POWER FACTOR CONTRSYNCHR

Murad Jafari Student Member IEEE

ABSTRACT This paper proposes a novel controimprove the power factor over a widespeed in a wind energy conversion sybased on a current source inverter electrically excited synchronous geneThe system consists of diode rectifier on thwhile a pulse wave modulated (PWM) CSside. In the proposed control schemeexcitation is controlled with the wind speegrid power factor. While the control freedomused to regulate the power output of the maximum power point tracking (MPPT) of deliver maximum power at any wind spanalysis was conducted to investigate thelimits of this approach, and computer simulthe validity of the theory.

Keywords — wind energy, current selectrically excited, synchronous generator,

1. INTRODUCTION Current Source Converter (CSC) based WEadvantages which include among others sigood grid side waveforms and protection adue to the high inductance in the dc choke [ Past research was done on several possiblwould meet grid requirements in terms of voltage and frequency as well as fault rsince wind speed is not constant, the systeable to extract the maximum possible energ(maximum power point tracking or MPPT)the power factor expectations within the mrange of wind speed [2] [3] [4].

Figure 1: Proposed system topology includdiode rectifier and a PWM C

ROL IN A WIND ENERGY CONVERSION SRONOUS GENERATOR EXCITATION

Bin Wu M

Fellow IEEE Studen

ol technique to e range of wind ystem (WECS) (CSI) and an

erator (EESG). he generator-side SI is on the grid-, the generator d to improve the ms of the CSI are

WECS to meet f wind energy and peed. Theoretical e feasibility and lations confirmed

source converter, power factor

ECS offer distinct imple topologies, against dc current 1].

e topologies that f support for grid ide through, but em must be also gy from the wind ) as well as meet

maximum possible

ding an EESG, a CSI

Discussed CSC based systems cpermanent magnet synchronouconnected with a PWM CSR on thPWM CSI on the grid side. The enough control freedoms to allooutput active power and keeping thneeded to maintain the required based on a diode rectifier and a bthe PWM CSR was also suggesteobjective. The buck converter servelevel, such that the PF is maintaingrid side requirement for dc currentoutput. EESG is widely used in prarely been published in previous lit The system proposed in this papdesign by eliminating the buck convexcitation control of an EESG to colevel. Figure 1 represents the topsystem with a diode rectifier on the on the grid side.

2. GENERATOR SIDE CON

RECTIFIER As figure 2 demonstrates, the maximextracted at each wind speed is prothe wind speed, whereas the torquproportional to the square of the win

Figure 2: MPPT power and torq

SYSTEM VIA

Mitesh Popat nt Member IEEE

onsisted primarily of a us generator (PMSG) he generator side, and a two PWM CSC’s have w controlling both the

he dc current at the level output PF. A topology uck converter instead of ed to achieve the same ed to boost the dc current ned and to decouple the t from the generator side

practice; however, it has terature.

er further simplifies the verter and, instead, using ontrol the dc link current pology of the proposed generator side and a CSI

NVERTER: DIODE R

mum power that could be oportional to the cube of ue or dc link current is nd speed [3].

que vs. wind speed [3]

2013 26th IEEE Canadian Conference Of Electrical And Computer Engineering (CCECE)

978-1-4799-0033-6/13/$31.00 ©2013 IEEE

For a fixed output power that corresponds to the MPP at each wind speed, the dc current value will vary depending on excitation. At lower excitation the output voltage drops and, consequently, the current increases. In a diode rectifier, due to the large inductance of the generator, current commutation is not instantaneous, and the commutation angle depends on the back emf in the generator, the generator inductance as well as the dc link current [5]. 1 2 , (1)

Where ω=2πf=377rad/s , L_s is the synchronous inductance. The diode commutation results in a voltage drop on the dc link side [5]. The voltage drop depends mainly on the generator inductance and the dc current. 1.35 3

(2)

Remembering that , we can substitute for

and solve for The result is the following quadratic equation: 3 3√2 0 (3)

The solution for this equation is in the form of:

2 (4)

Where √ , √ , and

The first solution where √ is rejected since it results in unreasonably high values of the dc current [3]. In order to be able to achieve a real value of the dc current ≥ 0 or: 3√2

≥ 12 (5)

Both and depend on wind speed. also depends on excitation [6]. It is possible at each wind speed, depending on excitation, that a real solution the dc current cannot be obtained. This depends largely on the value of the generator inductance. The implication from this is that higher values of the synchronous inductance could limit the range of wind speed within which a real value of the dc current that achieves MPPT can be obtained.

Figure 3: dc link current requirements for diode rectifier

with PWM CSI

3. GRID SIDE CONVERTER: PWM CSI

The main objective of CSI is to supply available dc-link power to grid and maintain desired dc-link current in the system. It is necessary for proper commutation of the CSI switches that a capacitor be connected at its grid side. The capacitor also serves to smooth the output signal and reduce the total harmonic distortion (THD) [7].

Since the diode rectifier on the generator side does not

offer any control of the output power, the control scheme of the CSI is designed in such a way that it tracks the dc current calculated based on MPPT at any given wind speed using the generator parameters. As stated above there are combinations of wind speeds and excitation values where a real reference dc current cannot be found. The control scheme has to account for that. Furthermore when calculating the grid side inverter current, we have to take into account the capacitor current, which is added to the grid current.

(6) At very low wind speeds the generator hardly produces

any active power. Consequently, the grid side current ( ) is very low. However, even at high speeds the voltage drop across the line inductance is minimal, which means that almost at all the speeds the capacitor voltage is close to the grid voltage, and therefore the capacitor current ( ) is almost speed independent. At very low speeds the grid side inverter current ( ) is equal to the capacitor current, and the minimal dc current required is equal to that current. For each desired grid side power factor (leading or lagging) it is possible to calculate the minimum dc current required by the grid at each wind speed.

Figure 3 represents grid side requirements for the dc current for different values of PF (dotted lines) as well as the MPPT value of the dc current from the generator side at different levels of excitation. To be able to supply the

required power factor to the grid, the generator must produce a dc current at least equal to the grid side requirement. To minimize dc losses, the dc current is kept as close to the grid requirement as possible.

We can also see from figure 3 that the lower the excitation current, the higher the dc current at each wind speed given that MPPT is observed. The control strategy is based on this idea.

4. PROPOSED SYSTEM CONTROL SCHEME

Simply put, the idea is to use the CSI to control the active power output of the generator to meet MPPT. The diode rectifier does not offer any control freedoms, so to control the dc current to the value required by the grid such that the grid side PF can be achieved (as explained above), the generator excitation is adjusted.

To calculate the reference active power at each wind

speed, the wind speed in pu is cubed and multiplied by the rated power, based on MPPT. The desired power factor is used to calculate the reference reactive power. The reference dc current is calculated using equation (4) above, and is compared with the measured value of the dc current to provide reference for the dc voltage, which then is multiplied by the dc current to provide the reference for the grid side active power ( _ ). Using _ , the reference for the inverter grid side d-axis current is calculated accounting for the capacitor d-axis current, as follows:

_ 1 _ _ (7)

The reference for the grid q-axis current can be calculated from the reference reactive power as follows:

1.5 (8)

Figure 4 represents the control scheme of the proposed

system. There are several ways to control excitation. One of them is to analytically calculate the value of the excitation current needed to produce the dc current which would be just slightly above the grid requirement.

Another method is to create a look-up table that defines

excitation vs. wind speed, such that the dc current stays just above the grid required value at that wind speed. Figure 5 illustrates this principle. The dark line represents the value of the dc current produced by switching excitation to stay above the dotted line, which represents the grid side requirement for unity PF.

Figure 5: Excitation control strategy

Figure 4: proposed system control scheme

Figure 6: dc link current tracking

Figure 7: active and reactive power tracking

Figure 8: PF with and without excitation control

For each value of the PF the grid required dc current changes, and therefore a different look-up table is needed. In the look up table for PF=1, in per unit, the excitation voltage had approximately the same value as the wind speed, so the value of the wind speed in per unit was used also for the excitation voltage. Simulations were run with and without excitation control as well as for step change in wind speed, we can see on figures 6 and 7 that the system tracks well both the reference dc current and the active and reactive power in response to a step in wind speed from 1 to 0.6 per unit. Meanwhile figure 8 illustrates the output PF while tracking for unity PF with and without excitation control (dotted line). We can see a significant improvement in the wind speed range within which the PF stays close to the desired value.

5. CONCLUSIONS

A diode rectifier and PWM CSI based topology is

proposed for EESG based WECS. The proposed configuration provides simple topology and a low cost solution. The main challenge under these conditions was overcoming the typically poor PF at lower wind speeds resulting from the lack of control freedoms in the diode rectifier as well as the impact of commutation in the diode rectifier on the dc link current.

After studying the impact of generator parameters on

commutation in the diode rectifier and excitation adjustment on the dc current, a novel control scheme is developed which uses generator excitation to improve the power factor over a wide range of wind speed. The feasibility of the proposed control scheme is verified by simulation results.

6. REFERENCES

[1] B. Wu, High Power Converters and AC drives, IEEE Press,

Wiley Interscience, 2006.

[2] J. Dai, B. Wu, D. Xu, N. R. Zargari and J. Wang, "Low Cost Current Source Converter Solutions for Variable Speed Wind Energy Conversion Systems," in IEEE International Electric Machines and Drives Conference, 2011.

[3] J. Dai, B. Wu and D. Xu, "A Novel Control Scheme for Current-Source-Converter-Based PMSG Wind Energy Cinversion Systems," Power Electronics, IEEE Transactions, pp. 963-972, 2009.

[4] X. Tan, J. Dai and B. Wu, "A Novel Converter Configuration for Wind Applications Using PWM CSI with Diode Rectifier and Buck Converter," in IEEE International Electric Machines & Drives Conference, 2011.

[5] N. Mohan, T. M. Undeland and W. P. Robbins, Power Electronics: Converters, Applications, and Design, New York: John Wiley & Sons, Inc., 2002.

[6] W. Quincy and C. Liuchen, "An intelligent maximum power extraction algorithm for inverter based variable speed wind turbine systems," Power Electronics, IEEE Transactions, vol. 19, pp. 1242-1249, 2004.

[7] F. Blaabjerg and Z. Chen, Power Electronics for Modern Wind Turbines, Morgan & Claypool Publishers, 2006.