Research On Single-Phase Parallel Active Power Filter

With the development of the science and technology, many different kinds of Power devices used in life bring more and more conveniences for people, but also bring kickbacks into grid. For the loads in the grid are always of pulse and non-linear, the reactive and harmonic current they engender will cause instantaneous impulse and the customer voltage distorted so that the normal use of the power is deeply influenced. Active Power Filter (APF) is the most common device to settle the harms of harmonics and influences of reactive current. Parallel Active power filter is the most elementary configuration. At present, Single-phase parallel APF mainly work as current source to compensate the grid current. On the real-time harmonics detection results form load current, PAPF send out the current with same amplitude and inverse direction compared to commands in order to eliminate the reactive current influences and harms of harmonics and then compensate the grid current. So, the power factor and current quality are improved.This paper starts from the derivation of compensation commands of PAPF, taking the adaptive filter theory as measures to get the commands. In allusion to the shortcomings that traditional fix step adaptive filter can not meet with the requirements of dynamic response and steady precision, analyze the theory of traditional variable step LMS adaptive filter. When the harmonics do not take a great proportion to the detected current (high Signal-to-Noise), the precision of steady figures and convergence speed can be met simultaneity. But, after the theory analyze and experiments validation, steady amplitude is not accurate at all. For this problem, optimized variable step LMS adaptive filter is brought forward. Under any detected load current condition, requirements of dynamic response and steady precision can both be met.This paper starts from the elementary unit of parallel APF----Single-phase PWM rectifier, and establishes the math model for it. Using the concept of current average model for reference, author constitutes the control system for single-phase PWM rectifier. It is a double closed-loop control system----Inner loop for current, outer loop for voltage. Inner loop realize the track for current commands, and outer loop make the DC voltage stable. After the PWM rectifier which is based on the digital PI controller stabilizes the DC voltage, join the compensation current commands into the inner loop to compensate the reactive and harmonic current.The work of this paper is a branch of the Unified power quality conditioner (UPQC). The APF in this paper stabilizes the DC voltage so that the DVR can be joined to the DC bus. After the compensation for reactive and harmonic current, the load is converted into the pure resistor, then the DVR can restrict the grid current by serial transformer in order to make the load rated voltage stable. Then the two parts will join together and form the whole UPQC.