Juha Jokipii

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.

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.

Influence of network voltage level, converter topology and integration of energy storage on the power losses of STATCOM devices

This paper presents an analysis of power losses in 2.1 MVA static synchronous compensator (STATCOM) devices and STATCOMs with integrated energy storage (ESTATCOM). The analysis was carried out by means of computer simulations, with the main focus on studying the influence of network voltage level (690 V - 2 kV), voltage source converter (VSC) topology (2-or 3-level) and energy storage integration on the losses of STATCOMs. Power loss models for LCL-filters, VSC bridges, DC/DC converter power stages and inductors, as well as supercapacitor banks were developed. According to the results the losses are minimized when a 3-level VSC is used, and the compensator is connected to the network voltage of 690 V, and the ESTATCOM energy storage is connected to the dc link with two separate DC/DC converters. This occurs because the system converters can be implemented with IGBTs of lowest rated voltage.

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.

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.

Comparison of converter losses in an LVDC distribution

Efficiencies of potential line and load converter topologies and their AC-filter inductors for a bipolar low voltage direct current (LVDC) 1500V distribution are compared. The LVDC distribution is a new low voltage distribution concept which is one possibility to realize the future smart grid. The target of the system is high cost-efficiency and reliability. Efficiencies of the converters and filters are analyzed by simulation and analytical calculations and with two different customers load characteristics to find out the total losses during a year.

Effect of conventional grid-voltage feedforward on the output impedance of a three-phase photovoltaic inverter

In this paper the small-signal model of a three-phase photovoltaic inverter is upgraded to include the grid-voltage feedforward. The feedforward is shown to increase d and q-components of the inverter output impedance which makes the inverter more insensitive to impedance-based interactions and explains the reported improvement in the output current harmonics. However, using a low-pass filter in the feedforward path is shown to deteoriate the effectiveness of the feedforward already at the frequencies one decade below the cut-off frequency of the low-pass filter. The effect of the feedforward is verified by extracting frequency responses from a switching model and from a prototype inverter operating at reduced voltages.

Dynamic characteristics of three-phase Z-source-based photovoltaic inverter with asymmetric impedance network

Three-phase Z-source inverter (ZSI) is a relatively new buck-boost-type inverter topology, which in photovoltaic (PV) applications offers more efficient power point tracking than traditional VSI-type inverter in a single power processing stage. This paper presents an accurate small-signal model for the impedance network present in three-phase grid-connected ZSI, which can be used to e.g. design its control system and predict its impedance characteristics. In this paper, the developed small-signal model is used to study the effect of the asymmetric impedance network to the behavior of the network. It is shown, that asymmetric impedance network will affect to the dynamic characteristics of the inverter, but does not significantly decrease its electrical performance or compromise its stability.