Thesis (PhD)--Stellenbosch University, 2001. / ENGLISH ABSTRACT: Accurate load models are required for the computation of load flows in MV distribution
networks. Modem microprocessors in recent times enable researchers to sample and log
domestic loads. The findings show that they are stochastic in nature and are best described
by a beta probability distribution. .
In rural areas two different load types may be present. Such loads are domestic and pump
loads, the latter may be modelled as constant P - Q loads. An analytical tool for computing
voltage regulation on MV distribution networks for rural areas feeding the mentioned loads
is therefore required. The statistical evaluation of the consumer voltages requires a
description of load currents at the time of the system maximum demand. To obtain overall
consumer voltages at any specified risk for the two types of the loads, the principle of
superposition is adopted.
The present work deals with conventional 22kV three-phase distribution (te:. - te:.) connected
networks as used by ESKOM, South Africa. As the result of the connected load, MV
networks can experience poor voltage regulation. To solve the problem of voltage regulation,
voltage regulators are employed. The voltage regulators considered are step-voltage
regulators, capacitors and USE (Universal Semiconductor Electrification) devices. USE
devices can compensate for the voltage drops of up to 35% along the MV distribution
network, thus the criteria for the application of the USE devices is also investigated. The
load currents are treated as signals when assessing the cost of distribution system over a
period of time due to power losses. The individual load current signal is modelled by its
mean and standard deviation.
The analytical work for developing general expressions of the total real and total imaginary
components of branch voltage drops and line power losses in single and three-phase
networks without branches are presented. To deal with beta-distributed currents on MV
distribution networks, new scaling factors are evaluated at each node. These new scaling
factors are derived from the distribution transformer turns ratio and the deterministic
component 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 the
distribution transformer turns ratio and the average value of the· signal treated as constant
real power load.
The evaluation of the consumer voltage percentile values can be accurately evaluated up to
35% voltage drop. This is possible by the application of the expanded Taylor series, using
the first three terms. The coefficients of these three terms were obtained using a search
engine imbedded in the probabilistic load flow. The general expressions for evaluating the
overall 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 the
modeling of voltage regulators.
The Newton-Raphson algorithm is applied to perform a deterministic load flow on singlephase
networks. A backward and forward sweep algorithm is applied to perform a
deterministic load flow on single and three-phase systems. A new procedure for modelling
step-voltage regulators in three-phase (te:. - te:.) connected networks is outlined. Specifying a
transformation ratio of 1.1 and 1.15 respectively, identifies the open-delta or closed-delta
configuration for three-phase networks. The algorithms and the developed general expressions for single and three-phase networks
without branches are presented in this work. A new algorithm is developed to enable the
developed general expressions to be applied to practical MV distribution networks. The
algorithms were tested for their accuracy by comparing the analytical results with Monte
Carlo simulation and they compared well. An illustrative example to show the application of
the present work on a practical MY distribution networks is presented. A criterion for the
application of the USE devices is outlined.
It is anticipated that, the work presented in this thesis will be invaluable to those involved in
the design of MY distribution systems in developing countries. / AFRIKAANSE OPSOMMING: Akkurate lasrnodelle word benodig vir drywingsvloei analises in MV distribusiestelsels. As
gevolg van nuwe digitale verwerkers is dit deesdae moontlik om huishoudelike laste te
monitor. Die lasdata dui daarop dat laste stochasties is en kan met behulp van die Beta
verdeling beskryf word.
In landelike gebiede is daar twee tipes laste. Hulle is eendersyds huishoudelike laste en
andersyds pomp-tipe laste wat as konstante P-Q laste beskou kan word. Dit is dus belangrik
om toepaslike analitiese metodes te gebruik om die spanningsvalle by hierdie laste te
bereken met inagname van die las-tipes. By die statistiese berekening van die verbruiker se
spanning moet 'n statistiese model van die lasstroom verskaf word op die tydstip van
maksimum aanvraag. Daarna moet die prinsiep van superposissie gebruik word om die
spannings by verskeie nodes by 'n gespesifiseerde vertrouensinterval te bepaal.
Hierdie proefskrif is gebaseer op konvensionele 22kV, drie fase distribusie (delta na delta)
netwerke, soos deur Eskom, Suid Afrika gebruik. Hierdie stelsels ondervind dikwels
nadelige spanningsvlakke en spanningsreëlaars word derhalwe aangewend. Hierdie reëlaars
is gewoonlik van tap-tipe of daar kan ook gebruik gemaak word van kapasitore en ook
elektroniese reëlaars soos die USE tipe toestelle. Laasgenoemde kan op LV vir
spanningsvalle tot 35% kompenseer. In hierdie werk word die werkdrywing verliese in die
geleiers bereken met behulp 'n seinmodel van die lasstrome. Die individuele lasstrome word
by wyse van gemiddeldes en variasies beskryf.
Om die algemene algoritmes vir die berekening van die reële en imaginêre spanningsvalle,
asook die verliese in enkelfase en driefase stelsels daar te stel word aanvanklik gebruik
gemaak van stelsels sonder vertakkings. Om die statistiese lasbeskrywing op die
laagspanningskant na die MV vlak oor te dra word van nuwe skaalfaktore gebruik gemaak.
Hierdie faktore word bereken op die basis van die transformator se verhouding en die
deterministiese komponent van die statistiese verspreide lasstrome, as konstante reële
drywingslaste beskou.
Met die ontwikkelde metode kan die verbruiker se spanning by 'n gegewe vertrouensinterval
akkuraat bereken word vir spanningsvalle tot 35%. Dit word moontlik gemaak deur die
Taylor-reeks tot drie terme toe te pas. Daar moet egter gebruik gemaak word van toepaslike
koëffisiënte
wat bepaal word deur 'n geprogrammeerde soektog. 'n Algemene stel vergelykings om die
spanning by enige verbruiker te bereken, ongeag die topologie van die netwerk, word ook
gegee.
Die Newton-Raphson metode word aangewend om die deterministiese drywingsvloei op
enkelfase stelsels te bereken. A truwaartse-voorwaartse metode is gebruik om die
drywingsvloei te bepaal vir driefase stelsels. 'n Nuwe prosedure is ontwikkel vir die
modellering van die spanningsreëlaars in driefase, delta-delta netwerke. Deur gebruik te
maak van 'n transformatorverhouding van 1.1 of 1.15 kan die oop-delta of toe-delta
netwerke voorgestel word.
'n Nuwe algoritme is ontwikkelom multi-vertakkings in 'n netwerk te hanteer. Al die
prosedures is deeglik met behulp van Monte Carlo simulasies getoets en die resultate is heel
bevredigend. Om die metodes te illustreer word 'n gevallestudie ingesluit waar die metodes
gebruik word om 'n netwerk te evalueer met en sonder die sogenaamde USE toestelle.
Kriteria vir die aanwending van hierdie toerusting word voorgestel. Daar word verwag dat die werk soos in hierdie proefskrifuiteengesit is die ontwerp van MV
distribusiestelsels, veral in ontwikkelende lande, heelwat sal verbeter.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52414 |
| Date | 12 1900 |
| Creators | Kundy, Beda Jerome |
| Contributors | Herman, Ron, Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 1 v. (various foliations) : ill. |
| Rights | Stellenbosch University |
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