May 2009

Requirements for the PV system technology of the future and solutions by SMA

The power supply of the future will require innovative system technology because it is the only means of guaranteeing grid and load management at all grid levels. SMA already provides tried and tested solutions.

The European Photovoltaic Industry Association (EPIA) has an ambitious target: in 2020, 12 percent of the European power requirement is to be generated by photovoltaics. At present, PV generates only 1 percent. This target is based on the "SET for 2020" study commissioned by the EPIA to investigate a paradigm shift scenario in the area of power supply. The clear result: if costs continue to drop and efficiency continues to rise, a 12 percent share of the European energy mix is absolutely feasible and will even be profitable at the business level.
As far as Germany goes, this target would also mean a significant increase in installed PV power. It is currently at around 5.5 GW, and would have to jump to around 80 GW in 2020. With its study on grid integration of up to 50 GW PV power, the Institute for Solar Energy Supply Technology (ISET) has clearly proven that there is a close correlation between PV power and grid load. Consumption and generation are subject to perfect timing, at this scale, solar power can be fed without requiring any extra measures.

Bild 1: Beispiel für einen Lastgang in Deutschland mit 50 Gigawatt installierter PV-Leistung (Quelle: ISET)

Paradigm shift required

However, a paradigm shift is required if 80 GW of PV power is to be integrated into the grid: consumers, generators and the power grid itself will have to be connected to an intelligent power supply system in the future. More and more power will be generated locally and at all voltage levels – which will require an overarching level of power grid management. Because most PV systems will feed into the low-voltage grid in 2020, the conditions there have become a primary concern. After it has been revised with regard to content and technology, the low-voltage directive[1] being discussed now could be the first step in this direction. "Load management" is also a prerequisite for the EPIA scenario, whether it is implemented by generation-dependent consumption or by using modern storage technology.

[1]Directive on the parallel operation of electrical equipment on the low-voltage grid

SMA provides solutions for future requirements

At this point, it would pay to take a closer look at the future requirements of power supply – and the variety of options provided by SMA systems technology. SMA already provides innovative solutions for these requirements. As a technology leader in the area of secure power supply for island systems, SMA has years of experience in controlling AC current grids quickly and precisely. SMA was significantly involved in adapting the new German Energy and Water Association Medium Voltage Guideline to the requirements of photovoltaics. Its Sunny Central 400/500/630HE already meets requirements that will not become binding until the middle of 2010. And the innovative SMA backup system ensures 100 percent supply security – but it provides functions for grid and load management as well.

1. Grid management at the low-voltage level

Reactive power helps to stabilize the grid voltage and compensate for the phase shift that exists at the grid connection point. Reactive power compensation relieves the grid infrastructure – an important task because low-voltage grids are often heavily loaded. But feeding leading reactive power can be used to lower the voltage in low-voltage grids as well.
The background: due to the ohmic resistance, feeding active power to the low-voltage grid leads to an increase in voltage. While its ohmic characteristic diminishes the voltage-reducing influence of the reactive power, it does compensate for 20 to 50 percent of the increase. Therefore, more active PV power can be fed in total.
The feeding of reactive power with a power factor between 0.9leading and 0.9lagging has already been included in the low-voltage directive draft. For SMA's current line of inverters, technological implementation is not a problem. Some Sunny Mini Centrals and the Sunny Tripower introduced at Intersolar can already handle a power factor of 0.8.

2. More grid security

Since a grid is unable to store energy, maintaining an ongoing balance between power generation and consumption is an important part of grid control. A power outage in a local low-voltage grid does not have an obvious effect on frequency, but frequency is the major control variable for the grid as a whole. Every imbalance between generation and consumption becomes apparent as a frequency deviation that has to be compensated for by conventional large power plants. For that purpose, the European UCTE grid is currently keeping three GW of rated power as a primary standby reserve. That is enough to compensate for the simultaneous failure of two large nuclear power stations.

However, the PV power installed in the German low-voltage grid today is already more than 4 GW and the EPIA scenario for 2020 assumes 40 GW in the low-voltage grid alone. That means if this PV power were simultaneously switched off due to a fault in the high-voltage grid, for example, the primary standby reserve could hardly cushion the impact today. A fault like this is entirely conceivable and when the grid is split to in several control zones, load compensation stops functioning. Grid regions with over-frequency or under-frequency are the result and according to modern regulations, all PV systems would immediately switch off in either case.
However, the PV power installed in the German low-voltage grid today is already more than 4 GW and the EPIA scenario for 2020 assumes 40 GW in the low-voltage grid alone. That means if this PV power were simultaneously switched off due to a fault in the high-voltage grid, for example, the primary standby reserve could hardly cushion the impact today. A fault like this is entirely conceivable and when the grid is split to in several control zones, load compensation stops functioning. Grid regions with over-frequency or under-frequency are the result and according to modern regulations, all PV systems would immediately switch off in either case.
To ensure grid supply security, it is essential that PV inverters that feed the low-voltage grid reduce their feed output when the grid frequency is rising. SMA's present line of central inverters includes this function and therefore already meets the requirements of the medium-voltage guidelines. In the AC-coupled island grid systems by SMA, PV power has been controlled via frequency for years. The relevant function has already been implemented in almost all SMA inverters, and can be easily adjusted to the conditions of the grid by changing a few parameters.

Bild 2: Abregelung von SMA Solar-Wechselrichtern in Inselnetzen. Diese Funktion kann bei Bedarf auch bei netzparallelen Wechselrichtern aktiviert werden

3. Grid relief via load transfer

Regardless of the reason for derating PV power, it is always the second-best solution because it means failing to use valuable power. The alternatives: using surplus power sensibly or storing it for later use.

Smart Grids – intelligent load control systems

In the future, load transfer processes will be playing a more and more important role in this context. Intelligent control systems based on Smart Grids are conceivable on the consumption side. They switch different power consumers on or off depending on the power supply, while taking the energy requirements of the relevant equipment as well as the forecast amount generated into consideration. For example, washing machines will be automatically switched on at noon or freezers will only cool during the day – the low temperature will be stored at night. All consumers that provide storage options or are not tied to specific operating times are suitable for participating in load management.

Bild 3: Mittlere Verbraucherleistung eines 4-Personen-Haushalts an einem Sommertag, überlagert mit der Tagesleistung einer 5 kWp-Solar-Anlage

New direct consumption regulation

The direct consumption regulation of the amended Renewable Energies Act (EEG, Germany) has incorporated the idea of sensible consumption. For the first time, it permits and supports the consumption of solar power "in the immediate vicinity of the plant." Because power that is directly consumed in the building that generates it relieves the grid and minimizes excess transport losses. At the instant when the solar plant delivers the required  
electricity, for example, your dishwasher is actually running on solar power – there is no need to draw normal-consumption power from the grid or feed the generated solar power into the grid.
Power suppliers are also becoming more and more interested in load management. Smart Meters, intelligent electricity meters, will provide a financial incentive for consumption control in the future. The price of electricity will no longer be fixed, but will be a reflection of supply and demand in the grid instead. At times when solar radiation is strong and loads are weak, a lower power price that should motivate customers (or their Smart Grids) to consume more will be the result. The mechanism will ultimately lead to a reduction in the load fluctuation in the grid, which in turn will relieve the grid infrastructure. At present, the use of Smart Meters is restricted primarily to transparent consumption monitoring. However, there are already devices that can record load profiles, bill power at different tariff rates and control consumption loads.
Studies relating to the direct consumption of solar power show that a direct annual mean consumption rate of 30 to 50 percent can be achieved when suitable incentive and load management systems are in place – an important step toward the scenario of a sustainable power supply in the future. Given the current price of electricity, direct consumption still does not pay as a rule. But with its latest Sunny Boy inverters, SMA offers the option of activating consumer loads via a multi-function relay[2] as soon as enough PV power is available.

[2] A free firmware update for this will soon be available

4. Useful supplement: Intermediate energy storage

According to the EPIA energy scenario for 2020, adjusting consumption and generation over time alone will not adequately compensate for generated surplus power. Because as soon as the installed feed-in power as per the German Renewable Energy Sources Act (EEG) is greater than the minimum consumption load in the grid, the power generated cannot be completely consumed at all times.
And since it is only engineered for an average rated power of approximately 1 kW per connected housing unit, the infrastructure of the low-voltage grid is also a limiting factor. At the assumed 40 GW PV power level, it could hit its capacity limits in radiation-heavy, load-weak periods. Instead of reducing the amount generated at that point, intermediate storage could be used as a supplement to load management. This solution can already be implemented using today's technology – and it is easily scalable, reliable, and tried and tested.
An intermediate power storage solution could cushion the impact of fluctuating levels of feed-in power and at the same time, enable continuous energy consumption independent of the point at which the power was generated. But the prerequisite for a sensible storage solution is that it takes place in close proximity to consumers and generator so that the grid can be relieved. In addition, since storage means costs, only peak generation should be stored.
Today, it already pays to store power that would otherwise be "given away" by derating. With current battery technology, it costs EUR 0.10 to EUR 0.20 per kWh – which is less than the price household customers pay for electricity. And the Sunny Backup system by SMA already provides a tried and tested solution for this.

The SMA Sunny Backup System: High tech for the future of grid management

Available in different expansion stages, Sunny Backup sets allow you to store solar power conveniently – but that is not all. The system can be used to optimally meet the demands made as part of grid and load management. It features three main functions:
First: a continuous power supply that switches to solar-supported battery power supply within milliseconds of a grid outage to secure the continued operation of important consumers.
Second: the option of selecting the consumption time of PV energy as required, as a result of intermediate storage. This means of load transfer helps to significantly increase the direct consumption rate again.
Third: every Sunny Backup system can provide the grid with valuable standby power – positive or negative – as required. The only limiting factors are the battery capacity and the rated power of the backup inverter. Above and beyond the low-voltage grid, the appropriate number of backup systems could contribute to the active power/frequency control of the grid – around the clock. Local primary and secondary control of this type would be as economical as it is reliable, but unfortunately there is no economic incentive for it right now.

Bild 4: Das Sunny Backup-System von SMA bietet mehr als nur Versorgungssicherheit

Sunny Backup sets by SMA also have the capability of performing other system services that are extremely interesting for the future of local power supply grids. For an obvious reason: if the grid fails, backup systems have to build up fully operational island grids. The battery inverter takes over the "master inverter" function and is responsible for the voltage, frequency, and power factor, filtering harmonic waves and feeding short-circuit current. If it can perform all of those functions in a comparatively fragile island grid, it can also provide outstanding support to the grid. Individual functions:

Feeding short-circuit current
Unlike "normal" grid-driven inverters, the devices can feed short-circuit current during grid voltage failures to support the grid when it is in trouble. A Sunny Backup 5000 delivers short-circuit current equal to four times the amount of nominal current for 60 ms.

Reactive power compensation
Sunny Backup inverters by SMA can provide as much reactive power as required. In other words, the power factor cos(φ) can be between 0 and 1 – even 100 percent reactive power feeding is possible. The devices are able to compensate for reactive power at the level of their nominal power and can therefore control the phase shift at the grid connection point to 1 (or any other value required).

Active filtering of harmonic frequencies
Simply put, harmonic frequencies are deviations from the ideal sinus wave. Harmonic frequencies in the electrical grid are caused by consumer loads such as simple rectifiers (chargers, computer power supplies, etc) or increasingly common energy-saving light bulbs that draw non-sinus wave electricity from the grid. Harmonic frequencies overburden the grid and lead to loss of power and equipment failures since the physics of AC current is based on a precisely sinus-shaped flow of current and voltage.
Sunny Backup inverters by SMA are able to detect existing harmonic frequencies in a household grid and provide 100 percent compensation by feeding corrective current in the opposite phase. The low-voltage grid is no longer overburdened by additional harmonic frequencies.

In conclusion: With SMA system technology, the EPIA scenario is not utopian

The capacity of the low-voltage grid is an important criterion for PV power expansion for the power supply of the future. Already today, a major share of generation takes place at this grid level. SMA provides a tried and tested solution for effectively relieving this grid level. Its innovative backup system functions as a grid-connected storage system to supplement useful measures such as direct consumption and load management. It also provides a fail-safe supply of power. In addition, the Sunny Backup inverter by SMA facilitates important system services such as reactive power control, short-circuit current feed and harmonic frequency filtering for low-voltage grid systems. As a leading manufacturer in the practical implementation of the German Energy and Water Association Medium-Voltage Guideline, SMA also supports PV inverter participation in coordinated low-voltage grid management. In addition to reactive power feeding, the  
frequency-dependent power control of grid-connected inverters is urgently required. Because the standby power available in Europe today would hardly be able to cushion the impact if all of the PV systems in the low-voltage grid were subject to frequency-related deactivation. Since almost all of the SMA inverters have been successfully used to control feed-in power in island grids for years, they are already equipped to deal with this type of frequency/active power control.
Grid-connected storage systems provide an additional advantage in regard to this point as well. A number of Sunny Backup sets distributed throughout the grid could function as a local energy storage system, providing around-the-clock positive or negative standby power to the grid – independently of the PV power.
The integration of other feed-in power as per the German Renewable Energy Sources Act (EEG) (such as wind power plants or small co-generators) ultimately rounds out this "power supply of the future" scenario in regard to seasonal distribution as well. Therefore, the EPIA target is not utopian – it could become reality. Today's system technology by SMA is making an important contribution to that.