Mostly, PLL is mostly employed to generate the phase angle of PCC voltage in grid-following controlled converter. However, the phase angle can also be directly computed from abc voltage (first do the clark transformation to alpha-beta than calculate the anti-trigonometric). The fluctuations of the result caused by harmonics can be filtered someway, i suppose. Isn't it an easier and more reliable way than PLL?
Hi Zhaoshun Deng,
The method of Clarke transformation can be used for grid synchronization assuming sinusoidal grid voltages. As you mentioned, low pass filters can be used for taking out the harmonics. The method can give the correct phase information in steady-state operation. However, this method as you can see is an open-loop technique with no feedback signal about the accuracy of the calculated angles.
For a converter operating in the grid following mode, it is very essential that the phase angle of the grid is precisely tracked at all times in spite of any harmonics and in the presence of transient disturbances. The Clarke transformation method suffers from a poor transient response during grid disturbances, mainly due to the use of low pass filters in the forward path without negative feedback.
The PLL essentially does a similar job, but it uses a closed-loop technique. The loop filter in the forward path of the PLL performs the job of eliminating the effect of unwanted harmonics. At the same time, the feedback from the output VCO in the PLL makes sure that dynamic response is not compromised.
Jose Titus Thanks very much for your answer! It would be better if there are some comparative experiment or simulation results.
simply, PLL is a close loop system which you can ensure that the phase angle error is driven to zero. using alpha beta frame component to calculate the angle can work only in ideal grid condition. it is not about harmonics but dips, phase jump, etc, is a concern for the open loop method. The is another reason which is little bit hidden, using the division (beta/alpha) could lead to an error at zero time due to delayed measurement of the grid voltage component. Another reason, if you use the alpha/beta to calculate the angle with pre-filter, a phase error will be introduced by the filter which is a bit difficult to compensate it.
I agree with Jose Titus. The calculation from three phase voltage and application of LPF has a pitfall. If you try to use low pass filter for filtering out unbalance as well as harmonics, the cut off frequency of the LPF should be much less than 2fs=100 Hz. In that case, the transient response of the overall system will be compromised. If we look at the overall system of GCC, delay in response at one stage will greatly affect the performance of system particularly in dynamic condition.
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