Nhlanhla Mbuli

Effect of the tap changer position of Zeus 765/400kV transformer on the steady state and voltage stability performance of the Cape system

Previously published research findings [1] on the impact of tap changer position on the steady state performance of distribution networks indicate that very little impact results from varying the tap changer position. In this paper, a similar investigation is carried out, but on a network with very high voltage transmission lines operating in parallel with a well-developed corridor of 400kV lines. The per-unit impedance of a line is reduced drastically as the line voltage is increased. When the design voltage of the transmission lines is high, the transformer impedance becomes dominant and plays a key role in the system. This paper investigates the effect of varying the impedance of a 765/400kV transformer on the steady-state (loadflow and active power losses) and voltage stability of the Cape corridor.

Some Considerations for Designing a Reactive Power Charge

Electricity tariffs that levy charges for electricity usage on the basis of only active power demand (in kW) and/or active energy (in kWh) do not offer customers incentive to control their reactive power demand (in kVAr) and/or reactive energy consumption (in kVArh). The consequence is that poor power factor, for the entire system, may result, leading to the degradation of the power system, The introduction of tariffs that have an apparent power charge (in kVA) does go some way in incentivizing customers to deal with their reactive power consumption, as this type of charge encourage them to improve their power factors.. However, further improvement in the utilization of power systems can be realized by having a tariff that directly charges for reactive power. In this paper, results of a survey on international practices on levying a reactive power charge to the customers are discussed.

Initial results of investigations into introducing new HVDC injections into the Cape network to increase transfer capacity

The early phases of the development of the Cape network entailed construction of 400kV lines to link supplied from the generation pool in the northern part of the country, in Mpumalanga, with the loads in the Cape, i.e., the Western and Southern grids of Eskom. Once the 400kV network was matured, it became apparent that further commissioning of 400kV lines led to lesser and lesser improvements in voltage stability limit improvements. This led to consideration of 765kV lines for further expansion. For the future, considerations are being made into introducing high voltage direct current (HVDC) systems into the Cape network to further increase its voltage stability limit to support future loads. This paper presents the results of the preliminary work to assess the techno-economic merits of introducing HVDC into the Cape. Some lessons learnt and further questions to be looked into are discussed.

Impact of circuit-breaker maintenance on life-cycle cost comparison for fixed and magnetically controlled reactors

Under light loading conditions, reactors provide an option for keeping voltages below steady state limits. In this paper, the impact of the number of operations of a circuit breaker on the choice of reactor technology (i.e. fixed versus magnetically controlled) is studied. In particular, the impact of circuit-breaker maintenance cost on the life-cycle cost of a reactor installation is evaluated. The study shows that when the expected number of circuit-breaker operations is high, it is beneficial to consider the use of a magnetically controlled reactor as this leads to very few operations. In addition, the savings in circuit-breaker maintenance cost can justify the extra cost of a magnetically controlled reactor over its life cycle. The economic life-cycle cost assessment can help to identify the lowest cost solution over the project life cycle.

Derating line voltage as an option for reliability enhancement of lightly loaded networks

Networks with light loadings can suffer from steady state overvoltages and this can lead to damaged insulation of the equipments, accelerated electrical aging of equipment due to frequent operation under exceeded voltage limits. Common solutions for this problems entail the use of shunt reactors, series reactors, using tap changing of transformers, and ultimately, switching off of lines. In this paper, the authors evaluate the potential of derating the voltage of lines in the load centre, using step down transformers, as a solution for overvoltages. Load flow studies are conducted for a healthy network, with and without step down transformers, and voltages in these two cases are compared. The results showed that it is possible to operate the network with acceptable voltages in the case with step down transformers. There is no need to disconnect lines, putting load at risk, thereby worsening reliability, to maintain acceptable voltages however, it is necessary in the case without step down transformers. The use of step down transformer, thus enable voltages to be kept within limits and improves the reliability of the network.

Impact of connecting a CSP plant in the upington distribution network on voltage dip performance

The Upington area within South Africa is well-endowed with solar resources, with relatively high annual solar radiation recorded compared to other parts of the country. The distribution network in the area comprises very long lines, has very low fault levels, and is very prone to voltage dips. Previous studies [1] have investigated the feasibility and network considerations of connecting concentrating solar power (CSP) plants of sizes 50 MW, 100 MW and 150 MW at Upington substation. In this paper, the previous work is extended to assess the impact of connecting the plant sizes described above on the voltage dip performance of this weak network. The studies show that embedding a CSP plant in such a weak network, apart from capacity-related benefits, can have positive on quality of supply of such a system by reducing the severity of dips due to disturbances.

Comparing the performance of HVDC schemes sourced from pluto to different receiving end points within the Western Cape Network

High Voltage Direct Current (HVDC) offer better capability for long distance, bulk power transmission compared to ac lines. Among the reasons for this is that HVDC does not suffer from stability limitations that ac lines experience. Also, the power that is required to be sent via HVDC lines can be scheduled as desired, whereas the power flowing through ac lines is directly influenced by the topology and impedances of equipment forming the system. In this paper, the possibility of introducing HVDC lines into the Cape system in order to improve the voltage stability limit of the system is investigated. In particular, the impact of the receiving end point in the load area chosen for the HVDC schemes is evaluated. Two receiving end points in the Cape network, namely Omega (scenario 1) and Kappa (scenario 2) substations are evaluated on the basis of loadflows, active power system losses, and voltage stability improvements. It is shown that the choice of the receiving end points can have profound impact of the techno-economic performance of the HVDC scheme commissioned to improve transfer capability.

Pre-Feasibility Investigation into Converting Fixed Line Reactors in the Cape System into Variable Types

To supply the Eskoms Western Cape portion of the transmission grid, a mature system of 400kV lines, in parallel with 765kV lines, is used. These lines move power from Mpumalanga in the northern part of the system, where a generation pool of coal-fired power stations is located, over a distance of about 1500km, to supply the loads. The above-mentioned transmission lines were designed with fixed line reactors, to protect them from being damaged by high voltages in case of fast and substantial voltage rises, e.g., due to sudden loss of load. The reactors installed in the power system consume capacitive reactive power from the system, and this can have adverse impact on the performance of the power system, e.g., problems in maintaining voltages within acceptable levels, adverse impact on total active power losses of the system, and adverse impact on the voltage stability margin. By making these reactors controllable, it will be possible to inject least amount of reactance required during peak conditions to ensure maximum performance, and to insert maximum reactance needed to operate the system during conditions of least loading. In this paper, the results of a preliminary investigation into converting the fixed line reactors in the Cape network into controllable type are presented.

Enhancing the Utilization of the Cape's 765kV Cape Corridor by Series Compensation

To enhance the voltage stability limit of the Western Cape network, in the 1980s Eskom introduced 765kV technology to build additional lines into the Cape. This was because introducing more 400kV lines seemed not to yield substantial, additional transfer, and, because of its transfer potential compared to 400kV, 765kV was considered a better option for future expansions. The challenge here was that the then existing 400kV system was very well-developed, with a number of lines built in parallel. As a result, when the 765kV lines were introduced, their utilization was far less than adequate. This paper reports the results of a study that evaluated the possibility of series compensating the lines in the 765k Cape corridor. The areas dealt with here are the impact of series compensation on (1) loading of the 400kV and 765kV lines, (2) the system active power losses, and (3) voltage stability limit of the system.

Optimal placement model of TCSC in power system network considering the budget available

This paper presents an optimal placement of TCSC which is a FACTS (Flexible Alternative Current transmission Systems) controller in order to increase the loadbility of the system. The optimization problem is solved using the genetic algorithm. In this study the availablity of the budget is taken in consideration. The result show that the increase in loadability can be restricted by the availability of budget and also that beyond a certain budget there will not be any further increase in loadability. Also beyond a certain number of TCSC there will be no further increase in system loadability.