Sensorless Control Technology for PMSG base on the Dead-time Compensation voltage

In order to improve the speed sensorless-control system of PMSG in low speed performance, this paper introduces a novel Dead-time compensation control method .Mathematical model is established according to the Dead-zone of the influence of the voltage source type inverter output voltage. At the same time, the given value of current regulator output voltage has been fixed based on the established model. Then the stator voltage after compensationed is applied to the flux estimation, which improves the performance of flux estimation. Finally, the position and speed of the rotor is estimated based on Back-Electromotive Force, which has Simple algorithm and good robustness. In order to verify the correctness of theoretical analysis, the experiment was done according to the new control method. The results proved the correctness and feasibility of this control method.

Wind energy is widely seen as a substitute for the traditional non renewable energy due to its large-scale development and utilization and no pollution.
Permanent magnet synchronous wind power generator has a wide application prospect, because of its compact structure, reliable performance and excellent power system fault adaptive [1][2][3].Sensorless control technology is to estimate the rotor position and speed by a specific algorithm, related to the use of easily measured variables which is in the motor winding, such as the stator voltage, stator current, winding flux.The purpose is to realize the high precision control of permanent magnet motor [4].A series of problems existing in mechanical sensor can be solved by using the speed sensorless technology.So that the reliability of the system is improved, the cost is reduced greatly [5].There are two kinds of coordinates in the generator vector control system of permanent magnet synchronous wind.One is two-phase rotating coordinate system (d-q coordinate) and the other is two-phase stationary coordinate system (alpha-beta coordinate system).In order to calculate the rotor position, We have to get the value of the stator flux linkage.A simple method of estimating the stator flux is the voltage model of stator flux in the alpha beta coordinate system and it is also the basis of other flux estimation method.This method only requires the use of a stator resistance parameter.So it is easy to be realized and has better robustness.When the power supply frequency is high, this estimation method has better estimation accuracy especially.
In alpha-beta coordinate system, vector induced electromotive force is defined as (1) The relationship between the stator flux vector and induced electromotive force vectoris (2) The equation(2)on both sides of integral The calculation process of stator flux using the formula(4)need use not only the stator current signal, but also the stator voltage signal.So the formula( 4)is called the voltage model in alpha beta coordinate of stator flux linkage.In the process of flux estimation, the estimation accuracy of the voltage model may be affected by the pure integrator.This is due to the voltage and current signal of DC component and the initial error and especially when the motor is in low frequency, this kind of influence is more serious.
In order to solve this problem, a low pass filter is introduced to replace the pure integral of basic voltage model.

The transfer function of the lowpass filter is
where c Z is the cutoff frequency of the lowpass filter.
According to equation( 4)can be obtained for the stator flux estimator based on the low-pass filter.
Fig. 1 The relationship between the coordinate system PMSG 07001-p.2

ICMEE 2015
According to the Fig. 1,the rotor position of PMSG can be expressed as [8] 1 Because the Back-EMF s e can be divided into type Where f \ is rotor flux, D s e and E s e is the Back-EMF of the alpha beta coordinate axis component.
Therefore the motor speed is expressed as according to the equation ( 7) In the Back-EMF method, the inverter output voltage value is used as the system state variables, but due to the influence of DTCV, the actual output voltage distortion, eventually lead to serious deviation of the estimated angle [9].
Because of the uncertainty caused by the current direction, the actual phase voltage of A,B,C three-phase is difficult to estimate.
Assuming the direction of the current does not change in a switching period, the actual voltage of A phase is expressed as follows, relative to the midpoint of the busbar O.
In the formula, Vdc is the DC busbar voltage of inverter, Vce is the tube voltage drop of power devices, Vd is the tube voltage drop of fly-wheel diode.In one switching period, put into the formula (12),the average voltage can be expressed as represent the current from the inverter to motor.Therefore, V AO has moved with the switch state and the current direction [10].
Similarly, B and C phase voltage can be expressed as: The above formula can be simplified as: ,T on stands for power device turn-on time, T off stands for power device turn-off time, T d stands for the dead time, V ce -V d is too small to be ignored.
Accurately estimate of the inverter output voltage, we must know the motor phase current direction from equation( 20).In this paper, we used the indirect method for determining the direction of the current, which is based on the concept of vector.The direction of the current is according to the angle between current and voltage vector (power factor) to indirectly determine.
The output of the inverter for three-phase current is CLARK transform is applied to three phase current and phase voltage.
Then, two phase currents D s i and E s i are transformed into the coordinate system, which is based on the stator voltage vector for the d axis synchronous rotating coordinates.(3) The formula ( 22) and the calculated voltage error can calculate the real value of A-B axis voltage.

°°®
Finally, the formula(23)and type( 6)-( 8)can be calculated by permanent magnet synchronous wind power generator rotor position and speed.This paper established the hardware platform of the speed sensorless vector control system for PMSG based on TMS320F28335 control board.Fig.5 shows the control block diagram and Fig. 6 shows motor control and test platform.
generator control system, flux estimation requires stator voltage.If a given voltage in the SVPWM algorithm is used as electronic voltage.It will cause error about the flux estimation and reduce the flux estimation accuracy.Finally it have a serious impact on the performance of low-speed permanent magnet motor, because the dead time effect [6].The Back-EMF method is widely used, because of its physical concept is clear, small amount of calculation [7].The combination of Back-EMF and dead voltage compensation is achieved the precise control of the sensorless permanent magnet synchronous generator.In this paper, a method is introduced about the sensorless permanent magnet DOI: 10.1051/ C Owned by the authors, published by EDP Sciences, 2015 the detection of three-phase voltage and three-phase current in the system.Then, according to the motor model, we calculated the rotor position deviation of motor and used Back-EMF method to track estimation of rotor position.Finally, an important means to study the actual physical object.In order to analyze and control of PMSG, the establishment of mathematical model with simple and feasible is very necessary.

Fig. 2
Fig.2 Three phase bridge inverter main circuit diagram Power device VT1-VT6 can be controlled on and off by various methods,VD1-VD6 is free wheeling diode.V dc is the DC bus voltage, n is a DC link neutral point potential, O motor winding neutral point.Therefore, relative to the neutral point O of the motor, A,B,C three points voltage and current equations respectively level of the A phase of the drive pulse.Because V ce and V d is increased with the rise of the current, effect of time of 1

( 2 )
According to the current direction and formula (20), we can calculate the voltage error AO V c BO V c CO V c caused by the dead time.
Fig.5 Consider the impact of dead zone sensorless PMSG