Synchronization

Synchronization is one of the important control functions in a grid-connected VSC. It involves obtaining information about the grid voltage such as the magnitude, the phase angle and the frequency. The phase angle of the grid voltage is useful in synchronizing the switching on and off of the semiconductor devices, reference frame transformation of the feedback variables and the determination and control of the active and reactive power flow [41]. The quality of the synchronization affects the quality of the control [41]. The main synchronization techniques found in literature are the zero-crossing detector (ZCD) [131],[132] and the phase-locked loop (PLL) [41], [44],[133],[134],[42].

The crossing detector is a simple synchronization method which detects the zero-crossing points of the grid voltage. By counting the number of zero zero-crossings, the frequency is estimated and by integrating the estimated frequency the phase angle is obtained. One of the drawbacks of the ZCD is that the zero-crossing can only be detected every half-cycle and there is no phase detection between the zero-crossing points which makes the dynamic performance of the ZCD poor [135]. Another drawback is that it is sensitive to noise and distortions in the grid voltage such as notches and low-frequency harmonics can negatively affect the output the output of the ZCD [132], [136].

A phase-locked loop is another method of grid synchronization. Originally used in telecommunications as a device whose output signal tracks its input signal, it has become very popular in grid-connected applications [137]. The most common PLL is the synchronous reference frame PLL (SRF-PLL) which is shown in Figure 4.7.

- + 

αβ

dq PI + +

0





v



v

gd

v

v 

v

 ^{~ }

Figure 4.7 SRF-PLL.

It consists of a reference frame transformation to obtain the synchronous reference frame voltages which are dc quantities. The lock is achieved by setting one of the synchronous

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frame voltages to zero. In Figure 4.7 v_gqis controlled to be zero by using a PI controller.

This aligns the grid voltage vector with the d-axis. The output of the PI controller is added to the nominal value of the grid angular frequency to obtain the grid angular frequency.

The frequency is integrated to obtain the voltage angle, which is fed back and used in the reference frame transformation. The SRF-PLL gives a satisfactory performance with balanced grid voltages, even in the presence of high-order harmonic distortion [138].

When a virtual flux based control technique is implemented, the virtual flux can be used for synchronization [81]. Due to the low-pass filters used for the virtual flux estimation, the virtual flux vector rotates more smoothly than the voltage vector and can be tracked more easily without using a PLL [81]. In this case, the virtual flux angle is calculated using the arctan function for the virtual flux components in the stationary reference frame. This is given by











 



  

 

g

arctan g (4.3)

The virtual flux vector lags the voltage by 90° and the voltage angle can be calculated from the virtual flux angle using

2

 

_v  _  (4.4)

The performance of virtual flux based synchronization can be improved for operation with distorted and unbalanced grid voltages by using a PLL [40], [88].

A virtual flux PLL (VF-PLL) is implemented in this study. The overall structure and operation of the VF-PLL is similar to the SRF-PLL. The block diagram of the VF-PLL is shown in Figure 4.8.

-+



αβ

dq PI



 ^~ 

0





gq g

 





Figure 4.8 VF-PLL.

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A notable difference between the SRF-PLL and the VF-PLL is that while for the SRF-PLL the q-component of the voltage is set to zero, for the VF-PLL the d-component of the virtual flux is set to zero. This is because of the -90° phase difference between the voltage vector and the virtual flux vector. With this orientation the virtual flux vector is aligned with the q-axis i.e. _gq

g 

  . With this orientation, the estimated angle of the VF-PLL is equal to the voltage vector angle. The orientation of the voltage vector and the virtual flux vector are shown in the vector diagram in Figure 4.9.



v





v



v





α-axis β-axis



d-axis q-axis

Figure 4.9 Orientation of voltage vector and virtual flux vectors.

The other difference is the performance of the two PLLs when the grid voltage is distorted with low-order harmonics. The performance of the SRF-PLL with distorted grid voltage is better when its feedback loop has a low bandwidth. This is because when the bandwidth is low, the PI controller which acts as the loop filter is able to reject the low-order harmonics. However, this reduces the dynamic response and the accuracy of the detected angle [29], [42]. In a high bandwidth SRF-PLL the effect of the low-order harmonics will be visible in the detected angle. The VF-PLL is more robust to low-order harmonics because its input is the virtual flux which is estimated using low-pass filters. Therefore, even with low-order harmonics present on the grid voltage, it gives a good performance.

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The performance of the SRF-PLL and the VF-PLL are compared in the simulation results below. In Figure 4.10 the performance of the two PLLs is shown for a purely sinusoidal grid voltage. This is the ideal case, and is rarely encountered in practice, though a strong grid does not deviate too far from this ideal case. The two PLLs are tuned to have the same bandwidth for comparison and both of them show good accuracy in tracking the grid voltage angle.

(a)

(b)

(c)

Figure 4.10 Performance of SRF-PLL and VF-PLL with balanced undistorted grid voltage (a) phase voltages (b) SRF-PLL angle (c) VF-PLL angle.

In Figure 4.11 a distorted grid voltage is applied to both PLLs tuned to the same bandwidth. The distortion is created by adding a positive-sequence fifth harmonic voltage of magnitude 20% of the fundamental grid voltage, and a positive-sequence seventh harmonic voltage of magnitude 15% of the fundamental grid voltage to the fundamental grid voltage. This is an extreme case of distortion which is not likely to be encountered in practice but it gives a good test of the robustness of the two PLLs. The SRF-PLL shows oscillations in its detected angle which show a deviation from the actual angle. The VF-PLL shows a better performance and the detected angle does not have any oscillations.

76 (a)

(b)

(c)

Figure 4.11 Performance of SRF-PLL and VF-PLL with distorted grid voltage (a) phase voltages (b) SRF-PLL angle (c) VF-PLL angle.

Therefore, the VF-PLL and the SRF-PLL have comparable performance in strong grids with almost sinusoidal voltages. However, in weak grids with highly distorted voltages consisting of low-order harmonics, the VF-PLL performs better.

The SRF-PLL and the VF-PLL where both implemented practically and the results are shown in Figure 4.12. The practical results verify the simulation results for undistorted grid voltage.

77 (a)

(b)

Figure 4.12 Experimental results for (a) SRF-PLL. Phase voltages (40 V/div); angle (120°/div) (b) VF-PLL. Phase voltages (40 V/div); angle (120°/div).