Background

With a growing proportion of electricity being generated from renewable energy sources, demands on the distribution grid are also rising. Not only consumption, but also a steadily increasing proportion of generator capacity is subject to temporal fluctuations and thus poses higher demands on grid management. Even if the daily capacity of PV plants can be well planned and they regularly reach their maximum at peak load times, large-scale PV systems can and must make a contribution towards grid security and participate in feed-in management. With SMA products this is made easy.

What are the requirements?

According to §6 of the German Renewable Energy Act (EEG) PV systems with an installed capacity of more than 100 kW must participate in feed-in and grid security management (NSM). The BDEW guideline „Generating Plants in the Medium-Voltage Grid“ stipulates this for all plants feeding in at the medium-voltage level. The issue here is that utility operators have the possibility of limiting the power of the plant by remote control, and even to reduce it to zero for a short period of time. The relevant control commands issued by the utility operator therefore need to be transmitted to the inverters promptly and reliably.

The NSM requirements in detail:

  • Remote-control reduction of feed-in capacity in grid overload situations
    (e. g. via a radio ripple control receiver)
  • Limitation of feed-in power in up to ten adjustable levels
    (typical levels e. g. 0 % / 30 % / 60 % / 100 % of the agreed installed active power PAV)
  • Setting of the required target value in less than one minute
  • Gradual increase of power at a rate of max. 10 percent per minute
  • Option of retrieving the actual feed-in power at any given time by the utility operator

The SMA solution

The SMA Power Reducer Box receives the digitally coded signal from the utility operator, e.g. via a radio ripple control receiver, and converts this into a control command for the Sunny WebBox which is connected by LAN. This then transmits the command via the respective field bus (LAN or RS485) to all the linked inverters. Each switching operation is logged both in the Power Reducer Box and in the Sunny WebBox – this means that the data is retrievable via Sunny Portal anywhere in the world. Alternatively to active power setpoints, the device is also able to process setpoints for reactive power or the displacement factor cos(phi), as laid down by the BDEW medium voltage regulation which becomes mandatory from July, 2010.

The solution in detail:

  • 4-bit digital input (e. g. for connection to a radio ripple control receiver)
  • 16 adjustable levels for active power limitation or setpoints for reactive power output
  • User-friendly configuration via web interface
  • Scalability: each Power Reducer Box serves up to 50 WebBoxes and thus a maximum of 2,500 inverters
  • Security: each utility operator target is recorded in a dual-log system and with an exact time stamp
  • Monitoring: via Sunny Portal the actual feed-in power of the plant can be retrieved on a 15-minute-average basis

Feed-In an Grid Management with SMA Power Reducer Box
Die Konfigurationsoberfläche der SMA Power Reducer Box

In general power limitation not likely

Around lunchtime, i. e. almost exactly when consumption load is at its highest, PV plants supply the most energy therefore, PV capacity does not increase fluctuations in grid load, but rather reduces them. Even if an assumed PV capacity of 30 GW is reached (in 2008 the installed capacity in Germany was only approx. five GW), one does not anticipate any adverse effects on grid stability as a result of solar power infeed. Thus, experience so far suggests that a regulation of PV plants by the utility operator will not be necessary. Moreover, according to §12 EEG (Renewable Energy Act, Germany) plant operators are entitled to compensation for yield losses sustained as a result of such power regulation.

SMA Solar Technology AG · Tel. +49 561 9522 4000 · Fax: +49 561 9522 4040 · E-Mail: Sales@SMA.de · www.SMA.de