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Energy Procedia 12 (2011) 799 807

1876-6102 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of University of Electronic Science and Technology of China (UESTC).doi:10.1016/j.egypro.2011.10.106

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Energy

Procedia Energy Procedia 00 (2011) 000000

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ICSGCE 2011: 2730 September 2011, Chengdu, China

The Requirements and Technical Analysis of Low Voltage Ride Through for the Doubly-Fed Induction Wind Turbines

Chen-guang Niu* , Guan-qi Liu Department of Electrical and Electronic Engineering, North China Electric Power University, Baoding, 071003 China

Abstract

With the continuing rapid increase of the installed capacity of the wind farms and the construction of the large-scale wind farms, utilities in every country have proposed more strictly grid codes for the wind farms. This paper will make a detailed instruction of the low voltage ride through (LVRT) requirement for the wind farms, based on the rules for the wind farms accessing to the power system. It will also make a comprehensive analysis and comparisons of the technologies to meet the LVRT requirements for the doubly-fed induction generator (DFIG), which is used universality. In the end, it will make an improvement of the technologies and come up with the future research direction.

Keywords: Wind farm; Doubly-fed induction generator; Low voltage ride though

1. Introduction

In the past 10 years, the installed capacity of the global wind power maintained a rapid growth momentum. Based on the current trend of accelerated growth, the World Wind Energy Committee have made a prediction: by the end of 2020, the total global installed capacity will be at least 1, 900, 000MW, at the same time, the world's electricity consumption will be 12% from wind power[1].

As the penetration level increases, grid operators in all regions have put forward higher standards in connecting the wind power with the power system [2-6], related to active and reactive power capability, voltage operating ranges, low voltage ride through (LVRT), and other aspects of control. The LVRT is considered to be the biggest challenge in wind turbines design and manufacturing technology[7]. To enhance the capacity of wind farms LVRT will definitely increase the cost of wind farm projects. Thus, according to the wind turbines' type, putting forward specific requirement of LVRT in the planning stage

* Corresponding author. Tel.: +86-13832298310. E-mail address: [email protected].

Open access under CC BY-NC-ND license.

2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of University of Electronic Science and Technology of China

Open access under CC BY-NC-ND license.

(UESTC)

800 Chen-guang Niu and Guan-qi Liu / Energy Procedia 12 (2011) 799 8072 Chen-guang Niu et al. / Energy Procedia 00 (2011) 000000

of wind farm construction has great economic significance [8]. The more common wind turbine implementations are variable speed, for it has the advantages of

higher energy capture, less mechanical stress, constant energy output and lower noise, compared with the constant speed turbines. So the mainstream model is based on variable speed constant frequency(VSCF) doubly fed induction control technology wind turbines(DFIG)[9], This paper will consider such model, according domestic and foreign researches, to discuss its LVRT requirements and technical measures.

2. Grid Codes

All around the world, the grid codes vary in scope and specific details from one jurisdiction to another. In order to regulate wind farms, all areas have developed specifications according to their relevant technical situations. The literatures [5, 6] have given Wind Power grid specifications in Europe and the Americas. The State Grid Corporation of China have established Wind Power System technical requirements(revised) in February, 2009. The main requirements are summarized below:

(i) Active power. It provides a clear requirement that wind farms must have the ability to regulate the active power, and to control their active power output in accordance with the dispatching department directives.

(ii) Reactive power. The wind farms are required, in any way, to guarantee a certain amount of reactive power regulation capacity. The power factor range is typically between 0.9(lag) to 0.98(lead).

(iii) Voltage range. When the voltage of the point of common connecting deviation between -10%-+10%, the wind turbines should be able to work properly. The wind farms should be able to control the points voltage between -3%-+7%, within its capacity.

(iv) Frequency operating range. The wind tuebines are expected to operate within typical grid frequency variations whitch the system frequency varies from 49.5 to50.5HZ.

(v) Low Voltage Ride Through (LVRT). In the event of a voltage drop, the turbines are required to remain connected for a specific amount of time before being allowed to disconnect. This requirement is to ensure that there is no loss of generation for normally cleared faults. Disconnecting a wind generator too quickly could have a negative impact on the grid, particularly with large wind farms.

3. LVRT Requirements

The most representative LVRT requirement was proposed in Europe in 2003 by E. On, (Fig.1). Since then, many jurisdictions have proposed their own version of the LVRT curve, the one applied in China is illustrated in Fig.2.

Fig. 1. E.On low voltage ride through requirement

The key basis for putting forward the LVRT requirement is the specific characteristics and configurations of the grid, whitch is the wind farms connecting with. In addition, the capacity of wind

Chen-guang Niu and Guan-qi Liu / Energy Procedia 12 (2011) 799 807 801 Chen-guang Niu et al. / Energy Procedia 00 (2011) 000000 3

farms also play a leading role.

Fig. 2. China low voltage ride through requirement

From the two diagrams something can be clearly found: China's low voltage ride through curve show that wind turbine can be removed from the grid, as low as 0.2p.u, in contrast, European standard is more stringent, requiring that wind turbines are allowed to disconnect as low as 0.15p.u.

4. DFIG LVRT Technologies

The DFIG has become the most widely used VSCF wind turbines. This article is only for the LVRT of DFIG. The model of DFIG is shown in Fig.3.

Fig. 3. The model of DFIG

Due to the strong coupling between the stator and rotor, the transient currents on the stator are reflected on the rotor windings, during a fault. The transients can also result in a rise in the dc bus voltage and the machine side converter current. The active and reactive power will oscillate, more seriously it will also cause the rotor circuit over-voltage and over-current. Over-current can damage converters, as well as, the generator's rotor windings may be damaged by the over-voltage.

Today, researchers achieve the LVRT for DFIG mainly use the following technologies[10-11]:(i) the active Crowbar protective circuit;(ii) the dc bus energy storage circuit;(iii) the rotor current control. The three solutions are compared and analyzed following.

4.1. The active crowbar protective circuit

The active crowbar circuit[12-14] is always installed between the rotor and the rotor side converter. When the grid in fault, the crowbar circuit will work if it detects the over-current in the rotor side and over dc link voltage. The specific measures are shorting the rotor, bypassing and interdicting the rotor-converter, with the effect of restraining over rotor current and protecting converter. This method is currently the most widely used, shown in Fig.4.


Fig. 4. DFIG equipped with active crowbar circuit

Two issues should be paid more attention when use the active crowbar protctive circuit [15-18]: (i) When the crowbar is in operation, the IGBT of converters will quit, and then the DFIG behaves as a

squirrel-cag induction machine, so the DFIG may lose its controllability. During the process of eliminating the question and the crowbar quiting work, may recur over-current and over-voltage,

(ii) In the design of active crowbar circuit, to select the reasonable value of the discharge resistance is more important. The larger the resistance is selected, the faster attenuation of transient component, but the larger resistance may cause rotor-side over-voltage and damage the rotor-side converter.

4.2. The DC bus energy storage circuit (ESS)

When the grid is in the low-voltage fault, generator terminal voltage is lower than normal. For the large inertia of wind turbine and the limited adjustment range of the wind turbine's own tone pitch system, there will be a part of excess obtained wind energy that cant be trasfered to grid, so there is a need to store this part of energy. Then release this part of energy to grid, after the fault is cleared. The researches in this area are little in China, some of the foreign literatures mentioned that the mainly used current energy storage device is super-capacitors [19], for the reason of super high energy density and high efficiency, Fig.5. DFIG equipped with energy system is shown in Fig.6.

Fig. 5. Topology of the DC bus energy storage circuit

Fig. 6. DFIG equipped with ESS

As can be seen from the figure, during fault, energy outflow from the rotor, after converter, the residual obtained energy is stored in the the ESS. Feasible measures are as follows:


i) The energy stored in the the resistors that connected in the DC bus. We should control the dc voltage in the range of its normal value's 10%;

ii) Short-circuit the rotor winding through a resistor, and enable generators to run as the traditional induction motor;

iii) Release the part of energy stored in the ESS to the power grid finally. The introduction of the ESS can solve the problem of the transient cuased by switching between

different running conditions, when using the active crowbar circuit, It is also a continuous control. However, to meet the LVRT requirements, the capacity of the device required to be determined resonable.

4.3. The rotor current control

Through the preceding analyses, We naturally think about that use the quick semiconductor converters to limite the current fed into the grid, thereby, restrict over-current. But actually, Since the rotor-side converter withstand over-voltage, so it loses the capacity of the over-current restraint . Some foreign literatures present a control method to limit the rotor current to meet the LVRT requirements.

The fundamental difficulty for the DFIG in ride-through is the electromotive force (EMF) induced in the machine rotor during the fault, which depends on the dc and negative sequence components in the stator-flux linkageand the rotor speed. The relationship can be shown in Eqa.(1).

2

rabcrabc

s rmrabc sabc rabc

s

rabcirabc

m

s

du Rdt

L L LL iL L

=

= +

+ (1)

Fig. 7. Control of the rotor current. The subscripts 1, 2 and 0 indicate the positive, negative, and zero (dc) sequence components in the flux linkage or current space vectors, respectively.

From this formula we know, the rotor current is decided by the injected rotor voltage and the EMF or the derivative of the stator-flux linkage with respect to time. Therefore, the difficulty of the rotor-side converter in constraining the current during a grid fault is associated with the induced EMF depending on the stator-flux linkage and speed. The measures are as following:make the direction of the rotor current on the opposite direction of the dc component and the negative sequence component of the stator flux (Fig.7). It can reduce or even eliminate the stator flux of the rotor flux, to some extent.

4.4. Compare and analysis

These are just a simple introduction of the principle of the technologies, The following aspects of the three technologies are compared.

(i) Generator Control. The use of the active crowbar or rotor current control method will change the operating conditions, so it may result in a loss of real and reactive power (P-Q) control over the machine


during the fault. Hence, at recovery, these two solutions will draw from the grid the reactive power required to magnetize the generator. However, it occasionally lead to over-voltage during the process of returning to normal as the controller adjusted its operation to the new conditions. In the ESS method similar sutuation will not happen.

(ii) Energy control. Different technologies handle the energy from the turbine in different ways. In the second method the energy stored in the energy storage; The third method had its energy stored in the rotor inertia and the dc link voltage, then subsequently sent to the grid; The first method had its energy stored in the rotor inertia, dissipated in the rotor resistance through the crowbar, in the dc link capacitor. For the three methods, the major part of the energy was stored in the rotor inertia, so the rotor speed during a fault should be controlled to avoid overspeed. The ESS method was able to maintain rotor speed more easily for remote faults, while the other two methods may experience greater acceleration.

(iii) Rotor current. The large currents mostly happened for faults located close to the wind farm. The latter two methods experienced large rotor current, in contrast, the first method can limite the rotor current rapidly.

(iv) DC bus voltage control. The closer the faults located to the wind farm, the larger variation in the dc link voltage was observed. The first two methods had an alternative path for the rotor current, effectively preventing them from affecting the dc link capacitor voltage significantly.

5. Improvement and New Technology

After a comprehensive analysis and in view of the advantages and disadvantages of each technologies, it will make certain improvements for each technology, to improve the capacity of LVRT for DFIG.

(i) The dc bus energy storage circuit. In addition to the ESS method is affected more by the fault severity, it has some distinct advantages compared with other two techniques. Such as the retaining control over the machine during the fault, it resulte in fast recovery after the fault is cleared. But this method can not effectively control the rotor current. In the future research, we should focus on how to control the rotor current while installing ESS.

To dissipate turbine power and control the dc bus over-voltage, we can combine the storage device with dump resistance. The use of ESS however requires that practical issues be considered, such as cost, size, and life cycle. To ensure the converter is not damaged by the rotor current, rotor side converters are required to use larger capacity IGBT.

(ii) The active Crowbar circuit. For the method that DFIG equipped with active crowbar circuit, it is uncomplicated and effective, but the actual results relay on the characteristics of the internal operating conditions and the fault.However, the rotor current can be well controlled. The dc bus voltage exhibited some oscillations, mostly due to the crowbar being removed before the fault is cleared. The loss of PQ control also resulted in reactive power consumption, hence a slower return of the terminal voltage to nominal value.The most important thing for using the crowbar circuit is finding the proper removal moment. So it is necessary to do a large numeber of experiments.

The control of the operation of the crowbar and the impact of the setting of the point at which it is operated should be considered. To restraint the ober-current and over-voltage produced during the process of eliminating the question and the crowbar quiting work, there is a need to switching the controller of network-side converter . The control scheme can be ajusted as following:In the reference of normal network-side voltage, the detected actual value compare with it, then by the PI, get the reference of the output reactive power. That is for the smooth switching process, we must set the actual value of this process as reference. When the crowbar circuit in operation, the network-side converter provide the maximum reactive power, to compensate the reactive power that the DFIG absorbed from the grid, then transit to the normal state slowly. Furthermore another new crowbar circuit should to be made.


After a number of experiments, the diode rectifier bridge plus IGBT and resistor has a better effect, as shown in Fig.8.

Fig. 8. Diode rectifier bridge plus IGBT and resistor

Besides research on new crowbar, there is a need to develop new control strategies.Since the control strategy can play a role in minor failure, at the same time, crowbar circuit can deal with serious fault conditions, so the combination of the two method is both effective and can reduce costs.

(iii) Rotor current control. The advantage of this method is that no additional hardware, constraining the fault current by controlling the EMF, while the transient can be better handled during the process of fault cleared. But the rotor current control method is also more affected by the fault severity. During the fault, the DFIG response is smooth, with little oscillation in the output power and terminal voltage. The recovery is similar to that of the crowbar method in most cases, the DFIG consume reactive power at grid recovery. The voltage recovery is slower than the ESS method.

Using a dc bus dump load (dc bus crowbar) or an ESS to handle the DC bus overvoltage. Besides the three techniques to meet the LVRT requirement previously mentioned, some literatures

also mentione that it is feasible to install power electronic devices or additional circuit at the side of wind farm.

(i) Use the Thyristor Controlled Series Compensation (TCSC) to improve the LVRT capability of the wind farm [22]. TCSC connect to wind farm is shown in Fig.9.

Fig. 9. TCSC connect to wind farm

The basic circuit of TCSC is shown in Fig.9. The Equivalent impedance can be shown in Eqa.(2), X X ( )X ( )

X ( ) XC L

TCSCL C

=

(2)

The is the firing angle. TCSC device has the characteristic that adjust its own firing angle to quickly change their impedance, so it can be accessed to wind farm as current limiter, to limit the output current during fault and improve the terminal voltage, ultimately meet the LVRT requirement.

(ii) Use the Series Dynamic Braking Resistor(SDBR) to improve the LVRT capability of the wind farms[23]. SDBR is consists of resistors, bypass switch and controller. It is can be centralized installed at wind farm exports, also separately at the unit exports. SDBR serial connect to wind farm is shown in Fig.10.

The action principle can be seen from Fig.11 When the power grid in serious fault and the voltage drop, use the large short-circuit current to generate significant voltage drop on SDBR to significantly improve


the terminal voltage, at the same time electromagnetic power will be substantially increased, ultimately meet the LVRT requirement.

This method is now mainly studied for constant speed wind turbine, VSCF wind turbines, widely used in wind farms, should be put more attation in the future.

Fig. 10. SDBR serial connect to wind farm

Fig. 11.The Principle of SDBR

6. Conclusions This paper start from the LVRT requirements for wind farms connecting to power grid and other

relevant provisions, based on researches in China and abroad, compare and analysis the performance of three LVRT solutions for DFIG.

In future researches, several issues are need to be noted: (i) The methods mentioned in this paper can all meet the LVRT requirements for DFIG, but they all

have their own advantages and disadvantages and some difficult issues to overcome. In the course of study could consider the combination of several methods or to improve the corresponding control strategy.

(ii) Improving the control strategy and better the DFIG model during fault analysis can play a role for minor fault. It can also be combined with other solutions to form a complete protection system.

(iii) The development and application of full-controlled power electronic device mature can better fulfill the LVRT requirements of wind farms. However, the capacity of power electronic devices as well as economic problems hindered its application. Therefore, there is a need to study the different situations in the wind farm projects planning stage, to propose specific LVRT from technical and economic aspects. This is in fact a higher level of requirements, the need for further deepening researches.

For the reasons that mentioned above, the LVRT requirement is remain some time a difficulty, but also a key, to wind power research. It need more technical personnels and researches to be achieved.

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