5-level Cascaded H-Bridge Multilevel Microgrid Inverter ... - IEEE Xplore

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DG inverter is to provide active power to the grid; however, by means of advanced control strategies, undesirable power components can also be compensated if ...
5-level Cascaded H-Bridge Multilevel Microgrid Inverter Applicable to Multiple DG Resources with Power Quality Enhancement Capability Ali Mortezaei,

M. Godoy Simões

Colorado School of Mines Golden, CO, USA

Fernando P. Marafão

Ahmed Al Durra

UNESP – Univ Estadual Paulista Sorocaba, SP, Brazil

The Petroleum Institute Abu Dhabi, UAE

Abstract – This paper presents a 5-level Cascaded HBridge Multilevel Inverter (CHMI) able to perform several tasks and applicable to a microgrid with multiple local power sources. The current control strategy is based on the conservative power theory (CPT) generating references for different modes of operation. The primary merits of multilevel cascade voltage source inverters are the possibility of having independent DC sources, such as fuel cells, solar cells and batteries and, the reduced dc link voltage compare to the traditional 2-level inverters. Hence, the voltage stresses and switching losses are also decrease. In addition to injecting the available energy from DC sources into the grid, the CHMI has also being used to compensate voltage and current disturbances and to improve the power quality at point of common coupling. Additionally, the paper presents the analysis and design of cascaded voltage control scheme based on capacitor filter current feedback to regulate load voltage in islanded operation mode. The principles supporting the proposed control strategy are presented and the inverter performance is demonstrated through digital simulations using PSIM. Keywords – Conservative Power Theory, Distributed Generation, Microgrid, Multilevel inverter, Power Quality Improvement. I. INTRODUCTION Considering Distributed Generation (DG) systems, the power electronic interfaces are experiencing an increasing evolution in terms of their functionalities, not only connecting local power source to the main grid, but also mitigating power quality disturbances. The main goal of a DG inverter is to provide active power to the grid; however, by means of advanced control strategies, undesirable power components can also be compensated if the converter power rating is available. Thus, the system would be able to operate as multi-functional compensator for improving power quality, instead of only providing active power to the grid [1]. Such functionalities are very important, particularly with increasing number of reactive, unbalanced and nonlinear loads and also, in any condition where the grid might not be strong enough, such as in isolated regions or rural systems. In such context, the power capacity of the interface converters is a key issue. Therefore, possible selective or cooperative strategies would be very useful, where the available interface converter capability determines the compensation performance in each inverter. A review of past studies using different current references for compensation

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shows that selective operation can be achieved by means of different approaches, such as: complex digital filters [2], harmonic damping strategies [3] or well-known power theories [4]. Moreover, the use of multilevel converters have some advantages, such as improved quality of generated current (with lower ripple), resulting in smaller filters and reduced switching frequency and voltage, making it applicable for higher power application and modularity [5], [6]. Considering different kinds of multilevel converters, the cascaded multilevel is attracting more and more attention because of its smaller number of components, straightforward physical layout, simpler modularity and higher reliability and independent dc sources, which makes this topology an ideal candidate for renewable energy systems in medium and high power applications [7]-[10]. The symmetrical CHMI is structured by a series of cascaded identical H-bridges. Each H-bridge as a power cell is capable of three different voltage levels at the output. The series connection of the H-bridges generates output voltage waveforms that are synthesized by the combination of each output of the H-bridges at certain switching states. Generally, when H-bridges are connected in series, the output waveforms contain 2 1 different voltage levels and a maximum output voltage of k times , assuming all cells have equal and constant voltage. In this paper, the study of a 5-level CHMI, able to perform various tasks in a microgrid application and relying on CPT for reference generation is presented. The 5-level CHMI is structured by two modules for each phase as shown in Fig. 1. The presented control method is modular and simple to adapt for any number of modules in series. The CPT is used as an alternative to generating different current references in the stationary frame, for selective disturbances mitigation and active power injection. The CPT current decompositions results in several current related terms associated with specific load characteristics under different supply voltages condition [11]. These current terms are orthogonal (decoupled) and could be used by the CHMI control system for multifunctional operation. Simulation results are presented to support the theoretical analysis. II. A BRIEF CPT OVERVIEW The CPT [11] proposes the decomposition of the instantaneous currents into different orthogonal current terms, which are related to load behavior. Thus, assuming a multiphase circuit where each phase of the system is denoted by the subscript “m”, one may have:

• • • •



balanced active current ( ) is the term related to the active power consumption and represents the load balanced equivalent conductance; balanced reactive current ( ) is the term related to the reactive energy circulation and represents the load balanced equivalent reactivity; void current ( ) is the term related to the nonlinear (distortion) behavior between voltages and load currents; ) is the term unbalanced current ( and related to the unbalanced load behavior. The are associated with the different values between balanced equivalent and phase conductance and reactivity, respectively; ) is the term that represents non-active current ( all the unwanted terms of the load current.

control purpose is the active current injection into the grid, as well as compensating the load current disturbances, improving power quality. In the islanded mode of operation the switch is open and the 5-level CHMI of Fig. 1 entirely supplies the load. Thus, the control aim is the regulation of the PCC/load voltage, i.e. , with the preservation of the grid-connected current control scheme. TABLE I - MICROGRID PARAMETERS. Nominal grid phase voltage, Grid frequency, Maximum power output Switching frequency, Output filter inductor, Output filter resitor, Output filter capacitor, DC link voltage, Carier amplitude,

By definition, the collective RMS current can be split into:

TABLE II - LOAD PARAMETERS.

(1)

Load inductor, Load inductor, Load capacitor, Output resistor, Output resistor, Output resistor, Output resistor, Output resistor, Load resistor, Load resistor,

indicating that all current components are orthogonal to each other and they could be controlled or minimized selectively. So, each decomposed current term can define a different control strategy, which can be included on the DGs in order to maximize its utilization and the power quality at the point of common coupling. III. CONSIDERED MICROGRID TOPOLOGY Figure 1 depicts the electrical diagram of the considered microgrid, including a 5-level CHMI with output filter and loads, which are connected to the distribution system at PCC via a power switch. and present the inductance is the and capacitance of the filter, respectively, while inductor internal resistance. The effect of local power source is represented by DC voltage sources, connected in parallel with the H-bridges DC-link capacitors. The parameters of the microgrid system and its loads are provided in Table I and II. The CHMI is controlled in a reference frame for both grid and islanded modes. In the grid-connected mode, is dictated by the grid representing the PCC/load voltage. The

50 mH 4 mH 220 uF 20 ohm 300 ohm 80 ohm 100 ohm 60 ohm 70 ohm 250 ohm

IV. CURRENT CONTROLLER FOR THE GRID CONNECTED MODE Figure 2 shows the block diagram of the current control scheme to regulate CHMI output current [12], [13]. Thus, the first step is to determine the transfer function and frequency response of the system, as following. (2) 2 1

iloadabc

Svgridabc

w

(3) (4)

PCC

~

127 V 60 Hz 4kVA 12 kHz 4 mH 0.15 ohm 5 uF 120 V 5V

a b c n L1

igridabc vtabc

Lf

Rf

R1

iabc vabc

Electrical Grid v(t)=180sin(wt)

VDC

VDC

VDC

L2 R2 R3

VDC

VDC

VDC

Control Strategy

Conservative Power Theory (CPT)

R4

iabc icapacitor

vpccabc iloadabc

Fig. 1. Considered microgrid topology with CHMIs, loads and switch to power grid.

R5 R6

C1

R7

C1

R7

|

Fig. 2. Current-control loop for grid connected mode.

In the control diagram of Figure 2, the open-loop transfer function is obtained as in (5), where is the current controller.

60

− 80

160 150

100 − 120 MagG.oi( f )

PhaseG .oi( f ) − 140

50

0 − 20

40

− 160 − 20

− 40 Mag.Plant ( f ) 20

− 90.952

− 100

0

60

(10)

Using the equations above for 2.4 , the parameters in the controller transfer function of (6) are 646. 27 , 8912.57 and calculated as 5102.81 . The frequency response of the open loop transfer function is illustrated at Fig.4. It can be seen that at cross 2400 ), the open loop gain of 0 over frequency ( dB and the phase margin of 60° are obtained.

(5) To design the controller, we first need to analyze the frequency response of the plant, as it is illustrated in Fig. 3.

|

Phase.Plant( f ) − 60

0

0.1 0.1

1

10

3

1×10

100 f

4

1×10

5

1×10

− 180 6 1×10 10

− 179.526

6

Fig. 4. Bode plot of the open loop current transfer function 0

− 20

− 80

− 20 0.01

0.1

1

10

0.01

1×10

100

3

f

− 100 4 1×10 10

− 100

4

.

Fig. 3. Gain and the phase of the plant

The crossover frequency of the current controller is chosen to be one fifth of switching frequency ( 2.4 ). Thus, a lead compensator is selected for the current controller , as in (6), resulting a fast dynamic response. is the controller gain. 1 1

⁄ ⁄

(6)

To yield a zero steady state error, it contains a pole at the origin, which introduces 90° phase shift in the loop transfer function. Based on the K-factor approach [14] and , we can calculate the poleknowing the phase boost zero locations to provide the necessary phase boost. In Fig. 90° , with the desired phase margin 3, °

°

60 and the required phase boost 60 . Consequently, the pole and the zero frequencies in the controller can be calculated as follows: (7) 45°

2

(8) 2 To compute the controller gain, from (5) at the crossover frequency , we have: 45

|

|

|

°

|

In Fig. 3, at 2.4 ,| | 0.795. Therefore, in (9) | Then, in (6) can be calculated at

|

|

1

(9)

| -1.984 dB = 1.984 dB = 1.256. as,

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.

The method used to switch cascaded H-bridge cells can be based either on the fundamental switching frequency, i.e., staircase modulation, or the pulsewidth modulation (PWM) technique [15]–[19]. In the first approach, the switching losses are lower, but the harmonics in the output voltage waveform appear at lower frequencies. The performance of the converter with different PWM techniques was evaluated in [18]. In this paper, PhaseShifted PWM (PS-PWM) is used. Since each H-bridge cell is a three-level converter, the traditional unipolar PWM switching schemes is adopted. The series-connected Hbridge cells of the converter are modulated with individual carrier waveforms, while sharing the same reference signals. A phase shift among the carrier waveforms is applied between adjacent H-bridge cells. The angle of the phase shift depends on the level of the converter and is tailored to the specific switching scheme, which is implemented in each H-bridge cell. V. MULTILOOP CONTROLLER FOR ISLANDED MODE For this mode of operation, the CHMI is set to regulate the instantaneous load line-to-neutral voltage magnitude, i.e. , and frequency . Hence, the cascaded control is employed to regulate load voltage/frequency in various load conditions. The cascaded control scheme could use either filter inductor or filter capacitor currents in inner loop as feedback variables with a higher bandwidth compared to the outer voltage loop [20], [21]. In practice, the choice of a particular feedback scheme depends also on other factors than the superiority of the control strategy, including issues such as the cost of the system, protection and the availability of current sensors [20]. In this paper, the cascaded control with capacitor current feedback control loop is employed. This control scheme only needs low cost sensors for capacitors current measurement, taking

advantage of overcurrent protection of the grid-connected inner current control scheme of Fig. 1. However, it should be noticed that the feedback variable for inner current loop may change from inductor current to capacitor current in islanded mode of operation. The cascaded control with capacitor current feedback control loop is shown in Fig. 5. From Fig. 5, is controlled by , where is the output of the voltage controller. Then, the transfer function between and is obtained as in (11). in (12) is the open-loop transfer function of is the controller of the voltage control scheme, where the outer voltage control loop. (13) shows the output voltage dependence on both, the reference voltage and the load current or system output impedance. The effect of system output impedance should be negligible so that the output voltage tracks its reference precisely. A non-ideal PR controller as in (14) is applied for the outer voltage regulation loop, ensuring almost zero fundamental steadystate error by introducing a high gain in the denominator of system output impedance in (13), where is the [22]. The bandwidth around the resonant frequency of ideal PR controller could lead to possible instability due to the infinite gain, whereas with (14) the PR controller has a finite gain at the resonant frequency , but it is still sufficiently large for tracking the fundamental output voltage with respect to its reference properly. The gain of the PR controller decreases at other frequencies and is not enough to eliminate the influence of the load harmonics; nevertheless, having CHMI with 5 levels terminal voltage before filtering, requires smaller output inductor filter, which reduces the significance of system output impedance. In (14), the parameters of and are the proportional and integral gains of the compensator, respectively. For 800 , phase margin of 50° , 20 / we can then compute that 377 / and 0.0196 and 2.7907. The frequency response of the open-loop voltage control system is illustrated in Fig. 6. 800 the gain of 0 dB At cross over frequency, and the phase of 50° are obtained.

Fig.5. Multiloop voltage controller based on capacitor filter current feedback.

(11) 1

(12) 2 2

(14)

1

80

0

80

0 60

− 50

40 MagG.ov( f )

− 100 PhaseG .ov( f ) 20 − 150

0 − 20

− 20 1 1

10

100 f

1×10

3

− 200 4 1×10

− 200

4

10

.

Fig. 6. Bode plot of the open loop voltage transfer function

VI. SIMULATION RESULTS AND DISCUSSION The dynamic performance of the control schemes for both modes of operation are evaluated in this section and the related performances are demonstrated through digital simulations using PSIM. Assuming the system of Fig. 1, Fig. 7 presents PCC and CHMI voltages, as well as load, inverter and grid currents, according to different conditions: Until t=0.3s, the VSC gating signals are blocked and all controllers are inactive, so that the entire load current is supplied by the grid. At t= 0.3s, the gating signals are unblocked, controllers are enabled, and the CHMI is set to supply the balanced active current component of the load current, ( ), which is practically sinusoidal, balanced and in phase with the voltages, such as in the case of an equivalent balanced resistive load. It can be observed that the grid is in charge of supplying all the undesirable characteristics of the load current (reactive, unbalances and harmonics), that is ( . At t=0.35s the CHMI is set to supply both balanced active and reactive currents components of the load, that is ( ). This means that the CHMI current is no longer in phase with the voltages. In this strategy the grid supplies unbalanced and void current components of the load . After t=0.4 CHMI is current), that is ( also set to supply the unbalanced current component of the ). In this case the load current that is ( CHMI current is still sinusoidal, but unbalanced. The grid supplies the remaining part of the load current, which is . void (non-linear) current component, that is ( Then, at t=0.45s the CHMI is set to supply entire load ). current, that is ( From Fig. 7, one can note that until t=0.5s, the CHMI is in grid-connected mode ( ). However, at t=0.5s, the CHMI is disconnected from the grid and switches with a smooth transition to islanded-mode, based on multiloop voltage control with capacitor filter current feedback. The task of CHMI in this mode of operation is the load voltage/frequency regulation. It is mentioned that the feedback variable for inner current loop needs to change from inductor current to capacitor current in the islandedmode of operation. (13) 1

Fig. 8 presents the waveforms of the load voltages and currents of islanded CHMI. It includes: the reference and load voltages, inverter terminal voltage of phase and CHMI and load currents, respectively. From Fig. 8, it is also possible to evaluate the dynamic performance of the islanded CHMI to step changes in . At t = 0.65s, is exposed to a step change from 180 to

vpcc vta

Va 200 100 0 -100 -200

200 100 0 -100 -200

Vb

140 V, then from 140 to 200 V at t =0.7s and finally, it goes back to 180 V at t =0.75s. Looking to the terminal voltage, , one can observe that under different reference voltage levels, the H-bridge modules cater together the output voltage. It is observed that the PCC/load voltage follows its reference value immediately, presenting a welldamped response.

Vc

vta

iload

IloadA

IloadC

0 -20 Ia

iinv

IloadB

20

Ib

Ic

20 0 -20

igrid

IgridA

IgridB

IgridC

20 0 -20 0.3

0.35

0.4 Time (s)

0.45

0.5

Fig. 7. Dynamic response of the CHMI unit based on CPT.

vref vload

VrefA 200 100 0 -100 -200 200 100 0 -100 -200

Va

VrefB

Vb

VrefC

Vc

vta

vta

200 0 -200

iload

IloadA

IloadB

IloadC

20 0 -20 Ia

Ib

Ic

iinv

20 0 -20 0.6

0.65

0.7 Time (s)

0.75

0.8

Fig. 8. Dynamic response of the islanded CHMI unit to step changes in the reference voltage based filter capacitor current feedback.

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VII. CONCLUSION Having multiple independent local power sources in a microgrid system needs an appropriate topology with simple modularity and good reliability. Among different topologies, the CHMI is drawing growing attention and may become an ideal candidate for renewable energy systems in medium and high power applications. Considering the rated capacity of the interface converters, any selective operative strategy can be used, where the available interface converter capability determine the compensation level. Thus, this paper presents a possible control strategy based on the conservative power theory (CPT) to generate references for different CHMI modes of operation. Moreover, the cascade control with capacitor current feedback in inner loop and PR controller in outer loop was developed and it has shown satisfactory steady state and transient performance. ACKNOLEDGEMENTS Dr. Marafão thanks to FAPESP (2012/14014-0) and CNPq (487471/2012-1) for supporting this research. REFERENCES [1] Singh, M.; Khadkikar, V.; Chandra, A.; Varma, R.K., “Grid Interconnection of Renewable Energy Sources at the Distribution Level with Power-Quality Improvement Features,” IEEE Transactions on Power Delivery, vol.26, no.1, pp.307-315, Jan. 2011. [2] Illindala, M.; Venkataramanan, G., “Frequency/Sequence Selective Filters for Power Quality Improvement in a Microgrid,” IEEE Transactions on Smart Grid, vol.3, no.4, pp.2039-2047, Dec. 2012. [3] Munir, S.; Yun Wei Li, “Residential Distribution System Harmonic Compensation Using PV Interfacing Inverter,” IEEE Transactions on Smart Grid, vol.4, no.2, pp.816-827, June 2013. [4] E. H. Watanabe, J. L. Alfonso, J. G. Pinto, L. F. C. Monteiro, M. Aredes, and H. Akagi, “Instantaneous p-q power theory for control of compensators in microgrids,” Przegląd Elektrotechniczny (Electrical Review), vol. 86, no. 6, pp. 1–10, 2010. [5] S. Kouro, M. Malinowski, K. Gopakumar, J. Pou, L. G. Franquelo, B. Wu, J. Rodriguez, M. A. Perez, and J. I. Leon, "Recent advances and industrial applications of multilevel converters," IEEE Transactions on Industrial Electronics, vol. 57, pp. 2553-2580, 2010. [6] J. Rodríguez, J. S. Lai, and F. Z. Peng, “Multilevel inverters: A survey of topologies, controls and applications,” IEEE Transactions on Industrial Electronics, vol. 49, no. 4, pp. 724–738, Aug. 2002. [7] M. Malinowski, K. Gopakumar, J. Rodriguez, and M. A. Perez, “A survey on cascaded multilevel inverters,” IEEE Transactions on Ind. Electronics, vol. 57, no. 7, pp. 2197–2206, Jul. 2010 [8] S. Khajehoddin, A. Bakhshai, and P. Jain, “The application of the cascaded multilevel converters in grid connected photovoltaic systems,” in Proc. IEEE EPC, Montreal, QC, Canada, Oct. 2007, pp. 296–301. [9] E. Villanueva, P. Correa, J. Rodríguez, and M. Pacas, “Control of a single-phase cascaded H-bridge

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multilevel inverter for grid-connected photovoltaic systems,” IEEE Transactions on Industrial Electronics, vol. 56, no. 11, pp. 4399–4406, Nov. 2009. [10] J. Dixon, L. Moran, J. Rodriguez, and R. Domke, “Reactive power compensation technologies: State-ofthe-art review,” Proc. IEEE, vol. 93, no. 12, pp. 2144– 2164, Dec. 2005. [11] P. Tenti, P. Matavelli and H. K. M. Paredes, “Conservative Power Theory, a Framework to Approach Control and Accountability Issues in Smart Microgrids”, IEEE Transactions on Power Electronics, pp. 664-673, Mar. 2011. [12] B. P. McGrath, D. G. Holmes, and W. Y. Kong, “A decentralized controller architecture for a cascaded Hbridge multilevel converter,” IEEE Trans. Ind. Electron., vol. 61, no. 3, pp. 1169–1178, Mar. 2014. [13] D. N. Zmood and D. G. Holmes, “Stationary frame current regulation of PWM inverters with zero steadystate error,” IEEE Transactions on Power Electronics, vol. 18, no. 3, pp. 814–822, May 2003. [14] N. Mohan, T. M. Undeland, and W. P. Robbins, Power Electronics Converters, Applications and Design, 3rd ed. New York: Wiley, 2003. [15] H. Sepahvand, M. Khazraei, M. Ferdowsi, and K. A. Corzine, “A hybrid multilevel inverter with both staircase and PWM switching schemes,” in Proc. IEEE Energy Convers. Congr. Expo., 2010, pp. 4364–4367. [16] Z. Du, L. M. Tolbert, B. Ozpineci, and J. N. Chiasson, “Fundamental frequency switching strategies of a seven-level hybrid cascaded H-bridge multilevel inverter,” IEEE Transactions on Power Electronics, vol. 24, no. 1, pp. 25– 33, Jan. 2009. [17] B. Diong, H. Sepahvand, and K. A. Corzine, “Harmonic distortion optimization of cascaded Hbridge inverters considering device voltagedrops and non-integer dc voltage ratios,” IEEE Trans. Ind. Electron., vol. 60, no. 8, pp. 3106–3114, Aug. 2013. [18] J. Rodriguez, J.-S. Lai, and F. Z. Peng, “Multicarrier PWM strategies for multilevel inverters,” IEEE Trans. Ind. Electron., vol. 49, no. 4, pp. 724– 738, Aug. 2002. [19] M. Calais, L. J. Borle, and V. G. Agelidis, “Analysis of multicarrier PWM methods for a single-phase five-level inverter,” in Proc. 32nd Annual IEEE PESC, Jun. 17– 21, 2001, vol. 3, pp. 1173–1178. [20] L. Poh Chiang, M. J. Newman, D. N. Zmood, and D. G. Holmes, “A comparative analysis of multiloop voltage regulation strategies for single and three-phase UPS systems,” IEEE Transactions on Power Electronics, vol. 18, no. 5, pp. 1176–1185, 2003. [21] M. Rizo, M. Liserre, E. Bueno and F.J. Rodriguez, “Voltage Control Architectures for the Universal Operation of DPGS,” IEEE Transactions on Ind. Informatics, vol. 11, no. 2, pp. 313–321, April. 2015. [22] A. Hasanzadeh, O.C. Onar, H. Mokhtari and A. Khaligh, “A Proportional-Resonant Controller-Based Wireless Control Strategy With a Reduced Number of Sensors for Parallel-Operated UPSs,” IEEE Trans. on Power Delivery, vol.25, no.1, pp.468-478, Jan. 2010.