Three-Level Boost Converter based Medium Voltage Megawatt PMSG Wind Energy Conversion Systems Venkata Yaramasu and Bin Wu Department of Electrical and Computer Engineering Ryerson University Toronto, ON M5B 2K3 Canada e-mail: [email protected]
(2300kW/400V), GWP82 (2000kW/690V), Kenersys K82 (2000kW/690V) and STX72 (2000kW/660V) . However, this design presents a number of technical concerns such as reduced reliability, low efficiency due to high component count and high cost. For instance, 3MW/690V WECS requires 4 power converters with 12 switches each to operate in parallel. The high line current (approximately 2500A) of this system makes cables running from nacelle to the bottom of the tower bulky and costly in addition to high power losses (I2R) in the cable . An alternative solution to address the above issue is to use medium voltage (MV) (3000-4000V) BTB neutral point clamped (NPC) converters which are most cost effective, compact, reliable and efficient -. This technology has been applied by two leading companies, ABB and Converteam (formerly Alstom) to Multibrid M5000 (5000kW/3300V) wind turbine -.
Abstract—For power rating of 2MW or higher, the medium voltage (MV) back-to-back (BTB) neutral-point clamped (NPC) converters are most preferable choice for wind turbine manufacturers as they reduce cost, size and complexity of the system compared to the BTB two-level converters. In this paper, a new MV topology using diode rectifier, three-level boost (TLB) and NPC converter has been proposed to further reduce the cost and size. The dc-link maximum power point tracking (MPPT) control scheme has been proposed with which the TLB performs MPPT and balancing of dc-link capacitors and thus provides a greater flexibility for NPC control. The simulation results for 3MW/3000V/53.33Hz nonsalient pole PMSG wind energy system validate the proposed topology and control scheme.
The research and development of megawatt turbines are of key attention in recent years for wind turbine manufacturers due to the increased wind energy market . The variable speed wind turbine technology using permanent magnet synchronous generators (PMSG) and fullscale power converters is rapidly growing due to higher efficiency, lower mechanical stress and reduction in installation and maintenance costs . Moreover, the leading/lagging grid reactive power control and fault ride through operation can be achieved without the need for additional equipment . Many power converter topologies are being developed for PMSG wind energy conversion systems (WECS) in a continued effort to reduce cost, increase reliability and improve wind energy conversion efficiency -.
Since PMSG doesn’t require magnetizing current, passive front-end converter can be used because the diode rectifier is inherently more reliable and cheaper than the PWM rectifier . The high power LV PMSG WECS using diode rectifier and boost converter has been analyzed in . This technology is being used by the Clipper Wind Power Liberty C89 (4*625kW/690V) . The boost converters can also be connected in parallel to increase power handling capacity -. A typical example of using the diode rectifier and multi-channel boost converter is the Vensys 70/77 (1.5MW/690V) wind turbine system . Considering the commercial success of passive front-end converters, a new converter configuration using diode rectifier, three-level boost (TLB) and NPC converter is proposed for MV PMSG WECS technology as shown in Fig. 1. This configuration simple, less costly and less in weight compared to BTB NPC converters. The application of this converter for LV photovoltaic systems has been analyzed in . The dc-link MPPT control technique is proposed and applied to 3MW/3000V/53.33Hz non-salient pole PMSG WECS to perform MPPT and balancing of dc-link capacitors. With the proposed control scheme a simple modulation scheme can be used for NPC control. The MATLAB simulation results validate the proposed method.
For most commercial PMSG WECS, low-voltage (LV) (0.5. In the region 2, the input voltage vin is greater than or equal to the center point voltage vdc 2 and thus duty cycle D ≤0.5. As shown in Fig. 4, the capacitors C1 and C2 are alternatively charged by inductor and thus they are theoretically balanced.
A. Symmetrical Operation The PMSG along with diode rectifier is represented by an input voltage vin and grid-side converter along with harmonic filter is represented by R1 and R2 as shown in Fig. 2. During symmetrical operation of TLB with the duty cycles D1 = D2 and capacitors C1 = C2 , the center point voltage can be maintained at vdc 2 .
B. Asymmetrical Operation When this converter is used in wind energy conversion systems, the load values R1 and R2 will never be same due to the switching actions of NPC converter ,  and thus leads to asymmetrical operation as shown in Fig. 5. By controlling the duty cycles D1 and D2 independently, the capacitors C1 and C2 can be charged or discharged independently to achieve neutral point balance. The independent generation of duty cycles can be accomplished by dc-link MPPT control as shall be discussed in the next section.
Figure 2. Configuration of three-level boost converter
With symmetry, this converter operates in four modes as follows -:
(b) Figure 4. Symmetrical operation of TLB:(a) D > 0.5 and (b) D ≤ 0.5
Mode1 Mode 2
Mode 1 Mode3
(d) Figure 3. Three-level boost converter modes of operation: (a) mode 1, (b) mode 2, (c) mode 3 and (4) mode 4 (b) Figure 5. Asymmetrical operation of TLB:(a) D > 0.5 and (b) D ≤ 0.5
PROPOSED CONTROL SCHEME
vdc 1 = vin 1 − D1
The block diagram of the proposed control scheme is shown in Fig. 6. Four variables need to be controlled tightly: the dc-link voltage, balancing of dc-link capacitors, active power and reactive power. The TLB performs MPPT (active power control of WECS) and balancing of dc-link capacitors, while the NPC controls the dc-link voltage and reactive power to the grid.
1 ρ A vw3 C p . 2
where, the input dc-voltage vin is uncontrollable and varies with respect to the generator speed (power). The second PI generates error duty cycle ΔD as follows:
ΔD = (k1 + k 2 / S )(vc1 − vc 2 ) .
A. dc-link MPPT Control The MPPT profile block provides power reference for the control system based on power versus wind speed curves using the following expression: P* =
for 0 ≤ D1 < 1 .
where (k1 + k 2 / S ) is the transfer function of the PI controller. The duty cycle D2 controls the NP balancing as follows:
D2 = D1 + ΔD .
B. Decoupled Voltage Oriented Control of NPC Converter The decoupled voltage oriented control (VOC) scheme is used to control the dc-link voltage vdc and reactive power
By controlling the dc-link current idc through a current feedback control using PI controller, the active power (MPPT) can be controlled as it is simply a product of dclink input voltage and current. In other words, the duty cycle D1 controls the WECS active power. The boost nature of the TLB can be defined similar to the conventional boost converter as follows:
Qg to the grid independently. There are three feedback
control loops in the VOC scheme: two inner current loops for the accurate control of the line currents idg and iqg, and
Figure 6. Proposed dc-link MPPT control for MV PMSG WECS
one outer dc voltage feedback loop for the control of dc voltage vdc.With a proper grid voltage orientation, the threephase line currents in the abc stationary frame, iag, ibg and icg, are transformed to the two-phase currents, idg and iqg in the dq synchronous frame, which are the active and reactive components of the three-phase line current, respectively. The independent control of these two components provides an effective means for the independent control of system dcvoltage and reactive power. The output of the decoupled controller can be expressed as
⎧vdi = −(k1 + k 2 / S ) − idg + ωLiqg + vdg ⎪ . ⎨ * ⎪⎩vqi = −(k1 + k 2 / S ) iqg − iqg − ωLidg + vqg * idg
Pg , Qg (pu ) 0
Figure 7. Grid active and reactive powers (base: S B )
I ag , I dg , I qg (pu ) 1.0 0.5
Since, the TLB controls the NP, a simple carrier based modulation scheme with 3rd harmonic injection is used. IV.
The MATLAB simulations are carried out on 3MW/ 3000V/53.33Hz PMSG WECS for different operating conditions such as WECS start-up and step changes in wind speed and are analyzed as follows.
Figure 8. Grid phase-a current and its dq components (base: I B ) 1.0
A. WECS Start-up
Fundamental (60Hz) = 567.2 (rms) , THD= 1.57%
I ag (pu )
The active power Pg starts to increase at t = 0.1 sec and
ramps up to its rated value (1pu) at t = 1.1 sec, and is then kept constant as shown in Fig. 7. The amplitude of the grid current iag (Fig. 8) is proportional to that of the active power
since its reactive component is zero. The FFT analysis for iag is shown in Fig. 9.
Figure 9. Harmonic spectrum of iag
The dc voltage vin decreases with the increase of the active power and falls to 1.63 pu at t = 1.1 sec as shown in Fig. 10. The decrease of vin is mainly caused by the voltage drop across the synchronous inductance of the generator, which increases with the generator current. The dc current idc (Fig. 11) varies proportionally with Pg , and reaches 1.84
vin (pu ) 1.9
pu at t = 1.1 sec, at which the dc power (Fig. 12) reaches the rated power (3pu) of the system. The dc voltage vdc is kept fairly constant by the NPC during the transient as well as in steady state as shown in Fig. 13. The TLB balances NP with fast transient response and low steady-state error (0.05pu) as shown in Figs. 14-15. As discussed before the TLB operates in asymmetrical mode to balance NP as shown in Fig. 16.
1.63 pu 0.4
Figure 10. Input dc-link voltage (base: V B ) idc (pu ) 1.6
B. Step Changes in Wind Speed Step increase from 0 to 1pu in active power Pg is applied
at t = 0.2 sec and step decrease from 1 to 0.5 pu active power as shown in Fig. 17. The TLB balances NP with faster transient response and lower steady-state error as shown in Fig. 18.
Figure 11. dc-link current (base: I B )
Pdc (pu )
Pg , Qg (pu ) 1
Figure 12. dc-link power (base: S B )
Figure 17. Grid active and reactive powers (base: * vdc , vdc (pu )
vc1 , vc 2 (pu )
Figure 13. dc-link voltage and its reference (base: V B ) vc1 , vc 2 (pu )
Figure 14. dc-link capacitor voltages (base: V B ) vc1 − vc 2 (pu ) 0.5 0 -0.05 -0.15 0
Figure 15. dc-link neutral point deviation (base: V B ) D1 , D2
The converter configuration is simple, less costly and less in weight compared to BTB NPC.
The NPC converter no longer needs to control the balancing of capacitors and thus a simple modulation scheme can be used for the NPC control.
The equivalent switching frequency of TLB is twice the conventional boost converter and thus offers lower input current ripple and output voltage ripple, faster dynamic response and better power handling capability. ACKNOWLEDGMENT
The authors gratefully acknowledge the financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada through Wind Energy Strategic Network (WESNet) Project 3.1.
In this paper, a new power converter topology using diode rectifier, three-level boost and NPC converter has been proposed for MV PMSG WECS. With the proposed dc-link MPPT control, the active power control from the wind turbine and balancing of the dc-link capacitors has been achieved with TLB under WECS start-up and step changes in wind speed condition. This configuration is promising for next generation WECS because of the following reasons:
vc 2 0.4
Figure 18. dc-link capacitor voltages (base: V B )
Figure 16. Duty cycles for three-level boost converter
Appendix TABLE I.
3.0MW/3000V/53.33HZ PMSG PARAMETERS
Rated output power
Rated mechanical shaft input power
Rated apparent power
Rated line-line voltage
3000 V (rms)
Rated phase voltage
1732.05 V (rms)
Rated stator current
700 A (rms)
Rated stator frequency
Rated power factor
Rated rotor speed
Number of pole pairs
Rated mechanical shaft input torque
Rated rotor flux linkage
4.3034 Wb (rms)
Stator winding resistance
d-axis synchronous inductance
q-axis synchronous inductance
1.0 pu 
GRID AND FILTER PARAMETERS
GSC apparent power S B
Grid phase voltage V B
1732.05 V (rms)
GSC rated current I B
577.4 A (rms)
GSC switching frequency
THREE-LEVEL BOOST CONVERTER PARAMETERS 
Input capacitor, Cin
Output capacitors, C1 & C2
Inducrors, L1 & L2
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