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 Three Phase AC System with RMS and Peak Lines

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.

Single Stage Inverter

If the minimum battery voltage cannot satisfy the 740VDC requirement there are two commonly used solutions:

  1. Utilize a DC-DC converter between the battery and the inverter DC rail to satisfy the 740VDC minimum requirement at the inverter input. 
  2. 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.

There are costs and benefits to each potential solution, we will consider a generic case as the exact requirements will vary depending  upon application and battery voltage.

Dual Stage Inverter with DC-DC

Here we have added another set of semiconductor switches and inductor create a bidirectional DC-DC converter. The converter will
typically run at a constant voltage output of Vinv = 800V to satisfy the 740VDC minimum requirement at the inverter DC bus. This enables  the direct use of a low voltage battery (typically on the order of 300-600VDC) to generate 480VAC at Va, Vb, and Vc. The additional of the DC-DC converter will increase the size of the inverter, increase the cost of the inverter, and decrease the efficiency of battery to grid and grid to battery power. This topology is commonly seen in lower power inverters, specifically where the low power requirements are driving down the battery voltage available in the market. It is typical for the inverter and converter to be packaged together and this integration is seamless to the end user. This type of topology is utilized in the Dynapower Solar Plus Storage inverters as it is very flexible for combining two DC sources (battery and solar) by paralleling DC-DC converters to a common output of Vinv. 

An alternate topology to the addition of the DCDC boosting stage is to add a transformer on the output of the inverter as shown above  in Figure 4. In this case the output voltage (Va, Vb, and Vc) of the inverter are limited by the battery DC voltage and the transformer is utilized to boost Va, Vb, and Vc to the grid voltage (Va’, Vb’, and Vc’). In this example we are showing a delta to wye (inverter to grid) transformer so there is now a neutral on the grid side. The addition of the neutral on the grid side has substantial benefits, specifically in microgrid applications. 

If we consider a case where the battery voltage range is 500-1,000 VDC and the rating of the energy storage inverter is 1,000 kVA  then Va=Vb=Vc = 500VDC/1.5 = 330VAC. For 1,000 kVA at 330 VAC the inverter output currents ia = ib = ic = 1,750 AAC. The transformer would then be rated for a generating step up application with a 300V delta secondary and a 480V wye primary. The inclusion of the transformer adds cost, increases size, and decreases efficiency when compared to single stage inverters. If we compare the topology of Figure 4 to the topology of Figure 2 where the battery voltage meets the 740 V requirement and Va=Vb=Vc= 480VAC at 1,750AAC = 1,450 kVA we can see that for the same inverter current of 1,750 AAC we can achieve an inverter rating increase of 45% (450 kVA). The purpose of the exercise is to identify the derating and premium paid when using a low voltage battery in utility scale energy storage applications. The topology of Figure 4 is common in high power application using lower voltage batteries where the cost of the DC-DC converter is substantially more than the cost of the transformer. It should also be noted that the reliability of a transformer exceeds the reliability of a converter.