A generalised harmonic linearisation method for power converters input/output impedance calculation

IEEE J. Emerg. Sel. Topics Power Electron.

et al. 2018

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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.

Section: Basic Theory For the Stability Analysis Of Ltp Systemsmentioning

confidence: 99%

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

IEEE J. Emerg. Sel. Topics Power Electron.

et al. 2018

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“…is evaluated (recall equations (13), (14), with jω substituted by p + jq), the modulus of each element of the matrix is calculated and the largest of these is stored and used in the contour plot. The idea is that as s approaches the location of a pole, the modulus of one of the elements of the matrix H 1 H −1 2 will diverge and it will be shown in red-yellow colour in the contour plot, whereas if s is away from a pole, the greatest modulus will be small.…”

Section: Ltp Nyquist Criterionmentioning

confidence: 99%

“…One of the most widely used is Harmonic Linearisation, where small-signal perturbations are used to determine the input-output small-signal impedances [12], [13], [14], and the Nyquist criterion is applied to their ratio. However, this method might be inaccurate in determining instabilities below the fundamental grid line frequency, as shown in [15].…”

Section: Introductionmentioning

confidence: 99%

Stability Assessment of Power-Converter-Based AC systems by LTP Theory: Eigenvalue Analysis and Harmonic Impedance Estimation

IEEE J. Emerg. Sel. Topics Power Electron.

et al. 2017

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Abstract-Stability analysis of power-converter-based AC systems poses serious challenges not only because of the non-linear nature of power converters, but also because linearisation is not generally applied around a steady-state operating point, as in the DC case, but around a time-periodic operating trajectory. Typical examples are single-phase and unbalanced three-phase systems. In this paper, two general methods to assess stability of the aforementioned systems are presented. Both are based on the Linear Time Periodic (LTP) systems theory. The first is model-based and relies on the evaluation of the eigenvalues of the linearised model, assuming a complete knowledge of the parameters. By contrast, the second proposes a set of smallsignal current injections to measure the Harmonic Impedances and applies the LTP Nyquist Criterion, so that stability of the system can be assessed with a black-box approach, without relying on knowledge of the system parameters. The basic LTP systems theory is reviewed in order to provide a mathematical justification for the second method. As case study, a simple network, consisting of a source full-bridge converter in AC voltage-control mode and a load full-bridge converter in AC current-control mode including PLL, is considered. Analytical results based on average modelling and simulations based on both average and switching models are presented, showing good accuracy in the identification of the stability thresholds for both the proposed methods.

“…The system is equivalently represented as a source and a load subsystem. Each system is described by its own small-signal impedances, which is calculated using Harmonic Linearisation techniques as shown in detail in [8], [9], [10]. Finally, stability of the system is assessed by applying the Nyquist criterion to the ratio of load to source small-signal impedances.…”

Section: Introductionmentioning

confidence: 99%

Stability Boundary Analysis in Single-Phase Grid-Connected Inverters With PLL by LTP Theory

IEEE Trans. Power Electron.

et al. 2018

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A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. Abstract-Stability analysis of power converters in AC networks is complex due to the non-linear nature of the conversion systems. Whereas interactions of converters in DC networks can be studied by linearising about the operating point, the extension of the same approach to AC systems poses serious challenges, especially for single-phase or unbalanced three-phase systems. A general method for stability analysis of power converters suitable for single-phase or unbalanced AC networks is presented in this paper, based on Linear Time Periodic (LTP) theory. A singlephase grid-connected inverter with PLL is considered as case study. It is demonstrated that the stability boundaries can be precisely evaluated by the proposed method, despite the nonlinearity introduced by the PLL. Simulation and experimental results from a 10kW laboratory prototype are provided to confirm the effectiveness of the proposed analysis.