Victor J Gosbell

Power quality (PQ) survey reporting: discrete disturbance limits

Discrete or event type power quality (PQ) disturbances mainly include voltage sags, swells, and the transients. An extensive literature survey suggests that there is no generally accepted method for characterization of these disturbances and suitable limits are not yet found in any international standard. One of the reasons for the lack of characterization methods is the difficulty of defining suitable site indices for each discrete disturbance type. In this paper existing characterization methods are reviewed and discussed. A new generalized approach is then given to show a better way of characterizing voltage sags, swells and transients. This is followed by a proposed new method of defining MV/LV distribution discrete disturbance limits for general utility networks and their suitability is shown by an examination of some Australian sites.

Unified power quality index (UPQI) for continuous disturbances

Power quality indices are being developed that attempt to quantify certain aspects of service quality. There has been considerable amount of work on the characterization of individual types of power quality disturbances and corresponding indices. However, there does not exist in the literature a standard approach that allows one to quantify the overall power quality. This paper proposes a unified power quality index (UPQI) as a useful tool for ranking sites as to their overall power quality.

The Australian Long Term Power Quality Survey project update

The Australian Long Term National Power Quality Survey (LTNPQS) is a large multi-utility power quality survey which has now been in operation for 8 years. This paper details the latest innovative developments in the reporting and analysis procedures developed for the LTNPQS. The paper also highlights the key power quality issues affecting Australian electricity networks at the present time. These include high voltage levels at LV sites and difficulties in measurement of voltage unbalance. The longevity of the survey has allowed better understanding of disturbance trend levels. Interesting results include an apparent reduction in harmonic levels in the past couple of years and no definite trend in sag levels even with 7 years worth of data. Some of the challenges involved in conducting a survey of this type are explored and indications as to solutions presented. With the implementation of smart grid concepts it is likely that the number of sites with instrumentation capable of supplying data to the survey will increase exponentially. This presents a new set of challenges and the survey must adapt to these. Future directions taken in response include the implementation of web-based reporting systems which will provide more flexibility for participants.

Harmonic allocation following IEC guidelines using the voltage droop concept

Present IEC guidelines are difficult to apply to realistic cases because of the data load, the number of assumptions required and the computational complexity. A new approach is given based on the concept of voltage droop, to be detailed in a companion paper. The benefits of the approach are a simple calculation with minimal data and no required assumptions. It applies to radial or meshed distribution systems where feeders are sufficiently short that line capacitance can be ignored. The approach can be applied at the installation (MV or LV) and the equipment level and can be used to give reference values for equipment emission standards.

Introducing power system voltage droop as a new concept for harmonic current allocation

“Voltage Droop” is a new concept that has been developed primarily for use in the harmonic current allocation process. Voltage droop offers the opportunity to both vastly simplify the procedures required by the present IEC 61000.3.6 [1] guidelines for Stage 2 allocation and provide a solid theoretical base to this work. The voltage droop concept has also been found to provide a usefull theoretical basis for the harmonic standard IEEE 519[2]. While this paper develops the voltage droop concept using diagrams and words, the companion paper “Harmonic Allocation Following IEC Guidelines Using the Voltage Droop Concept”[3] contains a full critical mathematical description including an example. The voltage droop concept can be applied to radial, meshed and SWER distribution systems where feeders are sufficiently short that line capacitance can be ignored and all capacitor banks are detuned to prevent resonances at harmonic frequencies. The concept has application in HV, MV and LV systems. Use of the voltage droop concept for flicker allocation and unbalance studies is also briefly discussed.

Design and implementation of a new three-phase four-wire active power filter with minimum components

This paper reports the development of a new three-phase four-wire active power filter which has no passive elements and only has four bidirectional switches. A new switching technique based on three dimensional vector treatment of unbalanced three phase circuits is used for the control of the APF. Experimental results from a prototype APF confirm the suitability of the proposed approach not only for the removal of harmonics, but also for the compensation of fundamental unbalance currents and reactive power consumed by the load. The experimental results show that current total harmonic distortion (THD) in each phase is reduced from 58% to less than 5% which is acceptable by most harmonics standards. The neutral current, including fundamental and harmonics, is reduced by a factor of 15.

Primary and secondary indices for power quality (PQ) survey reporting

Routine monitoring involves many PQ monitors at fixed sites with readings able to be interrogated remotely and downloaded to a central database. The amount of data for one year is large, and innovative methods are needed to ensure that it can give useful insights. Primary indices are proposed to show whether a site is acceptable to specified standards and are mainly based on 95% values. Secondary indices are proposed to give additional insights. For variations, indices are proposed to give a measure of the impact on customer equipment beyond that incorporated in standards. Sag secondary indices are defined to give insights as to the factors responsible for poor sag performance. Indicative maximum acceptable values are given for secondary indices based on a recent Australian survey of several distributors.

A new SAIFI based voltage sag index

Reliability measures of SAIDI (system average interruption duration index) and SAIFI (system average frequency index) are well established industry standards used world wide. While both measures have their limitations, they give a broad indication of average reliability that allows comparison within networks and across networks world wide. No such industry standard indices exist for voltage sags. The main reason being that voltage sags are multi-dimensional, involving retained sag voltage, sag duration, number of phases effected, phase angle jumps and the time between successive sags. This paper proposes a new voltage sag index that is dimensionally the same as SAIFI having units of equivalent interruptions per year, allowing a direct comparison with SAIFI. The proposed new index called ldquoSag SAIFIrdquo has been designed to allow voltage sag comparisons between sites, within networks and across networks. In addition, Sag SAIFI provides a means to directly compare the customer impacts of voltage sags with reliability (interruptions) and can assist in optimising expenditures on networks to maximise customer benefits of both reliability and voltage sag performance in their aggregate.

Analysis of harmonic distortion levels on a distribution network

It is generally acknowledged that harmonic distortion levels on distribution networks are slowly but steadily increasing due to the proliferation of non-linear loads. While customer complaints relating to harmonic distortion levels are relatively uncommon, and measured levels of harmonic distortion are typically well within the limits specified by national regulations and international standards, the increasing trend may mean that more active management by utilities will soon be required. Effective management of harmonic distortion levels will require knowledge not only of the level of disturbance, but also of how the disturbance levels vary with time, location, and the connected load. Accepted methods of assessing harmonic levels that rely on (typically 95%) cumulative probability statistics are inadequate for this level of insight. In this paper voltage total harmonic distortion levels (THDnu) from a distribution network power quality survey are analysed in terms of variation with time, the relationship of the measured THDnu levels to system load and the physical characteristics of the monitored sites.

The effect of data aggregation interval on voltage results

For various technical and operational reasons, many power quality surveys are carried out using non-standard data aggregation intervals. The data aggregation interval is the time interval that rapidly sampled data is reduced to by the monitoring instrument for subsequent analysis and reporting. Some of the rationales for using non-standard data aggregation intervals include instrumentation limitations, memory restrictions, a belief that more insights may be obtained from data captured at faster aggregation intervals and dual use of instrumentation (such is the case for many smart revenue meters). There is much conjecture over the effect which the data aggregation interval will have on the final outcomes of a power quality survey. IEC61000-4-30 which is the international standard describing power quality monitoring methodology suggests 10 minute data aggregation intervals are appropriate for routine power quality monitoring of most power quality disturbances including magnitude of supply voltage. This paper investigates the variation observed for magnitude of supply voltage monitoring when data is captured at a range of data aggregation intervals.