Projects:2020s1-1551 The Impact of Power Quality on Transmission Transformers
Sponsored by ElectraNet, this project aimed to analyse the impact of reverse power flow due to rooftop solar systems on power transformers in the transmission network. Inverters (used in rooftop solar systems) produce harmonics due to their non-linearity, which have the potential to cause overheating in transformers if occurring in high enough concentrations.
Contents
Introduction
Home solar power systems feed power back into the transmission system when they produce an excess in their distribution network. The power network was never designed to accommodate reverse power flow. It has changed the shape of the daily load profile in the SA network and continues to further change the shape as home photovoltaic (PV) systems grow. Stemming from this situation is concern that issues will occur in the network due to the lesser control over home power systems versus traditional large power generation systems.
Transformers' life spans decrease when overheated, which is caused by overvoltages including those from harmonics and interharmonics in the input power of the transformer. Power inverters used to convert power to AC for transmission into the grid produce harmonics and interharmonics due to their non-linearity. The potential effect of harmonics emitted from inverters, on transmission transformers, will be investigated.
International and Australian standards specify the allowed amount of harmonic emission from instruments in the network. These limits will be investigated to find whether there is a realistic potential for complying inverters to cause harmonic levels high enough that they are over the acceptable limits at the transmission transformer. Consideration will be given to the robustness of the limitation methods to see whether they are appropriately limiting harmonics.
This project uses data from ElectraNets transmission transformer power quality meters. The study is focused on the Adelaide Hills region and surrounds, with several transmission transformers being investigated.
Project Team
Students
Iman Atefi
Ellen Chinner
Supervisors
Mr David Vowles
Dr Gabriel Haines (ElectraNet)
Aims
This project answers the following research questions:
- What harmful effects could harmonics from rooftop PV have on transmission system transformers?
- Would these become an issue? If so, when?
- Are current regulations enough to mitigate these issues?
- How could these issues be solved by ElectraNet?
Research
Standards
Australian Standards and other guiding documents were reviewed to identify feasible gaps in the regulations on inverter emissions and transformer harmonics levels. Methods for evaluation, limits and assumptions are of interest to understand where issues could occur whilst inverters are still meeting regulations.
South Australian Power Network (SAPN) technical standards (TS) were also reviewed.
Key standards for this project were the 61000, 60076 and 4777 series, which cover harmonics, transformers and inverters respectively. These standards impose the following restrictions on harmonic levels in the network:
- Inverters must produce no more than 5% THD overall, with orders of harmonics to the 33rd restricted to below specific THD's (AS 4777.2:2015 Clause 5.6, Table 2 & 3).
- Transformers must have no more than 5% TDD (AS 60076.1:2014 4.2(d)).
- Equipment harmonic limits outlined in AS 61000 series.
Evaluation Methods
Different methods of evaluation exist for harmonic emissions in power transformers. The following evaluation metrics were used in this project:
Total Harmonic Distortion
THD evaluates harmonic distortion as a percentage of the current going through the system at that time. Thus, for small currents, harmonic distortion can appear significantly high, whilst in terms of a constant current value, harmonic distortion is not large.
Total Demand Distortion
TDD evaluates harmonic distortion as a percentage of rated current. Thus, regardless of the amount of current running through the system, TDD gives a consistent picture of the levels of distortion.
K-Factor
K Factor is a method for evaluating transformer rating requirements. It is a weighting of the harmonic load currents according to their effects on transformer heating. Higher order harmonics hold more weighting to the effect on transformer heating.
Transformers
Power transformers are affected by harmonic currents through heating of the windings. Harmonic currents cause additional heating within the transformers' windings, which is exacerbated by higher-order harmonics which have a larger heating effect due to higher energy levels. When harmonic levels (TDD) are too high (>5%), or high-order-harmonics are significantly high, overheating will occur.
Overheating causes the insulation materials within transformers (Kraft paper, oil, bushings) to age faster than planned. This is called 'ageing' and can cost hundreds of thousands of dollars in early maintenance and replacement of the affected power transformer.
Inverters
Inverters produce harmonic waveforms due to their non-linearity. Inverters commonly use switching frequencies of 18-20kHz. These high switching frequencies are used to reduce the power usage of inverters, making them more efficient. While limited by standards, it is possible harmonics in the distribution system could be amplified (or cancelled out) when interacting with one another. With enough harmonic sources in the network, it is possible to have total harmonic distortions over the threshold, even if individual inverters are operating within limits.
Analysis
Historic data measured from Power Quality Monitors (PQM's) at transmission stations across the South Australian power network was analysed to evaluate the effect of rooftop solar output on power quality coming into the transmission transformers. MATLAB was used for this analysis, with a range of plots developed to quantify changes in power quality over time and compare these with changes in rooftop solar generation capacity growth over time.
Plots generated included average harmonic current levels for summer and winter, and day and night, to compare at times where solar generation will be maximised (summer daytime) and minimised (night time), or reduced (winter daytime). Furthermore, K-Factor levels at these times were plotted.
The Australian Photovoltaic Institute (APVI) collects data on solar systems across Australia. Data from this source was used to evaluate the growth of solar generation capacity versus the growth of harmonic current levels in the system over time.
Conclusions & Recommendations
This study concluded that there is no evidence of a correlation between harmonic magnitudes seen at transmission transformers in the region, and the amount of solar energy being generated by photovoltaic systems. Furthermore, the harmonic levels as seen at these transmission transformers and the consequent k-factors are significantly lower than the 5% threshold, indicating no foreseeable threat to transformers.
The existing harmonic standards applied by Standards Australia and by SAPN are sufficient to mitigate the risks from harmonics due to rooftop solar systems.
It is recommended that other regions of South Australia's power network are investigated and that harmonic orders over the 50th are investigated. This study was limited by these two factors, and as inverters are using increasingly high switching frequencies, higher harmonic orders should be investigated to ensure no effects.
