J. Puukko

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.

Implementing current-fed converters by adding an input capacitor at the input of voltage-fed converter for interfacing solar generator

The concern on observed climate change has increased the utilization of renewable energy sources. The harvesting of solar energy is recognized as one of the key issues in reducing green house gas emission. Reliable solar-energy systems composing of solar arrays and their interfacing converters are of prime importance in uninterrupted solar energy production. The interfacing maximum-power-point converters are implemented usually by modifying the conventional voltage-fed converters. Actually, the modifications change the converter into a current-fed converter with corresponding steady-state and dynamic properties. The paper investigates the true properties of these transformed converters based on theory and practical measurements. As an example a direct-duty-ratio-controlled voltage-fed buck converter is shown to be transformed into a current-fed boost-type converter.

Effect of minimizing input capacitance in VSI-based renewable energy source converters

Most of single or three-phase converters used in interfacing renewable energy sources (RES) to the power grid originate from VSI-based topologies. A common practice is to connect a capacitor between the RES and the VSI switch matrix to filter the pulsating current drawn by the converter. When a converter is supplied by a RES, one is usually interested in controlling the converter input voltage for maximal power transfer. The VSI incorporates a RHP-zero in its control-to-output transfer function when the operating point is moved from the constant voltage to constant current region of the source. The frequency of the zero can be given based on the input capacitance, voltage and current. The RHP-zero will turn into a RHP-pole in the input voltage control loop, which leads to constraints between the capacitor sizing and control system design. This paper gives scientific insight into the capacitor sizing and control design, of which the latter is often ignored in the analyses but is an essential part of interfacing RES into the grid.

Change of paradigm in power electronic converters used in renewable energy applications

Recent investigations on the nature of power electronic converters in renewable energy applications have revealed that most of the converters are not what they are claimed to be. The voltage-type sources have dominated as power sources especially in the past. Therefore, most of the power electronic converters are developed and designed for such applications even if some of them are known as current-sourced converters. The rapidly growing number of renewable-energy-based distributed systems has forced to pay closer attention to the structure of those systems and especially on the nature of power electronic converters within the systems. This paper summarizes the recent findings and states that most of the power electronic converters within the renewable energy systems are real current-fed converters, which have totally different static and dynamic properties compared to the conventional voltage-fed converters.

Appearance of a RHP-zero in VSI-based photovoltaic converter control dynamics

A typical grid-connected voltage source inverter (VSI) in its simplest form consists of an input voltage source, switch matrix, output inductor and grid, which acts as a voltage type load. This arrangement yields a converter with first-order control dynamics, a buck-type converter behavior and discontinuous input current. A photovoltaic (PV) array can provide a specific maximum current depending on the irradiation conditions. This implies that if the peak value of the pulsating input current of the converter tries to exceed the PV array current, the array voltage will collapse to a value dictated by the output voltage of the converter while the output current is determined by the array current. Furthermore, when the discontinuous input current of the converter is zero, the PV array operates in an open circuit. As a consequence, it may be obvious that an energy storage element, i.e. an input capacitor, is needed in parallel with the PV array in order to filter the pulsating input current drawn by the VSI. A vast majority of VSI analysis in PV applications assumes a constant voltage input source and current control dynamics depending only on the filtering elements at the output, e.g. first order dynamics with a mere inductor. However, this assumption is not well justified as will be proven in this paper.

Photovoltaic generator as an input source for power electronic converters

A photovoltaic (PV) generator is internally a power limited non-linear current source having both constant current and 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 one commercial solar array simulator as an example in terms of the defined properties. Investigations are based on extensive practical measurements from dc-dc as well as three- and single-phase dc-ac converters.

Designing MLBS Excitation for the Frequency-Response Measurement of AC-Connected Power Electronics Systems

Abstract Renewable energy, such as solar and wind, is usually connected to a power grid through grid-parallel inverters. The impedance mismatch between the grid and the interfacing circuit often generates harmonic resonances which leads to reduced power quality. Recent studies have shown that the problem can be approached through impedance models that may be obtained by broadband excitation and cross-correlation technique. However, the inverters are affected by the sinusoidal (AC) grid voltage, which necessitates modifications to the state-of-art techniques designed for DC systems. This paper considers the modifications and proposes methods for obtaining impedance models for AC-connected systems.