June 2010

SMA inverters as grid managers

Overview of the grid management functions of SMA inverters

It started in Germany in January 2009, following France in 2008: Since then, largescale PV plants have had to participate in grid management, and provide grid services to an increasing extent. Experts unanimously agree that such measures are essential. The installed PV power increases continuously, and individual projects are growing larger and larger. Increasing size and power also mean greater responsibility for the electricity grid, as only a stable grid permits unrestricted expansion of renewable energies. This was enough reason for SMA to commit fully to participation from the outset. SMA, technology leader in the field of solar inverters, is now also a pioneer in grid management.

Netzmanagement-Funktionen von SMA Wechselrichtern

Powerful grid management functions

For example, the SMA Power Reducer Box was the first market ready solution for utility interaction management as defined in Art. 6 of the German Renewable Energies Act (EEG) – and is also compatible with virtually all SMA inverters. Inverters with an automatic power reduction feature in case of increased frequencies make a valuable contribution to stabilizing the grid frequency if more energy is generated than can be consumed. Reactive-power compatible inverters help keep the grid voltage constant, but can also be used to compensate for undesirable phase shifts. And the dynamic grid support features support the grid in the event of faults, and can also prevent, or at least restrict, the fault from spreading further.

1. Generation and grid stability management

Sixty seconds: Inverters in Germany have just one minute to implement possible specifications of the utility operator. If a section of the grid is overloaded temporarily, the utility operator can and must limit the power from decentralized generating systems The plants make a major contribution to the stability of the grid, which can hardly keep pace with the rapid expansion of power from renewable energies in some cases. The SMA Power Reducer Box von SMA translates the incoming setpoint specifications of the utility operator into control commands for the Sunny WebBox. This in turn forwards the commands to the connected inverters via fieldbus, and logs the external setpoint specification – which is important for legally compulsory compensation for possible resulting yield shortfalls.

2. Automatic active power frequency control

The frequency in alternating current grids is kept constant within strict limits – typically at exactly 50 Hz or 60 Hz. The frequency drops if more energy is consumed than the generators feed-in. The opposite occurs if there is an energy surplus – the grid frequency increases. SMA inverters can react to this with an automatic active power reduction: The higher the frequency is above the setpoint, the more they derate the active power output. This stabilizes the grid and prevents mass deactivation of systems due to excessive grid frequency.

Die Netzebenen im Wechselstrom-Verbundnetz

3. Static voltage support based on reactive power

In order to protect the connected loads, the voltage must be kept within defined limits – that applies to the distribution grid in particular. With their ability to provided controlled inductive or capacitive reactive power, PV inverters can help guarantee the voltage quality required in EN 50160. Reason: the increasing feed-in at low and medium-voltage levels can result in voltage increases which caused problems for the grid operation method used so far (flow of energy from central generator to decentralized loads). Using reactive power, PV inverters can now reduce undesirable voltage increases significantly. Reactive power-compatible inverters can also be used to compensate for existing phase shift. As even reactive power reduces efficiency, this reduces the load on the grid infrastructure, whose capacity limits may already have been reached. Typical causes of phase shifts include transformers, large motors or even simply long cable sections.

There are several options for adjusting the percentage of reactive power: Plant operators can use set points specified by utility operator, or various reactive power values can be set via an agreed schedule or remotely programmed via the SMA Power Reducer Box. The third option is regulation of the percentage of reactive power via a characteristic curve depending on the grid voltage measured at the connection point or the active power output of the inverter.

4. Dynamic grid support

Until now, PV plants had to disconnect from the grid immediately, even during temporary drops in grid voltage. This meant that many plants were deactivated in the event of grid failure, unbalancing the grid even further. Inverters with dynamic grid support functions act within milliseconds in such events, preventing the grid failure spreading further. The so-called restricted dynamic grid support ensures that the inverter is ready to feed energy into the grid immediately after a drop in the grid voltage. Devices with full LVRT or FRT behavior (Low-Voltage or Fault Ride Through) can also feed reactive power into the grid during grid voltage drops. LVRT is already supported by the new Sunny Central CP series.

With SMA technologies equipped for the future

Providers of decentralized grid services and an intelligent interface to the supply grid: That describes the solar inverter of the future. The utility operators have also recognized that inverters are particularly predestined for the pending grid management responsibilities – and demand these system services to an ever greater extent. SMA makes an important contribution to this with its products.