Stability Assessment of High-Bandwidth DC Voltage Controllers in Single-Phase Active Front Ends: LTI Versus LTP Models

Abstract

In recent years, a considerable effort has been made to minimise the size of DC-link capacitors in single-phase activefront-ends (SP-AFE), to reduce cost and to increase power density. As a result of the lower energy storage, a high-bandwidth outer DC voltage control loop is required to respond to fast load changes. Linearised modelling is usually performed according to the power-balance method and the control is designed using LTI techniques. This is done assuming negligible voltage ripple at twice the grid frequency, and the model is considered valid up to the grid frequency. However, its precise validity limits are usually unknown and the control design becomes empirical when approaching these boundaries. To overcome this drawback, Linear Time Periodic (LTP) theory can be exploited, defining the range of validity of the LTI model and providing precise stability boundaries for the DC-link voltage loop. The main result is that LTP models more accurately describe the system behaviour and provide superior results compared to the LTI ones. Theoretical analysis, simulations and extensive experimental tests on a 10 kW converter are presented to validate the claims.

An Adaptive Multi-Resonant Current Controller for a Single-Phase Grid-Tied Converter With Grid Disturbance Rejection Capability

“…1. The voltage and current control loop are cascaded [32] The outer loop is the dc-bus voltage control loop to track V*dc. The inner loop is the current control loop to track I*ref generated by the voltage control loop [33].…”

An Improved Linear Model for High-Frequency Class-DE Resonant Converter Using the Generalized Averaging Modeling Technique

“…This method maintains all the information of the converter, but is not very pratical for most analysis and control design. Another modeling approach which has recieved a lot of attention in the modeling of active front ends is the Linear Time-Varying Periodic (LTP) model [14][15][16][17][18]. This method uses a harmonic state space model to descripe how each of the harmonic components of the input impact each of the harmonic components of the output.…”

Experimental Validation of Harmonic Impedance Measurement and LTP Nyquist Criterion for Stability Analysis in Power Converter Networks

“…Another option is the Harmonic Linearisation defined using signal-flow graphs [17], providing a visual tool in order to understand the various harmonic interactions. Harmonic State Space, which is based on the Linear Time Periodic (LTP) systems theory [18], [19], [20], also provides a straightforward relationship between the various harmonic components of the system.…”

“…ω inj has been selected from[2,3,4,5,6,8,10,12,15,20] Hz, which identifies 10 groups of independent measurements. Each single group comprises 2M + 1 separate injections, with M chosen equal to 27.…”