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Abstract—The control strategy for interface inverters or grid- tied inverters to optimize power quality and power sharing in micro-grids with varying dynamics and ...
Distributed Co-Control of Interface Inverters for Harmonic-Loss Mitigation and Power-Sharing Accuracy in Micro-grids Ahsan Shahid, Student Member IEEE Department of Electrical and Computer Engineering University of Illinois at Chicago Chicago, Illinois [email protected]

Abstract—The control strategy for interface inverters or gridtied inverters to optimize power quality and power sharing in micro-grids with varying dynamics and load demand is a distinct issue. Intuitively, the control problem termed here as Co-Control (CoC) is to regulate and distribute the power flow at grid-micro-grid interface with the help of inverters so as to support system inertia at any time instant. A flexible and seamless inverter control paradigm with desired modulation and a degree of operability is the need of hour for next generation distributed power systems such as micro-grids. This paper presents a joint control strategy for interface inverters in microgrids leveraging on subsystem controllers for accurate power sharing and harmonic filtering. Control loops are improvised for better power quality by potentially reducing the harmonic loss to make the system efficient, reliable and power dense. Simulation results and parametric analysis validate the usefulness and contribution of the proposed approach. Index Terms—Co-Control, Distributed, Harmonic filtering, Inverters, Power dense.

I. INTRODUCTION Distributed generation (DG) being the generation system of tomorrow, might be an under-statement but not an overstatement. DG marks an internet of power in which the nonconventional renewable energy resources are scattered throughout the distributive network capable of interacting with the loads. Micro-grids, also named mini-grids, are becoming an important part of the concept to integrate and manage DG systems. This concept has been developed to cope with the penetration of renewable energy systems to an extent that the final user is able to generate, store, control, and manage the energy that will be consumed. This change of paradigm, allows the final user to be not only a consumer but also a part of the supplier or utility [1]. Apart from all its benefits and services, distributed generation disrupts power quality and power sharing within a power network and thus poses a direct threat to the domestic appliances and

Syed Muhammad Arsal Department of Electrical Engineering COMSATS Institute of Information Technology Islamabad, Pakistan [email protected]

sophisticated machinery at industrial level. Therefore, the stress is with the potential negative effects on power quality caused by the installation of DG systems. A micro-grid is a local grid consisting of distributed generators, energy storage systems, and dispersed loads, capable of operating in both grid-connected and islanded modes. DGs are often connected to the micro-grid through power electronic interfaces which consist of power converters and power filters. For such an inverter based micro-grid system, the prime responsibility of an interface converter is to regulate the power injection to the main stream power system [1]-[2]. In the grid-connected operation, the microgrid is interfaced with the main grid at the point of common coupling (PCC), and each DG unit generates proper real and reactive power which is regulated by the subsystem controllers. In off-grid or islanding operation using the voltage mode control, the DG voltage should be properly controlled and the DG units should share the total load demand according to their respective ratings [3]. This manuscript presents a simple yet seamless and flexible approach to solve the problem of harmonic compensation in micro-grids, keeping into account proper power sharing in case of multiple sources and loads. Space vector pulse-width-modulation (SVPWM) has been chosen for micro-grid inverters as it utilizes advance computational switching technique to reduce total harmonic distortion (THD) and switching losses because of the changing of any one switching state which results in one single phase voltage change every time. The proposed distributed co-control strategy with power flow management and power sharing through customized droop functions, eliminates the need of separate controllers in dq (synchronous) and αβ (stationary) reference frames and additional phase-locked loops (PLL). In the simulation results section, a parametric analysis is provided to demonstrate that the proposed design is robust, low cost and effective.

II. RELATED WORK Harmonic mitigation approaches of [4] and [5] are based on making the DG units of a power distribution system replicate a resistance at harmonic frequencies. For micro-grid islanded operation, fundamental control strategies have been defined in [6]-[7]. A real-time method for compensation of voltage harmonics in an islanded micro-grid has been presented in [8]. This method is also based on the resistance emulation and applies a harmonic power droop characteristic in order to share the compensation effort among DGs. An autonomous voltage unbalance compensation scheme which works based on the local measurements is proposed in [9] for each DG unit of an islanded microgrid. The control system design has been discussed in detail and the proposed approach is validated by experimental and simulation results provided that control systems are implemented in dq (synchronous) and αβ (stationary) reference frames. The control method presented in [10] is based on a modular approach in order to control power flow of a microgrid system to compensate for the voltage harmonics and uses droop curves for proper power sharing. In [11], a distributed networked control system is used in order to implement a distributed secondary control thus avoiding the use of a micro-grid central control, similarly [12] uses the distributed control strategy to enhance current sharing accuracy and restore dc bus voltage in a dc micro-grid. This work provides an extension to the contemporary techniques for power quality improvement and power sharing accuracy in microgrids by implementing a co-control strategy. The novel contribution of this work is a compact design with desirable, efficient and power dense modulation and control. III. POWER QUALITY AND MICRO-GRIDS In three-phase systems, the generated voltages are sinusoidal and their magnitudes are equal with 120 degrees apart from each other. From the harmonic modeling and simulation standpoint, a distributed generator in a micro-grid is usually a converter-inverter type topology and can therefore be treated as a non-linear load injecting harmonics into the distribution feeder [13]. The type and severity of harmonics depend on the power converter technology and interconnection configuration. The percentage of harmonics relative to fundamental and Total Harmonic Distortion (THD) allowed by IEEE (519-1992) can be seen from figure 1.

A. Modulation Technique: Space vector pulse-width-modulation is used for microgrid inverters because of its fast real-time response, low harmonic content and high efficiency. SVPWM determines the on and off states of six switches used in inverter to control the output voltage and frequency. Reference voltage vector always remains inside the polygon. Maximum value of reference voltage is limited to case when inscribed circle of polygon is obtained [14]. There are eight switching patterns for three switches i.e. 000, 100, 110, 010, 011, 001, 101, and 111. Sequence 000 and 111 short circuit the load terminal therefore zero voltage across load while hexagon formed with six non-zero vectors in figure 2 represents voltage because of other switching sequences in d-q frame.

Fig. 2. Switching states and sectors in space vector modulation [14]

This digital modulating technique has the objective of generating PWM load line voltages that are in average equal to a given (or reference) load line voltages [15]. This is done in each sampling period by properly selecting the switch states from the valid ones of the voltage source inverter (VSI) as shown in table 1 and by proper calculation of the period of times they are used. The selection and calculation times are based on the space vector transformation. Table. 1. Switching sequence in space vector pulse-width-modulation Vector

A+

B+

C+

A−

B−

C−

V0 = OFF OFF OFF ON ON ON {000}

VAB VBC VCA 0

V1 = {100}

ON OFF OFF OFF ON ON +Vdc

V2 = {110}

ON ON OFF OFF OFF ON

0

0

0

zero vector

0

−Vdc

active vector

+Vdc −Vdc

active vector

V3 = OFF ON OFF ON OFF ON −Vdc +Vdc {010} V4 = OFF ON ON ON OFF OFF −Vdc {011} V5 = OFF OFF ON ON ON OFF {001}

Fig. 1. Percentages of allowed harmonics set by IEEE

0

0

active vector

+Vdc

active vector

−Vdc +Vdc

active vector

0

V6 = {101}

ON OFF ON OFF ON OFF +Vdc −Vdc

0

active vector

V7 = {111}

ON ON ON OFF OFF OFF

0

zero vector

0

0

Any three-phase set of variables that add up to zero in the stationary abc frame can be represented in a complex plane by a complex vector that contains a real (α) and an imaginary (β) component. For instance, the vector of three-phase lineυ υ υ T can be represented modulating signals V by the complex vector v = v = υ υ following transformation

T

by means of the

0.5

(3.1)



(3.2)

If the line-modulating signals are three balanced and angular sinusoidal waveforms with an amplitude frequency , the resulting modulating signal in stationary frame becomes a vector = of fixed module which rotates at frequency . The angle between two non-zero adjacent vectors is 60 degrees. Voltages are made to follow reference voltage by calculating the proper on and off duration of sequences (voltage vector) represented by adjacent sides of sector [15]. To ensure that the generated voltage in one sampling period is on average equal to the vector , following expression should hold v ·T

v ·T

v

·T

vZ · T

(3.3)

The solution of the real and imaginary parts of above equation gives T T T

T · υ · sin π⁄3 T · υ · sin θ T T T

in the same way as the traditional power system frequency adjustment.

θ

(3.4) (3.5) (3.6)

For different voltages, T1 and T2 are changed while Ts remains the same. To change the frequency, rate of change of angle β is varied. This implementation is shown in figure 3.

Fig. 4(a). Pulse calculations subsystem

Fig. 4(b). SVPWM based inverter with 2nd order filter

System voltage and frequency regulation can decouple active power-frequency and reactive power-voltage droop curve. The VSI in a micro-grid configuration acts like a voltage source with the magnitude and frequency of the voltage controlled by respective droops and therefore emulating the behavior of a synchronous machine. This methodology is described in detail in [16]. The applicability of the control scheme in low voltage grids with predominantly resistive power network has been proven in [17]. In general, the following equations characterize frequency and voltage droops ω=ω P k V=V Q k where, kp: frequency droop coefficient kq: voltage droop coefficient ω0: nominal frequency V0: rated phase voltage magnitude P: real power output of micro-grid inverter Q: reactive power output of micro-grid inverter

(3.7) (3.8)

IV. PROPOSED CONTROL DESIGN Fig. 3. Space vectors time calculations subsystem

Time it takes the reference vector to complete one revolution over the circle decides the output frequency. MATLAB implementation of SVPWM based micro-grid inverter is shown in figure 4. B. Power Sharing: To maintain proper accurate power sharing among the loads, droop control is required. Droop control mainly refers to the frequency tuning of micro-gird inverter which is done

The system under consideration consists of a utility grid with bulk generation from a large generator rated at 500MW, 11KV and two micro-grids connected through a PCC to the utility grid and distributed loads. Micro-grids have DC microsources rated at 400V DC. Circuit breakers are installed for interfacing micro-grids with the utility grid forming different zones. When the main grid is out of service, micro-grids are switched to supply the distributed loads. Each micro-grid has a voltage source inverter provided with the droop control model that provides a voltage reference signal at the output.

This reference signal depends on the actual active and reactive power output of the inverter and maintains power sharing. It is recommended to add additional gain for maintaining system stability. The model of main grid alongwith the transmission and distribution transformers, circuit breakers, micro-grids and distributed loads, has been developed in MATLAB/Simulink as shown in figure 5. In grid connected mode, power is being supplied to loads through the main utility grid. The feeders A and B are connected to both the utility grid and distributed generation systems. In case of a fault or unavailability of the grid, power demands of feeder A and B are fulfilled by the fuel cells.

Fig. 5. High level model of the system under test

P=

cos

Z EV

Q=

cos

Z

V

cos

Z V Z

sin

EV Z EV Z

sin

sin

(4.1)

sin

cos

(4.2)

Where, E is the amplitude of inverter output voltage, V is the common bus voltage, is the power angle and Z and θ are the magnitude and phase of impedance, respectively. When the network is predominantly resistive, we substitute Z=R, θ=0 in equations (4.1), (4.2) to give the following of equations EV

P=

cos

Z EV

Q=

R

sin

V R

V. SIMULATION RESULTS Three phase voltages injected by micro-grid inverter (figure 6) show smooth waveforms leading to power dense operation. Table 2 shows the parameters used in the model. Table. 2. Micro-grid inverter simulation parameters DC-Link Voltage

Vdc=400V

Fundamental Frequency

f=60Hz

Switching Frequency

fZ=10kHz

Sampling Time

TZ=10msec

Modulation Index

a=0.85

Output Filter

2nd Order Low Pass

Cut-off Frequency

f0=500Hz

500

The active and reactive powers injected by the DG units to the grid bus can be modeled by the following equations EV

ω ω k Q (4.7) E E k P P (4.8) Where, (P0, Q0) and (P, Q) are active-reactive powers with their nominal and rated values, respectively. kq is frequency boost coefficient and kp is the voltage droop coefficient.

(4.3) (4.4)

0

-500

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0

0.01

0.02

0.03

0.04

0.05 Time

0.06

0.07

0.08

0.09

0.1

500

0

-500

500

0

-500

Fig. 6. Three phase voltages of standalone micro-grid inverter

Without the proposed control strategy, the harmonics present in the system are more than 5% which violate IEEE 519-1992 voltage limit. Waveforms in this case are shown in figure 7. 10

For practical applications, power angle is usually small, so P/Q decoupling approximation (sin = , cos θ=1) can be assumed for the simplification of control design. Hence the simplified equations will be

Voltage (V)

5

0

-5

-10

P ≈ (E – V) R

Q=

EV R

(4.5) (4.6)

From equations (4.5) and (4.6), it is evident that increase in output voltage magnitude gives high real power and increase in power angle gives low reactive power. This is known as inverse-droop condition [17]. In such condition, P-V droop and Q- ω boost functions given in equations (4.7) and (4.8) lead to active and reactive power sharing accuracy.

0

0.01

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0.03

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0.1

0

0.01

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0.03

0.04

0.05 Time

0.06

0.07

0.08

0.09

0.1

0.015 0.01 Current (A)

V

0.005 0 -0.005 -0.01 -0.015

Fig. 7. Voltage and current profile of the system before proposed control

Applying the proposed control strategy, the harmonic content is mitigated to significantly lower numbers in grid-connected

as well as in islanded mode. The results are shown in figure 8. 400

Voltage (V)

200

0

-200

-400

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

1

Current (A)

0.5

switching or interfacing of DG units with the utility grid. Simulation goal is achieved by implementing a co-control strategy which mitigates harmonics present in the system due to above mentioned reasons and improves power-sharing accuracy at the same time. The design is seamless, optimized and reliable as it results in improved power quality and power sharing at system level. The flexibility of manipulation and digital implementation of SVPWM render it useful in microgrid inverter applications. With these features, the system contributes to the ongoing research in terms of performance, sustainability and economics. REFERENCES

0

-0.5

-1

0

0.01

0.02

0.03

0.04

0.05 Time

0.06

0.07

0.08

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[1]

Guerrero, J.M. ; Chandorkar, M. ; Lee, T. ; Loh, P.C, “Advanced Control Architecture for Intelligent Microgrids-Part I: Decentralized and Hierarchical Control”, IEEE Trans. Industrial Electronics, 2013.

[2]

Guerrero, J.M. ; Poh Chiang Loh ; Tzung-Lin Lee ; Chandorkar, M, “Advanced Control Architecture for Intelligent Microgrids-Part II: Power Quality, Energy Storage and AC/DC Microgrids”, IEEE Trans. Industrial Electronics, 2013. Yan Li ; Yun Wei Li, “Power Management of Inverter Interfaced Autonomous Microgrid Based on Virtual Frequency-Voltage Frame”, IEEE Trans. Smart Grid, 2011. Savaghebi, M. ; Jalilian, A. ; Vasquez, J.C. ; Guerrero, J.M., “Secondary Control for Voltage Quality Enhancement in Microgrids”, IEEE Trans. Smart Grid, 2012. Zhou Niancheng, Chi Yuan, Wang Qianggang; “Control Strategies for Microgrid Power Quality Enhancement with Back-to-back Converters Connected to a Distribution Network”, 15th International Conference on Harmonics and Quality of Power, 2012. Lopes, J.A.P. Moreira, C.L. Madureira, A.G. “Defining Control Strategies for Micro-grids Islanded Operation”, IEEE Trans. Power Systems, Volume: 21, Issue: 2, page (s) 916-924, 2006. Yongqin Gu , Peiqiang Li , Yuan Pan, Hui Ouyang , Dong Han, Yuanzhao Hao, “Development of Microgrid Coordination and Control Overview”, IEEE PES Innovative Smart Grid Technologies, Asia, 2012 Shabestary, S.M.A. Saeedmanesh, M. Rahimi-Kian, A. Jalalabadi, E. “Real-time Frequency and Voltage Control of an Islanded Mode Microgrid”, Iranian Conference on Smart Grids, 2012. Savaghebi, M. ; Jalilian, A. ; Vasquez, J.C. ; Guerrero, J.M., “Autonomous Voltage Unbalance Compensation in an Islanded Droop Controlled Microgrid”, IEEE Trans. Industrial Electronics, 2013. Shahid, A; Azhar, H; “A Modular Control Design for Optimum Harmonic Compensation in Micro-grids considering Active and Reactive Power Sharing”, 16th IEEE International Conference on Harmonics and Quality of Power, 2014. Shafiee, Q. ; Vasquez, J.C. ; Guerrero, J. M., “Distributed Secondary Control for Islanded Micro-grids-A Networked Control Systems Approach”, 38th Annual Conference on IEEE Industrial Electronics Society, 2012. Lu, Xiaonan ; Guerrero, Josep M. ; Sun, Kai, “Distributed Secondary Control for DC Micro-grid Applications with Enhanced Current Sharing Accuracy”, IEEE International Symposium on Industrial Electronics, 2013. Shahid, Ahsan, "Modeling and control of distributed generation based micro-grids for power quality studies" M.S. dissertation, Department of Electrical and Computer Engineering, Univ. Illinois, Chicago, 2014. Kashif, S.A.R. ; Saqib, M.A. ; Zia, S. ; Kaleem, A., “Implementation of Neural Network based Space Vector Pulse Width Modulation InverterInduction Motor Drive System”, ICEE Third International Conference on Electrical Engineering, 2009. Rashid; M. H., “Power Electronics, Circuits, Devices and Applications”, Prentice Hall, 2004. Piagi P,Lasseter R, H. “Autonomous, Control of Microgrids”, IEEE Power Engineering Society General Meeting, pp.1-8, 2006. Savaghebi, M. Jalilian, A, “A New Control Strategy for Distributed Generation Interface Converters to Compensate Micro-grid Harmonics”, International Symposium on Power Electronics, Electrical Drives, Automation and Motion, 2010.

0.1

Fig. 8. Voltage and current profile of the system after proposed control

FFT analysis of output waveforms is provided in figure 9. [3]

Fundamental (60Hz) = 392.9 , THD= 0.27% 0.15

[4]

0.1

[5]

0.05 0

0

500 1000 1500 Frequency (Hz)

2000

Fig. 9. Harmonic analysis of voltages injected by micro-grid inverter

Results validate the effectiveness of proposed control strategy in terms of harmonic-loss mitigation and power-sharing accuracy. Table 3 shows a parametric analysis of different control strategies applied to micro-grids where CoC approach has emerged as a powerful control algorithm. Table. 3. Parametric analysis of harmonic-loss in different control strategies

Control Strategy PQ Control SPWM + PPF + Droop Control SVPWM + PPF PQ + APF + SVC CoC

Total Harmonic Distortion (%) Grid Islanded Connected Mode Mode 8.07 1.95 4.37 1.13 0.27

3.57 2.76 0.95

VI. CONCLUSIONS In this paper, we deal with the issue of power quality in general and power sharing in case of multiple source-load combinations in distributed generation systems such as microgrids. Power quality issues may arise due to the intermittent nature of distributed energy resources (DERs), internal switching of power converters and external grid level

[6] [7] [8] [9] [10]

[11]

[12]

[13] [14]

[15] [16] [17]