Download NETWORK VOLTAGE MANAGEMENT WITH DISTRIBUTED SOLAR

NETWORK VOLTAGE MANAGEMENT WITH DISTRIBUTED SOLAR
PHOTOVOLTAIC GENERATION
Network Tasman has experienced significant growth in distributed solar PV generation on its network
over the past last years and there is an expectation that high numbers of applications for distributed
generation will continue to be received into the future. Nelson being the sunny area that it is, is attractive
for PV generation. We have the highest penetration of PV generation (generating sites as a percentage of
total connections) of any lines company in the country.
The effects of distributed generation on the electricity network are to raise voltage at the consumers point
of connection and more generally at extremities of the network on summer days. At times of peak solar
generation, background network loadings in many areas are at a minimum.
As power is drawn from the network by consumers, impedance in the supply system causes the voltage at
the consumer’s point of connection to fall. Conversely as power is injected, the voltage at the consumer’s
point of connection tends to rise. EDB’s are required under the Electricity regulations to keep voltage at
the consumer’s point of connection to within +/- 6% of the nominal supply voltage of 230V.
The electricity distribution network is not generally designed for the reverse flow of power. It is primarily
designed for distribution of power to consumers rather than for the transportation of power from remote
producers.
In remote rural areas in particular, the distribution network is designed and operated such that the
maximum voltage approaches the upper limit under the lightest loading conditions, and approaches the
lower limit under the heaviest loading conditions. Taking advantage of the regulatory tolerances means
that the capital invested into the network is put to greatest use. Rural capital asset utilisation is thereby
maximised. Larger solar generation installations in these areas (or many small installations) will therefore
cause the regulatory upper voltage limits to be breached.
In urban areas, the supply impedance is generally significantly lower, and as a result voltage rise is
generally much less of an issue for low levels of PV saturation. Many more installations occur in the
urban area than the rural area however and eventually urban solar PV penetration will reach a point where
summer time voltage management becomes an issue. In some overseas cities where there are high levels
of PV generation, voltage management has become a significant problem for lines companies. Rear guard
responses are difficult and expensive to implement.
This upper voltage exceedance can be rectified either by further capital investment into the network to
reduce the supply impedance or by reducing the amount of generation into the network when the voltage
has risen to the regulatory limit. Capital investment into the network for the primary purposes of
accommodating solar generation is generally uneconomic. Relatively few consumers are generators and
the capital servicing costs of reducing supply impedance are typically orders of magnitude higher than the
benefits derivable from solar generation.
In order to address the voltage management issues both in rural areas now and in urban areas in the future,
it is proposed that EDB’s introduce a requirement for all solar PV inverters connected to their networks
that they automatically shut down generation if the voltage has risen to such a limit that the regulatory
supply voltage has been exceeded.
This will automatically cap the voltage at all points of supply. There is a disadvantage for PV generation
owners in that their generation will not be operating during times of high network voltage and this may
decrease the total amount of energy that they generate, and decrease their return on investment. However,
as EDB’s are required by law to hold the voltage within the regulatory limits, then it is not unreasonable
that they have this control as a condition of connection of the PV generation to the network.
All grid connected solar inverters in NZ must meet the standard AS4777. This standard incorporates a
voltage control facility that allows for this automatic overvoltage shutdown function. It is a matter of
programming the correct voltage limits into the inverter at the time the inverter is installed.
Network Tasman recommends that regulatory limit based automatic over voltage shutdown be
mandatorily set up and operational in all grid connected solar PV inverter installations in New
Zealand.
Murray Hendrickson
Network Manager
Network Tasman