Introduction
The modular multilevel battery (M2B) is a novel approach to integrating batteries into the power grid. Conventional stationary Battery Energy Storage Systems (BESS) consist of a central inverter and hard-wired battery modules that form the high-voltage battery rack. In contrast to this topology, the M2B has a different structure: The battery modules are each equipped separately with their own power electronics. This approach offers a number of advantages, including higher efficiency compared to conventional inverters.
Contents
Based on a low-level simulation model, a very high efficiency was achieved for the entire power range with a maximum of 99.4%. This is mainly due to the low switching frequency and the use of low-impedance transistors. Even at 10% of the rated power, the efficiency is 97.5%, compared to an efficiency of 93% for a reference system with conventional architecture.
Based on the low-level efficiency curve, the analysis for a BESS that maintains primary control power showed that the increased efficiency of the M2B technology increases the cash flow by 15% compared to a reference system. Due to the higher efficiency, especially in the partial load range, less energy is lost during charging and discharging. As a result, less energy needs to be purchased on the Intraday Market (IDM) to manage the state of charge of the energy storage system.
Basics principles
Understanding the approach of our efficiency demonstration requires an understanding of how battery energy storage systems (BESS) can act as a primary reserve (Frequency Containment Reserve (FCR)), their typical topology, a rough understanding of the power electronics in BESS and finally the functionality of M2B & its expected benefits.
Standard topology of stationary battery energy storage systems
Figure 1 shows the standard topology of a stationary BESS. In addition to auxiliary components for thermal management, system control and monitoring, they consist of two main hardware parts: the battery pack and the power electronics unit.
The battery pack typically consists of several modules made up of different cells. The modules can then be connected in a series-parallel configuration to achieve the desired capacity and output voltage depending on the project or customer specification.
The power electronics serve as the connection point between the battery and the grid and convert the DC battery voltage bidirectionally into the AC grid voltage. This can be realised in several ways. Either each battery pack – if there is more than one – is equipped with its own inverter, or several battery packs are connected to a DC bus with a common inverter.
Depending on the voltage levels on both sides, the battery and the power electronics can be connected to the grid with or without a transformer. If the output voltage and the grid voltage match, the transformer can be omitted [5].
Standard technologies for power electronics
The standard technologies are based on the two-stage voltage source inverter, which is the standard topology for stationary energy storage systems [1, 2, 3, 5]. A three-phase voltage source inverter, shown in Figure 2, consists of three half-bridges. Within a half-bridge, two switches are connected in series with a terminal in between.
AC/DC conversion is achieved by switching on and off the six fully controlled semiconductors, typically insulated-gate bipolar transistors (IGBTs). To avoid short circuits, the switches of a half-bridge must never have the same state at the same time.
As the output voltage can only assume discrete states, a continuous sinusoidal curve cannot be generated. Therefore, the average value of the output voltage must be set over a switching cycle so that it corresponds to the continuous, sinusoidal setpoint [12]. This is the task of modulation methods such as pulse width modulation (PWM): By varying the time of the switch-on and switch-off state, the so-called duty cycle, of the transistor during a switching cycle, the average voltage can be controlled and a sine wave generated.
Although this topology is simple in design, it has some disadvantages [13]: The output voltage for one phase is limited to DC voltage. For applications that require higher AC output voltages, an additional DC/DC step-up converter must be installed, which increases costs and reduces efficiency.
Another disadvantage is the relatively high harmonics of the PWM voltage. A large output filter is required to smooth the generated PWM voltage in order to fulfil the standards for electromagnetic interference (EMI) and grid standards. This component entails additional power loss and costs [3, 13].
In addition, the failure of a battery module when used as a stand-alone converter for an energy storage system requires an external bypass or replacement, otherwise the entire system becomes unusable.
Multilevel technologies
To address the above problems, multilevel technologies have been introduced as they can synthesise an AC voltage from multiple levels of DC voltages [1]. This is achieved by connecting identical sub-modules and an inductor in series to form a so-called inverter leg, as shown in Figure 3. The inductance enables voltage differences between the individual legs and limits the current increase when the power electronic blocks are connected [3].
The maximum output voltage and the number of possible voltage stages depend on the number of submodules per inverter leg.
With each additional stage, the output voltage becomes more similar to the sinusoidal grid voltage and the harmonic content decreases [3, 14].
If the number of stages is high enough, the harmonic content is so low that no additional filter is required [15].
There are various multilevel converter topologies that differ in the way the inverter legs are connected. One widely used implementation is the cascaded H-bridge converter (CHB), which serves as the basis for the M2B [3].
In addition to the low harmonic content of the output voltage, other advantages of the CHB are the low switching voltage and frequency, both of which contribute to lower switching losses [16]. Its modular design enables greater reliability, as failed modules or cells can be easily bridged. According to [17], the CHB is more practical than other multilevel topologies in terms of cost and performance.
The modular multilevel battery
The M2B is a novel approach to integrating batteries into AC systems and was developed at the University of the German Armed Forces in Munich [3], while STABL (formerly m-Bee) is focusing on the commercialisation of the technology. Several publications describe its use for stationary and automotive applications [4, 18-20].
In contrast to the conventional system topology of stationary BESS, the M2B has neither a central inverter nor a centralised battery management system. In addition, the battery pack is divided into modules. Each module consists of one or more battery cells and a power electronics module.
The main task of the M2B is to convert the DC battery voltages into AC bidirectionally.
In the simplest case, the sinusoidal waveform is generated by switching all modules on and off in sequence.
According to [3], the batteries can have different voltages, SOH (State of Health) values and chemicals; the only change that needs to be made if a battery needs to be replaced is a software update of the master controller. However, different module voltages limit the use of parallel connections of the batteries.
Advantages of the M2B approach:
According to [3, 18], the M2B has several advantages over a conventional stationary BESS, most of which can be attributed to its higher efficiency. This is achieved by eliminating four loss mechanisms:
- Losses due to SOC equalisation
- switching losses
- conduction losses
- Static losses
Determining the efficiency
Estimates of the profitability and competitiveness of the M2B can only be made if the efficiency of the M2B is known. To determine the efficiency, an existing simulation model created by Singer [3] was used. It can represent an M2B system with any number of modules at a semiconductor detail level and was modified to calculate the efficiency at each operating point.
The original model was developed in Matlab/Simulink and includes the physical system of the BESS and its connection to the grid. A controller that regulates the current and SOC is also implemented.
A detailed description of the formulas and modelling used is not provided here.
The corresponding values for the individual operating points are shown in Figure 6.
For a power proportion (p) > 0.2, the efficiencies assume high values greater than 99 %. For both cases, the maximum is at p ~ 0.4 (99.39 % and 99.41 % for charging and discharging respectively).
At p > 0.45, the efficiencies begin to fall slightly due to the greater influence of high currents. In partial load operation, at p < 0.2, the efficiencies drop rapidly.
In low power ranges, the system consumption is the dominant factor for the losses, as it is independent of the output power and remains constant at all power ratios.
Due to the low resistance (R) of the MOSFETs, the conduction losses only account for the majority of the losses for p > 0.65. As the line and PCB resistance losses increase with the square of the current, they together account for over 80 % of the losses at maximum power.
The switching losses do not play a significant role. The maximum absolute value for them is 0.195 W at a power ratio of p = 1.2. The use of MOSFETs with shorter rise and fall times than IGBTs is one reason for this effect. The low switching frequency also plays a role. Due to the higher number of stages and the lack of a superimposed PWM, the maximum switching frequency in the simulation was only 147Hz. However, the main reason for the low switching losses is the lower switching voltage due to the multi-level approach. As the switching losses correlate with the frequency and the squared voltage, the losses are minimised by the factor m² (m = number of modules). With four modules, the switching losses would therefore be reduced to 1/16 of a central inverter, simply by reducing the voltage.
In order to compare the efficiency with modern inverters, we have simulated an M2B system with eight modules that is capable of generating a 230V output voltage. An inverter curve modelled by Notton et al [22], which is characterised by “low standby and load-dependent losses”, was chosen as a reference. Figure 7 shows both efficiencies as a function of the power ratio.
The M2B has a higher efficiency at every operating point, with the difference being greatest in the partial load range. At p = 0.05, the reference inverter achieves an efficiency of 87.8 %, while the M2B efficiency is 95.1 %. This corresponds to a relative difference of over 8 %. The maxima are 99.41 % for the M2B and 96.94 % for the reference. At high power ratios, the low resistance of the MOSFETs is responsible for the slow drop in efficiency of the M2B compared to the reference.
Case study: Frequency limiting reserve
Using the efficiency curve generated with the low-level simulation model, we evaluate the long-term performance of an M2B system based on a case study for a stationary BESS used for primary control power.
The open-source software tool SimSES [23, 24] was used for this purpose.
Technical Results
The BESS achieves a round-trip efficiency of 90.71 % compared to 79.63 % for the reference system. The efficiency of the inverter is higher than that of the reference both when charging and discharging M2B. Looking at the power distribution of the delivered AC power for both systems in Figure 8, it is clear that FCR does not exceed 30% of the nominal power more than 1% of the time.
Together with the relative difference in conversion efficiency of the M2B and the reference, we can explain the high difference in round-trip efficiency. Almost 90 % of the AC power delivered is in the range of 0 to 10 % of the rated power, with the M2B efficiency being on average 13.5 % higher than that of the reference.
Economic Results
Figure 9 shows all cash flows for the simulated year. The expenses of M2B through IDM transactions are 53.94% lower than those of the reference. The income from FCR is the same for both systems, as they can provide their required service all the time. The cost savings from the IDM result in an annual cash flow of EUR 99,669 for the M2B, which is 15.8% higher than that of the reference.
The reason for the cost savings is the higher efficiency of the M2B system, whereby differences in partial load operation in particular come to bear. The higher efficiency means that less energy is lost during charging and discharging. Consequently, less energy has to be purchased via IDM transactions in order to remain within the valid operating range. This leads to lower costs and thus to a higher annual cash flow.
We discount the cash flow for 20 years assuming the same revenue each year and use the NPV and IRR methods to determine the time in which the project becomes profitable.
M2B reaches break-even after the 13th year, while the NPV of the reference becomes positive three years later, after 16 years of operation. After 20 years, the reference achieves an NPV of EUR 461,606. The NPV of the M2B is EUR 797,957, which represents a relative increase of 72.9 %. The IRR shows a similar picture and becomes positive after 13 and 15 years for M2B and the reference respectively. The IRR for the reference becomes positive one year earlier than the NPV. This means that for the period when the NPV is negative and the IRR is positive, the capital costs are higher than the IRR and the project should be rejected for this expected period. After 20 years of operation, the IRR is positive for both projects: M2B is expected to generate a return of 5.8% compared to the 4% of the reference project.
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