Reactive power usually occurs any time that energy is transferred via alternating current. Its importance for solar engineers and PV system operators is increasing, for larger and for smaller systems. Most important realization: reactive power is no problem at all. It is actually a solution for some problems.
On July 1, 2010 things will get serious: PV systems that feed into the grid at medium-voltage level will then have to be able to provide reactive power. This is stated in the 2008 edition of the Medium-Voltage Guidelines of the German Federal Association of the Energy and Water Industry (BDEW). For the low-voltage grid, even stricter requirements are being discussed. So what is reactive power actually? What is it good for? What are the requirements for PV systems? And which solutions does SMA offer?
As a model for explaining reactive power we can look at the revenues and expenditures of a fictional enterprise: in January, it will earn €10,000, in February it will incur expenses of €10,000. This will repeat over the following months. Despite a monthly account turnover of €10,000, the average profit equals zero – you could compare this to pure reactive power. But how does this come to happen in an AC grid?
For DC, the equation is quite simple: electric power is the product of voltage and current. However, for AC things are a little more complicated because the intensity and direction of a current and voltage change regularly here. In the public grid both have a sinusoidal trajectory with a frequency of 50 or 60 Hz. As long as the current and the voltage are "in phase", i.e. moving at the same rhythm, the product of these two oscillating factors will also be an oscillating output with a positive average value – pure active power (Fig. 1a).
Fig 1a: When there is no phase shift, the product of current i and voltage u is an oscillating, yet always positive output – pure active power.
However, as soon as the sinusoidal trajectories of the current and the voltage are shifted against each other, their product will be an output with an alternating positive and negative sign. In extreme cases the current and the voltage are phase-shifted by a quarter period: the current always reaches its maximum intensity when voltage is equal to zero – and vice versa. The result: pure reactive power, the positive and negative signs completely neutralize each other (Fig. 1b).
Fig 1b: At a phase shift of 90 degrees between the current i and the voltage u, an alternating positive and negative output with an average value of zero will be the result – pure reactive power.
This phase shift can naturally occur in two directions. It occurs when coils and capacitors are in the AC circuit – which is usually the case: all engines or transformers have coils (for inductive shifts); capacitors (for capacitive shifts) are also commonly found.
Multi-conductor cables also function like a capacitor, while high-voltage overhead lines can be seen as extremely long coils. Therefore, a certain degree of phase shifting, i.e. reactive power, cannot be avoided in AC grids. The measurement parameter for phase shifting is the shift factor cos(φ), which can have a value between 0 and 1. It can be used to easily convert output values (see info box).
Only active power is actually usable power. It can be used to power machines, make lamps glow, or operate electrical heaters. Reactive power is different: it cannot be consumed and can therefore not power any electric devices. It simply moves back and forth in the grid and thereby acts as an additional load. All cables, switches, transformers, and other parts need to additionally consider reactive power.
This means that they need to be designed for apparent power, the geometric sum of active and reactive power. The ohmic losses during energy conduction occur based on apparent power; additional reactive power therefore leads to greater conduction losses.
Fortunately, existing phase shifts can be compensated. You only need a phase shift in the opposite direction via compensation coils or compensation capacitors – or through inverters. This not only reduces conduction losses, it also leads to the grid only being loaded with active power. The freed capacities can be used for transferring more active power.
Capacitive or inductive phase shifting also has another effect: it increases or reduces the grid voltage. Therefore, large power plants already generate energy with a capacitive phase shift in order to compensate for the voltage-reducing influence of inductive overhead cables and transformers. So monitoring phase shifts or reactive power is extremely important for regulating the grid – not only for large power plants but also for PV systems in the medium and low-voltage grid.
According to the BDEW's Medium Voltage Guidelines, grid operators will be able to demand that inductive or capacitive reactive power be fed into the grid with a shift factor of 0.95 from July 2010. However, some already demand reactive power feed-in today when connecting systems to given grid links – some even with a shift factor of 0.90. An according guideline for the low-voltage grid is already being worked on. A study of the Technical University of Munich also proposes a shift factor of 0.90.
Background: For physical reasons, feeding in active power will lead to a voltage increase, especially in the low-voltage grid, which might become a problem (see Fig. 3). At the same time, a large amount of reactive power is necessary here to lower the voltage again.
SMA is already offering a range of reactive-power-compatible products: all newer central inverters, the Sunny Mini Central inverters with reactive power control, and the new Sunny Tripower are all designed for producing reactive power. The current central inverters of the HE series already meet all requirements of the Medium Voltage Guidelines, which will become effective in mid-2010, and offer shift factors of up to 0.90; the other devices even offer factors of up to 0.80.
Power Reducer Box – the all-rounder
The SMA Power Reducer Box is an additional communication solution for presetting the shift factor: in addition to remote-controlling the feed-in power, the device also lets you remote-select from up to 16 freely definable shift factors or reactive power values (the maximum values of the used inverters are to be considered here).
High tech with added value
The innovative Sunny Backup Sets by SMA take it one step further: in the case of public grid failure, a backup system will establish a fully functional island grid with the help of batteries and solar power. In this case, the battery inverter takes over the grid generation task and regulates the voltage, frequency, and reactive power compensation, while also filtering harmonics oscillations. It is able to deliver its entire nominal power as reactive power and can thereby regulate the phase shift in the island grid to any given value. After a software modification the Sunny Backup inverter would also be able to do this even with voltage on the line - and thereby relieve the low-voltage grid accordingly.
Of course reactive power must be considered when designing the PV system. The desired or required shift factor plays a decisive role here: it determines the amount of apparent power and thereby the additionally necessary inverter power. This way, an apparent power of 105.26 percent of the available PV active power will occur at a cos(φ) value of 0.95. In order to feed in 100 kW of active power with this phase shift, an inverter with at least 105 kVA of nominal apparent power will be required (see Fig. 2). Note: The active power received by the inverter will be fully preserved. The according reactive power will be produced in the inverter, which is why it needs to be sized accordingly. The free SMA design program Sunny Design version 1.50 and up will also let you calculate all possibilities for feeding in reactive power.
Fig. 2: The required reactive power is produced in the inverter – additionally to the received PV active power. The geometric sum of both is the apparent power; it is decisive for the inverter design.
In certain constellations feeding in reactive power can be quite advantageous for the PV system operator. For example, if a system can only feed in very little active power because the grid voltage would otherwise exceed the permissible values and the inverters would disconnect from the grid. Especially when feeding into the (mostly ohmic) low-voltage grid this case is quite realistic, since feeding in active power will noticeably affect the grid voltage. In Germany, for systems with an output of less than 30 kWp this is a problem of the grid operator, since they must provide an accordingly sized feeding point. However, for larger systems it is important to find the most economical overall solution – no matter if at the expense of the system operator or the grid operator.
Stabilizing the voltage by feeding in reactive power might be the most economical option here. With an according phase shift through the inverter, the voltage increase at the grid connection point might be compensated to such an extent that a breach of the voltage criteria will definitely be avoided (see Fig. 3). This can help to avoid an unnecessary grid expansion (at the expense of the grid operator) or choosing a more distant grid connection point (at the expense of the system operator).
Additional investments for inverter power will have to be made in order to feed in reactive power. However, the effort will pay out since far less or no active power at all would be fed in otherwise, or a different grid connection point would have to be chosen.
Fig. 3: The voltage increase in percent at 27 kW active power feed-in, depending on the grid impedance angle and the shift factor
Providing reactive power through solar inverters is an important step for integrating photovoltaics into the grid control, but it can also be attractive for operators. The good news: Due to their mode of operation inverters are excellent for this.