Tuomas Messo

Photovoltaic Generator as an Input Source for Power Electronic Converters

A photovoltaic (PV) generator is internally a power-limited nonlinear current source having both constant-current- and constant-voltage-like properties depending on the operating point. This paper investigates the dynamic properties of a PV generator and demonstrates that it has a profound effect on the operation of the interfacing converter. The most important properties an input source should have in order to emulate a real PV generator are defined. These properties are important, since a power electronic substitute is often used in the validation process instead of a real PV generator. This paper also qualifies two commercial solar array simulators as an example in terms of the defined properties. Investigations are based on extensive practical measurements of real PV generators and the two commercial solar array simulators interfaced with dc-dc as well as three- and single-phase dc-ac converters.

Determining the Value of DC-Link Capacitance to Ensure Stable Operation of a Three-Phase Photovoltaic Inverter

Grid interfacing of photovoltaic generators using three-phase inverters offers the advantage of constant power flow allowing smaller capacitance values to be used in the dc-link compared to single-phase inverters. Electrolytic capacitors, used in the dc-link, are often considered to decrease reliability. Reliability could be improved by using film capacitors, but their usage is limited by high cost and low capacitance. Much research has been done to minimize the dc-link capacitance value, particularly, in the field of drives and wind turbines. It has been shown that motor drive in regenerative mode contains a right-half-plane (RHP) pole in its control dynamics having a significant effect on the required dc-link capacitance. The RHP pole can cause instability as has been observed in wind turbine applications. Photovoltaic inverters have been reported to suffer from instability of the dc-link-voltage control, but the origin of the observed problems is poorly understood. This paper shows explicitly that an RHP pole is present in the control dynamics also in photovoltaic inverters affecting the minimum required dc-link capacitance. The paper proposes a minimum value for the dc-link capacitance that is required for stable operation. Design rules are given for single- and two-stage inverters. Moreover, it is shown that a source having constant power output effectively removes the RHP pole from the dc-link-voltage control dynamics.

Modeling the grid synchronization induced negative-resistor-like behavior in the output impedance of a three-phase photovoltaic inverter

Photovoltaic power has to be converted from DC into AC in grid-connected applications. The conversion is done by using a single or three-phase inverter. Phase angle and frequency of the injected current and the grid voltage have to match to achieve unity power factor. This has been commonly accomplished by using a phase-locked-loop (PLL). The PLL has a tendency to make the output impedance of the inverter to appear as a negative resistor which can introduce harmonics in the grid current or even make the inverter-grid interface unstable. This paper presents a general small-signal model of a PV inverter in the synchronous reference frame which includes the PLL. Due to the negative resistance, the inverter can become unstable when the grid has high inductance. The derived small-signal model can be used to predict the exact conditions where the instability will take place by utilizing the impedance ratio and Nyquist stability criterion.

Generalized multivariable small-signal model of three-phase grid-connected inverter in DQ-domain

This paper proposes a multivariable small-signal model of a three-phase grid-connected renewable energy inverter in dq-domain. The model allows predicting the shape of inverter output admittance with an arbitrary power factor and includes the dynamic effects of DC-voltage control, AC-current control, phase-locked-loop and decoupling terms. There is no need to simplify the model by neglecting any cross-couplings between d and q-components because the model is developed by recognizing the multiple-input multiple-output (MIMO) nature of the inverter. The output admittance matrix of the inverter including the crosscoupling admittances can be accurately predicted which allows precise evaluation of small-signal stability with finite grid impedance. It is shown that neglecting the crosscoupling admittances gives inaccurate predictions on small-signal stability and the full admittance matrix should be used in stability analysis instead.

Dynamic Characterization of Power Electronic Interfaces

Voltage-type sources such as storage battery, ac grid, and constant-output-voltage-regulated converters have dominated as input sources for power electronic interfaces for a long time, leading to the development of different power stages dedicated to voltage-type input sources. Recent penetration of renewable energy sources has initiated the use of current-type sources as well as input voltage regulated converters as input sources for power electronic interfaces. While the power electronic converter topology remains unchanged in both the cases, steady state and dynamic properties of the coupled source-converter system are quite different. Moreover, the voltage/current nature of the load contributes to the complex dynamics as well. This paper investigates the factors determining the dynamic properties of a power electronic converter in a specific arrangement. The findings show that the open-loop converter (without any internal or external feedbacks) automatically adapts its dynamic properties to the properties stipulated by source and load if certain terminal constraints are satisfied. If internal or external feedback is activated, the dynamic properties of the converter may be varied as desired; however, the control design process is substantially different for each source/load arrangement. The findings presented in this paper have not been presented in the literature by far. A buck-power-stage converter is used as an illustrative example. Experimental results are given to validate the analytic investigation outcomes.

An accurate small-signal model of a three-phase VSI-based photovoltaic inverter with LCL-filter

Three-phase photovoltaic inverters are usually equipped with an LCL-type output filter to reduce cost and size of the converter compared to a simple L-type output filter. The LCL-filter has an inherent resonance which has to be damped by a passive or active method to avoid instability. This paper presents an accurate full-order small-signal model of the three-phase VSI-based photovoltaic inverter with LCL-type output filter. The model is developed in the dq-domain, where the steady-state operating point can be solved. The developed small-signal model has been verified by extracting frequency responses from a scaled-down prototype. The model is shown to give accurate predictions on the shape of inverter transfer functions such as control loop gains and output impedance. Thus, the model can be used for control design, impedance shaping and impedance-based stability analysis.

Steady-state and dynamic properties of boost-power-stage converter in photovoltaic applications

Dc-dc interfacing of photovoltaic (PV) modules into the downstream system is usually done by using a boost-power-stage converter with an added input capacitor. Its dynamic properties are often assumed to be equal to those of a conventional boost converter. The input voltage of the converter is most often feedback controlled to achieve maximum power transfer in PV applications, which actually changes the converter to be a current-fed converter. This paper will show that the boost-power-stage converter with an added input capacitor has thoroughly different dynamic properties than those of the conventional voltage-fed boost converter. The effect of input-side control on the output impedance and the mode of the output port are also discussed.

Power hardware in-the-loop laboratory test environment for small scale wind turbine prototype

This paper presents power hardware-in-the-loop (PHIL) laboratory test environment for wind turbine (WT) prototype. The environment utilizes dSPACE and RTDS real time simulators, converter controlled grid emulator (GE) and the prototype. The analysis is carried out using frequency response measurements. The main contribution is the detailed analysis of the grid emulator performance. The main performance limitations are revealed and it is shown that the performance of the GE is dependent on the hardware of the WT prototype in addition to the design of the emulator itself. The comparison of the operation of the GE with open loop and closed loop control modes is carried out. It is shown that the accuracy of the environment is clearly sufficient for symmetrical voltage dips when closed loop control system is used. However, the open loop controlled GE cannot execute correct point of common coupling voltages independent on the operation point of the WT which is relative to the wind conditions. If the GE is controlled at open loop or at closed loop, the network asymmetrical voltage dips are not generated accurately. The reason is that the system bandwidth is not wide enough to execute negative sequence 50 Hz component without gain and phase error. However, the accuracy of the PHIL environment is clearly enough for testing the operation of WT control functionalities under asymmetrical network voltages. These functionalities may include PCC voltage balancing or positive sequence reactive power injection.

Simple method for measuring output impedance of a three-phase inverter in dq-domain

This paper introduces a simple method to measure the output impedance of a three-phase grid-connected inverter in dq-domain. The impedance measurements are most often used for model verification purposes but they can be also utilized to study impedance-based interactions between three-phase converters and the utility grid. Therefore, the methods to model and measure three-phase impedances can provide to the inverter manufacturers valuable information about their products. Implementation of the proposed method requires a three-phase voltage source, a digital signal processor, and a frequency response analyzer which are equipment typically available in most power electronic laboratories. The algorithms used in the method are explained, and the performance of the method is verified by means of frequency response measurements from a small-scale laboratory prototype.

Design of grid-voltage feedforward to increase impedance of grid-connected three-phase inverters with LCL-filter

Grid-connected three-phase inverters are increasingly used to interface renewable energy sources to distribution networks. From the perspective of power quality the output impedance of the inverter should be as large as possible since inverters are usually operated under output-current control mode. Feedforward from grid voltages has been demonstrated to improve power quality which is an indicator for better impedance behavior. However, the effect of feedforward has not been verified by accurate impedance measurements in the literature. This paper demonstrates the effect of grid voltage feedforward on the output impedance of a three-phase inverter with LCL-filter and proposes an accurate small-signal model to predict the shape of inverter output impedance in the dq-domain. The model can be used in impedance shaping and for evaluating instability caused by impedance-based interactions. Moreover, the ideal feedforward transfer functions are derived and it is shown that the proposed method has the potential to increase the inverter output impedance significantly at the LCL-filter resonant frequency.