A Novel UPF Control Strategy of PMSG Based on Stator Flux Orientation

XuZhen Shen, Yi Wang, YongGang Li School of Electrical and Electronic Engineering

North China Electric Power University, Baoding, China E-mail－yi.wang@ncepubd.edu.cn

Abstract—The traditional control strategies of the permanent magnet synchronous generator (PMSG) based on rotor flux orientation have unconquerable drawbacks, such as the reactive power exchange by the id=0 control and the complicated calculation by the unity power factor (UPF) control. A novel UPF control strategy of the PMSG using stator flux orientation is proposed in this paper. The simulation results of these control strategies of the PMSG indicate that the proposed control strategy can achieve the performances of UPF and decoupling control with simplified calculation.

Keywords— PMSG; wind generation;Stator flux orientation; unity power factor (UPF)

I. INTRODUCTION The market of the MW-level wind power system has

been dominated by the doubly fed induction generator (DFIG) currently. The directly-driven permanent magnet synchronous generator (PMSG) wind turbine’s market share is increasing every year with the advantages that no gear box and with a better low voltage ride through ability (LVRT) when grid voltage drops[1-3], The permanent magnet synchronous generator for wind generation is normally driven by a “AC-DC-AC” converter. Compared with the “diode rectifier + boost chopper + PWM inverter”, the back-to-back PWM voltage source converter can be controlled more flexible. It significantly reduces the harmonic current and torque fluctuation, and improves the power factor of the generator. Therefore, the back-to-back converter achieves more attention in PMSG-based wind farms.

The permanent magnet synchronous motor (PMSM) based on rotor flux-orientation always uses the following control methods: zero direct-axis current control (id=0), unit power factor (UPF) control, constant flux control, weaken flux control and maximum ratio of torque and current control [4].

In wind power generation, it always sets d-axis current zero or UPF to achieve different control requirements under rated wind speed. The former has a simple control algorithm. However, the generator needs to absorb reactive power which increases converter capacity. The latter limits the reactive power of the generator-side,

This work was supported by the National Natural Science Foundation

of China (No. 50977028 and No. 50107085) and by the Fundamental Research Funds for the Central Universities.

however the control algorithm is complex, and the active power is coupled with the reactive power. In order to resolve these problems, the paper proposes a new UPF control strategy of generator-side converter, based on the reorientation of the stator flux. It can not only achieve the active power and reactive power decoupling with UPF, but also simplify the calculation. The feasibility of the proposed strategy is verified by the simulation comparing these methods during the maximum power point tracking (MPPT).

II. THE MODEL OF WIND TURBINE Wind energy captured by wind turbine from the air

can be expressed as:

),(21 32 λβρπ pwm CVRP = (1)

whereρ is air density, R is rotor radius of the wind turbine,βis pitch angle, tip speed ratioλ=Rωw/Vw,ωw is the speed of wind turbine, Cp is power coefficient related with the pitch angle and tip speed ratio. When pitch angle and wind speed are fixed, only operated at a particular Cp the output power can achieve the maximum. By connecting the maximum power point corresponding to different wind speeds, the curve of wind turbine power characteristics can be achieved as Fig. 1.

III. CONTROL STRATEGY BASED ON ROTOR FLUX ORIENTATION

The AC-DC rectifier on the generator-side controls the torque and the reactive power of stator by adjusting the currents of d and q axis in the stator. The grid-side DC-AC converter controls the DC voltage and the reactive power to the grid by adjusting the currents of d and q axis to the grid, which can realize decoupling control of the output power. The damp load is used to consume the redundant energy accumulated in DC-side under grid faults.

Fig.1 Power characteristics curve of wind turbine

978-1-4244-6255-1/11/$26.00 ©2011 IEEE

wind turbine

generator-sideconverter

grid-sideconverter

PMSG

C

L

GRID

Fig.2 Wind power system using PMSG with full back-to-back

converter

A. The model of PMSG According to the generator convention, the PMSG

model in the synchronous rotating reference frame oriented on the rotor flus is given as[5-7]:

sq

sd s sd e sq

sqsq s sq e sd e f

diu R i Li L

dtdi

u R i Li Ldt

ω

ω ω ψ

⎧= − + −⎪⎪

⎨⎪ = − − − +⎪⎩

(2)

where isd、isq、usd and usd are the d, q axis component of stator current and voltage, respectively, Rs is the generator resistance which is very small, ω e is the electrical angular frequency of the generator,ψf is the flux of rotor. Generally without considering saliency effects of the rotor magnetic, Ld=Lq=L. p is the pair of poles. The electromagnetic torque and power are expressed as

sqfsqfsqsdqde ipiiiLLpT ψψ23])[(

23 =+−= (3)

⎪⎪⎩

⎪⎪⎨

⎧

−=

+=

][23

][23

sqsdsdsq

sqsqsdsd

iuiuQ

iuiuP (4)

B. The control strategy of zero d-axis current Below the rated wind speed, the speed is controlled by

the generator-side converter to achieve maximum power tracking. Ignoring the dynamic characteristics, this kind of control strategy is the same as the control strategy of maximum torque /current ratio. At this point the equation of PMSGs can be written as below:

sd s sd e sq

sq s sq e sd e f

u R i Liu R i Li

ωω ω ψ

= − +⎧⎪⎨ = − − +⎪⎩

(5)

⎪⎪⎩

⎪⎪⎨

⎧

=

=

sqsds

sqsqs

iuQ

iuP

2323

(6)

Fig.3 shows that: A rising speed leads to the increase of active and reactive power on generator-side too. By the instantaneous power theory, the output power is shown as above, and the power factor of the generator-side decreases.

d

us

jxis

jxis'

fdψsi

'si

'su

Fig.3 Vector control chart of id=0

C. The control strategy of UPF While the generator operates in the UPF state,

neglecting dynamic changes in the stator resistance and current, the stator voltage can be expressed as[8-10]:

⎩⎨⎧

+−==

fesdesq

sqesd

LiuLiu

ψωωω

(7)

If Q=0, the following relationship can be obtained according to (4).

fsd

sq

sq

sd

sq

sd

LiLi

uu

ii

ψ+−== (8)

A quadratic equation on isd above can be derived from the proportional relationship, where isq can be expressed by Te, as a braking torque.

)4(21 222

sqffsd iLL

i −+−= ψψ (9)

sisu

0e sjxi

Fig.4 Vector control chart of UPF

Fig.4 shows that: A rising speed leads to the increase of active power on generator-side only. The power factor of the generator-side is kept at zero.

IV. THE CONTROL STRATEGY OF UPF BASED ON STATOR FLUX.

Name the angle from A axis in three-phase stationary reference frame to the rotating rotor flux asθ, the angle from stator flux to rotor flux asθs, which is getting from us=Rs is+ωeψs. So the equations based on stator flux orientation can be expressed as follows:

sd s sd

sq s sq e sd

u R iu R i ω ψ

= −⎧⎨ = − −⎩

(10)

⎪⎩

⎪⎨⎧

=

=

023

s

sqsqs

Q

iuP (11)

Fig.5 shows that: The active power output of the system increases and the reactive power keeps zero value when turbine speed increases. So they are controlled to be decoupled. The control scheme of generator-side converter is showed in Fig.6.

fψ

sψ

0 e fe ω ψ=

sθ

Fig.5 Vector control chart of UPF based on the stator flux

fsddiL ωψω +'

'sqqiLω

dtdθ

se θ−

ω θ

'sdi

'sqi

0* =sdi 'sdu

'squ*ω

ω

θ

sθθ +

*sqi

Fig.6 Control scheme of generator-side converter

V. SIMULATION STUDIES The simulation and operation control system is built

based on Matlab/Simulink. There are five PMSGs with thirty-eight pairs of poles each in a wind farm. The rated output power is 2MW; the rated armature voltage is 690V; the rated DC voltage is 1200V and the rated generator speed is 16.7r/min. The power devices are IGBTs with a 2 kHz switch frequency. Simulation examples and results are shown as follows, when wind speed is 12 m/s during the first 3 seconds and then down to 9.6 m/s in a 8 seconds’ simulation time.

Simulation example 1: Generator-side converter is controlled by the strategy of zero direct-axis current. The operation characteristics curve is shown in Fig.7. As wind speed jumps from 9.6m/s to 12m/s, the output mechanical power rises from 0.38pu. to 0.73pu, that means along the MPPT curve (Fig.7(a)). Reactive power absorbed reduces correspondingly. id keeps to be zero, iq reduces while the rotor speed decreases(Fig.7(b)). It can be seen the DC voltage is hold on the value of 1200V (Fig.7(c)).

Simulation example 2: Generator-side converter is controlled by the strategy of UPF based on the rotor flux orientation. The operation characteristics curve is shown in Fig.8. With the change in wind speed, the active power of PMSGs is the same as above, however the reactive power keeps to be zero (Fig. 8(a)). id and iq are coupled, so they both reduce (Fig.8(b)).

Simulation example 3: Generator-side converter is controlled by the proposed UPF strategy, the second orientation based on stator flux. The operation

characteristics curve is shown in Fig.9. With the change in wind speed, the active power of PMSGs is the same as above, however the reactive power keeps to be zero (Fig.9(a)). id and iq are decoupled. The former is kept at zero while the latter reduces with change of the wind speed (Fig.9(b)).

(a) Active and reactive power of the PMSGs

(b) Stator current of the PMSGs

(c) DC voltage

Fig.7 Dynamic performance curves of id=0

(a) Active and reactive power of the PMSGs

(b) Stator current of the PMSGs

Fig.8 Dynamic performance curves of UPF

VI. CONCLUSION This paper has presented the system model of

directly-driven PMSG based wind turbines, and set up the simulation model using Matlab/Simulink. All of the three kinds control strategies are proved to be effective and have a good dynamic performance, as well as they can track the maximum power point.

Strategy of zero direct-axis current based on the rotor flux orientation has a simple control algorithm and is easy to be realized on hardware. So it is widely used. However, the generator needs to absorb reactive power, thus the converter capacity increases. The strategy of UPF based on the rotor flux orientation has to consume active current to achieve the control objectives, however the algorithm is too complex. With the same size of stator current, it produces less torque than the former.

The proposed UPF strategy, the reorientation based on stator flux, is not only easy to be realized with a simple algorithm, achieving the maximum ratio of torque and current, but also can limit the reactive power on generator-side and decouple it and the active power. In practical applications, those control methods should be considered together according to the different conditions on wind farm.

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P(MW) and Q(MVar)

(a) Active and reactive power of the PMSGs

id a

nd iq(KA)

(b) Stator current of the PMSGs

Fig.9 Dynamic performance curves of UPF based on stator flux