A coordinated control strategy for battery/supercapacitor hybrid energy storage system to eliminate unbalanced voltage in a standalone AC microgrid

Purpose – A standalone microgrid (MG) is able to use local renewable resources and reduce the loss in long distance transmission. But the single-phase device in a standalone MG can cause the voltage unbalance condition and additional power loss that reduces the cycle life of battery. This paper proposes an energy managementstrategyforthebattery/supercapacitor(SC)hybridenergystoragesystem(HESS)toimprovethetransientperformanceofbusvoltageunderunbalancedloadconditioninastandaloneACmicrogrid(MG). Design/methodology/approach – The SC has high power density and much more cycling times than battery and thus to be controlled to absorb the transient and unbalanced active power as well as the reactive power under unbalanced condition. Under the proposed energy management design, the battery only needs to generate balanced power to balance the steady state power demand. The energy management strategy for battery/SC HESS in a standalone AC MG is validated in simulation study using PSCAD/EMTDC. Findings – The results show that the energy management strategy of HESS maintains the bus voltage and eliminates the unbalance condition under single-phase load. In addition, with the SC to absorb the reactive power and unbalanced active power, the unnecessary power loss in battery is reduced with shown less accumulate depth of discharge and higher average efficiency. Originality/value – With this technology, the service life of the HESS can be extended and the total cost can be reduced.


Introduction
With the fast development of electrical technologies in recent decades, modern power systems normally use a lot of distributed resources (DRs), including both the renewable energy generations, such as wind turbine, photovoltaic (PV), geothermal and tidal, and micro resources, such as fuel cell and micro-turbine (Blaabjerg et al., 2004). Apart from that, the high-voltage direct current transmission and even the mixed usage of AC and DC networks also quick developed. These achievements make the development of power system use increasing more power electronics in power generation, transmission, distribution and enduser side (Blaabjerg et al., 2013). Thus, the power-electronics-enabled power systems (PEEPS) has high potential to become the main solution in future power systems.
The PEEPS technologies have been used in some special applications, such as electric aircrafts (Maldonado and Korba, 1999;Rosero et al., 2007;Wheeler and Bozhko, 2014), fuel cell ships (El-mezyani et al., 2012) and low-voltage (LV) islanded microgrid (MG) (Lasseter, 2002;Peas Lopes et al., 2006). However, as the power electronics based distributed resources are flexible and fast reaction in operation, it makes the PEEPS has less stiffness and inertia than the traditional power grids that use large generator as main sources (Diaz et al., 2009;Guerrero et al., 2011). In addition, because the power of renewable power generation is affected by the uncertainty of the weather and the randomness of end-user power consumption, it is difficult to ensure that power generation and power consumption can always be balanced. Therefore, when the renewable energy DR generates power less than the demand in the MG, alternative resources are needed as a backup power source.
The controllable micro-sources are chosen as the alternative resources for MG applications. The most common choices are micro-turbines and fuel cells. The microsources can response in seconds, which is faster than the traditional backup power sources, such as diesel engine. But it is still not fast enough for the low stiffness microgrid under disturbance. In addition, the micro-sources are transferring the chemical energy into electrical energy and its operation only support uni-directional power flow. Therefore, energy storage systems (ESS) are often required in PEEPS systems to ensure the balance of power generation and consumption. The most common ESSs include batteries, flywheels and supercapacitors (SC). Because of their fast response speed, bi-directional power flow characteristics and independency to weather conditions, they can be used independently to balance the power difference between the generation side and the demand side (Li et al., 2010;Tummuru et al., 2015;Zhou et al., 2011;Zhang et al., 2010).
As the two most commonly applied energy storage devices, the battery and SC have their own advantages and disadvantages. The battery has higher energy density but lower power density and less cycling life, normally up to several hundred cycles. The SC has higher power density and cycling life than battery but much lower energy density. Thus, the battery can be used as long time storing of energy while the SC can be used for short time high power supply and frequent energy recycling. A hybrid energy storage system (HESS) combining battery and SC then become a typical solution to provide both high energy capacities and high power density (Li and Joos, 2008a;Dogger et al., 2011). In recent years, this application is widely applied in electric vehicle (Camara et al., 2008(Camara et al., , 2010(Camara et al., , 2012Zhang et al., 2017;Golchoubian and Azad, 2017;Song et al., 2017Song et al., , 2018Yodwong et al., 2020), tramway (Han et al., 2017;Li et al., 2017), fuel cell ship (Chen et al., 2020), wind/PV power generation system (Li et al., 2010;Tummuru et al., 2015;Abdelkader et al., 2018;Jing et al., 2018), other renewable energy sources (Jayasinghe et al., 2011) and DC microgrid (Thounthong et al., 2007;Zhou et al., 2011;Jing et al., 2016;Enang and Johnson, 2020).
The LV DC network has some disadvantages of its low efficiency in long distance power transmission and requires large number of DC/AC converters to join distribution AC network. A typical AC microgrid normally with components of renewable power generation, energy storage, and power consumptions, as shown in Figure 1. In order to use LV DC energy storage devices in the AC microgrid, the coordinated control of the HESS is required to transfer their characteristics from DC to AC. As the power of renewable energy is unpredictable and changes rapidly with weather conditions, a large number of DRs used in MG will cause frequent charge and discharge cycles on ESS. As the battery usually has only hundreds of cycles of life, if the battery is only sources in ESS to balance the power of the DR and the load in JIMSE 1,1 the MG, the cumulative depth of discharge (DOD) will increase rapidly, resulting in battery aging and reduction in cycle life (Li and Joos, 2008a). To solve this, many researchers have proposed the methods of using batteries and SCs together in a hybrid energy storage system (HESS). Some literatures have proposed HESS control strategies in microgrids, such as using Fourier Transform to decompose the frequency information of load power to allocate load power to batteries and SCs (Mao et al., 2013); or use SC to compensate the harmonics and imbalance of battery current output to improve power quality (Zhu et al., 2014).
However, in actual application of MG, the power consumption and generation are not three-phase balanced. Most power consumption devices, such as washing machines, and power generation systems, such as PVs, are connecting directly to a single-phase AC power line. This causes the high risk of three-phase unbalance conditions in AC bus in Microgrid. The previous control strategies have not been designed to control the battery and SC individually with different purposes for optimizing their usage efficiency based on their features. The control strategies for battery and supercapacitor should be designed to coordinate their performances under the unbalance condition considering the low stiffness feature of a standalone AC microgrid (Ren et al., 2016). This paper proposes an energy management strategy for the battery/SC HESS in order to improve the transient response of the MG bus voltage and reduce the accumulate DOD of battery under unbalanced voltage conditions. The SC is controlled to absorb the transient and unbalanced active power as well as the reactive power, while the battery is controlled only generate balanced power for the steady state power demand. Thus, the power oscillation caused by unbalanced voltage condition will be compensated by SC and this unnecessary power loss can be eliminated from the battery to reduce the accumulate DOD and thus increase the battery usage period.
2. Model of energy storage system in microgrid 2.1 Battery model and state of charge In recent decades, with the popularity of Li-ion battery, the modelling of battery has been widely researched. There are two main categories of battery models, the electrochemical A coordinated control strategy for battery model, and the equivalent circuit model. The electrochemical model represents the behaviour of internal chemical reaction with modelling the movement of electrons. This type of model includes detailed information of battery and can be used to predict the performance, aging, thermal behaviour, and even failure rate of battery cells. However, these models are very complex and with extremely high orders, most are more than three hundred orders (Li et al., 2019). To simplify the model of battery, the equivalent circuit model uses the ideal battery, resistor, and capacitance to emulate the electrical behaviour of battery. Thus, the parameters of equivalent circuit model have no direct relationship with physical components inside the battery cells. And it is difficult to use the equivalent circuit model to simulate the aging and thermal behaviour of battery (Hu et al., 2011). This paper uses the electrical equivalent circuit model of battery that neglecting the temperature impact and battery aging performance. The terminal voltage of a battery is presented from the Thevenin battery model as shown in Figure 2a. The ideal battery model is used to present the open circuit voltage (OCV) V ocv . Battery OCV is corresponding to its state of charge (SOC), which is normally defined as the integration of charging current to presents the power absorbed by battery or produced from the battery (Bae et al., 2014). The power loss during battery operating is presented by the battery discharging current I flow through an internal resistance R i . It transient behaviour referring to the varying load is presented by a parallel RC circuit with a transient contact resistor R T and capacitor C T whose voltage is known as V T . The dynamics of battery equivalent circuit model can be presented as (Tremblay et al., 2007) Equivalent circuit of (a) battery, and (b) supercapacitor where E 0 is the nominal open circuit voltage of battery, Q 0 is the rated capacity of battery in Coulomb, the integration of current. This paper does not consider the battery aging, which decreases the battery fully charged capacity and voltage. Thus E 0 and Q 0 are set as constant value. K is the polarization voltage, A is the exponential amplitude and B is the related time constant (Tremblay et al., 2007). The parameters are varying referring to different battery types. The battery SOC and DOD are presented as (Dogger et al., 2011) SOC

Supercapacitor model
The SC has the feature of very fast charging and discharging speed that can reach its peak current immediately as a conventional capacitor and with a very large capacity than conventional capacitor (Abbey and Joos, 2007). In addition, similar with traditional capacitors, the SC has much higher self-discharging rate than battery. Even without external load, it can self-discharge to consume one-third its stored energy within a month of time. Thus, it is normally only used for short-term power exchange.
As the electrical behaviour of SC is similar with a capacitor, its main component is a fixed value ideal capacitor. Its self-discharge behaviour is presented by an equivalent parallel resistor (EPR). The SC can be model by a RC loop and an equivalent series resistance (ESR) as shown in Figure 2b, accurately without considering the effect by temperature and other operation conditions (Li and Joos, 2008b). The dynamic model of SC is presented as where V sc is the terminal voltage of SC, V c is its initial open circuit voltage depending on its state of charge, i C is the discharging current, C is the capacitance of SC, R EP and R ES are the equivalent parallel and series resistor of SC.

Model of voltage source converter (VSC)
The voltage source converters (VSCs) are used to inverse the DC voltage of ESS to synchronize the AC voltage of bus in the MG. The VSC will need to be controlled to regulate the current flowing into the inductors and transfer it between DC format and AC format. The normal modulation methods of VSC are pulse-width-modulation or space-vector modulation. Thus, it needs a filter loop to smooth the PWM current flowing from the ESS into a sinewave current. This paper uses the VSC model with a LC filter in d-q axis as (Mohamed and El-Saadany, 2008;Bidram et al., 2013) di A coordinated control strategy for battery where v od ; v oq ; i od ; i oq , are the output voltage and current in d-and q-axis, respectively. i Ld ; i Lq , are the currents flow into the inductor L s . v d and v q are the control output voltage reference in d-and q-axis. ω is the synchronous electrical angular speed in the AC voltage for Park transformation. R s , L s and C s are the resistance, inductance and capacitance of the VSC.

Power efficiency and voltage unbalance factor
The power loss on resistance of either battery or converter can reduce the total efficiency of the HESS system. The power consumption on the battery internal resistance will not only decrease the efficiency but also increase the ambient temperature that causes more risks in battery failure. The efficiency of the whole HESS system during charge and discharge cycles is defined as To combine the charging and discharging efficiency in one equation, the efficiency can be represented by the ratio between the min value and the max value of battery terminal power and cell power as η battery ¼ minðjP cell j; jP term jÞ maxðjP term j; jP cell jÞ Under unbalanced load condition, the voltage of AC bus is not balanced as a perfect threephase sinewave. The index of presenting the level of unbalance condition is using the voltage unbalance factor (VUF). The VUF is defined as the ratio of the negative-sequence component over the positive-sequence component of the target voltage waveform as (Savaghebi et al., 2013): where v þ bðRMSÞ and v À bðRMSÞ are the root-mean-square (RMS) value of positive-and negativesequences of bus voltage in AC grid.

Energy management strategy for HESS
The proposed energy management strategy with coordination control of battery and SC is designed to use the supercapacitor (SC) to generate transient unbalance power for eliminating the unbalanced voltage in the AC bus. The control strategy of HESS aims to improve the transient performance of MG bus voltage under unplanned sudden load, or generation, and reduce the battery power loss under unbalance condition.
The energy management strategy controls the battery to provide the three-phase balanced current to balance the steady-state power demand. And the SC is controlled to compensate the unbalanced power demand, provide the reactive power and transient power demand before the battery reaches the steady state. Then the oscillated power caused by unbalanced voltage is compensated by the SC with the mean energy of zero to avoid the battery to perform the frequent recharging that reduces its accumulate DOD and cycle life.

Controller for battery VSC
In the battery/SC HESS, the battery aims to generate three-phase balanced power under unbalanced voltage condition. Thus, the control algorithm of battery extracts the positive components of bus voltage and generate the current reference to synchronize with the positive-sequence bus voltage with power factor equal to one When the unbalanced condition happens, in order to decouple the positive and negative sequence components, the common method is to synchronize them in the same frequency but opposite direction. Rotate the positive-sequence frame under the frequency of ω and the negative-sequence frame under the frequency of −ω. Then at the rotating frame synchronizing to the positive-sequence, the negative-sequence components voltages and currents are presented with double fundamental frequency in opposite direction as −2ω.
The instantaneous active power is presented as where P presents the average power andp presents the oscillation power flowing into the grid; v odþ , v oqþ , v od− and v oq− are the positive-and negative-sequence components of bus voltage in d-q frame; i dþ , i qþ , i d− and i q− are the positive-and negative-sequence components of output current in d-q frame flowing from energy storage device to the grid. To eliminate the power oscillation, letp ¼ 0, and assume v oqþ ¼ 0 in steady state. Then the negative-sequence output current in d-axis can be presented as Then the reference d-axis current reference can be designed as A coordinated control strategy for battery Similarly, the instantaneous reactive power can be obtained as and the reference q-axis current can be designed as The reference current of the battery VSC is presented as The controller block diagram of energy management strategy for battery system is shown in Figure 3a.

Controller for VSC of supercapacitor
As SC is a high-power density device storing less energy capacity, it is not used for long-term (tens of minutes or hours) power supply. The functionality of SC in the HESS is to balance the transient high-power demand. Therefore, the proportional gain is used in the positivesequence voltage and current regulation for a rapid dynamic response. In addition, as its low energy capacity feature, it is necessary to avoid the over voltage and under voltage issue of the SC. This aims to maintain the performance of SC via recharging it to its nominal voltage level. Therefore, additional charging loop should be added on the energy management strategy. The current control is designed under the dual synchronous reference frames (SRF), which splits the positive-and negative-sequence components in different frames. The control algorithm is presented as where i * ds cþ and i * qscþ are the positive-sequence reference current in d-q frame with synchronizing frequency at ω; i * dsc− and i * qsc− are the negative-sequence reference current in d-q frame with synchronizing frequency at −ω; k m and k n are the proportional control gain for positive-and negative-sequence voltages; k dc is the charging gain of SC to maintain its voltage.
The current references generated above are feeding an inner current loop that uses PI controllers to regulate output current tracking its reference. Then transfer the control output back into stationary frame using inverse Part's transformation, the final control output can be presented as where v α , v β are the reference voltage in stationary frame, v dþ , v qþ , v d− and v q− are the reference voltage in positive-and negative-sequence d-q frame; T −1 dqþ and T −1 dq− are the inverse Park transformation. Finally, the space vector modulation (SVM) is used to produce switching signals to drive the VSC. The control block diagram of energy management strategy for SC system is shown in Figure 3b.

Simulation results
The designed energy management strategy is validated on a battery/SC hybrid system in a standalone MG, as the power stage shown in Figure 4, which is modelled and simulated in PSCAD/EMTDC software. To validate the energy management system that controls battery and SC in different objectives, the simulation cases are designed to test the coordinative operation performance of the HESS under unbalanced disturbance, including the singlephase impedance load and variable power generation. The parameters of the target LV AC A coordinated control strategy for battery MG are given in Table 1, and the control parameters of the energy management system are given in Table 2.

Flexible combination of energy and power capacity
Under the sudden single-phase load, the grid bus voltage drops and show obvious unbalance condition. Figure 5a shows the bus voltage waveform under single-phase resistive load disturbance. To show the indices of the bus voltage rather than the three-phase waveform, the   As the power capacity of the plug-and-play DRs and load depend on the customers in the MG, the ESS in the MG also need the ability of plug-and-play flexibility in combination of energy and power capacity. One superiority of the energy management method is the flexible design of the energy and power capacity. As an example of the HESS application, if the MG scheme is designed for an industry region, more power density is required for large number of Source Voltage control Current control Battery k p 5 0.5 k p 5 5 k i 5 50 k i 5 1,000 SC k m 5 20 k p 5 5 k p 5 50 k i 5 1,000 k dc 5 5  To compare between different energy and power capacities in flexible design of the HESS, different number of SCs are used to find out the performance improvement in bus voltage response. Thus, the pure battery ESS, and the HESSs include a battery and 1, 2, or 3 SCs are compared under the same system conditions and disturbances. The simulation compared under three-phase resistive load connected and disconnected, induction motor starting-up and single-phase load connected.
When sudden resistive load connected to the MG, the load absorbs power from grid. If the transient power generation is less than the power demand, the bus voltage will drop to reduce the power drain from grid to the load. Simulation results of sudden resistive load connected and disconnected with different number of SCs are shown in Figures 6a and c. Without the support of SC, the battery based ESS cannot provide power fast enough to follow the power demand. If using SC, the bus voltage drop is only one-third the drop of using pure battery ESS. Figure 6b shows the bus voltage response during inductive load connected. The performance improvement of using SC is faster in transient power generation. Figure 6d shows the bus voltage response under single-phase load. Because of the battery output power control is designed without unbalance voltage eliminate ability. If only uses the battery in the ESS, the bus voltage has oscillation under single-phase load has oscillation. The results show that with increasing number of SC, the bus voltage has increasing faster transient response in all load conditions. Under the sudden load disturbance, the three-phase bus voltage will have transient unbalance condition referring to its phase angle. The VUF under previous disturbance is shown in Figure 7. As the result in Figures 7a-c Figure 7d. The result also indicates that the greater number of SC in use, the quicker the VUF at bus voltage will be eliminated. That validates the plug-and-play performance of the HESS system using the proposed energy management strategy. With this design, the usage of SC can be flexible referring to different objectives in the AC MG application.

Case 1: single-phase impedance load
The negative-sequence voltage components in bus voltage caused by unbalanced disturbance can cause more power loss in the ESS. The unwanted power loss leads more accumulated DOD in battery and that will reduce the battery cycle life. The designed energy management strategy aims to use SC absorbing the unbalance power for reducing the unnecessary accumulated DOD of battery and enhance the average efficiency. Comparing the HESS with energy management strategy with battery only ESS, the proposed method has obvious improvement in bus voltage maintenance. The comparison of bus voltage waveform between the battery ESS and hybrid ESS are shown in Figures 8a  and b. When the single-phase impedance load connects to the bus voltage of AC MG, the bus voltage drops immediately due to the sudden load. In the HESS with proposed energy management strategy, the SC responds first to produce transient power to compensate the unbalanced power demand. Then the battery slowly increases the output balanced power to tracking the steady state power demand and charging the SC back to its nominal voltage level. The results show the real voltage in three-phase sinewave directly. It can clearly show the difference but not easy to compare them to find the numerical improvements.  Figures 9a and b. From which it is easy to find that the SC produced the full reactive power while the battery produces the active power in steady state. This is to take the advantage of SC with its much more cycle life to avoid the battery frequent discharging cycles under unbalance voltage condition. To compare the current outputs, Figures 9c and d show the three-phase output current generated from the battery and SC separately using the energy management strategy. The current generated from battery is three-phase balanced and slowly increased to its steady state. And that from SC is obviously unbalanced to compensate the unbalanced power flow in the bus caused by single-phase load.
When single-phase load connected to the grid, the power is provided by the ESS to balance the power demand. In the battery ESS, the battery is the only sources to provide the unbalanced power to the single-phase load. This causes the unbalanced voltage on grid and then power oscillation in the battery. Figure 8e shows the battery accumulated DOD that indicates the usage of battery and Figure 8f shows the battery efficiency under the singlephase load. One can find that the battery efficiency has obvious oscillation cause by unbalanced condition. But in the hybrid ESS with proposed control strategy, the SC is designed to support the unbalanced power and charging itself from battery. Thus, in the hybrid ESS, it can be known that the SC is used to absorb the three-phase balanced power from battery and support the single-phase power to load as an energy buffer. From the results, the battery accumulated DOD is reduced and efficiency is smoothed due to the help of SC to filter the unbalanced power demand to balanced power demand that reduced the battery internal power loss. To compare the improvement numerically, the indices of control performance of battery ESS and hybrid ESS are shown in Table 3 and the indices are also shown in the bar chart in Figure 12.
It shows that the hybrid ESS using the proposed strategy has faster transient response in bus voltage that leads to less bus voltage drop and integrated absolute error (IAE). From the result, the peak voltage drops on bus maintained by battery ESS is 0.26, while that of hybrid ESS is 0.04, 85% less than that of battery ESS. The voltage IAE of bus maintained by battery ESS is 0.13 while that of hybrid ESS is 0.036. The improve on voltage IAE is about 72%, smaller than that of peak voltage drop. This verifies the effectiveness of using

Case 2: single-phase power generation
The renewable power generation is always unpredictable and changes rapidly referring to weather conditions. It cannot always match the power demand from end-user. Thus, the power difference will be balanced by the ESS. This will cause frequent charge and discharge cycles. If the renewable power generation is small and low power, it normally connected directly to the single-phase of bus voltage. In this case, the performance of battery ESS and hybrid ESS has been compared in Figure 10. Due to the single-phase power generation is time-varying, the bus voltage of battery ESS has obviously worse power quality and unbalanced voltage than the hybrid ESS, as shown in Figures 10a and b. This can also be found in Figures 10c and d that the VUF of hybrid ESS is much lower than that of battery ESS. The indices comparison in Table 3 and Figure 12 show that peak voltage drop of using hybrid ESS is about 84% lower than that of the battery ESS while the voltage IAE reduction is about 70% less. The VUF reduction of bus voltage under single-phase power generation is about 79% in average and 95% for the peak. The comparison of active power and reactive power generation in hybrid ESS is shown in Figures 11a and b. The battery is controlled to generate averaged and slowly changed active power and zero reactive power in steady state. The output and absorbed current of battery is three-phase balanced while that of SC is unbalanced to support the single-phase load or generation as shown in Figures 11c and d. The objective of using SC in a HESS is to reduce the usage of battery to increase its lifetime. This is verified in Figure 10e that the battery accumulated DOD of hybrid ESS is much lower than that of battery ESS. In the numerical comparison, the reduction of  accumulate DOD is about 52% less of using hybrid ESS. The battery efficiency is shown in Figure 10f and shows that it has about 0.2% improvement in average (see Figure 12).

Conclusion
This paper proposed an energy management strategy for a battery and supercapacitor (SC) hybrid energy storage system (HESS) in order to improve the transient performance of bus voltage under unbalanced load condition in a standalone AC microgrid (MG) and reduce the usage of battery. The energy management strategy controlled the battery and SC with coordinated design to balance the power demand and generation and maintain the bus voltage. In AC MG, the single-phase load and generation can easily cause the unbalance voltage issue. It not only reduced the power quality but also caused unnecessary power loss in battery to reduce its cycle life. With the proposed energy management strategy, the SC is used to absorb the reactive power and transient and unbalanced active power, while the battery is used to generate balanced power in three phase to the grid to balance the steady state power demand. The energy management strategy of battery/supercapacitor HESS in a standalone AC MG is validated in simulation using PSCAD/EMTDC. The results show that performance of bus voltage under unbalanced disturbance has been improved with faster transient response and less voltage drop. The energy management strategy is suitable for the plugand-play requirement in standalone MG application with flexible number of SC referring to different operation requirements. The reduction of battery loss has also been validated as lower accumulate DOD and higher average efficiency.