Electric Spring for Voltage and Power Stability and Power Factor ...

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 53, NO. 4, JULY/AUGUST 2017

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Electric Spring for Voltage and Power Stability and Power Factor Correction Jayantika Soni, Student Member, IEEE, and Sanjib Kumar Panda, Senior Member, IEEE

Abstract—Electric spring (ES), a new smart grid technology, has earlier been used for providing voltage and power stability in a weakly regulated/stand-alone renewable energy source powered grid. It has been proposed as a demand-side management technique to provide voltage and power regulation. In this paper, a new control scheme is presented for the implementation of the ES, in conjunction with noncritical building loads like electric heaters, refrigerators, and central air conditioning system. This control scheme would be able to provide power factor correction of the system, voltage support, and power balance for the critical loads, such as the building’s security system, in addition to the existing characteristics of ES of voltage and power stability. The proposed control scheme is compared with original ES’s control scheme where only reactive power is injected. The improvised control scheme opens new avenues for the utilization of the ES to a greater extent by providing voltage and power stability and enhancing the power quality in the renewable energy powered microgrids. Index Terms—Demand-side management (DSM), electric spring (ES), power quality, renewable energy, single-phase inverter.

I. INTRODUCTION ENEWABLE energy sources (RESs) like solar and wind are indispensable components for a sustainable microgrid of the future. However, their intermittent and unpredictable nature poses an issue of power and voltage instability in the grid. Various methods have been proposed both at the source side and load side to mitigate this intermittency. Demand-side management (DSM) has been used actively as a method to attenuate the effects of renewable energy source intermittency. Various methods such as direct load control, load scheduling, energy storage, etc., are used to implement the DSM. However, they cannot be used in real time like load scheduling or might be intrusive to customer like direct load control. A new approach to DSM namely, electric spring (ES) was introduced by Rui et al. in [1]

R

Manuscript received May 16, 2016; revised October 11, 2016 and January 9, 2017; accepted March 4, 2017. Date of publication March 15, 2017; date of current version July 15, 2017. Paper 2016-SECSC-0353.R2, presented at the 2015 9th International Conference on Power Electronics and Energy Conversion Congress and Exposition (ECCE) Asia, Seoul, Korea, Jun. 1–5, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Sustainable Energy Conversion Systems Committee of the IEEE Industry Applications Society. This work was supported by the Republic of Singapore’s National Research Foundation through a grant to the Berkeley Education Alliance for Research in Singapore (BEARS) for the Singapore–Berkeley Building Efficiency and Sustainability in the Tropics (SinBerBEST) Program. The authors are with the Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576 (e-mail: jayantika. [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIA.2017.2681971

and [2], which is able to provide voltage and power stability in real time. The authors in [1]–[8] utilized only reactive power compensation to provide voltage support in real time and load shedding for noncritical loads. In an ac system, a unity power factor operation is desirable to improve efficiency, reduce losses, increase active power delivery, economical advantages on grid-side equipment, etc. [9], [10]. Power factor correction (PFC) methods like passive capacitors and shunt condensers work perfectly in the conventional grid. Their placements are determined by the reactive load and losses in the distribution system. With increase in nonlinear loads and advancement in power electronics, devices such as DSTATCOM [11], [12] are being employed to improve the power quality. In future microgrids with substantial distributed renewable energy sources, it is desired that we look at PFC as a DSM issue. Buildings are going to be quintessential elements in such future microgrids. They have great potential to implement the concept of ES as illustrated in [6], [13], and [14] through various noncritical loads such as electric heaters, air-conditioners, and refrigerators. The concept of ES can be extended further to improve the power factor in a renewable energy powered microgrid. Since the ES is implemented through an inverter and by utilizing its potential for both active and reactive power compensation [15] this could be achieved. The real power compensation has been utilized to improve power balance in a three-phase system [16] and to improve the power factor without any voltage or power regulation [17]. The radial chordal decomposition (RCD) control [18] and novel δ-control [19] are some of the control techniques to incorporate PFC. Electrical parameters of the system and grid voltage (input voltage) are required to implement the control scheme introduced in [19] and the control strategy will not be a demand-side solution. Control scheme in [18] decouples grid voltage regulation and PFC of the ES-associated smart load. In this paper and in [14], we demonstrate implementation of the ES through an improvised control scheme to provide the power and voltage stability and overall PFC, an aspect that has not been explored yet in the literature. Also, a comparative study of this scheme versus the conventional control scheme of ES is also carried out and presented. In Section II, the characteristics of a conventional and improvised ES are illustrated. Also, single-phase to d-q transformation is discussed. The modeling of ES and the improvised control scheme are explained and proposed in Section III. In Section IV, through simulation study first the performance of the conventional ES is discussed. The improvised control scheme

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Fig. 1.


Fig. 2. Phasor diagrams of voltage and current for PFC and voltage support in (a) undervoltage conditions and (b) overvoltage conditions.

ES in a circuit.

is applied to the same system and results are presented. Also, a comparative analysis is performed between the two control schemes, i.e., the conventional input-voltage control and the improvised input-voltage-input-current control scheme. The control scheme verification through hardware-in-loop (HIL) is presented in Section V. Final remarks and conclusions are drawn in Section VI. II. BRIEF OVERVIEW OF BUILDING BLOCKS OF IMPROVISED ES A. Operating Principles of ES The concept of ES was introduced by drawing parallels to a traditional mechanical spring [1]. In an RES powered microgrid, it could be realized through an inverter and is attached in series with the noncritical load, such as electric heaters, refrigerators, and air conditioners, as shown in Fig. 1, to form a smart load. In parallel to this smart load, critical loads like a building’s security system are connected. Earlier versions of ES implemented an input-voltage control scheme to generate reactive power compensation in order to provide voltage and power regulation to critical loads in steady state. As a result, the noncritical load voltage and power vary in accordance with the fluctuations in the weakly regulated grid due to intermittent power from RESs [1]. In order to provide only reactive power compensation from the ES, the compensation voltage, i.e., ES voltage, Ves should be perpendicular to noncritical load current, Inc [14], [16]. The ES voltage is governed by (1), where Vs is line voltage, Vnc is the noncritical load voltage, and Ves is ES voltage. Vs = Vnc + Ves .

(1)

In a distribution system, with various inductive and capacitive loads, a substantial reactive power injection can worsen the power factor of system and lead to reduced power efficiency. Thus, a feature of PFC can be incorporated in the ES along with the existing characteristics of voltage and power regulation. By utilizing a dc source such as a battery to power the inverter, as illustrated in Fig. 1, both active and reactive power compensation could be obtained from an ES. This property of an ES could be utilized to shape the line current, Iin , to be in phase with line voltage, Vs . Phasor diagram in Fig. 2 demonstrates how the ES compensation voltage, Ves , could help improve the power factor in the distribution system and provide voltage and power support

in steady state in a system with resistive-inductive loads, i.e., it has an overall lagging power factor. The ES needs to operate under two major scenarios: 1) when the line voltage Vs is less than the reference line voltage Vref [root mean square (RMS) value of 230 V] called the undervoltage case; and 2) when the line voltage is more than the reference line voltage called the overvoltage case. In the undervoltage case, as shown in Fig. 2(a), the ES injects a combination of capacitive and real power in the system, so as to boost the line voltage Vs to the reference value of 230 V and to regulate that the line voltage Vs and the line current Iin remain in phase. In the overvoltage case as depicted in Fig. 2(b), the ES injects a combination of real and inductive power in the system, to perform similar functions of line voltage regulation and PFC. In Fig. 2, Vin , Vx , Vs , Vnc , and Ves are the input voltage, voltage across line impedance, line voltage, noncritical load voltage, and ES voltage, respectively and Is , Inc , and Iin are the critical load current, noncritical load current, and line current, respectively. Also, Rx + jXx is the line impedance of the power circuit, where Xx = ωLx and Lx is the line inductance. B. Single-Phase d-q Transformation The rotating frame d-q transformation is widely used for the three-phase system for analysis and control. It is used for transformation of the d-q variables between rotating and stationary frames. The concept has also been extended to a single-phase system to achieve a simpler control and analysis. However, at least two independent variables are required to create a d-q system. Thus, the concept of orthogonal imaginary circuit was introduced [20]–[22]. Two variables, i.e., the real (voltage or current) and the imaginary, are utilized for the transformation. The imaginary variable is identical in characteristics to real variable but has a 90° electrical phase shift with respect to real variable. If the real sinusoidal signal, Xr is given by (2), then the imaginary sinusoidal signal, Xi would be as indicated in (3); A is the amplitude of the signal and θ is the phase of the real sinusoidal signal. Xr = A cos (ωt + θ) π Xi = A cos ωt + θ − . 2

(2) (3)

SONI AND PANDA: ELECTRIC SPRING FOR VOLTAGE AND POWER STABILITY AND POWER FACTOR CORRECTION

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III. MODELING AND CONTROL OF THE IMPROVISED ES A. Model of the ES

Fig. 3.

The ES can be realized through an inverter as shown in Fig. 1. The model of an ES [26] can be realized using KVL and KCL. Effective series resistances (ESR) of the filter inductor Lf and the capacitor Cf are neglected and it is assumed that all the devices of the inverter are lossless. The voltage across the filter inductor is indicated by VL f and the current through it is indicated by Ies , the voltage at the inverter terminal is indicated by Va , and the critical load impedance is Zc . KVL and KCL are applied on the ac side of the inverter and are written as

Orthogonal signal generation using an SOGI.

dIes Va − Ves = VL f = Lf dt Vs = Zc Is = Zc (Iin − Inc ) Cf Fig. 4.

Internal PLL using d-q transformation properties.

The orthogonal signals could be generated using transport delay block [20], the inverse Park transformation [23], and the Hilbert transformation [24]. However, these methods have some shortcomings such as frequency dependency, high complexity, nonlinearity, poor or no filtering [25]. A system based on the second-order generalized integrator (SOGI), as shown in Fig. 3, is used to generate the two orthogonal signals Xr (real variable) and Xi (imaginary variable) [25]. The signal Xr has the same magnitude and phase as the fundamental of the input signal (X). The clean orthogonal signals Xr and Xi are generated due to resonance of the SOGI at ω (grid frequency). This structure generates orthogonal signals, filters orthogonal signals without delay, and is frequency adaptive [25]. The single-phase d-q transformation for the signals, Xr and Xi is performed using a transformation matrix, Tr given in (4) and the d-q components rotating in a synchronous reference frame are generated using (5). An internal phase locked loop (PLL) could be generated by using properties of the single-phase d-q transformation, which would be employed in the proposed improvised control scheme. (For the purposes of the control scheme, the line voltage, Vs would be used as signal for generation of internal PLL as shown in Fig. 4; rated angular frequency (ωf f ), in this case 100π r/s, is feed-forwarded in the internal PLL). The internal PLL ensures that the d-q transformation is independent of frequency variations and is able to generate clean real and imaginary signals. Thus, by using the single-phase d-q transform, a time-varying ac signal can be converted to dc values and an appropriate controller could be designed for the inverter. Tr =

cos(ωt)

sin(ωt)

−sin(ωt) cos(ωt) Xr cosθ Xd = Tr =A . Xq Xi sinθ

(4)

(5)

Vs dVes = Ies + Inc = Ies + Iin − . dt Zc

(6) (7) (8)

Because of the high frequency Lf Cf filter, only the fundamental component would pass through. For mathematical simplicity, it is assumed that only the fundamental component, Va,1 is available at the inverter terminal voltage and is as given by (9), where m is the modulation signal and Vdc is the dc-link voltage of the inverter. Va = Va,1 = m × Vdc .

(9)

Thus, (6) can be rewritten as Lf

dIes − Ves . = Vdc m(t) dt

(10)

Using (8) and (10), the state-space equations of the system are given as ⎡ ⎤ ⎡ 1 −1 ⎤ 0 ⎢ Lf ⎥

d Ies ⎢ Lf ⎥ Ies ⎥ mVdc Vs =⎣ 1 +⎢ ⎦ ⎣ ⎦ 1 dt Ves Ves 0 Cf Zc Cf ⎤ 0 + ⎣ 1 ⎦Iin . Cf ⎡

(11)

The state-space equations can be written in the real and the orthogonal imaginary form as (12) and (13), where the subscript r denotes the real variable and the subscript i denotes the orthogonal imaginary of the real variable. The transient phase error as a result of SOGI [27] is ignored and the analysis below is carried out in steady state. −1 Ves,r 1 mr d Ies,r (12) = + Vdc dt Ies,i Lf Ves,i Lf mi Vs,r d Ves,r 1 Ies,r 1 Iin,r 1 = + − . dt Ves,i Cf Ies,i Cf Iin,i Zc Cf Vs,i (13)

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Using (4) and (5), the d-q transformation for (12) and (13) can be obtained and state-space equations would be (14) and (15), where the subscript d denotes the d-component of the signal and the subscript q denotes the q-component of the signal. 0 ω Ies,d d Ies,d 1 Ves,d Vdc md = − + dt Ies,q Lf Ves,q Lf mq Ies,q −ω 0

d Ves,d dt Ves,q

ω

1 Ies,d Cf Ies,q Ves,q −ω 0 Vs,d 1 Iin,d 1 + − . Cf Iin,q Zc Cf Vs,q

=

0

Ves,d

(14)

+

(15)

B. Improvised Control Scheme An advantage of using the single phase d-q transformation is that the parameters of the converter are dc in steady state [22]. Thus, from analysis point of view, the rates of change in the inductor current in d-q axes would be zero, i.e., (14) would be zero. And similarly, the rates of change in the capacitor voltage in d-q axes would be zero, i.e., (15) would be zero. After solving these two equations and making them equal to zero, (16) and (17) are obtained. Further, solving these two equations, (18) is obtained, which gives the d-q components of modulation signal, md and mq . Using the inverse d-q transformation given by (19) and (20), the modulation signal m could be obtained, which would generate the compensating voltage, i.e., ES voltage, Ves given by (21) in steady state. md 1 Ves,d − ωLf Ies,q = (16) Vdc Ves,q + ωLf Ies,d mq ⎡ ⎤ Vs,q − + Ies,q + Iin,q ⎥ Ves,d 1 ⎢ ⎢ Zc ⎥ (17) = ⎦ ωCf ⎣ Vs,d Ves,q − Ies,d − Iin,d Zc ⎡ ⎤ Vs,q ⎢ − Zc + Iin,q + Ies,q ⎥ md 1 ⎢ ⎥ (18) = ⎦ ωCf Vdc ⎣ Vs,d mq − Iin,d − Ies,d Zc mr md = T −1 (19) mi mq m =m r Ves = mV dc .

(20) (21)

Vs,d and Iin,q are chosen as control parameters because mq is directly linked to Vs,d and md is directly linked to Iin,q . The internal PLL property of the single-phase d-q transformation is utilized in this control scheme; internal PLL is generated by using the line voltage Vs as depicted in Fig. 4. The control scheme is shown in Fig. 5 where the phases are synchronized with the line voltage. Thus, the q component of line voltage Vs,q becomes zero and is used to generate the reference ωt for the

Fig. 5.

Improved control circuit for PFC and voltage support using ES. TABLE I SIMULATION SYSTEM SPECIFICATIONS System Voltage and Line impedance Line voltage, Vs (RMS) Undervoltage: 218 V Overvoltage: 238 V Line impedance 0.1 Ω, 2.5 mH Load Specifications Noncritical load Critical load

6.11 + j 0.44 Ω 11 + j 11 Ω

ES Power Circuit Inverter topology Switching frequency DC bus voltage

Single-phase H -bridge 20 kHz 400 V

Output Low-Pass Filter Inductance Capacitance

1.92 mH 13.2 μF

control loop. We regulate the d component of line voltage Vs,d and the q component of line current Iin,q while the d component of line current Iin,d is allowed to vary dynamically. The direct (d)-axis reference voltage signal Vs,dref is calculated so as to regulate the RMS of the line voltage to 230 V and the quadrature (q)-axis reference line current Iin,qref is zero so that maximum PFC for the system is achieved, such that the line current Iin is in phase with the critical load voltage, Vs . A restriction is incorporated in the controller so that the ES does not become primary source in the system. IV. SIMULATION RESULTS—CONVENTIONAL ES AND IMPROVISED ES The system shown in Fig. 1, with specifications as shown in Table I is considered. It was simulated on a MATLAB Simulink platform. The reference line voltage is set to be 230 V (RMS). The system has an effective resistive-inductive load, and thus, a lagging power factor. In this system, two cases are recorded: 1) with the conventional ES, providing only reactive power compensation; and 2) with the improvised ES, providing both real and reactive power compensations. Both systems are compared with each other on voltage and power regulation and PFC capabilities in steady state. The results are discussed in the following subsections. The control scheme for the conventional ES is the one used in [1] and for the improvised ES as shown in Section III-B. Proportional-integral (PI) controllers are used in both control schemes and the controller parameters are depicted in Table II. A. Case A: Conventional ES The ES with only reactive power compensation capabilities is used to provide voltage and power regulation. In the overvoltage


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TABLE II VALUES OF PI CONTROLLER Type of ES

Conventional ES Improvised ES d-axis q-axis

PI Values Kp

Ki

1 3 7

14 10 10

Fig. 8. Overvoltage, conventional ES: Active and reactive power across critical load, noncritical load, and ES (ES turned ON at t = 0.5 s).

Fig. 6. Overvoltage, conventional ES: RMS line voltage, ES voltage, and noncritical load voltage (ES turned ON at t = 0.5 s).

Fig. 9. Undervoltage, conventional ES: RMS line voltage, ES voltage, and noncritical load voltage (ES turned ON at t = 0.5 s).

Fig. 7. Overvoltage, conventional ES: Power factor of system (ES turned ON at t = 0.5 s).

scenario, the RMS line voltage is kept at 238 V, i.e., above the reference value. The ES, when turned ON at t = 0.5 s, reduces and maintains the RMS line voltage at the reference value of 230 V by injecting inductive power in the system as shown in Fig. 6. Also, the voltage across the noncritical load decreases to 215 V (RMS). The ES injects 2100 inductive VAR (Qes ) in the system (see Fig. 8) and as a result, the power factor of the system worsens from 0.965 (lagging) to 0.895 (lagging) as depicted in Fig. 7. Thus, the power quality of the system worsens. In the undervoltage scenario, the RMS line voltage is kept at 218 V, i.e., below the reference value. The ES is turned ON at t = 0.5 s and boosts the line voltage to 230 V as shown in Fig. 9. The voltage across the noncritical load decreases to 190 V (RMS). The ES injects a reactive power, (Qes ) of −4300 VAR (i.e., capacitive VAR) in the system Fig. 11. As a result,

Fig. 10. Undervoltage, conventional ES: Power factor of system (ES turned ON at t = 0.5 s).

the power factor of a resistive-inductive system improves from 0.965 (lagging) to 0.985 (lagging) as shown in Fig. 10. Although, the power factor has improved, there is potential for further improvement.

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Fig. 11. Undervoltage, conventional ES: Active and reactive power across critical load, noncritical load, and ES (ES turned ON at t = 0.5 s).

Fig. 13. Overvoltage, improvised ES: Power factor of system (ES turned ON at t = 0.5 s).

Fig. 14. Overvoltage, improvised ES: Active and reactive power across critical load, noncritical load, and ES (ES turned ON at t =0.5 s). Fig. 12. Overvoltage, improvised ES: RMS line voltage, ES voltage, and noncritical load voltage (ES turned ON at t =0.5 s).

B. Case B: Improvised ES The improvised ES, with the proposed control scheme, is subjected to similar scenarios as the conventional ES. This ES would be able to inject both real and reactive power in the system. Similar to the previous subsection, the RMS line voltage is kept at 238 V in overvoltage scenario and the ES is turned ON at t = 0.5 s. The ES reduces the line voltage to the reference value of 230 V shown in Fig. 12. It injects real power (Pes ) of 1500 W and 1500 inductive VAR (Qes ) in the system (see Fig. 14). The power factor of the system reduces from 0.965 (lagging) to 0.93 (lagging) as shown in Fig. 13. To maintain the line voltage to the reference value of 230 V, the ES injects a combination of real and inductive power in a highly inductive system, thus, the power factor is reduced from 0.965 to 0.93. However, the conventional ES worsens the power factor from 0.965 to 0.895. Though the performance with the improvised ES is not an optimal unity power factor, it is better than the conventional ES, which worsens the system power factor in the overvoltage scenario; a 1.5% improvement in the power factor is observed with the proposed control scheme as compared to the conventional ES. The conventional ES injects only inductive reactive power as shown in Fig. 8 to balance the line voltage. The improvised ES is equipped to provide both voltage support

Fig. 15. Undervoltage, improvised ES: RMS line voltage, ES voltage, and noncritical load voltage (ES turned ON at t =0.5 s).

and power factor improvement, and thus, has to inject a small amount of active power to maintain the balance. As a result, the voltage and active power consumption of the noncritical load increases as shown by Figs. 12 and 14. In the undervoltage scenario, the ES boosts the RMS line voltage from 218 to 230 V when it is turned ON at t = 0.5 s as shown in Fig. 15 The ES absorbs 1100 W (Pes ) and injects reactive power (Qes ) −2750 VAR (i.e., capacitive VAR) in the system as depicted in Fig. 17. The power factor of the system


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TABLE IV HIL SYSTEM SPECIFICATIONS System Voltage and Line impedance Line Voltage, Vs (RMS) Undervoltage: 227 Vst3 Overvoltage: 233 V Line impedance 0.1 Ω, 2.5 mH

Fig. 16. Undervoltage, improvised ES: Power factor of system (ES turned ON at t = 0.5 s).

Load Specifications Noncritical load Critical load

30 + j 3 Ω 60 + j 57 Ω

ES Power Circuit Inverter topology Switching frequency DC bus voltage

Single-phase H-bridge 20 kHz 200 V

Output Low-Pass Filter Inductance Capacitance

1.92 mH 13.2 μF

visioned that multiple of such ES when distributed across the future renewable powered grid, with poorer power factor and unstable voltage and power, should not just be able to support the grid but also provide PFC. The tasks associated with the stability and efficient and economic viability in such a grid could be solved using a single type of device, i.e., ES. V. HIL VERIFICATION

Fig. 17. Undervoltage, improvised ES: Active and reactive power across critical load, noncritical load, and ES (ES turned ON at t = 0.5 s).

TABLE III HARMONIC CURRENTS IN SIMULATIONS

Harmonic Third (3rd) Fifth (5th) Seventh (7th)

Standard (IEC-61000-3-2) (in Amp)

Overvoltage Scenario (in Amp)

Undervoltage Scenario (in Amp)

2.30 1.44 0.77

0.50 0.30 0.05

0.2 0.1 0.1

improves from 0.965 (lagging) to almost unity (see Fig. 16). The voltage and power consumption of the noncritical load are reduced as visible from Figs. 15 and 17. In the overvoltage scenario, a 4% improvement in the power factor from the conventional ES is observed. The conventional ES injects only inductive power in the system, whereas the improvised ES injects both real and inductive power. While in the undervoltage scenario, a 1.5% improvement is observed; the conventional ES injects only capacitive power and improvised ES injects both capacitive and real power in the system. Also, the harmonic injection limits in the system is maintained as dictated by IEC-61000-3-2 standards [28]. The 3rd, 5th, and 7th harmonic currents in both the scenarios and their respective standards are shown in Table III. Although the percentage improvement is not substantial for a single ES, it could be en-

For the purpose of validation and testing of the proposed new control scheme and devices HIL could be used to emulate power electronics in real time [29], [30]. In HIL testing configuration, the power electronic converter is replaced by an ultrafast realtime emulator that interacts with the controller via high-speed physical input/output interfaces [31]. The real plant is emulated so coherently that from the controller point of view, there is no difference between the two [27]. The system of Fig. 1 is emulated on a Typhoon HIL 602 system and the controller was implemented through a PI controller module of dSPACE 1104. The system characteristics are indicated in Table IV; for emulating practical conditions and to limit current in the setup, the load impedances were increased from simulation. Since, the ability of ES’s compensation range is dependent on load impedances [26], the overvoltage and undervoltage limits are reduced. Two different scenarios of undervoltage (227 V RMS) and overvoltage (233 V RMS) were tested. The line voltage and current, voltages across the ES and the noncritical load, power factor of the system, and line current harmonics were observed and recorded. In the undervoltage scenario, the RMS line voltage is boosted from 227 V to the reference voltage of 230 V. The power factor of the system is boosted from 0.968 to 0.996. Figs. 18 and 19 illustrate line current Iin , line voltage Vs , ES voltage Ves , and noncritical voltage Vnc before and after ES is turned ON, respectively. The harmonics of the line current are also measured and illustrated in Fig. 19 and they are found to pass the harmonics limits of Class A equipment [28]. Similarly in the overvoltage conditions, the line voltage was reduced form 233 to 230 V; Figs. 20 and 21, respectively, display the measured signals Iin , Vs , Ves , and Vnc before and after ES is turned ON. Similar to the overvoltage scenario of the simulation study, the power factor

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Fig. 18. HIL System in undervoltage scenario; ES not turned ON. Ch1: Line current, Iin (10 A/div), Ch2: Line voltage, V s (200 V/div), Ch3: ES voltage, V es (200V/div), Ch4: Noncritical Voltage, V nc (200 V/div).

Fig. 21. HIL System in overvoltage scenario; ES turned ON. Ch1: Line current, Iin (10 A/div), Ch2: Line voltage, V s (200 V/div), Ch3: ES voltage, V es (200 V/div), Ch4: Noncritical voltage, V nc (200 V/div).

Fig. 19. HIL system in undervoltage scenario; ES turned ON. Ch1: Line current, Iin (10 A/div), Ch2: Line voltage, V s (200 V/div), Ch3: ES voltage, V es (200 V/div), Ch4: Noncritical voltage, V nc (200 V/div).

grids. Further in this paper, by the implementation of the proposed improvised control scheme, it was demonstrated that the improvised ES maintained line voltage to reference voltage of 230 V, maintained constant power to the critical load, and improved overall power factor of the system compared with the conventional ES. Also, the proposed “input-voltage– input-current” control scheme is compared to the conventional “input-voltage” control. It was shown, through simulation and HIL emulation, that using a single device voltage and power regulation and power quality improvement can be achieved. It was also shown that the improvised control scheme has merit over the conventional ES with only reactive power injection. Also, it is proposed that ES could be embedded in future home appliances [1]. If many noncritical loads in the buildings are equipped with ES, they could provide a reliable and effective solution to voltage and power stability and in situ PFC in a renewable energy powered microgrids. It would be a unique DSM solution, which could be implemented without any reliance on information and communication technologies. REFERENCES

Fig. 20. HIL System in overvoltage scenario; ES not turned ON. Ch1: Line current, Iin (10 A/div), Ch2: Line voltage, V s (200 V/div), Ch3: ES voltage, V es (200 V/div), Ch4: Noncritical voltage, V nc (200 V/div).

is reduced from 0.968 to 0.843. It is in coherence from the inferences drawn from simulations, where due to injection of inductive power to maintain line voltage, power factor worsens. Also, like the simulations, the noncritical load-voltage increases. Thus, the HIL emulation validates the claims and simulation results proposed in the previous section. VI. CONCLUSION In this paper, as well as earlier literatures, the ES was demonstrated as an ingenious solution to the problem of voltage and power instability associated with renewable energy powered

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Jayantika Soni (S’14) received the B.Tech. degree in electrical engineering from the Indian Institute of Technology (Banaras Hindu University), Varanasi, India, in 2011. She is currently working toward the Ph.D. degree in the Department of Electrical and Computer Engineering, National University of Singapore, Singapore. Her research interests include power electronic technology in smart grid, control of renewable energy sources, and microgrids.

Sanjib Kumar Panda (S’86–M’91–SM’01) received the B.Eng. degree from South Gujarat University, Surat, India, in 1983; the M.Tech. degree from the Indian Institute of Technology (Banaras Hindu University), Varanasi, India, in 1987; and the Ph.D. degree from the University of Cambridge, Cambridge, U.K., in 1991, all in electrical engineering. He received the Cambridge-Nehru Scholarship and the M. T. Mayer Graduate Scholarship during his Ph.D. study (1987–1991). Since 1992, he has held a faculty position with the Department of Electrical and Computer Engineering, National University of Singapore, Singapore, and is currently serving as an Associate Professor and the Director of the Power and Energy Research Area. He has published more than 200 peer-reviewed research papers, co-authored one book and contributed to several book chapters, and holds six patents. His research interests include high-performance control of motor drives and power electronic converters, condition monitoring and predictive maintenance, building energy efficiency enhancement, etc. Dr. Panda is an Associate Editor of several IEEE TRANSACTIONS, such as the IEEE TRANSACTIONS ON POWER ELECTRONICS, the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, the IEEE TRANSACTIONS ON TRANSPORTATION ELECTRIFICATION, and the IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS.