Development of Wind IPMSG Based Bipolar DC Microgrids

This paper presents the development of a wind Interior Permanent-Magnet Synchronous Generator (IPMSG) based bipolar DC microgrids along with various switch-mode rectifiers. Firstly, the wind IPMSG is established and adequately controlled to possess satisfactory generating characteristics during different driven speeds and loads. Later, the boost switch-mode rectifier (SMR) based bipolar DC bus is established. Further, three-phase single-switch (3P1SW) boost SMR, three-phase two-switch (3P2SW) three-level boost SMR, and a three-phase three-switch (3P3SW) Vienna SMR are comparatively evaluated. Along with the proposed robust voltage and current controls, a well-regulated microgrid DC-bus voltage is established. Moreover, the voltage balancing control is proposed to minimize the imbalance in the bipolar DC-bus voltage. For the wind IPMSG having Vienna SMR, the commutation angle setting is adjusted to use the reluctance power component effectively.


Introduction
Microgrids with renewable sources can be operated independently to reduce fossil energy consumption effectively [1][2][3][4][5]. Microgrids are classified into AC microgrid, DC microgrid, and AC/DC hybrid microgrid. Comparatively speaking, the DC microgrid has a few merits, such as a longer bus length, simpler and fewer interface converters used for sources and loads, etc. In constructing a DC microgrid, one can adopt a unipolar or bipolar bus structure. Compared with the former, bipolar shows higher reliability because of an open line fault, fewer power switches used to form load converters, etc.
Any electric machine can be operated as a generator and applied to construct a wind generator [6]. However, due to volumetric and weight limitations, AC machines are still the mainstream actuators. Compared with other machines, the permanent-magnet synchronous machine (PMSM) has the advantages of a simple structure, brushless without excitation-winding, high power density, etc. [7][8][9][10]. Owing to high-performance permanent magnet-material manufacturing technology improvement, wind power generation systems tend to use PMSG, especially the interior PMSG (IPMSG), for its rigid rotor and higher efficiency [11][12][13].
The AC/DC converter is an essential front-end for an electronic power plant powered by the utility grid. To have a better quality of line-drawn power and a well-regulated DC output voltage, one can adopt a suited SMR having power factor correction (PFC) control. The surveys for the existing SMRs can be found elsewhere [14][15][16]. A three-phase SMR has to be equipped as its following converter for a wind generator using three-phase AC machines. Normally, the boost type SMR is adopted to establish the common DC-bus voltage of the microgrid. Considering the schematic and the control, the possible SMRs for the wind generator may include: (i) three-phase single-switch (3P1SW) DCM SMR [17]; (ii) 3P1SW CCM SMR [14,15]; (iii) three-phase two-switch (3P2SW) SMR [18]; (vi) three-phase three-switch (3P3SW) Vienna SMR [19,20]; and (v) three-phase six-switch (3P6SW) full-bridge SMR. Since only unidirectional power capability is required for a wind generator, the Vienna SMR is considered to be the best schematic in making compromised considerations in switch number, the sinusoidal input current waveform PWM switching ability, and the switching losses. More importantly, the bipolar DC bus with three-level voltage switching can be formed naturally.
To integrate various harvested sources, energy storage devices, loads, and utility grids with the DC microgrid system, the proper power electronic converters based on their specific characteristics need to be employed. The AC/DC converter is used for the AC wind generator. The survey of the commonly used SMRs has been presented above. The function of the DC/DC converter is to adapt the input/output voltage levels. The DC/DC converters can be broadly classified into (i) isolation and non-isolation; (ii) single-port and multi-port; (iii) step-up, step-down and step-up/down; (iv) unidirectional and bidirectional; and (v) hard switching and soft switching [21][22][23][24][25][26][27][28][29]. While the unidirectional converters are applied for photovoltaic and solid-state fuel cells, the energy storage devices must adopt the bidirectional ones for achieving charging/discharging operations [24][25][26][27][28][29]. As for the local load and utility grid, they need to be interfaced with the DC microgrid common DC-bus via PWM inverters [30][31][32][33][34].
This paper presents a wind IPMSG-based bipolar DC microgrid followed by different SMRs. Compared to other topologies, the Vienna SMR offers acceptable performance regarding power quality, control flexibility, and unidirectional power flow. Furthermore, it is indicated that the commutation instant shift should be properly adjusted to extract more power from the IPMSG, which can be considered for implementation in future wind IPMSG systems. Apart from the introduction section, the organization of this paper is listed as followed: (i) Section 2 presents the system configuration and problem statements. After providing the topologies and control schemes of the adopted SMRs, the used machines for the prime mover and the wind generator are introduced; (ii) The non-sinusoidal SMR based bipolar DC microgrid is provided in Section 3. It is found that the inherently large inductance of the IPMSG does not allow the SMR to operate normally under the DCM; hence two SMR topologies are used for evaluating comparatively; (iii) Section 4 introduces the three-phase Vienna SMR based bipolar DC microgrid. Through the proposed control schemes, it possesses a better line drawn power quality with a higher utilization rating. The commutation instant shift for the IPMSG with followed Vienna SMR can also be applied to increase the energy conversion efficiency; and finally, (iv) Section 5 provides the conclusion for this paper. Figure 1 shows the conceptual block diagram of a wind IPMSG based bipolar DC microgrid. A three-phase AC/DC converter is used as the following converter for the AC wind generator. A suitable type of SMR can be adopted to have good AC input and DC output characteristics. Further, the bipolar DC bus having well-regulated voltages is established using suitable schematics. The power stages being interfaced to the microgrid common DC bus may include battery energy storage system, local loads, utility grid, and other parts. The unidirectional SMR can be employed for a wind IPMSG. The simplest schematic of a wind IPMSG with 3P1SW DCM SMR for establishing the microgrid bipolar DC bus is depicted in Figure 2(a). The AC side energy storage inductors must be small enough to ensure the DCM operation for the overall load range [17]. The PFC function is then inherently preserved without applying current control. Figure 2(b) shows the voltage-mode direct duty control scheme. The bipolar DC-bus is established using a passive capacitor leg. If the inherent voltage imbalance needs to be avoided, one can use the IGBT voltage balancer as an alternative to replacing the capacitor leg.  For a utility grid-powered 3P1SW DCM SMR, the energy storage inductors are added externally, and they have certain limitations. However, analytic and experimental studies indicate that the limitation for low inductance of the energy storage inductor generally cannot be satisfied for the inherent PMSG armature winding inductance. The quantitative analyses will be presented later in Sec. 3.1.2.

The Studied Plants
To avoid the limitations of the energy storage inductor possessed by the 3P1SW DCM SMR, one can adopt the 3P1SW CCM SMR depicted in Figure 3(a). It is formed by inserting a CCM DC/DC boost converter between the diode rectifier and the capacitive filter. The standard cascade control scheme consisting of outer-voltage and inner-current loops is shown in Figure 3(b). Similarly, the bipolar DC bus is also formed by a capacitor leg. The voltage transfer relationship under various driving speeds of the developed wind IPMSG is depicted in Figure 3(c).
The wind IPMSG with 3P2SW CCM SMR and its control scheme are shown in Figures 4(a) and 4(b), wherein the proposed voltage balancing scheme is also arranged. The three-level bipolar DC-bus is inherently established.
Although the topologies of Figure 2 to Figure 4 are simple, they possess non-sinusoidal armature winding current with a low power factor and thus attain a low generator rating utilization. To solve this problem, one can adopt the three-phase three-switch (3P3SW) three-level Vienna rectifier depicted in Figure 5(a). Owing to its 3P3SW structure, the three-phase Vienna SMR can control three-phase currents separately to have a sinusoidal line drawn current having high rating utilization. Moreover, one can obtain higher power generating capacity for the IPMSG by adjusting the commutation shift angle. The control scheme of the wind IPMSG with Vienna SMR is shown in Figure  5 Figure 5 Wind IPMSG based bipolar DC microgrid with 3P3SW Vienna boost SMR: (a) schematic; (b) control scheme.

Governing Equations
The electromagnetic developed torque or retarding torque T e of a PMSG in the dq-domain can be written as: where P = pole numbers, B = total damping coefficient, J = total inertia constant, T i = input torque. Let β be the phase angle between a-phase current phasor and the q-axis, i.e., ̃= √2 ∠ − , one can obtain the following expressions for the d-q current components and the developed torque: Further, the electromagnetic developed power can be expressed as = .
Page 7/23 The first and second terms of equation (4) are known as magnet torque and reluctance torque, respectively. It should be noted that < for an IPMSG, and the proper setting of β is required for maximizing T e and P e . For an SPMSG, only the magnet torque exists because of = , and = max = 0 ∘ can be set for yielding maximum values of T e and P e . On the contrary, in a wind IPMSG, one can adjust the value of β to reduce power generation when there is a surplus of energy for issuing the power conditioning control.

Key Parameters of the Employed SPMSM and IPMSG
In the studied experimental wind IPMSG, as shown in Figure 1, an inverter-fed SPMSM drive is employed as an alternative to the wind turbine. The key specifications and parameters of the used SPMSM and IPMSG are listed in Table 1. The switch Q and diode D in Figures 2(a) and 3(a) and the devices (Q 1 , Q 2 ) and (D 1 , D 2 ) provided in Figure 4(a) are obtained using off-the-shelf IGBT Module CM100DY-12H (100 Amperes, 600 Volts) made by Powerex Inc.

Feasibility of the Employed IPMSG
The operation under DCM for the SMR is depicted in Figure 2(a); the inherent power factor correction function is preserved without an extra current controller. The control scheme with only a voltage loop is shown in Figure 2(b). Although the control is simple, the limitation of inductance for DCM operation must be considered.
To allow the successful operation of the SMR, the energy storage inductances must be small enough to ensure the DCM operation for the overall load range of the selected switching frequency and a complete half AC cycle. According to Table 1, it is understood that the phase back-EMF peak E m under 3000 rpm is 116.47 V, and the maximum inductance ,max = 6.83 mH. For the given = 400 V ( = (400 − 116.47 × √3)/400 = 0.4958), = 1 kW, the maximum value of the phase current can be calculated as ,max = 11.45A. Thus the estimated switching frequency for preserving the boundary condition (Upper bound of DCM) can be derived as = ( )/( ,max ) = 738 Hz . Hence the allowed is unreasonably low compared to the fundamental frequency of the generated voltage by the employed IPMSG to build the 400 V DC bus. Thus, the 3P1SW DCM SMR is not suitable to be the follow-up converter of the employed IPMSG, which has a large embedded winding inductance.

Control Scheme
By applying the state-space averaging technique, the equivalent control block of the current-loop is derived and shown in Figure 6 Current Control Scheme. The current feedback controller is chosen to be the proportionalintegral (PI) type having: The design methodology lies in: (i) By applying the large-signal stability criterion, the upper limit of the P-gain is first determined to be < 6.62 ; And (ii) Choosing the I-gain = 100 , the simulated Bode plots of the loop-gain for the current-loop shown in Figure 6(a) are used to determine the final P-gain = 1.5 with the crossover frequency of = 2.2 kHz, accordingly: Voltage Control Scheme.
(a) Voltage feedback controller After having a well-designed current-loop, ′ = ( ) ≈ * can be reasonably assumed to yield the outer voltage-loop control block, as shown in Figure 6(b). The voltage sensing factor is given as: The voltage feedback controller, which is shown in Figure 6(b), is chosen to be the PI type, with: The desired voltage response without overshooting due to the step load power change is also sketched in Figure 6(b). The design procedure is briefly described as follows: (i) At the given operating point ( = 183.84 V, = 400 V, = 300 Ω), the step-response approach is applied to yield the estimated plant model parameters:  (b) Robust controller To enhance the control robustness, a simple, robust control scheme, as shown in Figure 3 is added. The robust weighting function is set as: It can be shown that: (i) The voltage tracking error yielded by PI control only can be reduced to (1 − ) by adding the robust control; (ii) However, the controlled efforts would be magnified. Taking the compromised considerations, = 0.6 is chosen.

Experimental Results
The inverter-fed SPMSM driven IPMSG shown in Figure 3 is operated under the condition of ( = 142.65V/150Hz/3000rpm, = 400 V, = 150 Ω). The measured ( , , , , ) are shown in Figure 8  The measured and * at ( = 400 V, = 300 Ω ) without ( = 0 ), and with robust control ( = 0.6) of the boost converter powered by the DC power supply at the DC-link with due to a change in the step load of = 300 Ω → 150 Ω are shown in Figure 9. The effectiveness of robust control in enhancing the regulation response can be understood from the results. From the experimental results (not shown here), one can observe the imbalance between 1 and 2 as the bipolar DC-bus loads are not balanced. To improve this, one can add a voltage balancer across the DC-bus, or the following two SMRs are adopted.

Efficiency Assessment
To evaluate the efficiency of the established 3P1SW CCM SMR, the energy conversion efficiency assessment facility is arranged in Figure 11. Three power analyzers (using Yokogawa WT500 and HIOKI 3390 power analyzers) were used to measure the load power and the two armature powers and of the SPMSM and the IPMSG, respectively.

Figure 11
System configuration of efficiency measurement. The output DC voltage = 400 V is set, and the load resistance is varied from 800 Ω to 160 Ω (P d = 200 W to 1000 W). The related efficiencies were defined as 1 ≜ ⁄ , 2 ≜ ⁄ (the efficiency of SMR), 3 ≜ ⁄ = 1 2 , and ≈ ≜ √ 1 . Since the machine shaft power cannot be measured directly, the IPMSG efficiency is approximately estimated from and by assuming ≈ . The power-efficiency curves of and 2 are plotted in Figures 12(a) and 12(b).

Figure 12
Efficiency curves of the developed wind IPMSG followed by 3P1SW CCM SMR under varied and driven speed at = 400 V: (a) -curve; (b) 2 -curve. ( ) = + = 4 + 22 (16) Robust Controller: Same as (14).  Figure 4(a) one can observe that the charging periods of 1 and 2 depends upon their PWM control voltages. Hence as shown in Figure 4(b), the voltage balancing of 1 and 2 can be achieved by modifying the control voltage for 2 as 2 = 1 + , along with being generated from a PI voltage balancing controller ( ). The parameters of ( ) are determined via the method of trial-and-error are as follows:

Wind IPMSG with 3P2SW Three-Level CCM Boost SMR
To observe the balanced voltage characteristics of the established bipolar DC-bus, let 1 = 75 Ω and 2 be changed in the step between 75 Ω and 100 Ω. Figure 13(a) shows the measured , , ( 1 , 2 ) of the 3P2SW CCM SMR due to the step resistance change of 2 = 75 Ω → 100 Ω → 75 Ω at ( 1 = 75 Ω, 2 = 75 Ω ) without the voltage balancer, and Figure 13(b) shows the measured , , ( 1 , 2 ) of the 3P2SW CCM SMR with voltage balancer. One could note the correctness of the voltage balancer.

Experimental Results
The wind IPSMG is driven at 3000 rpm, and the measured ( , , , , ) of the developed Wind IPMSG along with the followed 3P2SW three-level boost SMR are shown in Figure 14(a). And the measured , ( * , ′ ) and ( * , ′ ) of the three-level boost converter are plotted in Figure 14

Efficiency Assessment
The energy conversion efficiency measurement facility arranged in Figure 11 is also employed in this case. To measure the steady-state power characteristics, the output DC voltage is set as a constant (400V). The load resistances 1 and 2 are set at five values from 400 Ω to 80 Ω ( = 200W to 1000W). The power-efficiency curves of and 2 are plotted in Figures 16(a) and 16(b).

Figure 16
Efficiency curves of the developed wind IPMSG followed by 3P2SW three-level boost SMR under varied and driven speed at = 400 V: (a) -curve; (b) 2 − curve.

Vienna SMR Based Bipolar DC Microgrid
To improve the limitations possessed by the non-sinusoidal SMRs presented in the previous section, the three-phase three-switch Vienna SMR could be adopted as the wind generator followed converter. The armature currents can be PWM controlled to have a sinusoidal waveform with a higher rating utilization. Meanwhile, the micro-grid bipolar DC bus is inherently established. Through careful derivation, one can express the phase current ripple as:

Power Circuit
The maximum current ripple occurs when = 2 ⁄ . Normally, to operate in the CCM, the current ripple is needed:

Current Control Scheme
The current feedback controller is selected to be the proportional-integral (PI) type with: To enhance the dynamic response of the DC-bus voltage regulation, the robust controller is also added with the weighting function:

Voltage Balancing Controller
In Figure 5

Effects of the Commutation Angle Shift
For an IPMSG, the commutation shifting angle β can be included to improve the energy conversion efficiency. To observe the effectiveness of the shifting angle, the voltage loop of the Vienna SMR shown in Figure 5(b) is removed. By letting the output resistive loads be = 400 Ω and 200 Ω without 1 and 2 , the current command is manually tuned as ̂= 2A and 3.98 A with = 0° to result in the output voltage of 400 V. Starting from the initial point ( = 0°), the shifting angle is set between -30° to 30° with = 2°. Figure 18 shows the relationships between the shifting angle β and the output voltage . One can observe the influence of d v and thus the = 2 / by varying the β angle.

Figure 18
Relationships between the shifting angle and under two loads.

Operating Characteristics
The voltage loop is applied with * = 400 V. The steady-state characteristics of the IPMSG followed by the Vienna SMR with shifting angles = −30° and = + 30° under two resistive loads are listed in Table 2. From Figure 18 and Table 2, one can understand the effects of commutation angle on the IPMSG generating characteristics. Let the wind IPMSG be driven at = 3000 rpm with a load of ( = 400 V, = 150 Ω). The measured ( , , ), , , are shown in Figure 19. Compared to the of Figure 19 to the ones depicted in Figures 8 and 14, one can find that the slightly better waveform is yielded by the Vienna SMR. Meanwhile, the measured waveforms of , , and at ( = 400 V, = 150 Ω) of the wind IPMSG system under the varying speed of = 3000 → 2500 → 2000 → 2500 → 3000 rpm are shown in Figure 20. The well regulated d v under varying driven speed can be observed.   To measure the steady-state power characteristics, the output DC voltage is set as a constant (400 V). The load resistances 1 and 2 are set according to five values from 400 Ω to 80 Ω ( = 200 W to 1000 W). The power-efficiency curves of and 2 (Efficiency of the SMR) are plotted in Figures 21(a) and 21(b). While comparing the results plotted in Figure 21 to those shown in Figures  12 and 16, one can observe the slightly higher and 2 by applying the Vienna SMR as the wind generator followed converter.

Conclusions
This paper has presented the development of wind interior IPMSG based bipolar DC microgrids consisting of various switch-mode rectifiers. Some interpretation can be concluded from the experimental explorations: (i) Through the proposed robust voltage and current controls, a wellregulated microgrid DC-bus voltage is established; (ii) 3P1SW DCM and CCM boost SMRs having a passive capacitor leg to form bipolar DC-bus do not have voltage balancing capabilities. To improve this, an extra voltage balancer leg must be added; (iii) Normally, the 3P1SW DCM SMR is not suitable to be the followed converter of the IPMSG having a large embedded winding inductance; (iv) Both, the 3P2SW three-level boost SMR and the 3P3SW Vienna SMR can naturally form the bipolar DCbus, and the voltage balancing can be preserved via proper control. Compared to the former, the Vienna SMR has a slightly better power quality on the AC side and efficiencies of the SMR schematic.