October 2009

Grid in safety

With the Power Reducer Box from SMA grid safety management is more simple than thought

Since the beginning of 2009 large solar systems must participate in the so-called feed-in- and grid safety management. SMA is the first manufacturer to offer a reliable product solution for this with the Power Reducer Box.

Feed-in- and grid safety management (GSM) - cumbersome terms for a simple issue: grid operators in Germany should, since the start of this year, have the ability to reduce the feed-in power of PV systems whose output is greater than 100 kW via remote access. Mind you, only short-term and also then, only if the safety of the grid demands it. The basis for this is §6 of the amended EEG (German Renewable Energy Act) as well as the 2008 revised guideline "generation systems on the medium voltage grid" from the BDEW (German Federation of Energy and Water Industries). The solution from SMA: the Power Reducer Box specially developed for this. It transmits the control commands of the utility operator quickly and reliably to the inverter and is easily integrated into existing as well as new solar systems.

The customers are happy

The device has been on the market for more than nine months - and is successfully operational in over 50 systems. The range stretches from medium sized solar systems to the currently second largest PV power station in Germany. The customers are happy, but also the respective utility operator: the Power Reducer Box was accepted smoothly in all cases as solution for the GSM.

Operating Principles

As a rule a radio-ripple control receiver is used, which receives the nominal value specifications from the utility operator via long-wave radio and transmits this via relay contacts. The Power Reducer Box continually evaluates this signal, records every change and sends respective control commands via network to the SMA Sunny WebBox data logger. There, the nominal value specifications are also recorded and transferred to the inverter via fieldbus. The solution from SMA provides many advantages:

1. Safe control
All status changes are recorded twice. If required, the data from the SMA Sunny WebBox data logger is automatically transmitted to Sunny Portal and is therefore viewable globally.

2. Easy Installation
Upon delivery the four standard limitation steps (100, 60, 30 and zero percent of the agreed connection power) are already set, all you have to do is enter in the IP addresses of the Sunny WebBoxes to be controlled. Like the Sunny WebBox you can also configure the Power Reducer Box via an integrated web interface - with the relevant network connection via the internet from anywhere in the world. In addition up to 16 signal combinations of chosen power values of the inverter can be allocated - for the case when the utility operator uses more than four control levels or other values.

3. Simply scalability
With the current firmware the Power Reducer Box can control up to 50 Sunny WebBoxes, which in turn can communicate with up to 50 inverters. Thus with a single device PV power plants with up to 2,500 inverters can be controlled.

4. Suitable for reactive power setpoints
As an alternative to the remote controlled power limitation, the 16 possible status combinations can also be allocated with values for the displacement factor cos(φ) or the reactive power. Thus the Power Reducer Box also makes the external reactive power setpoint possible, which within the framework of the medium voltage guideline becomes legislature from July 2010.

15 Megawatts under control

Since January 2009 the Power Reducer Box has been installed in over 50 systems across Germany. One of the first devices went to Klaron Sechste Solar GmbH, the operator of a 15 Megawatt solar park close to Köthen, Saxony Anhalt: fourteen concrete stations each with two SMA central inverters and medium voltage transformer are distributed over the site, the AC spinal cords run together in a node station. Together with the radio ripple control receiver the Power Reducer Box is mounted there in a switch cabinet and controls the entire solar park, if GSM measures are demanded.

More convenience and control

A further example: the solar system of the operating association GSG Ingenieurbüro and Photovoltaik GmbH in Trüben, also Saxony Anhalt. It accomplishes 536 kilowatts with 54 Sunny Mini Central inverters from SMA. The Power Reducer Box here has replaced a provisional GSM solution, as explains Managing Director Klaus-Dieter Gräf: "At first we had three contactors, which were directly controlled by the radio ripple control receiver and could only separate the system from the grid completely. Indeed, there was no communications capability and no recording of the reduction times. For re-connection, someone had to drive out every time - that only worked manually." With the Power Reducer Box all of these limitations are removed - reason enough for the operating association to also fit out the 350 kWP system in Cobbelsdorf with it.

The future can come

Currently it is still open, how system operators will be compensated for possible yield losses in the event of GSM reductions. The compensation is legally bound in §12 EEG, but the EEG does not make any concrete specifications on determination of the yield losses. The German Federal Network Agency in Bonn has to decide - they already have suggestions from utility operators. Fact is: The participation of large PV systems in the feed-in and grid safety management functions trouble-free and is just as simple as convenient with the Power Reducer Box from SMA. Through the possibility of remote controlled reactive power setpoints the systems are also perfectly prepared for the upcoming requirements of the medium voltage guideline.