware.

Substation control system

Distribution networks (medium voltage and low voltage) are subject to changes caused by re-regulation of the energy supply, economical and environmental constraints more sensitive equipment, power quality requirements and the increasing penetration of distributed generation. The latter is seen as one of the main challenges for today’s and future network operation and design. In this thesis it is investigated in what way these developments enforce intelligent distribution network design and new engineering tools. Furthermore it should be investigated how a new design and control strategy can contribute to meet the power quality and performance requirements in distribution networks in future. This thesis focuses on network structures that, typical for the Netherlands, are based on relatively short underground cables.Managing current and voltage in such networks both during normal and disturbed operation, requires a good network design and an adequate earthing concept. The limited size of Dutch distribution networks has a positive effect on power quality aspects and reliability. The use of impedance earthing for medium voltage (MV) cable networks reduces the risk of multi-phase faults that cause large fault currents and deep dips. It also reduces the risk on transient overvoltages due to re-striking of cable faults. A TN earthing system for the low voltage (LV) network reduces the risk of damaged apparatus and it maintains safety for people. However, care must be taken for the earthing of devices of other service providers, which requires a co-operative solution. The fast developments of computation techniques and IT equipment in the network opened the possibility to perform many calculations in short time based on both actual and historical data. Examples are the on-line distribution loadflow and the short-circuit calculation for protection coordination and intelligent fault location. In LV and MV network calculations the accuracy of the models and the availability of data are the main obstacles. Because of the unsymmetrical nature of load and generation in LV networks a multiple conductor model is needed. For safety calculations also the earth impedances have to be modelled as well as the neutral and protective earth impedances and their mutual interactions. The protection philosophy in MV networks must take into account the changing requirements regarding safety and power quality. An overall philosophy concerning both network and generator protection is necessary. New developments in substation automation benefit future upgrade and refurbishment of substation control and protection. As a result, also cheap,accurate and fast fault location becomes feasible, reducing the outage time of the customers. Next the influence of distributed generation on the above subjects is investigated. The increasing magnitude of short-circuit currents and the increasing voltage variations in the network are seen as a major challenge for the network planners. Conventional measures for reducing voltage problems may introduce problems with the short-circuit current level and vice versa. In networks which contain a large amount of both load and distributed generation, adverse voltage problems may occur, especially when the generation is located in the LV network. In order to reduce this, specific control strategies need to be developed. The last part of the thesis is related to these control strategies as a solution for operating future distribution networks. By introducing storage and power electronics, networks can be transformed into autonomously controlled networks. These networks remain an inseparable part of the electricity network but may behave in a fairly autonomous manner, both internally and externally, with respect to the rest of the network. The focus in this thesis is on maintaining an optimal voltage for all customers during all combinations of load and generation. Because of the autonomous behaviour of the control systems, their operation must be based on local measurements. A suggested approach is to replace the normal open point between MV feeders by a so called "intelligent node". This node is able to control the power flow in several feeders by means of power electronics and, if provided, by electricity storage. The voltage profile can be improved further, by introducing an intelligent voltage control on the HV/MV transformer feeding the distribution network. The simulation studies in this research have been performed on a realistic model of a typical Dutch MV/LV distribution system. Based on the results the following conclusions are drawn: • The HV/MV transformer control must be based on line drop compensation. This compensation must use the load situation instead of the measured exchange signal. The compensation factor must differ between cases of high load and of high generation. • The optimal control of the intelligent node is a voltage control, based on a linear dependence of the voltage at the node and the power flow towards that node. This method can be improved when the voltage of the MV bus bar in the substation is taken into account. • Methods to obtain a perfect voltage profile will lead to a storage device that is not available for this voltage level yet. • A voltage control based on a fixed value at both terminals of the intelligent node and at the MV bus bar of the HV/MV substation does not result in the optimal voltage profile, although guarantee a good voltage quality and might therefore be a good alternative

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Intelligent distribution network design

This paper presents a method for estimating the location of faults on a radial MV system, which can include many intermediate load taps. In the method nonhomogeneity of the feeder sections is also taken into account. Performance of the technique was investigated using data obtained from EMTP/ATP simulations, based on Dommel (1986). The data recorded during faults in a real MV network were also used. A distribution utility's MV networks are used as an example. The voltage and current data samples obtained from EMTP/ATP simulations or delivered by DFR were converted to a MATLAB format. The 1 kHz sampled data were then used in the MATLAB fault locator model. The basic block diagram of the proposed fault locator is presented. The paper also describes some evaluation results.

Fault location method for MV cable network

Various kinds of distributed generations (DGs) are being increasingly connected on the distribution systems and computer network-based controls are introduced to assist the system operations This paper proposes a new network-based adaptive strategy for protection of distribution-system feeders connected with DGs The proposed strategy provides an intelligent network-enabled protection for feeders and overcomes the DGs-imposed technical challenges such as increase of fault current, change of prescribed fault flow paths, sympathetic tripping, unintentional islanding operation, continuous non- interruptible fault current, etc, as well as non-DG-caused problems such as undetected high-impedance ground faults This paper illustrates effective real-time determination of correct protection operations for feeders with dispersed DG-connections against faults and surges resulting from lightning, switching, equipment short-circuit, etc Four typical case studies of the proposed network-enabled adaptive protection strategy for feeders in the distribution system connected with DGs are provided in the paper

Network-based Adaptive Protection Strategy for Feeders with Distributed Generations

The framework of supervisory control of discrete event systems (DESs) is applied to agent-based power protection systems for achieving high reliability and selectivity. The DES-based supervisory control uses feedback information of events occurring in systems to achieve the set of desired event sequences. The large-scale and complex power systems need systematic tools for protection and control on a more abstract level. The supervisory control technique on the event-based abstract level can be used as a formal method for protection of large-scale power systems. Specifically, a design procedure of a supervisor that coordinates the behaviour of relay agents to isolate fault areas through the minimum operations of circuit breakers is presented.

Modelling and control of agent-based power protection systems using supervisors

This paper describes a new system developed by Ontario Hydro to integrate the protection, control and monitoring functions at transmission and distribution substations. The implementation of each of the system functions is discussed, with an emphasis on the protection functions. The focus is on the processing of power system information at each level of the system architecture. The system has been successfully operating in Ontario Hydro's power system.

G.R. Allen

Papers

Integration of protection, control and monitoring functions for transmission and distribution substations