Analysis and improvement of digital resonant controller for harmonic control of grid system

Abstract

If harmonics are added to the input of the grid-connected inverter, the efficiency of the inverter decreases, which adversely affects the system. Due to these harmonics, the technical document IEEE 1547 proposed the proportion of harmonics allowed in the gird as an acceptable proportion to the rated current. The purpose of this paper is to control harmonics within the harmonic ratio defined in IEEE 1547 using controls when harmonics occur. Several methods have been suggested to remove harmonics. In this thesis, harmonics are controlled and simulated using a proportional resonant controller that acts as an integrator and compensates for harmonics in a specific frequency band that is different from the general proportional-integral controller. The weakness of the resonant controller is that the controlling frequency of the resonant controller is shaken, the gain of the resonant controller is decreased and the performance of the controller is rapidly deteriorated. Due to these characteristics, it is important that the resonator controller accurately adjusts the frequency that it controls. Typically, when converting a system from analog to digital, the amplitude and phase are different. To compensate for this, a compensation method of about 1.5ωTsamp(ω=angular frequency, Tsamp=switching period) is used. However, this 1.5ωTsamp delay compensation is not discussed in detail. In this paper, we analyze the problem that occurs when converting the controller to digital based on the resonant controller and propose compensation method and compensation value. There are two main considerations when implementing digital resonant controllers. First, analog resonator controllers implemented in s-domain use four representative analog-to-digital conversion methods to solve the problem of different amplitude and phase when converted to z-domain by a digital resonant controller. This is solved when implemented with the characteristics of the implemented controller with the frequency characteristics most similar to analog. Second, there is a problem that occurs when a general digital system is implemented. There is a unit time delay in which a result of a value input to the system is output after one sampling, and an additional delay occurs in a system control by PWM. This time compensation is proposed as a way to compensate for the changes that occur in the delay through the lead compensator