Power quality of an electrical system is used to describe the quality and inherent compatibility of power use and distribution in a network. For site connections, power quality is typically assessed in terms of harmonic voltage distortion, harmonic current distortion, voltage fluctuation, voltage unbalance and flicker.
Power quality should be an area of concern for all consumers and generators of power as poor quality can result in increased electricity costs, nuisance tripping, disruptions to operations and damage to equipment. In addition, power quality limits are required in connection agreements by Distribution Network Service Providers (DNSP), where a customer modifies an existing point of connection or creates a new connection.
Poor power quality is inherently difficult to diagnose for existing installations using equipment already installed or available onsite. Rather, Power Quality Monitoring (PQM) involves a process of utilising specialist equipment to monitor and record power system data. Analysis and interpretation of measurement data should be conducted by engineers with sufficient expertise to correlate resultant power quality issues with the site installation, such that issues can be remedied efficiently.
Australian Standards for Power Quality
In Australia, power quality of electrical systems is governed by the National Electricity Rules and AS/NZS 61000 series. Within the NER and AS/NZS 61000, a set of system standards are presented with requirements for the following aspects of power quality.
Harmonic Voltage Distortion – refers to the disruption of the voltage waveform, because of non-sinusoidal currents in the network. Voltage harmonics are caused by nonlinear loads, primarily those loads which utilise switching electronics to convert voltage between AC and DC or vice versa. For industrial applications this includes power supplies, inverters, variable speed drives and PLC systems. The presence of harmonic voltage distortion results in harmful outcomes, such as overheating of transformers and motors, increased system losses, reduced power system capacity and nuisance tripping.
Harmonic Current Distortion – identical to harmonic voltage distortion, but for current waveforms.
Voltage Fluctuation – refers to variations in voltage, expressed as a percentage of the nominal system voltage. For industrial applications voltage fluctuations are normally a symptom of varying current draw from loads, such as DOL starting of motors. Fluctuation in system voltage may result in nuisance tripping, disruption to processes and damage to equipment. Connection voltages at medium voltage or higher are limited in the number of allowable voltage fluctuations per day.
Voltage & Current Unbalance – defined as the ratio of negative or zero sequence components to the positive sequence voltage component. Unbalance is typically a symptom of unequal load configuration per phase. Voltage unbalance in an electrical system creates a disproportionate unbalance in system current, resulting in excess heating of motors and hence, reduced motor efficiency and lifespan.
Flicker – occurs when there are rapid changes in system voltage, such that supplied lights flicker in a random manner. Flicker can also cause nuisance tripping of protection devices and unwanted switching of UPS system from mains to battery. Flicker is primarily caused by large loads with rapidly varying load profiles, such as welders and boilers.
Although these national references present a set of system standards for power quality, the practical implementation of power quality standards is typically by the local electricity DNSP. Within NSW, the three electricity DNSP’s each have a unique set of power quality planning limits and levels as follows:
- Ausgrid - NS238
- Endeavour Energy – MDI 0050
- Essential Energy – CEOP8026
Understanding Responsibilities for Power Quality of New or Modified Connections
With reference to the above Standards, the individual power quality limits for each unique connection are typically provided by the DNSP as a part of the connection agreement between the DNSP and customer. It is the customer’s responsibility to ensure and provide evidence that their connection satisfies the provided limit, usually by engaging external contractors to undertake PQM.
It is worth noting that many of the discussed power quality aspects can, and usually are, inherited from the local DNSP’s network. As such, PCE recommends that power quality monitoring is conducted for a sufficient period (typically 2 weeks) before modification of an existing point of connection or installation of a new site supply. In PCE’s experience existing background information has proved to be invaluable in the process of achieving acceptance of connection by the DNSP.
Methods of Power Quality Monitoring
The process of PQM involves using specialist Power Quality Analysers (PQA) to record system voltage and currents. For measurement of a site’s overall power quality the PQA is connected at the point of connection, preferably using the PCC main switchboard metering and protection cubicle test links and CT secondaries, which are at extra low voltage. The image below is from a previous PQM undertaking by PCE, with the PQA measurements at the 11kV RMU switchboard metering and protection cubicle. PQA’s can also be used for downstream PQM of a customer’s site, such as the output of transformers or UPS units.
PCE has extensive experience and expertise in power quality monitoring, where we have assisted a range of clients to diagnose and rectify onsite PQ issues, as well as meeting DNSP requirements for new and modified connections.
Should you have an interest in modifying your existing electricity supply or creating a new network connection, reach out to PCE today - and remember - before you kick off your next project, be sure to organise pre-connection PQ recordings to benchmark the existing supply network.
For further information on how PCE can assist you with this complex, yet crucial aspect of your electrical system, call (02) 4961 3344, or visit www.pceng.com.au