Power distribution and transmission networks (ie grids) face multiple challenges as the percentage of renewable energy in the grid increases. Traditionally photovoltaic (PV) power plants, and more specifically the solar inverters, need to shut down when the grid becomes unstable or even fails. However, as PV power today can be a considerable local share on the power grid, sudden stoppage of all PV power plants could trigger much more dramatic grid problems than the initial event.

Therefore, this larger share of variable renewable power fed to grids via fast-response electronic power converters has put utilities in a tight spot. The fear of increasing network disturbances and instability in power transmission has led to rapid development of grid codes, which are new sets of connection and operation rules and requirements. In practice, this means that solar inverters must take greater responsibility for keeping the grid in operation.

Why it matters

Distribution and transmission networks must be operated in agreed voltage and frequency windows without disruptions. The voltage is controlled locally at power plants and sub stations, while frequency is controlled by balancing production at power plants to match consumption.

Normally all production plants and substations are under the direct control of the electrical utilities and the grid owners' central control facilities. PV and renewables, however, are not typically under such central control and as they grow they may stress the grids and make balancing difficult.

Grids generally were designed long ago for rotating generators and are not optimal for today's rapidly-developing, decentralised, semiconductor-based, fast-reacting PV power. Refurbishment is needed in the higher-voltage base grid as well as the lower-voltage community level substations. Additionally, this may also require controlling and balancing of loads in the network with respect to renewable energy production.

Stability challenges

Advanced grid-support features, on top of voltage and frequency window limits and anti-islanding functions, are now needed to meet obligations set by grid codes. These include:

• LVRT (Low Voltage Ride Through) and HVRT (High Voltage Ride Through), with and without current feeding to manage short voltage dips and peaks (and not to shut down).
• Reactive power support of the grid by utility remote control signal, grid voltage dependent function or with fixed value.
• Active power limitation to support the grid frequency automatically or with utility remote control.

All these can be met with the flexible ABB central inverters. Required functions can vary widely by utility, and even by location of the site as well as by time, so the adjustable and remotely-controllable ABB central inverter ensures all grid-support functions are met.

Whose responsibility?

However, these ever-increasing demands are financially controversial for many reasons. Inverters that can handle the requirements cost a bit more, at the same time as the market is expecting falling equipment prices as feed-in tariffs gradually go down. Extensive reactive power support can demand de-rating the inverter's nominal power, requiring more inverters and thus higher investment. Reactive power support and active power limitation can also reduce the energy fed to the grid, resulting in less income.

From a wider perspective, would it perhaps be better to ensure grid stability with purpose-made equipment for easier central control, thereby letting the solar plants produce energy at highest effectiveness? This carefully-located separate equipment could be in critical places like substations and transmission/distribution lines. Also, possible energy storage could be located here to be more cost-effective.

Another concern is that distribution networks with old on-load-basis designed substations might not be able to withstand all the power coming from distributed power plants. Grids no doubt need to be strengthened but these modernisations have long been delayed due to economic pressures. These issues may sometimes be used unnecessarily as a “barrier” and limiting factor in an early stage of renewable energy penetration to slow down its development.

Problem or solution? The opportunity

PV and renewable energy can be a great addition to the grid if potential complications are carefully considered. Large ground-mounted PV power plants provide bulk energy in large quantities but medium-sized strategically-located PV plants can reduce the distribution needs and even reduce day time peak-power load. Residential PV systems normally produce most energy when there is minimum load in the house, so it goes to distribution.

When the share of renewable energy increases above a certain limit there is an obvious need for additional advanced grid support, either by the solar inverter or special purpose-built plants. In addition to grid support, energy storage might be needed to even out and “move” the daytime production.

Should this storage be part of the PV plant or the grid? Economically and environmentally it seems best to have large storage entities at the community level, operated by the grid owner. At the same time, all grid-support measures could be integrated into combined bulk-storage/grid-stability plants.

It all comes down to a question of how costs are shared and whether energy production infrastructure is viewed comprehensively, or only from individual players' own limited perspective. Renewable energy is growing. Grids and grid control therefore must, and will, develop.

PV and renewable energy is not a threat but offers a way to support grids, with the optimum solution of either centralised or distributed grid support depending on each particular case.

About: ABB Solar Inverters is part of ABB, a leader in power and automation technologies that enable utility and industry customers to improve their performance while lowering environmental impact. The ABB Group of companies operatesin around 100 countries and employs about 145,000 people.