Harmonic current detection based on synchronous coordinate transformation

Preface

The load current is converted to d,q Coordinate system , Through low-pass filter (LPF) After filtering out the high-order harmonics, we get the d,q Shaft DC component , And then with this angular frequency from d,q The coordinate system is inversely transformed to abc Coordinate system , Thus, the component of this harmonic is obtained .

Principle of harmonic detection

For better research and analysis SRT Harmonic detection . First of all, we need to understand its principle .d-g The principle block diagram of the detection method is shown in Figure 1.

Based on the d-q The analysis of the detection method shows , This method is first carried out by the grid voltage PLL phase lock . Get the fundamental positive sequence synchronous rotation angular velocity of the grid voltage wt, set up ia,ib,ic Is the harmonic current of three-phase load , Conduct three-phase load harmonic current Park Transformation , Change to d,q The current behind the coordinate system is ：

The principle of this method is to set the current at the fundamental angular frequency wt
Change to d,q After the coordinate system , because wt It can track the change of fundamental frequency of power grid voltage in real time ． The fundamental component of harmonic current of three-phase load becomes direct flow in the angular frequency rotating coordinate system ． The harmonic frequency becomes the AC flow in the angular frequency rotating coordinate system , namely ：

therefore , Will be like ,‘ adopt LPF After filtering, its DC minute is obtained
The amount id,iq, Conduct the DC component Park After the inverse transformation, it is the value of the fundamental component , The above is d-q The principle of detection . This method can only detect the fundamental current of power grid . On this basis ． take wt Conduct n Frequency doubling posterior meridian park Transformation . The income is equal to nwt Fundamental component of the same frequency , namely n Subharmonic content , Can be realized n Arbitrary extraction of subharmonics , That is to say SRT Harmonic detection .

SRT

In the above d-q On the basis of law , Put forward SRT, It detects harmonics
Principle diagram 2 Shown . Three phase load current passes Clarke Transform to two-phase stationary coordinate system ． And then to PLL The angular velocity obtained by phase locking is measured by a specified number of times ( Is assumed to be n Time ) After frequency doubling, the synchronous rotation is transformed to d,q In coordinate system , among ,PLL The extracted current can be kept in phase with the grid voltage , After spinning d,q After low-pass filtering of axis component , Then inverse transform to abc In coordinate system , It is the detected n Subharmonic current reference signal .