Report on Rotor-Speed Stability Improvement of Dual Stator-Winding Induction Generator-Based Wind Farms by Control-Windings Voltage

Abstract of Rotor-Speed Stability Improvement of Dual Stator-Winding Induction Generator-Based Wind Farms by Control-Windings Voltage

The is about the stability of rotor-speed improvement of the dual stator windings induction generator which is based on wind farms under the fault conditions of power system. The wind turbines based on DWIG characteristics are discussed in this study. It analyzes the factors that affects the stability of rotor speed, as well as the novel control winding of drive voltage oriented together with the braking resistor (BR) is also projected to handle both reactive power as well as the active power of DWIG at the time of fault. Furthermore, the additional supplementary loop of control is provided by the proposed control method to conventional control of voltage oriented winding drive. To adjust the value of BR, it was devoted by the supplementary control loop in way that BR absorbs the kinetic energy of turbine as well as generator. Moreover, the slip of DWIG is forced by the orientation of voltage control winding to be constantly approximately, limiting the consumption of reactive power during the fault. The effectiveness of the control scheme is demonstrated by the experimental as well as simulation works from the prototyping the system of 1.2-KW three / three phase wind power of DWIG as the important BR influence to improve the stability of rotor-speed of DWIG.

Table of Contents

Abstract. 2

Introduction. 4

Literation review.. 5

System stability of DWIG-based wind turbines. 5

Rotor-speed stability of the system.. 7

Control winding voltage of induction generator. 8

Control-winding oriented control with the help of supplementary control loop. 9

Problem statement. 9

System model presentation. 9

Simulation results. 11

Discussion. 12

Conclusion. 13

References. 15

 Introduction of Rotor-Speed Stability Improvement of Dual Stator-Winding Induction Generator-Based Wind Farms by Control-Windings Voltage

            Because of increasing environmental concerns, wind energy as well as fossil fuels consumption as on of the most widely used renewable energy has been attracting more attention. Furthermore, the wind turbines majority is placed on the ground because of the increasing demand for wind turbines to place offshore, where the condition of wind is better, as well as the noise issues and the landscape impact are slightly upgraded. The connection of electrical cable is very common issue to all of the offshore conversion of energy systems to the onshore substation. It must be buried the cables undersea as well as it can also increase the distance problems because all ac cables have the current line charging as well as high capacitance for long cable run scan be so high. Although, the technology of high voltage use is one of solutions prospectively for the large offshore farms wind. The strong coordination is exhibited by dual stator-windings induction generator (DWIG) through the transmission of HVdc as well as an economical and the attractive candidate would the strong coordination for large offshore wind farms with the transmission of HVdc. So, two winds sets are in the stator slot in the generators. It may also be fed directly by referred to the winding power with the diode bridge rectifier in the link of HVdc as well as other are known as the control winding or excitation. Due to possibility of controlling the voltage wind control frequency, the DWIG has better capability of slip control fortunately. Furthermore, it connects with the source cover voltage as well as the supply of reactive power is provided in order to generate the flux magnetizing in DWIG. The dual construction has the benefit of regulation effectively of the load frequency as well as magnitude voltage for the ships applications and aircrafts while eliminating or reducing the harmonics the converter-induce as well as power output in load along with the increasing power output possibility as well as the efficiency system for optimization. Furthermore, the DWIG has several benefits such as wide speed range, demand of low maintenance, construction brushless innate as well as dc voltage output better performance with the power converter in small size under the load and speed variations. The fact is showing that it can use the DWIGs in the systems of wind power, especially as the solution potential for wind offshore farms through using transmission of HVdc. One of the main issues associated with the irrespective employed type in farm wind and technologies of wind turbine is the stability of rotor speed. The important speed may be caused by the transmission which is also known as the faults to increase generator rotor and turbine. It may speed rotor of the turbine wind so high that the stable value is not returned (Basak et al , 2017)

The transient stability nature is analyzed as well as identified by this study related the DWIGs as well as proposed techniques for increasing the speed of the rotor stability of DWIGs. The instability rotor speed issue is because of lack of active output power for the synchronous wind farm based on generator. In machine parameters, the control of voltage-oriented control winding is highly robust according to change. The DWIG during the fault could keep close the DWIG slip to minimize the absorption of reactive power as well as nominal value by control method. To control the rotor speed, BR is proposed during fault at the time of the wind tribune kinetic energy continually to enhance due to the acceleration of rotor (Kavousi et al , 2018).

Literation review of Rotor-Speed Stability Improvement of Dual Stator-Winding Induction Generator-Based Wind Farms by Control-Windings Voltage
System stability of DWIG-based wind turbines

The configuration of system offshore wind turbine based on DWIG is illustrated in figure 1. It connected to grid mainland through transmission Hvdc. The electrical power is fed by the power in the link of HVdc through diode rectifier ( Li et al , 2009) .

Figure 1: Schematic diagram of a grid-connected DWIG-base wind turbine

To the AC transmission network, it connects with the HVdc transmission system through ac/dc converter as well as transformer or setup power. To occur on ac transmission line near the transformer power, it assumes the fault. To provide significant proportion of the total reactive power consumption, it connects the fixed capacitor bank excitation through the power winding terminal. To generate power reactive variables, it connects the static excitation capacitor on control winding side across the power reaction regulation in other working condition for achievement. DWIG has several benefits such as wide speed range, demand of low maintenance, construction brushless innate as well as dc voltage output better performance with the power converter in small size under the load and speed variations. As a voltage source UcDC to the SEC, the capacitor dc-bus is used. It connects SEC to handle winding through filter inductor for the minimization of harmonics high frequency inserted into the machine. With the control winding-oriented control voltage of DWIG has three benefits during the fault in the system on the induction generators conventionally which are given below.

·         The generators of conventional induction are associated directly with energy system, as well as when the issue happens, the voltage at the terminals of the generator drops significantly, causing the electromagnetic torque and electric force yield of the generator to be incredibly decreased. Be that as it may, the mechanical-input torque is practically consistent during normal nonpermanent flaws and this makes the machine quicken. The recurrence of the force framework is steady and speeding up of generator makes the slip of generator increment. Fig. 2(a) shows how the receptive force utilization by enlistment generator increments as slip increments with the goal that when the issue cleared, these generators draw a lot of responsive force from the framework (Muyeen et al , 2014).

Figure 2 (a) : Reactive power-slip curve of a DWIG

The reactive power consumption can be reached by the generator of induction squirrel-cage more than twice to turbine of wind based on induction squirrel cage after a severe fault occurrence. The operating area of SEC is shown in figure 2 (b) as well as it has also assumed that maximum reactive Qmax power is generated by SEC in the figure.

figure 2 (b) : operating area of its SEC disturbance

 

If the fault was on for the long period as well as it does not clear till slip exceeds the S_max critical value as shown in figure in the server disturbance case in system for power. Furthermore, it wills able SEC to produce DWIG required reactive power. The slip of DWIG is forced by the orientation of voltage control winding to be constantly approximately, limiting the consumption of reactive power during the fault. The worsening of reactive power balance may be caused into the DWIG electric torque decrease in flux ( Krauce et al , 2002). Thus, it may be led by according to the instability rotor speed. Due to possibility of controlling the voltage wind control frequency, the DWIG has better capability of slip control fortunately. Full control of frequency is allowed by control winding SEC as well as although, DWIG speed synchronization. Controlling the recurrence of the control winding will make it conceivable to keep the synchronous speed of DWIG near the rotor speed, limiting the slip, and, subsequently, the receptive intensity of the DWIG. As a control drive methodology, control-winding focused control is chosen to accomplish both a speedy reaction and constraining of the adjustments in slip. To control both the reactive power of the SEC as well as the electric torque, it is made it putting all voltage winding control possible in the strategy. Furthermore, it chooses the frequency voltage control winding in the different way in which the DWID operation of constant slip is attained (Shi et al , 2009)

Rotor-speed stability of the system of Rotor-Speed Stability Improvement of Dual Stator-Winding Induction Generator-Based Wind Farms by Control-Windings Voltage

The next advantages are related to the rotor-speed stability of the system. Therefore, there is a need for active power compensators like BR. This is because it is capable of absorbing nominal active power in case of any disturbance present in DWIG. For that case, a fixed capacitor bank is used and it is involved in generating nominal reactive power. On the other hand, BR is such a kind of resistor that contains high power dissipation capacity for a short time.

Moreover, it is only seen as a form of fast load injection and it is involved in absorbing excess transient energy. The brakes are attached between the terminals of the generator and they will control the excess amount of energy gained by a generator during that period. This absorbed electrical energy is directly proportional to the voltage square. Moreover, during the fault in a conventional induction generator, the terminal voltages are dropped in no time. Due to this case, for improving the transient stability BR resistor is not an efficient choice in the induction generator. On the other hand in conventional induction generators, this stability is improved through the help of BR resistor. It can be seen that there is no connection between the control and power windings of the induction generator. This shows that whenever a fault occurs at the winding side will not affect the control wining of the induction generator. The control winding of the generator is responsible for providing reactive power that will be used for generating output voltage. It can be noted that the best advantage of this feature is that the SEC of this induction generator will also ensure that the magnetic field is always active due to this it will always induce EMF in the winding. This can be explained in other words through the example of reactive power.

It can be seen that the dual stator winding induction generator is quite similar to the synchronous generator. In these generators, only dc excitation winding is ensuring the output voltages and reactive power. At the time of the fault, if the power side of the winding is disconnected from the HVDC transmission then there will be no change of affecting the output voltages of the generator. It will maintain the output voltage because there is no electrical connection present. Due to this case, BR is playing a major role in improving rotor speed stability in a proper way.

 Another thing is that at the time of fault the control voltages and power winding terminal voltages are completely constant. The next thing is that the fixed capacitor bank is used for generating the nominal reactive power in the induction generator. But the main problem is that during the fault the terminal voltages of the generator will drop when a fixed capacitor bank is used. Therefore, it is completely useless to use a fixed capacitor bank at the time of the fault.

Moreover, there is another advantage in the induction generator and it is related to the control winding voltages. It is an important property of this generator. The next thing is that during the faulty problem as the rotor speed is increasing in the induction motor the frequency of the SEC is also increase, but the reactance of the capacitor band is decreased.

Previously, it can be seen that the voltages of the winding remain the same at the time of the fault. Due to this, it can be seen that there is a linear relationship present between the reactive power and reactive power generated through the capacitor bank. Moreover, as the speed is increased there will be more power is generated through the capacitor bank. It is one of the most important features of the induction generator and it will prevent the overload of the reactive power and there will be proper power balance in the induction generator.

Control winding voltage of induction generator

According to the power theory, it can be seen that the reactive power and active power of the generator winding is expressed as

According to this above equation  is considered as active power and on the other hand

 It is considered as reactive power. Now, there is a need to use the phase-locked loop for the system and put all control winding values in the d-axes. It will become like this

In the above equation,  is considered as the angular stator of the electrical frequency. Moreover, this equation is showing that is producing instantaneous reactive power and on the other hand is generating absolute power. The next thing is that reactive power is able to regulate the flux of control winding at any speed and load. Now the next thing is that when actual power is regulated then the torque of the induction generator is also regulated.

But for the case of induction generator, if there is any change of magnetic flux at the winding will cause extreme variation in the air gap flux. Now according to the dc-bus voltage of control winding, it must be properly under control for effective results from the induction generator. This can be done by the regulation of the control mechanism of the induction generator. It can be oriented in this form given below

It can be noted that for any induction generator that contains control winding, there will be   and also . Now apply the Kirchhoff law and the voltages are given below as

This will show that the control winding voltage of the induction generator contains no relationship with the machine parameters. There is also a need for stator flux observer, flux oriented algorithm and speed estimation control.

Control-winding oriented control with the help of supplementary control loop

The next thing is that for the sake of stable operation of the induction generator and it can be implemented for power applications. It will also control the active power as well as the reactive power with perfection. Another thing is that due to the imbalance power of the wind turbine that contains an induction motor will cause some changes in the rotor speed. Due to this just a little bit of changes in the moor will case rotor speed problem and it will become unstable and not easy to stop the turbine. This can be explained in other words, for stable turbine operation there will be precise input voltages with control. Due to this, its mechanical stability will be increased. According to the fact, in steady-state such balance in the active power is obtained through the coordinate control of the turbine. This shows that if it coordinates are control then the wind speed of the induction motor is stable.

Problem statement of Rotor-Speed Stability Improvement of Dual Stator-Winding Induction Generator-Based Wind Farms by Control-Windings Voltage

The problem statement of this paper is built the DWIG model based on Wind farm. The improvement of the DWIG of the rotor speed based on Wind farm. These entire models are based on the control winding of voltage oriented control.

 System model presentation of Rotor-Speed Stability Improvement of Dual Stator-Winding Induction Generator-Based Wind Farms by Control-Windings Voltage

The operation of DWIG system may also be threatened by further enhancement in the slip in these conditions. The additional supplementary loop of control is provided by the proposed control method to conventional control of voltage oriented winding drive.

The first model is built in the MATLAB/Simulink about the DWIG based on wind turbine;

Its simulation result is shown in the below simulations part;        

The next model is about the “winding oriented control of the HVdc-connected DWIG” which is also built in the MATLAB/Simulink

It will also control the active power as well as the reactive power with perfection. Another thing is that due to the imbalance power of the wind turbine that contains an induction motor will cause some changes in the rotor speed. It can be seen that the induction generator is connected with the AC system through the help of the HVDC transmission system. Due to this control system, the decoupling control strategy is applied to pulse modulation.

The voltage control of the DWIG power wind system;

 

 Simulation results

The below simulate result is about the winding oriented control of the HVdc-connected DWIG there are different graphs which is required;

The below simulate result is about the DWIG based on wind farm; there are different graphs which is required;

 

The below simulation part; is about the voltage of the dwig power wind system;

 Discussion of Rotor-Speed Stability Improvement of Dual Stator-Winding Induction Generator-Based Wind Farms by Control-Windings Voltage

Moreover, the next thing is that at the time of the fault, the protective equipment of the HVDC will disconnect itself from the ac system. Due to this case, there will be no electric power is transmitted to the wind turbine. This shows that if there is a need to stable the rotor speed and its stability then apply BR resistor. Another thing is that wind power is not constant, it will vary with time. Due to this fact, the speed of the induction generator is affected. It is not easy to predict wind power. For that case, the BR resistor must be changed according to the wind condition and speed in a proper way. At this moment, when any fault occurs the active power of the BR resistor will be the same according to the dual stator winding induction generator.

According to this fact, there is a need to apply a supplementary control loop at the control-winding that will remove the different present between the input and output power of the turbine. It must be added to the voltage oriented drive of the induction generator. It will provide a fast and active power balance at the time of the fault. This technique is almost the same as the VAR compensator. In this technique, the reactive power is obtained only through the value of the capacitive bank. But it can be noted that the active power of the induction generator is only obtained through controlling the value of BR. The next thing is that the BR is considered as the active shunt element and this is the reason why its control strategy is only applied for controlling BR resistors. The whole arrangement is extremely expansive because high current rating thyristor switches are used in it. Furthermore, this control scheme is slow because the rate of change of BR is about 20 ms. The next thing is that for controlling this problem there is the use of a supplementary control loop with control winding voltages and it will be extremely helpful in the faulty situation. The next thing is that such a control method is extremely helpful for the stand-alone application of the induction generator. In this case, a switch is used and it contains a duty cycle of . According to this fact, when the value of k is greater than the ramp signal then it means that Diode D is on and this BR resistor is giving the minimum value of R. on the other hand, when its value is less than the ramp signal then the circuit is open. At this stage, the BR resistor will provide maximum value to the system. This whole system is given in the figure below

Figure: Control system of the induction generator

 Conclusion of Rotor-Speed Stability Improvement of Dual Stator-Winding Induction Generator-Based Wind Farms by Control-Windings Voltage

Summing up all the discussion from above it is concluded that the BR resistor is extremely effective for the stability of the induction generator. This control strategy is extremely effective for the induction generator used in wind farms. It can be seen that the wind is not constant every time. Due to this case, it is not easy to predict the input voltages. In that case, it will affect the internal winding of the generator. Then it will become extremely difficult to control the speed of the rotor. In this paper, the main idea to combine conventional control of the generator with BR.

The next thing is that the fixed capacitor bank is used for generating the nominal reactive power in the induction generator. But the main problem is that during the fault the terminal voltages of the generator will drop when a fixed capacitor bank is used. The next thing is that during the faulty problem as the rotor speed is increasing in the induction motor the frequency of the SEC is also increase, but the reactance of the capacitor band is decreased. Due to this just a little bit of changes in the moor will case rotor speed problem and it will become unstable and not easy to stop the turbine. This can be explained in other words, for stable turbine operation there will be precise input voltages with control.

Then after the theoretical calculation, the whole system is presented on the MATLAB Simulink. During the simulation, the orientation of the control wining voltage forces will become fixed. This is because it will prevent the turbine from rotor acceleration. The results are showing that there is no backswing instability in the generator when the BR switch is on for a long period.