Prabodha Paranavithana

An improved methodology for determining MV to LV voltage unbalance transfer coefficient

The International Electrotechnical Commission (IEC) has recently released a technical report (IEC/TR 61000-3-13, Ed. 1.0, 2008) in relation to the assessment of voltage unbalance emission by installations connected to MV, HV and EHV power systems. As in the cases of harmonics and flicker, this requires a quantitative measure of propagation of voltage unbalance from upstream (higher voltage) to downstream (lower voltage) systems in terms of transfer coefficients. Naturally, these transfer coefficients depend on the downstream load composition. The existing method for determining MV to LV voltage unbalance transfer coefficient suggests a value of unity in relation to passive loads in general. However, this paper reports that MV to LV transfer coefficient of unity is conservative in the presence of commonly prevailing constant power loads. Further, the paper reports on an improved methodology for estimating MV to LV voltage unbalance transfer coefficient taking system and load characteristics into account.

A systematic approach towards evaluating voltage unbalance problem in interconnected sub-transmission networks: Separation of contribution by lines, loads and mitigation

The levels of voltage unbalance which exist in some interconnected sub-transmission systems in Australia have been observed to exceed the stipulated levels. The causes of the problem are not well known to system operators, i.e. whether the problem mainly arises as a result of system asymmetry or loads or both. Hence, system operators have found difficulties in identifying potential mitigation approaches which provide the optimum level of correction at various busbars. This paper reports, employing a study network, a suitable theoretical basis that can be used to identify the level of contribution made by the asymmetrical transmission lines and the unbalanced loads in relation to the problem of voltage unbalance and a systematic approach for identifying the optimum mitigation options.

Location of sources of voltage unbalance in an interconnected network

Identification of principal contributors to voltage unbalance and hence the implementation of suitable corrective measures has become an issue of concern for some network providers. In order to comply with stipulated limits, these network service providers require the development of quantitative measures that are reliable. For simple radial networks, the identification of sources may be seen as a trivial task. However, for interconnected networks which contain untransposed transmission lines and unbalanced loads, the identification of sources of unbalance is a non-trivial task. This paper gives a systematic theoretical approach that can be used to study the voltage unbalance behaviour exhibited by line and load asymmetries in interconnected network environments. A study network is initially analysed, and the outcomes are employed to develop a new concept termed ‘voltage unbalance emission vector’ to ascertain the overall influence made by an asymmetrical line or a load on voltage unbalance in a global sense. Using the voltage unbalance emission vectors of individual lines and loads, a technique has been developed which enables the identification of dominant contributors to voltage unbalance levels. Assessments made employing the above technique on the study system are confirmed using unbalanced load flow analysis.

Analysis of Network Asymmetry of Interconnected 66kV Sub-transmission Systems in relation to Voltage Unbalance

The levels of voltage unbalance that exist in some interconnected sub-transmission networks in Australia has been observed to be above the code requirements (e.g. 1% in Victoria). There is a perception that these high levels arise as a result of asymmetrical loads. However, it is identified that the network asymmetry in relation to transmission lines also play a significant role in the lead up to this situation, employing an interconnected 66 kV sub-transmission system as the study case. The individual transmission lines of the study network are analysed in order to observe their possible influence arising as a result of line impedance characteristics and line loading levels. A methodology is developed to quantify the contribution of individual transmission lines to the role played by the network at nodal level. The leading contributors to the problem in the study network are explored employing the proposed approach.

Voltage unbalance emission limits for installations - general guidelines and system specific considerations

Guidelines for developing voltage unbalance emission limits for installations connected to EHV, HV, and MV power systems have recently been published as a technical report by the International Electrotechnical Commission (IEC). These guidelines, based on the work of Joint Working Group CIGRE/CIRED C4.103, are intended to provide a common basis for the development of similar guidelines or standards in specific countries. This paper reviews these international guidelines and the background to the development of specific recommendations made in the technical report. It also considers elements of these guidelines that may relate to specific network considerations, and therefore may be of interest to countries wishing to develop local guidelines or standards. Additional information relating to the evaluation of such specific considerations is referenced.

A robust voltage unbalance allocation methodology based on the IEC/TR 61000-3-13 guidelines

The International Electrotechnical Commission (IEC) has introduced approaches for managing continuous power quality disturbances (harmonics, flicker and voltage unbalance) in power systems through the allocation of emission limits to customer installations, which are based on a common philosophy. However, it has been found that these harmonics and flicker allocation methods lead to planning levels being exceeded even when no customer exceeds the allocated emission limit. Subsequently, an alternative allocation policy which is referred to as the ‘constraint bus voltage’ (CBV) method has been developed to overcome this problem. This paper examines the application of the recently introduced IEC voltage unbalance allocation procedure (IEC/TR 61000-3-13) which involves an additional aspect ie. the emission arising from system inherent asymmetries. Paper identifies that this voltage unbalance allocation method also leads to a problem similar to above employing a simple 3-bus test system. A new voltage unbalance allocation policy based on the CBV method is suggested. It is demonstrated that this new voltage unbalance allocation technique satisfies the key allocation objective of complying with the set planning levels.

A review of recent investigations with reference to IEC/TR 61000-3-13 on voltage unbalance emission allocation

The Technical Report IEC/TR 61000-3-13:2008 provides guiding principles for coordinating voltage unbalance between various voltage levels of a power system through the allocation of emission limits to installations. This report is based on widely accepted concepts and principles in relation to voltage unbalance. With regard to some of the key ideas used in this report, investigations have been carried out which have enabled the development of deeper insights making the voltage unbalance allocation process more comprehensive. The key aspects which have been considered in detail include: voltage unbalance which arises as a result of lines and voltage unbalance propagation in HV-MV, MV-LV power systems. In addition, a robust voltage unbalance allocation method has been developed which overcomes some difficulties associated where a uniform voltage unbalance planning level is adopted across all bus bars with the same voltage level classification (ie, MV or HV or EHV). With regard to voltage unbalance emission assessment a novel technique has also been developed which was verified through the application to an interconnected power system where the methodology allows identification of the contributors to voltage unbalance at a selected bus bar.

Management of Voltage Unbalance through Allocation of Emission Limits to Installations

Abstract The International Electrotechnical Commission (IEC) has recently released the Technical Report IEC/TR 61000-3-13 for the assessment of voltage unbalance emission by installations connected to medium voltage (MV), high voltage (HV) and extra high voltage (EHV) power systems, which is expected to provide the basis for developing a compatible Australian Standard. The philosophy of the approach described in this report is similar to the IEC recommendations for harmonics and flicker allocation. In addition, this involves a unique aspect, the contribution to global voltage unbalance by system inherent asymmetries, noting the fact that the voltage unbalance at a busbar can arise due to both load and system (essentially lines) asymmetries. Although the concepts and the basic principles used in this approach are widely accepted, the Technical Report IEC/TR 61000-3-13 is seen to require further refinements and original developments in relation to some of the key aspects. This paper reports, in summary, the work that has been carried out addressing: (a) propagation of voltage unbalance from MV to low voltage (LV) levels; and (b) global emission in MV systems due to line asymmetries, with the view to make contributions for further improving the present technical report.