When specifying an energy storage inverter there are a variety of high level requirements customers consider such as power rating, AC output voltage, features, and included over current protection. A parameter that can be overlooked by some system integrators and not appreciated by all energy storage system vendors is the impact of battery DC voltage on inverter cost, efficiency, and size. This paper will provide a comparison of cost, efficiency, and size of the most common and reliable, and efficient energy storage inverter topologies, while providing a review on how the battery DC voltage range influencing viable energy storage inverter topologies. We will only consider three phase systems for this discussion but the principles are transferable to single phase systems.
Figure 1 depicts a 480VAC three phase AC system as is common in North America. Here we see the three phases (a phase, b phase, and c phase), each phase shifted by 120ᵒ from one another. The horizontal red line indicates the RMS value of the AC voltage as 480VAC while the green line indicates the peak of the 480VAC voltage at 678.82 V (480*√2). The inverter must be able to replicate the entire AC voltage waveform across its full peak to peak voltage (-678V to 678V) in order to provide low harmonic content on the AC output. Note this is at nominal line voltage (PU = 1.0), however, the requirements in North America call for inverter operation from 0.88PU voltage to 1.10PU voltage, or 422 – 528 VAC.
The most common inverter topology is the single stage inverter simplified below in Figure 2. The topology consists of a DC input, here a battery, a DC fi lter capacitor, a set of six semiconductor switches, and an output AC filter (here made of inductors La, Lb, and Lc). The output current of the inverter is denoted on a per phase basis as ia, ib, and ic with corresponding output voltages Va, Vb, and Vc. In order for the inverter to provide AC output current (either positive or negative) with low harmonic distortion the inverter must able to replicate the AC grid voltage. In this case if Va = Vb = Vc = 480 VAC the system will have a peak voltage (at nominal line voltage) of 678V, however, there is a voltage drop across the output inductors (La, Lb, and Lc) so additional voltage is needed beyond 678V to account for the voltage drop. The exact voltage drop will depend upon the PWM technique used to switch the IGBTs, as well as the inductor design, and operational power factor. The typical minimum battery voltage to utilize a single stage inverter in a 480VAC system is 740 VDC this is a relatively common requirement shared among the leading inverter manufacturers, specifically for utility scale energy storage inverters. Note this is roughly a 1.5 ratio (minimum battery voltage/RMS line voltage).
This topology is utilized in the Dynapower Compact Power System series of utility grade energy storage inverters as it provides high reliability coupled with high efficiency and low cost.
If the minimum battery voltage cannot satisfy the 740VDC requirement there are two commonly used solutions:
- Utilize a DC-DC converter between the battery and the inverter DC rail to satisfy the 740VDC minimum requirement at the inverter input.
- Use a single stage inverter, however, the AC output voltage (Va, Vb, and Vc) will not be 480VAC they will be something less and then utilize a transformer to boost Va, Vb, and Vc to the voltage at the point of common coupling of the storage system.
Here we have added another set of semiconductor switches and inductor create a bidirectional DC-DC converter. The converter will