Performance Evaluation of Three-Phase Grid-Connected Photovoltaic Inverters Using Electrolytic or Polypropylene Film Capacitors

Abstract

In grid-connected photovoltaic (PV) power stations, improving the life expectancy and long-term reliability of threephase PV inverters is urgently needed to match the significantly higher lifetime of the PV modules. A key contribution toward such improvement is replacing the conventional electrolytic film capacitors by metallized polypropylene film ones. This paper presents a detailed evaluation of a conventional three-phase grid-connected PV inverter performance when replacing the electrolytic capacitor with a minimum value of metallized polypropylene film capacitor. Although the minimum dc bus capacitance leads to higher voltage ripples, such ripples were found to be within acceptable limits to operate the inverter satisfactorily. Simulation and experimental results are presented for a 5-kW grid-connected inverter prototype with a nominal dc voltage of 457 V to confirm the theoretical considerations.Index Terms-Electrolytic capacitor, grid-connected inverters, metallized polypropylene film capacitor, photovoltaic (PV) systems.

A Control Scheme without Sensors at the PV Source for Cost and Size Reduction in Two-Stage Grid Connected Inverters

“…The gains of G ILf are shown in Table 3. A reduced DC-link capacitance leads to the fast dynamics of the V DC control loop at the expense of an increase of the THDi of I G [16,17]. In Reference [16] a notch filter in the DC-link voltage control loop was implemented to reduce the low-frequency harmonics of I G .…”

“…The estimation of P G , P G_est , depends on I G and on the grid voltage RMS value (V G ) so that variations of V G perturb the calculations of P G_est . To overcome this issue, the value of the signal v' pk is used to calculate the estimation of P G as it is shown in Equation (17). The signal v' pk is the amplitude of the fundamental of V G and is provided by the FLL-SOGI, not requiring additional computational resources.…”

“…The algorithm increases or decreases V PV_Ref in perturbation steps of a value ∆V PV_Ref to move the operation point along the I-V curve. In the sensorless implementation shown in Figure 10, instead of the measurements of V PV and I PV , the value of P G_est expressed by Equation (17) was used. In Reference [11] it was proposed to manage the duty cycle of the switches (D F ) of the DC-DC to move the operation point along the I-V curve.…”

“…There is a trend to reduce the required capacitance at the DC-link between the DC-DC converter and the inverter stage, which allows replacing electrolytic capacitors by film capacitors, which are more durable [17][18][19][20][21][22]. Two major effects of the DC-link capacitance reduction are the increase of the voltage ripple at the capacitors and higher transient variations of that voltage under dynamic operation point changes of the inverter.…”

Modeling and Controller Design of PV Micro Inverter without Using Electrolytic Capacitors and Input Current Sensors

“…Some devices include a decoupling circuit or ripple port circuit to absorb the reactive power [10,11]; however, this increases fabrication costs and results in switching losses that reduce the efficiency of the MI. As shown in Figure 1a [12][13][14], a film capacitor with a longer lifespan can be used as a substitute for an E-cap in typical single-phase two-stage MI systems [15]. However, the capacitance of a film capacitor is less than that of an E-cap with the same power rating, which increases voltage ripple at the 2nd harmonic frequency on the DC bus and can have three serious side-effects.…”