Projects:2015s1-07 Remote AVR Control for Embedded Generation

Solar panels are more annoying than you think

Project Information

Background

Project Team

William Declan Schuller

Sebastien Gray

Yiju Ma

Daniel Tang

Supervisors

Commercial

Mark Doherty

Erin Hart

Academic

Rastko Zivanovic

SA Power Networks

SA Power Networks is South Australia's privately owned but heavily regulated power distribution network operator. SA Power Networks is charged with delivering power from the transmission grid to consumers around South Australia. The company is split into a regulated monopoly and a competitive infrastructure and maintenance company.

Over the past decade South Australians have taken up rooftop solar power in a big way, recent statistics suggest that SA has at least 575 MW of installed solar capacity. As solar levels have risen SA Power Networks has begun to receive complaints of solar panels tripping off where penetration is at it's highest. The solar panels are tripping off because the voltage on their output terminals has risen above Australia's regulatory level. SA Power Networks began looking into the issue in more in late 2014 and has asked a team of Adelaide University Electrical and Electronic Engineering students to take a look at the problem from a more academic direction.

The South Australian Grid

The system of grid management in South Australia has existed in a relatively static fashion for the past decade. Power is delivered from large sites via Electranet's transmission grid to connection point substations at 275kV. These substations distribute power down to standard substations embedded in the suburbs at sub-transmission voltage 66kV. The suburban substation then sends it's power down 11kV feeders to pole top transformers at the end of the street. The mini-feeders on the 415V side of the pole top transformer then deliver power to individual premises.

Voltage Regulation Today

Dynamic voltage regulation in the form of On-load tap changing transformers exists at the substation level between 275kV/66kV and 66kV/11kV. This allows easy regulation of voltage when all the generation comes from a source at high voltage and is consumed at low voltage. In simple terms it's easy to regulate when there is an on load tap changer between the generation and consumption.

Voltage regulation at the 11kV to 415V level has always been a problem but has been easily solved with off load static tap changers on pole top transformers (a much cheaper option). With the introduction of solar we introduced a dynamic generation component to our 415V network. It appears that the static tap changing properties of pole-top transformers prevent them from being used to cope with daily fluctuations in solar output.

Existing Research

The problems associated with high PV penetration have been experienced worldwide. A wide variety of studies have previously been conducted in the field of reducing the negative impacts of high PV penetration. The Electrical Power Research Institute (EPRI) has completed a significant amount of research in the field. Studies have been conducted into the effects of high PV penetration, the effectiveness of current solutions, and proposed effectiveness of alternative solutions. It is important that previous research is acknowledged to ensure that further research shall be beneficial.

Many of these studies have been conducted by researchers located in the US, in particular in the state of California, where there is a sunny climate similar to our own. For this reason, there will be a focus of studies in California due to the similarities, for example, Los Angeles’ electrical grid is considered a larger version of the Adelaide grid.

The number of PV systems installed at the residential level has been and will continue to be on the rise [1] [2]. Every kWh of energy generated by a South Australian residential PV system saves approximately 610g of CO2 from being released into the atmosphere [3], as the technology is emission free. However, large concentrations of PV systems at a certain point on a distribution feeder begin to cause issues with voltage regulation and system protection [4].

There are many ways of expressing the amount of PV systems installed on a feeder. One way to express the amount of PV installed on a feeder is to express the installed capacity of PV (in kW) as a percentage of peak load demand. This is a good indicator of the significance of the impacts that the PV systems will have on that particular feeder. Suggestions have been made on maximum penetration limits of installed capacity as a percentage of peak load demand for PV systems on distribution feeders, all of these suggestions have been below 50% [4].

Conventional electricity grids have been designed for unidirectional power flow from source to load. “Reverse power flow is the main cause of voltage rise in distribution feeders.” [5]. Regions with high PV penetration may have sunny days where generated power exceeds demanded power, causing power to flow from the end user back towards substations. Managing reverse power flow will prevent voltage regulation issues and allow for more PV systems to be effectively integrated into the grid.

Voltages can become dangerously high during times of high PV generation and low load causing overvoltage. When overvoltage occurs, voltages can become high enough to damage household electrical appliances if not properly regulated. Cases of overvoltage occur more frequently on feeders with more PV installed closer to the end-of-line [2] causing power to flow from the end-of-line back towards the substations. It is important that the location of a PV system on a feeder (close to substation/close to end-of line) is considered while modelling the PV system.

It is also important to consider the time varying nature of generation from PV systems due to clouds and other shadows. The amount of solar irradiance in W/m2 can drop by over 90% over a period of 10 seconds [6]. It is also important to consider the layout of PV systems on a feeder and whether PV systems are centralised and clustered or distributed and well spread. A large storm cloud passing over a rather centralised group of PV systems could cause a substantial drop in distributed generation power in a matter of seconds, having a more severe impact on voltage regulation than if the PV systems were well-spread.

In order to ensure the safety of household electrical appliances, modern inverters for PV systems are capable of active power curtailment (APC). APC involves stopping an inverter from feeding active power to the grid after the voltage across its terminals become excessively high. APC is a method employed here in SA by SAPN. One study found that APC can be used to allow PV power to generate around 30% more of the neighbourhood’s energy needs for the year without causing over-voltages [7]. Although this strategy allows for a higher integration of PV generation into the grid, it makes owners of PV systems dissatisfied as they are not reaping the full benefits of their systems once their inverters cut out.

Dissatisfaction of the owners of PV systems with the APC strategy has led to research into alternative solutions. These solutions were discussed during the 2013 DOE/CPUC High Penetration Solar Forum, a two forum that was held in San Diego, Florida, USA. Presentations were made on alternative solutions involving energy storage, solar forecasting, Volt/VAr control inverters and other advanced power electronics systems.

Research into the Volt/VAr control inverters appears to be the most promising at present.

Volt/VAr control inverters can be used to absorb/generate reactive power when required, in order to regulate voltage. Generation of reactive power by inverters can reduce the required amount of reactive power flow on a feeder, reducing voltages for a feeder. Volt/VAr control inverters have been modelled and simulated using multiple power flow simulation software packages, and demonstrated the ability of the inverters to mitigate over-voltages for a feeder [8] [9].

Modelling and simulation play a vital part in finding solutions to the negative impacts of high PV penetration. Previously, while modelling distribution circuits and the integration of PV into the electricity grid, power flow modelling has typically been conducted using “snapshot” style solutions, simulating a series of different conditions that are considered important. Important “snapshot” solutions would include situations where maximum PV power generation coincides with minimum load, maximum load coincides with minimum PV generation, the “average” case and a few other typical situations. Simulation of this style does not fully capture the characteristics in PV generation and load caused by seasonal variations. Great benefits can be reaped using simulation data for an entire year rather than data from a few single “snapshots” due to the time-varying nature of both PV generation and electricity load [2].

Aims and Objectives

The project aims to provide SA Power Networks with a feasibility study of which voltage regulation solution will work best to solve their voltage regulation issue due to embedded solar generation. The study must compare 4 proposed solutions based on their ability to solve the problem (as solar continues to increase in penetration), their ease of implementation and development and their cost.

Proposed Approach

The project will begin by