Design and control of voltage source converters with LCL filters

Voltage source converters use filters to connect to the grid in order to limit the harmonic content injected to the grid. Compared to a simple inductor (L-filter), LCL-filters result in a lower total inductance value. Losses are reduced and the dynamics are preserved, but stability problems can arise in the current-control loop if the LCL-filter resonance is not properly damped. Passive damping adds dissipative elements to the LCL-filter capacitors. The control software (nested controllers for current and DC voltage) does not need to be modified, but there are additional losses and encumbrances. In spite of its disadvantages, passive damping is widely used in the industry because of its simplicity. Active damping modifies the control algorithm to obtain current-control stability. The control procedure becomes more complex and sometimes additional sensors are necessary. Literature presents many options for the LCL-filter design, passive damping design, and active damping design, and this chapter will present well-known practical methods. In this chapter, the LCL-filter design uses a step-by-step procedure with simple formulas that avoid trial-and-error iterations. Different configurations for passive damping are shown with design and stability considerations along with simple formulas for estimating the passive damping losses. The procedures presented for active damping preserve the nested controller configuration, and they are feedback type and filter based. Feedback-type procedures result in a robust design against line inductance variations. The capacitor-current feedback method requires an additional sensor and the lead-lag network avoid additional sensors by using the capacitor voltage also for synchronization. The filter-based procedure presented in the chapter uses a notch filter at the voltage reference output to the modulator. The procedure is straightforward, but it may require an estimation of resonance frequency when large grid inductance variations are present. The techniques explained in this chapter constitute the basis of the past and the ongoing research on LCL-filter-based grid converters.