Development of Hybrid Energy Storage System Testbed with Instantaneous Discharge Controller for Shunt Active Filter Application

Abstract

The high penetration of renewable energy sources has necessitated the use of more energy-storage devices in Smartgrids. The proposed work addresses the development and implementation of an Instantaneous Discharge Controller (IDC) for a hybrid energy storage system. The discharge control algorithm manages the discharge of the battery and supercapacitor and protects the battery from transient currents. Hybrid energy storage systems (HESSs) are well known for providing ideal attributes such as high-power density and high-energy density for many application areas, including electric vehicles and renewable energy-supported microgrids. However, the application of HESSs for supporting shunt active filters and protecting low power density storage systems from fast variations in load has not been proposed yet. In this context, a hybrid energy storage system (HESS) is proposed here to eliminate harmonics and to support the grid by providing real and reactive power supervened by varying load conditions. This paper proffers an innovative controller for a shunt active filter unified with an HESS to effectively manage storage devices for a microgrid connected to a grid.

1. Introduction

Power systems are commonly outfitted with power electronic devices to improve the performance characteristics of electric power grids. Power electronic devices enable the manipulation of grid power to conform to varying load demands. Power quality is compromised by the natural non-linearity of power systems affected by power electronic components. Active Filters (AFs) are the most coherent schemes that eliminate harmonics as well as support the power grid by furnishing reactive power. One of several controller algorithms can generate the requisite reference compensating currents for active filters.

1.1. Background and Motivation

The large-scale integration of renewable energy (RE) sources into the grid is called for to save the earth from anthropogenic environmental pollution and compliance with increasing demands for electricity. These RE sources require power conversion and storage devices for smoothening and maintaining regulated output. The integration of renewables and power electronic devices engenders harmonics that precipitate system unreliability and inordinate perturbations in grid power quality. Divergent load conditions are one of the major challenges for grid operators; active filters can furnish the real and reactive power support needed by the grid, as well as improving the quality of power, with the help of a hybrid energy storage system (HESS) coupled to them.

The hybridization of two storage devices with high energy density and high power density can meet transient and steady variations in loads. The efficiency and life of storage units are increased by the hybridization of the storage units. The management of an HESS to meet the load demands and its charge and discharge control are the main constraints in managing an HESS. There are several control strategies and algorithms to manage the charging and discharging of each of the storage units in the system.

In this paper, a novel discharge controller for an HESS is developed and implemented for power quality improvement and reactive power compensation in the grid. Harmonic and reactive power compensation is achieved with the IcosΦ controller by generating reference currents for compensation. A new discharge controller is proposed to generate reference signals for the battery and supercapacitor.

1.2. Hybrid Energy Storage Systems for Efficient Energy Management in Power Systems

The integration of renewable energy sources with power grids leads to instability in the power sector due to its intermittent nature. In order to support the grid and microgrids, energy storage devices are deployed on a large scale. Energy storage technologies can be classified as chemical, electrochemical, mechanical, and electrical. As the technical parameters and operating characteristics are unique to the selected storage technology, hybrid storage systems with supplementary operating characteristics can be utilized for grid-level applications [1]. HESSs minimize the cost of individual storage units. With hybridization, a storage unit characterized by high power density can withstand transients and load fluctuations, unlike storage units set apart by high energy density that are expected to manage only the required average power. The total system efficiency increases [2], and dynamic losses are reduced with the hybridization of storage units.

HESSs have a wide range of applications—autonomous renewable energy supply, assistance for renewable sources in grid-connected modes of operation, reduced power quality issues, etc. Frequency fluctuation is a nasty limitation in stand-alone microgrids with renewable sources [3]. As a recourse, energy storage is widely used to regulate frequency fluctuations. Batteries and supercapacitors (SC) are commonly used for hybridization. SCs are electrochemical energy storage devices with unique features of high specific power, long life cycle, and environmentally friendly nature built-in. SCs, which retain the charge for up to 40 min, can be re-charged within 20 s. Diverse batteries with different energy densities were used with SCs to enhance the life of batteries [4,5]. With the hybridization of supercapacitors and batteries, supercapacitors could smoothen current drawn from batteries, especially for electric vehicle applications [6].

HESSs can be outfitted with diversified sets of storage devices—battery, hydrogen storage, SC, and superheated water—so that power flow can be regulated using a suitable energy management strategy [7]. For solar PV modules, hybridizing battery and hydrogen fuel cell subsystems could reduce costs and improve performance. Electronic power converters, specific to each energy storage device, were used to derive optimal operation (charge–discharge characteristics) so that controllability is high [8], Ref. [9] employed a 4-Leg 3L-NPC topology as an HESS/RES interface because of its low THD and management capability for unbalanced loads.

The configurations of HESSs vary with the adopted control strategy. Storage devices are connected to a common AC or DC bus. The control of the DC link voltage is essential for an HESS system integrated with active and passive topologies. In passive topology, the energy storage devices are connected directly, whereas, in active topology, storage devices are connected via power converters.

In active topology, all the storage devices can be controlled actively despite high complexity. Multilevel converters are also used as hybrid energy storage power converters. In Reference [10], an energy management scheme was proposed to minimize the cost of operation of HESSs, using battery and SC. It has been stated that the HESS combination of battery and SC is less costly compared to the combination of different batteries. A DFT-based coordinated methodology was used in [11] to control power sharing between batteries and pumped hydro storage to mitigate wind energy fluctuations. An efficient energy management system developed for electric vehicles, with an HESS support for different load conditions, is illustrated in [12]. Although there are different HESS combinations for applications like power smoothening, peak shaving, and load leveling, an HESS has not been devised for power quality improvement as well as real and reactive power support for the grid. This paper proposes a novel control scheme for an HESS, covering power quality improvement and grid support.

1.3. Current Harmonics Elimination and Power Factor Correction in the Grid Using Active Filters

Active filters are used in place of passive filters to inject current or voltage, depending on how they are connected in the network. Passive filters are used to eliminate harmonics, and capacitors help to improve the power factor. Restrained by factors of resonance and fixed compensation, the necessity for dynamic and adjustable power quality improvement techniques has arisen. Active filters inject current and voltage signals equal in magnitude but in phase opposition to the harmonics in the system. AFs marshal numerous power electronics components to conduct the filtering on the AC as well as DC sides of the network. In [13] it is mentioned that Active filters eliminate harmonics and supply the reactive power needed by the network. The author of [14] proposed a control strategy for energy management in a renewable energy sources integrated grid. DSTATCOM is also used for power injection, but the system’s complexity is greater.

Based on the control strategy and configuration, AFs’ characteristic functions include the elimination of voltage harmonics, terminal voltage regulation, and voltage flicker suppression. Developing and implementing control strategies are especially important for properly working AFs. The required voltage and current signals are sensed, and compensating commands are derived in terms of voltage or current. Compensating signal extraction is the essence of the AF’s performance in steady state and transient operations. Compensating commands are generated in the frequency domain and current domain correction techniques. Fourier transforms are used in the frequency domain. In the time domain, compensating signals are generated from distorted and harmonic polluted voltage or current signals. Based on topology, AFs are classified into shunt, series, and hybrid active filters. Shunt active filters (SAFs) are used for reactive power compensation and harmonics elimination [15]. In [16], a unified control algorithm was proposed to compensate for harmonic current. The authors of [17] proposed an SAF with a modified IcosΦ controller to control the real power supplied by a power grid. IRPT was modified in [18] to obtain a unity power factor. The performance of battery energy storage systems integrated with solar PV and a unified power quality conditioner (UPQC) is investigated in [19]. Battery energy storage systems improve voltage support capability and reduce the complexity of the DC link voltage regulation algorithm. A combination of shunt active power filters and static var compensators is used to improve the power quality of islanded microgrids [20]. But grid integration of microgrids is not performed. In Reference [21]], a distribution system with a shunt active filter and battery energy storage is used to eliminate harmonic current and low-frequency oscillating power inserted by the load generation group.

Many research works demonstrate that the battery–SC hybrid energy storage system can improve battery life by reducing charge–discharge stresses. Most of the mentioned papers use general aspects of energy management systems for HESS. However, this paper proposes a novel instantaneous discharge controller for energy management in HESS. In comparison to already reported work on shunt active filters and HESS applications, this paper proposes an HESS-supported shunt active filter which can provide real and reactive power support to the grid. An HESS-supported shunt active filter is a novel methodology that has unique features of power quality improvement as well as protecting batteries from transients. The proposed energy management scheme with a novel instantaneous discharge controller is discussed in detail.

2. Proposed Hybrid Energy Storage System–Active Filter Topology

The proposed scheme uses combinational storage devices, i.e., battery and supercapacitor to support the grid by delivering the transient and reactive components of a grid-connected load. The control strategies for battery energy storage are described in [22]. Ref. [23] proposes offer an instantaneous real power calculation method for reference current generation for producing switching pulses for shunt active filters. A novel instantaneous discharge-control scheme was developed to differentiate the real and reactive components of load current so that the steady part of the load would be supported by the storage unit having high energy density and the transient part of the load would be supported by the storage unit having high power density. The discharge of the storage units is controlled by generating reference currents for the transient and steady parts of the grid load. The combinational storage device powered by renewable sources would support the filter to eliminate harmonics. Hence, the integration of renewable sources can actively support the grid without compromising the power quality of the system.

In the system shown in Figure 1, a three-phase AC source was connected to a nonlinear load, a three-phase rectifier with R load. This rectifier causes harmonics in the system; these harmonics can be removed by the connection of an SAF in parallel with the grid. An SAF is a VSC that is switched by controller 1, and phase voltage and load current are the inputs. The algorithm used is IcosΦ. The outputs of the controller are the six switching pulses for the VSC. The DC link of the VSC was connected to the battery and SC through two boost converters connected in parallel. The switching pulses were generated using an instantaneous discharge controller. The instantaneous discharge controller has filter currents and filter voltages as inputs. It transforms these voltages and currents into α, β components and generates the reference currents necessary to generate switching pulses for the two boost converters.

2.1. Controller for Active Filter in HESS–AF Topology

In the proposed system, an SAF was used to reduce power quality issues and to provide the reactive power required by the load. With the IcosΦ algorithm, filter current references are generated. Figure 2 shows a block diagram of the IcosΦ controller. The IcosΦ controller facilitates retrieval of the active portion of the fundamental load current, which is drawn from the source. The mains current is the product of the magnitude of IcosΦ and a unit amplitude sine wave, in phase with source voltage. The IcosΦ component is extracted at the negative-direction zero-crossing of the phase voltage, which a zero-crossing detector detects. The difference between the desired source current and load current yields the filter reference current.

2.2. Generation of Reference Currents for Battery and SC Using Instantaneous Discharge Controller

A unique controller was developed to generate reference currents for the battery and SC. The instantaneous harmonic currents are decomposed in to two parts—one depending on the real power, and the other depending on imaginary power. The reference currents for the discharge controllers for the battery and SC were generated by the requisite into this: filter voltage inputs (Vf_a, Vf_b, Vf_c) and filter currents (if_a, if_b and if_c). These inputs were transformed to α and β components using Park’s transformation. Equations (1) and (2) represent the results for Park’s transformation of three-phase voltages.

$$V_{f\propto }=0.816V_a-0.408V_b-0.408V_c$$

(1)

$$V_{f\beta }=0.706V_b-0.706V_c$$

(2)

Similarly, Equations (3) and (4) represent Park’s transformation of three-phase currents.

$$I{f}_{\alpha }=0.816I_a-0.408I_b-0.408I_c$$

(3)

$$I{f}_{\beta }=0.706I_b-0.706I_c$$

(4)

where $V_{f\propto }$ is the alpha component of the filter voltage

$V_{f\beta }$ is the beta component of the filter voltage

$I{f}_{\alpha }$ is the alpha component of the filter current

and $I{f}_{\beta }$ is the beta component of the filter current.

The active and reactive powers are given by Equation (5)

$$\left[\begin{array}{c}{P}_f\\ Q_f\end{array}\right]=\left[\begin{array}{cc}{V}_{f\propto }& V_{f\beta }\\ -V_{f\beta }& V_{f\propto }\end{array}\right]\left[\begin{array}{c}I{f}_{\alpha }\\ I{f}_{\beta }\end{array}\right]$$

(5)

$P_f$ and $Q_f$ can be split into constant and varying components, as shown in Equations (6) and (7)

$$P_f=P_{dc}+P_{ac}$$

(6)

$$Q_f=Q_{dc}+Q_{ac}$$

(7)

where $P_{dc}$ and $Q_{dc}$ are the direct components of real and reactive power; similarly,$P_{ac}$ and $Q_{ac}$ are the varying components of real and reactive power.

The battery provides the steady-state part of the current, and SC should provide the transient and harmonic parts. The steady-state part is derived as in Equation (8) and the transient and harmonics components are extracted as in Equation (9).

$$\left[\begin{array}{c}{i}_{bat\propto }\\ i_{bat\beta }\end{array}\right]=\left[\begin{array}{cc}V{f}_{\propto }& V{f}_{\beta }\\ -V{f}_{\beta }& V{f}_{\propto }\end{array}\right]-1\left[\begin{array}{c}P{f}_{dc}\\ q{f}_{dc}\end{array}\right]$$

(8)

Similarly, for supercapacitor,

$$\left[\begin{array}{c}{i}_{sc\propto }\\ i_{sc\beta }\end{array}\right]=\left[\begin{array}{cc}V{f}_{\propto }& V{f}_{\beta }\\ -V{f}_{\beta }& V{f}_{\propto }\end{array}\right]-1\left[\begin{array}{c}P{f}_{ac}\\ q{f}_{ac}\end{array}\right]$$

(9)

The magnitude of the battery reference current and SC reference current is obtained using Equations (10) and (11). These references are compared with the actual battery and SC currents for generating the switching pulses for the boost converters for the battery and SC.

$${|i}_{bat}|=\sqrt{{\left|i_{bat\propto }\right|}^2+|i_{bat\beta }}|2$$

(10)

$${|i}_{sc}|=\sqrt{{\left|i_{sc\propto }\right|}^2+|i_{sc\beta }}|2$$

(11)

2.3. Simulation of Proposed HESS–AF Supported Grid

A simulation of the proposed HESS–AF-supported grid system was simulated in Simulink. The battery and supercapacitor were hybridized, and their output was boosted and connected to the DC link.

The boost converter used for the battery was rated at 5400 W and 6100 W for the SC, with a constant output voltage of 650 V.

Table 1. Boost converter parameters.

Boost Parameters	Rating
Capacitor	0.3 mF
Inductor	0.55 mH

presents boost converter parameters;

Table 2. Boost converter parameters for the battery.

Parameters	Rating
Output voltage Vo	650 V
Input voltage Vd	325 V
Input current Id	16 A
Output current Io	8 A
Switching frequency fs	10 kHz
ΔV	1 V

gives boost converter parameters for the battery, and

Table 3. Boost converter parameters for SC.

Parameters	Rating
Output voltage Vo	650 V
Input voltage Vd	365 V
Input current Id	17 A
Output current Io	9 A
Switching frequency fs	10 kHz
ΔV	1 V

gives boost converter parameters for the SC.

Table 4. Battery and SC parameters.

Module	Rating
Battery	325 V, 15 Ah
SC	360 V, 500 F

shows rating of the battery and SC used. The DC side capacitor serves two main purposes. It maintains the DC voltage with small ripples in steady state and serves as an energy storage element. The capacitor serves to supply real make-up power—covering any shortages between the load and source during the transient period.

During steady-state operations, the source supplies real power drawn by the load and power to compensate for active filter losses. The DC link voltage can be maintained at a reference value. The change in load injects disturbances into the real power balance between the electrical mains and the load; the DC link capacitor compensates for the differences in the real power.

Selection of Filter Inductor

As the DC link capacitor is employed as an energy source, the output of the inverter is the voltage that must be filtered by inductance or a higher-order filter. The inductor attenuates ripples of the inverter current, triggered by the switching actions of the inverter. The design of the filter inductor is based on the principles of harmonic current reduction. The design equation is given by Equation (12).

$$L=\frac{\mathrm{V}\mathrm{s}}{2\sqrt{6}fs∆Ifppmax}$$

(12)

The ratings used for the simulation are given in Table 3. The system’s RMS voltage was V = 230 V, and the frequency was 50 Hz. The SAF switching pulses were obtained via an IcosΦ controller. Figure 3 shows waveforms of the source voltage and current (before connecting the SAF) with a rectifier load. The source currents were imbued with harmonics, and the rectifier load was changed from 1800 W to 3600 W at 0.4 s and back to 1800 W at 0.6 s.

Table 5. Other system parameters.

System Parameters	Ratings
RMS voltage	230 V AC
Link voltage	650 V DC
DC link capacitance	5 mF
Filter	L = 1.5 mH, f = 10 kHz

shows the system parameters adopted for simulation. The system’s RMS voltage was V = 230 V, and the frequency was 50 Hz. The SAF switching pulses were obtained via an IcosΦ controller.

Figure 4 shows the source voltage and current when SAF was connected to the grid. The source current became sinusoidal and THD was reduced from 30 to 5%.

Figure 5 shows the actual filter current generated by the SAF using the IcosΦ algorithm. The DC link voltage was maintained at a constant voltage of 650 V.

Figure 6 shows the SC alpha beta component generated by the instantaneous discharge controller. SC must provide the transient and harmonic part of the load. Figure 7 shows the battery current alpha beta component generated by controller 2. The battery must provide only the steady-state component.

Figure 8 shows the battery and SC reference generated by controller 2. At 0.4 and 0.6 s, due to load change, there were transients that needed to be compensated for by SC.

The reference current generated by the instantaneous discharge controller for the battery had a transient part during load changes, but it was made steady by limiting the current drawn from the battery to a specified value.

The harmonics and reactive components in the load current were filtered out with SAF, and it was differentiated into steady and transient parts. The battery provides the steady component, and the SC provides transient components.

The extracted alpha and beta components were used to generate reference currents for the battery and SC, as shown in the figure.

Figure 9 shows the DC link current, battery current, and SC current. The DC link current was shared by the battery and SC. At 0.4 s, a change in load triggered a transient change in the current drawn. The reference current generated for the battery was not constant, so in the proposed controller, battery current limiter was also implemented so that it remained constant even though the load changed at 0.4 to 0.6 s. This transient change was managed by the SC by maintaining the constant battery current. Figure 10 shows the DC link voltage waveform.

Figure 11a,b show the harmonic analysis of the HESS-supported grid system. The total harmonic distortion was 30.29% and was limited to 4.96% with the support of a shunt active filter.

3. Implementation of a Hybrid Energy Storage System Coupled with Shunt Active Filter on a Testbed

Figure 12 shows the hardware implementation of the HESS-supported SAF connected to the grid. The input to the system was given by three single-phase transformers of ratings 12 V AC, RMS. The load kept was a three-phase rectifier with an R load. For the IcosΦ controller, the inputs were phase voltage and load current. Phase voltages were inputs to the ZCD, while load current was an input to the second-order filter. The load current was fed into the current sensor, which sensed the current and gave a voltage the same as the current waveform. The output of the current sensor was given to the subtractor circuit, which removed the offset and added some gain and its output was given to the second-order filter. The outputs of ZCD and second order filter were fed into the sample-hold IC, whose output was fed into the multiplier along with the unit sine wave. The unit sine wave was generated using a resistor divider network. The output of the multiplier IC was subtracted from the load current to give a reference filter current. From this, PWM pulses were generated. For controller 2, the inputs were filter voltage and current, which were given to the microcontroller and current sensor output of two boost converters. Execution of the code in the microcontroller provides reference currents for the boost converter, which are compared with the actual current to generate PWM pulses. Figure 13 shows the input, which was a sine wave, and outputs, which were pulses at negative zero crossing.

The load current and reference filter current are shown in Figure 14. Figure 15 shows reference filter currents for all three phases.

Figure 16 and Figure 17 show the magnitude of the alpha and beta components for the battery and supercapacitor generated by the instantaneous discharge controller. Figure 18 gives the magnitude of the battery, SC, and DC link currents. Battery current (green) was constant. If the battery current exceeded this value, then the SC would oversee the additional load current requirements, thereby protecting the battery from transients.

4. Conclusions

Typical hybrid energy storage systems are used to meet the transient and steady load variations in load current. Power sharing is carried out by batteries and supercapacitors in a hybrid energy storage system so that batteries can meet the steady load demand without compromising their service life. The supercapacitor, which has a high power density, can meet the transient demand. This paper proposes a novel control methodology, namely the instantaneous discharge control scheme, for an HESS to support the DC link of a shunt active filter connected to the grid. The application of a hybrid energy storage system for improving power quality as well as extending the life of low power density storage devices from fast variations in load has been explored in this paper. The proposed instantaneous discharge controller could effectively extract the steady and transient portions of the shunt active filter current so that the discharge of the battery and supercapacitor could be controlled efficiently. The shunt active filter could effectively eliminate harmonics from the source current and limit it to 5% as specified in IEEE Standards. The proposed controller could effectively manage power sharing between the battery and the supercapacitor. A shunt active filter provides active power capability to the network when integrated with an HESS. Moreover, it makes the system capable of supplying active and reactive power to the network. The future scope of the work includes the application of instantaneous discharge controllers in microgrids with renewable energy sources for powering an HESS, and its grid integration for real and reactive power compensation with power quality improvement.