In 2010, the New South Wales Government introduced the Solar Bonus Scheme, which paid customers 60 cents per kilowatt hour (kWh) of electricity produced by compliant embedded generation (EG) systems that was fed back into the grid.
Since 1 January 2010, the total solar panel capacity installed on the Ausgrid network has grown from around 8 megawatts peak (MWp) to over 100 MWp, with over 50,000 residential customers installing small-scale solar systems under 10 kilowatts (kW).
The two other NSW electricity distribution networks – Endeavour Energy and Essential Energy – saw a similar growth in capacity.
Within the Ausgrid network, the residential solar take up was particularly high in the Central Coast and Hunter Valley regions, with some local government areas seeing 5-7 per cent of residential customers installing photovoltaic (PV) systems.Article continues below…
The large number of PV systems added to the network in recent years has brought to light a number of issues relating to power quality, such as voltage rise and current unbalance. Both of these factors can contribute to voltage unbalance (i.e. when the phase voltages are unequal) on the grid.
Traditionally, a low-voltage distribution network is designed as a one-way radial system, with the transformer being the single source of energy. The network designers set the voltage at the transformer to allow for voltage drop caused by customer loads, so that under all loads and at all points of the distribution network, the voltage at each customer point of supply is still within the specified range of 216-253 volts (V) for a 230 V nominal range.
With EG, there are now multiple sources of energy distributed randomly at various customer installations connected along the low-voltage distribution mains. If the EG is of sufficient capacity or if the impedance of the service wires is high, then there will be a rise in voltage at the point where the EG connects to the network. Furthermore, at times of light load coincident with high solar insolation, the voltage can rise above that of the distribution transformer.
Voltage rises caused by EG may deliver steady voltages within customer installations exceeding the 253 V limit of the Australian Standard 60038 for a 230 V range, resulting in concerns over potential damage and/or reduction in product life to customer equipment.
If the voltage goes too high, then the inverter will cut out as a safety measure resulting in loss of energy generation (and revenue) for the customer. In cases seen on the Ausgrid network, voltage rises of over 10 V (to 265 V and higher) have been observed on customer installations. It is recommended that the overvoltage trip on inverters be set to no higher than 260 V in order to protect customer equipment.
A 10 kW single phase inverter will result in six times the voltage rise on the same size service mains than if a 10 kW three phase inverter is installed.
Ausgrid customer voltage complaints relating to PV systems in 2011 were generally resolved by minor works such as adjusting the tap setting on the distribution transformer, upgrading the service wire to the customer or adjusting the phase connections of the customer loads.
The key to a successful PV connection is to manage the impedance of the connection to the grid, particularly inside the customer’s installation and their individual connection (i.e. service line) to the street mains, and for larger inverters, to ensure a true three-phase inverter is used.
In late 2011, Ausgrid commenced migration from a nominal voltage of 240 V to 230 V in line with the new standard AS 61000.3.100:2011, entitled ‘Limits—Steady state voltage limits in public electricity systems’. Over time the average Ausgrid network voltage will be reduced from around 250 V to around 240 V, and will allow for additional ‘headroom’ for voltage rise due to EG.
Three-phase motors operate most efficiently on a balanced three-phase voltage supply. Small amounts of voltage unbalance will result in shortened equipment life due to heating in the motor. An unbalance of even 2 per cent can result in a service life of the motor only reaching 5 years compared to 20 years with 0 per cent unbalance.
If large single-phase inverters are installed and not balanced across the phases, then a voltage unbalance situation can occur. In one case observed, a 10 kW PV inverter was installed on one phase only, and this – combined with a high impedance service wire and consumers mains – resulted in a voltage unbalance of over 3 per cent at the customer’s premise during inverter operation.
Ausgrid is currently working with other Australian network providers to clarify guidelines for installers to improve the integration of embedded generation with the grid. This includes requirements on limiting the voltage rise in the service mains to 1 per cent and similarly in the consumers mains to 1 per cent at the maximum inverter rating, and encouraging the use of three phase inverters to minimise current and hence voltage unbalance between phases.
Service and maintenance
As PV systems get older there is a greater likelihood that the system will experience performance issues, often a result of solar panels dropping in efficiency due to the aging process of the silicon cells, physical damage or inverter failure.
When a property with an existing PV system is sold, the new customer may not have the same level of engagement with the PV system as the previous owner, and as such the system may not be kept in good order. This is exacerbated if the financial reward is low, as is the case in NSW, where once a property is sold, the Solar Bonus Scheme dictates that the new owners will not receive the 60 cents per kWh feed-in tariff.
A survey completed in 2010 by Ausgrid, which looked at PV systems installed for the Sydney 2000 Olympics in the suburb of Newington, revealed that up to 23 per cent of installations may have been non-operational.
The next challenge
It is likely that the cost of a typical PV installation will decline in coming years, and even more PV installations will be connected to Australian networks.
The next challenge for networks will be to manage the aggregation of the generation from these installations, particularly residential PV, as the peak generation from such PV installations often coincides with relatively low loads being drawn by residential customers from the network. It is important that network power quality is not adversely affected as PV generation in Australia grows.