Probabilistic analytical methods for evaluating MV distribution networks including voltage regulating devices

[摘要] ENGLISH ABSTRACT: Accurate load models are required for the computation of load flows in MV distributionnetworks. Modem microprocessors in recent times enable researchers to sample and logdomestic loads. The findings show that they are stochastic in nature and are best describedby a beta probability distribution. .In rural areas two different load types may be present. Such loads are domestic and pumploads, the latter may be modelled as constant P - Q loads. An analytical tool for computingvoltage regulation on MV distribution networks for rural areas feeding the mentioned loadsis therefore required. The statistical evaluation of the consumer voltages requires adescription of load currents at the time of the system maximum demand. To obtain overallconsumer voltages at any specified risk for the two types of the loads, the principle ofsuperposition is adopted.The present work deals with conventional 22kV three-phase distribution (te:. - te:.) connectednetworks as used by ESKOM, South Africa. As the result of the connected load, MVnetworks can experience poor voltage regulation. To solve the problem of voltage regulation,voltage regulators are employed. The voltage regulators considered are step-voltageregulators, capacitors and USE (Universal Semiconductor Electrification) devices. USEdevices can compensate for the voltage drops of up to 35% along the MV distributionnetwork, thus the criteria for the application of the USE devices is also investigated. Theload currents are treated as signals when assessing the cost of distribution system over aperiod of time due to power losses. The individual load current signal is modelled by itsmean and standard deviation.The analytical work for developing general expressions of the total real and total imaginarycomponents of branch voltage drops and line power losses in single and three-phasenetworks without branches are presented. To deal with beta-distributed currents on MVdistribution networks, new scaling factors are evaluated at each node. These new scalingfactors are derived from the distribution transformer turns ratio and the deterministiccomponent of the statistically distributed load currents treated as constant real power loads.In the case of an individual load current signal, the transformation ratio is evaluated from thedistribution transformer turns ratio and the average value of the· signal treated as constantreal power load.The evaluation of the consumer voltage percentile values can be accurately evaluated up to35% voltage drop. This is possible by the application of the expanded Taylor series, usingthe first three terms. The coefficients of these three terms were obtained using a searchengine imbedded in the probabilistic load flow. The general expressions for evaluating theoverall consumer voltages due to statistical and non-statistical loads currents are also given.These non-statistical currents may be due to constant P - Q loads, line capacitance and themodeling of voltage regulators.The Newton-Raphson algorithm is applied to perform a deterministic load flow on singlephasenetworks. A backward and forward sweep algorithm is applied to perform adeterministic load flow on single and three-phase systems. A new procedure for modellingstep-voltage regulators in three-phase (te:. - te:.) connected networks is outlined. Specifying atransformation ratio of 1.1 and 1.15 respectively, identifies the open-delta or closed-deltaconfiguration for three-phase networks. The algorithms and the developed general expressions for single and three-phase networkswithout branches are presented in this work. A new algorithm is developed to enable thedeveloped general expressions to be applied to practical MV distribution networks. Thealgorithms were tested for their accuracy by comparing the analytical results with MonteCarlo simulation and they compared well. An illustrative example to show the application ofthe present work on a practical MY distribution networks is presented. A criterion for theapplication of the USE devices is outlined.It is anticipated that, the work presented in this thesis will be invaluable to those involved inthe design of MY distribution systems in developing countries.

[发布机构] Stellenbosch University