Improved ip-iq Reactive Current Detection Method Without PLL

As the traditional d-q transformation method is not sufficient in detecting the three-phase asymmetric components, a new detection method in which a single phase voltage is input to a differentiator to get an orthogonal differential is proposed. The synchronous rotary angle can be obtained by using the orthogonality relationship based on mathematical relationship. The new detection method is suitable for both asymmetric load and asymmetric supply voltage systems. The effect of time delay caused by phase-locked loop (PLL) is eliminated without using PLL circuit to obtain synchronous rotation angle in the detection process. The phase shifting deviation is solved without shifting the phase of three-phase current. The unbalance degree of input voltage in Static Synchronous Compensator (STATCOM) is effectively reduced by using this method. Simulation and experiment results show that the proposed method is valid and feasible.


INTRODUCTION
With the development of modern industrial technology, more and more power electronic device has been applied into the power grid, therefore a large number of harmonics and reactive current have been produced which have brought great hidden danger and harmonic pollutions to the normal operation of the power grid [1] . The reactive power and harmonics can be effectively reduced by active filter and reactive power compensation technology, as the safety and stability of the power grid operation can be improved. The key to active filter and reactive power compensation technology is that the harmonic and fundamental current can be detected quickly and accurately [2] .
In recent years, a variety of detection methods have been put forward, such as the method based on adaptive cancellation, fast Fourier transform, wavelet transform, fuzzy rules and neural network [3] [4] . In practice, the mainstream is still the detection method based on instantaneous reactive power theory [5] [6] . An improved design method based on the traditional ip-iq detection method is proposed, in which a function is constructed to replace the phase-locked loop (PLL). The design scheme has the advantages of simple structure, strong anti-interference, fast response and high precision. Simulation analysis based on MATLAB is in positive to the analysis showing the accuracy and the feasibility of the improved method [7] .

TRADITIONAL IP-IQ DETECTION METHOD
The methods of detecting reactive current are consist of p-q detection method and ip-iq detection method. Where, the p-q detection method is only suitable for power grid voltage with no distortion. The ip-iq detection method is suitable for both distorted power grid voltage and asymmetric power grid voltage.
The sine signal sinωt and cosine signal cosωt in phase with the a-phase power voltage fundamental component are used in traditional ip-iq detection method which is obtained by a PLL and a signal producing circuit. The diagram of traditional ip-iq detection method is shown in Fig.1.
The detection principle is as follows: The ip and iq can be calculated according to the transformation of α-β coordinate. The DC  Fig.1 Diagram of traditional ip-iq detection method As the Fig.1` shown, we can obtain: The objects controlled by STATCOM are reactive current, harmonic current, negative sequence current and zero sequence current. Let be the iq=0, the active component iam, ibm and icm can be calculated. The harmonic and reactive current component can be obtained by subtracting the active component from the detection current. Three-phase current to be detected is as follows: Suppose the sine signal and cosine signal are Low pass filtering is used to ip and iq, the low-frequency component p i and q i are given by Where, DC component can be obtained when ω1 is equal to ω. In practice, the frequency of the power grid voltage is 50±0.2 Hz, and its maximum can be 50±0.5 Hz with a certain amount of AC, and at this time, ω1 is not equal to ω. Thus, it is necessary to ensure the pass-band higher than │ω1-ω│, and the stop-band frequency must be lower than the minimum of ω1+ω and │ω1-2ω│. The obtained low frequency component Where, C 23 is 1 23 32 1 0 From the consequence we can find that although the sine signal and cosine signal are different with the frequency and phase of power grid voltage, the fundamental of resonant current can be detected and the detection signal can be obtained by designing reasonable low-pass filter.

IMPROVED IP-IQ DETECTION METHOD WITHOUT PLL
From the research of the traditional detection ip-iq method, it can be seen that sampling power grid voltage is actually to find the relationship among power grid voltage, current frequency and phase. Although the fundamental frequency and initial phase angle of three-phase current can be obtained by using PLL, it is often affected by the signal. Moreover, the area of the circuit board will be increased with PLL circuit, its designing and debugging is complicated. Whether the function of PLL can be replaced by mathematical calculations is a question. The improved diagram is shown in Fig.2. The feasibility will be proved as following: From the defined analysis, it can be seen that we just need to know the direction of the current needs to be devoiced, as the power grid voltage and current phase angle φ can be obtained. The relationship between angle φ and power grid voltage is not tight. Thus, the power grid voltage Substitute (10) into (12) and (13) Where, C is The influence of PLL traditional detection method is overcame and the computational complexity of synchronous rotary angle is decreased by the method of single-phase power voltage.

SIMULATION ANALYSIS
To verify the feasibility of the proposed method, it is simulated on MATLAB.
Overall block diagram of the simulation on MATLAB is shown in Fig.3. The simulation model has 3 parts: the source and load module, the reactive extraction module and the PWM generator module.  Fig.3 Overall block diagram of the simulation on MATLAB The Improved ip-iq detection method module which is shown in Fig.4 is built. The main parameters are as follows: the power line voltage is 380V, the switching frequency is 10 KHz, the grid frequency is 50 Hz, the load inductance is 0.025H and the load resistance is 5Ω.

Symmetrical Three-Phase Voltage With no Higher Harmonics
For a symmetrical three-phase voltage is shown in Fig.5, the reactive current waveform of the improved ip-iq detection method is shown in Fig.6. The improved ip-iq detection method is accurate of a-phase reactive current and traditional method with no higher harmonics.

Distorted Three-phase Voltage
System voltage distortion occurs at 0.1 s. is shown in Fig.7 Reactive current of a-phase detected by improved ip-iq detection method is shown in Fig.8.  Fig.6 and Fig.8, it can be seen that the improved ip-iq detection method can always detect the reactive current accurately whether the three-phase voltage distorted or not. The advantages of the improved ip-iq detection method on speed and complexity degree are in positive to the feasibility of the design.

Three-phase Asymmetric Load
Supposing the other simulation conditions maintain the original value, the load of a-phase is 5Ω, b-phase is 10Ω, c-phase is 15Ω, and put it into the STATCOM device at the time of 0.1s. The wave of the current is shown in Fig.9. Fig.9 Current wave before and after putting it into STATCOM device From Fig.9, it can be seen that three-phase current is symmetric after putting into STATCOM device for one cycle and it has a good effect on compensating for three-phase asymmetric. It is in positive to the effectiveness and rapidity of the SATACOM device using the proposed detection method.

CONCLUSION
Traditional synchronous detection method is proposed based on three symmetric systems, in practice, the three-phase asymmetry occurs frequently. Thus, the synchronous rotation angle is constructed by using a single-phase voltage in the new detection method. The influence of PLL in traditional synchronous detection method has been overcome, and the complexity of compute synchronous rotation angle is also reduced. Simulation result shows that the proposed detection method has a high precision and fast response characteristic when detecting reactive current. The desired effect can also received by applying the method to STATCOM device.