Benefits of power quality monitoring outside substations

Australian utilities possess some of the longest feeders in the world, and long feeder lines exacerbate the issues presented with power quality and bushfire risk. However, with recent developments in capabilities of automatic circuit reclosers (ACRs), it is possible to alleviate this headache.

Utilities operating in Australia face many adversaries with regards to power distribution; challenges which would drive most international counterparts to alternative markets. Australian utilities are a much more resilient group, and with modern developments in pole-mounted switchgear, the obstacles faced are not quite as insurmountable as they used to be.

Further updates to recloser control capabilities have caused the advent of remote ACR power quality monitoring, and this new capability has opened the door for greater reliability on long feeders as more accurate, local and relevant power quality data can be gathered. All of this information can be remotely interrogated, retrieved and then manipulated to grant utilities unprecedented resources to improve their reliability of supply. Additionally, with new capabilities to control and manipulate reclosing sequences remotely without having to edit settings, along with simple self-diagnosing communications systems to ensure reliable network reporting and awareness, it is possible to reduce and manage the bushfire risk.

This article outlines the recent developments in bushfire mitigation strategies through the use of pole-mounted ACRs.

ACRs' humble beginnings as hydraulic devices in the mid-1900s have evolved greatly through the years through to semiconductor controlled switches. These switches have proliferated across networks all over the globe, driven by the immediate reliability benefits and protection offered on a reasonable budget. Australia itself has had a fair share of manufacturers, including NOJA Power.

NOJA Power's RC10 control system allows users to update and integrate bushfire mitigation strategies using commissioned NOJA Power assets, with a simple firmware update and a network integration strategy. By using the onboard voltage and current sensing capabilities of these reclosers, the RC10 now has the capability to conduct complete power quality monitoring and reporting. This functionality is now available out on the feeders, using the exact same protection devices installed years ago, providing power quality feedback from the shores of the Gold Coast to Uluru.

Reclosers for bushfire safety

Bushfire risk is a fundamental issue of concern for most Australian utilities, with events such as the February 2009 bushfires in Victoria being blamed on a local utility resulting in lengthy court proceedings and fines. As responsible DNSPs (distribution network service providers), all utilities are interested in mitigating their risk of causing fires. Recent developments in recloser technology allow for the simple integration of bushfire risk management strategies using the current install base of NOJA power reclosers.

Recloser strategy for system reliability basically relies on interrupting faults and restoring supply after a specified open time at the recloser. A reclose sequence may have multiple different close attempts, but from a bushfire mitigation standpoint the more reclose operations in a sequence, the greater the risk of ignition at a fault point on the feeder. While for low fire risk days a longer recloser sequence will result in less customer lost minutes, the multiple reclose attempts will each increase risk of ignition.

Previous strategies implemented in recloser schemes involved complete disabling of the reclose functionality on bushfire risk days. This can be achieved by remote SCADA control and toggling of the global control of the recloser converting it essentially to a single short circuit breaker. This practice compromises the economic performance and is a brute force method of addressing the risk of bushfire ignition. It is far more elegant to have a remote capability to modify the reclose sequence, by applying global controls which can be toggled to reduce the length of the reclose sequence in different ways, without completely compromising system performance like using the 'Auto Reclose OFF' method.

NOJA Power has worked closely with utilities to establish global control points for the R10 recloser system which will allow for the mitigation of bushfire risk.

Power quality

Power quality is an important concept to understand in modern electrical service provision. By ignoring this issue, we allow opportunity for devastating harmonics to freely travel through our networks, destroying our assets and interrupting our customer service. Only through protection and monitoring of these issues will it be possible to improve network performance, safety, reliability and economic bottom line.

New developments in reclosers allow for comprehensive power quality monitoring and protection features using the current install base. These reclosers now have the ability to measure harmonic distortion, interruptions, and sags and swells, and it is important to develop an understanding of these features for optimum use of the resource.

Harmonics

Within a power transmission system, all power is delivered at a set frequency, which in Australia is 50 Hz. Harmonics are 'contaminants' within the power supply, which have a frequency that is a multiple of the baseline or fundamental frequency. These contaminant harmonics enter the power system through many different means, but ultimately the bottom line is they are a nuisance, and should be protected against.

Harmonics on the network can be devastating. Since these harmonics are essentially carrying unusable superfluous energy across the network, they put excess strain on any devices connected. These harmonics cause damage to insulation and the very power electronics which cause them, along with excess transmission losses. The major issue is that harmonic damage is insidious. There are usually very limited symptoms of harmonic issues, until a catastrophic event such as the loss of a transformer or motor, which is usually accompanied by an inherent fire risk. These risks are something which DNSPs are taking a great interest in, and in Australia the harmonic limit of contamination is limited as low as 8% at the point of common connection. It is our responsibility in the energy industry that harmonics are prevented from travelling through the network. And this in turn means that any responsible DNSP needs to be able to provide protection against these damaging harmonics.

The single greatest source of harmonics within the power system is the semiconductor. Most modern loads which use some sort of power electronics to transform the grid energy to usable energy for the device cause harmonic distortion. This is a result of what is known as nonlinear current draw, meaning that the devices do not take in the full natural sine wave. There are other causes, such as transformer saturation, or large industrial loads such as arc furnaces, or even fluorescent office lighting. Additionally, the large-scale installation of solar photovoltaic arrays and their semiconductor inverters are a notorious source of harmonics. With the proliferation of power electronics into the network, it's easy to see how a minor issue of the past is becoming progressively more prevalent as technology advances.

Simplicity of calculation is lost when starting to consider harmonics, which is the initial challenge of interpreting harmonic content. While these unattractive waves look terribly complicated to understand, there are two major mathematical ideas which make the concept quite simple. These are the principle of superposition and fourier transforms.

The basic idea is that any complex wave can be represented as a sum of individual simple waves. This is the concept of superposition. Fourier transforms are the mathematical method for working out what these individual waves are. The main difference between these waves are their magnitude and their frequency – which just so happens to be the two main features of harmonic identification.

Harmonics are waves which have frequency multiples of the fundamental frequency. Since any complex wave can be represented by a combination of these waves, it is then possible to understand what the harmonic content of a power supply is. The RC10 system uses Fast Fourier Transform to analyse the energy flowing through the device and provide exact values for both the harmonic frequency and its magnitude.

Harmonics are measured in two separate methods, known as total harmonic distortion (THD) and total demand distortion (TDD). THD is a ratio between the fundamental voltage wave and all the voltage harmonics. This is expressed as a percentage. Total demand distortion is calculated in a similar way, except the ratio is based on the peak current demand, rather than instantaneous voltage used by THD. TDD is used to calculate current distortion relative to the peak demand.

Interruptions

One of the greatest indicators of power quality issues is the measure of customer minutes lost. This value is obviously of high interest to DNSPs as it directly relates to their economic bottom line. The RC10 allows for user-configurable settings to determine the difference between a short and long interruption and can log all the information relevant to each of these interruptions separately.

Sags/swells

Sags and swells are characteristic of the ebb and flow of a power system as the energy demand shifts through the day. In times of low demand, the end-user voltage can begin to creep up, and vice versa. Just like the interruptions monitoring, sag and swell monitoring fills the data void left by our overvoltage and undervoltage protection. Also, since it evaluates a smaller deviation, data which is usually missed by protection levels is still recorded. Sags and swells can be indicators of greater issues present in the network and also allow utilities to better prepare for the mitigation of ill effects caused by periods of overvoltage and brownout.

Oscillography

The final piece of the monitoring puzzle is to actually capture a direct copy of the current and voltage waveforms passing through the recloser. By directly capturing the waveforms, a wealth of possibilities for analysis, interpretation and network improvement are provided. As the reclosers already possess all measuring devices required as well as the capability to interpret all data at a high sample rate, it is a logical extension to be able to plot this data in the IEEE format of COMTRADE. This oscillography data can then be retrieved and imported into many different software packages for analysis.

In order to capture data that is worthwhile, it is important to be able to trigger the correct capture point. This depends greatly on the installation, but selecting the correct prefault capture and trigger point is paramount in effective use of this technology.

One of the most interesting applications of this technology is to capture fault events at reclosers and import these COMTRADES into relay test sets such as an Omicron or Doble. These test sets allow for simulation of the real faults present on a network, granting the capability for evaluation of performance of network assets, along with the optimisation of network protection.

Remote retrieving of data

All power quality data including oscillography gathered from the NOJA Power RC10 can be gathered remotely. Given the challenges of geographic distribution within Australia, many DNSPs have progressed towards an engineering access approach to managing their smart reclosers. By using this same port on the RC10 reclosers, it is possible to remotely gather the PQDIF and COMTRADE files from the reclosers.

Conclusion

Through bushfire mitigation and power quality monitoring, it is possible to grow the performance and revenues of DNSPs, and in a time where efficient, safe network operation is paramount it is negligent to disregard the capabilities available within the switchgear of choice on distribution networks, the semiconductor controlled ACR.

Image credit: ©iStockphoto.com/Thomas Dixon.