Effective Use of Film Capacitors in Single-Phase PV-inverters by Active Power Decoupling

Transcription

1 Effective Use of Film Capacitors in Single-Phase PV-inverters by Active Power Decoupling Fritz Schimpf and Lars Norum Norwegian University of Science and Technology (NTNU), Department of Electrical Power Engineering O.S. Bragstads Plass 2E, 7491 Trondheim, Norway Abstract The lifetime and reliability of PV-inverters can be increased by replacing electrolytic capacitors by film-capacitors. Film-capacitors have a lower capacitance per volume ratio; therefore a direct replacement leads to very large and expensive solutions, especially for single-phase applications. This paper presents an active circuit which acts as an interface between the DC-link of a PV-inverter and an additional storage capacitor. The voltage ripple in the storage capacitor can be increased compared to the DC-link capacitor, allowing a more efficient use of the stored energy and thus a massive reduction of the overall installed capacitance. An especially promising application can be found in moduleintegrated PV-inverters, because here the most efficient and cheapest topologies suffer from big electrolytic capacitors which deteriorate the lifetime. The paper focuses on different possible control schemes of the decoupling circuit. Results from simulations are used for discussing the proposed control methods. Also results from an experimental efficiency comparison between systems with electrolytic and film-capacitors are given. Finally a lab-prototype of the decoupling-circuit is presented which will be used for further experiments. I. INTRODUCTION It is a well known problem that in single-phase inverters the DC-link-capacitance needs to be relatively large (typically ca. 0,5 mf per kw of output power) to decouple input and output of the inverter. The DC-source at the input (i.e. the PV-generator) delivers a constant power, while the AC-output leads to a fluctuating power with double grid frequency. The DC-link-capacitor acts as a buffer and delivers or receives the difference in instantaneous power. Normally electrolytic capacitors are used in the DC-link because of their good capacitance per volume ratio and low price. By using electrolytic capacitors it is easy and affordable to install very high capacitances for decoupling between input and output of the inverter. This comes with a price: A severe drawback of this type of capacitor is a limited lifetime. Electrolytics are affected by ageing effects more than other electronic components and are therefore a bottleneck for inverter reliability and lifetime. Over long time the liquid electrolyte evaporates through the rubber seals of the capacitor, degrading the capacitance. The effect can be compensated by oversizing the capacitors by design, but a limit in lifetime will still exist. Many efforts have been made to replace electrolytic capacitors by film capacitors (metalized polyester or polypropylene films) because this type has a much higher lifetime and can even be self-healing in case of minor isolation breakdowns. The disadvantage of film-capacitors is a low capacitance per volume ratio (approximately 20 times lower than for electrolytics) and a much higher price. A direct replacement is therefore not feasible in terms of cost and size. A change from single-phase to three-phase topologies is a simple way to reduce the required capacitance for the DClink, because for a given input power the output power will be constant. Therefore the decoupling capacitor can be much smaller than in the single-phase case. But for low-power PV-inverters, especially module-integrated inverters for ACmodules this would be a costly solution. Additional current sensors, power semiconductors and increased overall complexity could annihilate the lifetime advantage of the film capacitor. Therefore single-phase topologies are advantageous in the power range of several hundred watts up to some kw. The problem to be solved is to reduce the required capacitance while still having sufficient power decoupling between input and output. In PV-applications the voltage ripple at the inverter input has to be kept small in order to assure stable operation in the maximum power point of the PV-modules. This is especially critical in single stage inverters, there the DC-link is connected directly to the input from the PV-panel(s) and a large DC-link capacitance is required. It cannot be reduced without compromising the overall efficiency by de-stabilizing the operating point. Several solutions have been proposed to solve the problem of large capacitances in single-phase PV-inverters. Some of them are based on known topologies like flyback or push-pull converters with additional switches and storage elements to actively reduce the voltage ripple at the input. Examples are described in [1] (flyback converter with an additional power decoupling circuit) and [2] (push-pull type with additional decoupling paths). Another interesting solution is to use the DC/DC stage of a two-stage inverter for decoupling between the input and the DC-link between the two stages. This is described in [3]. A general problem of low power inverters is their low efficiency compared to inverters at higher power levels. To make them competitive, high efficiency is the key factor in addition to acceptable lifetime and reliability. The highest efficiencies in the PV-market are reached with transformerless

2 I PV I decouple C DCl Fig. 1. S 1 S 2 D 1 I load D 1 C store DC AC U store I inverter Decoupling circuit Concept and topology for parallel decoupling ~ single-stage inverters. In addition to their very good efficiency these inverters are simple, light and relatively cheap. If the capacitance requirement of single-stage single-phase inverters can be reduced to allow the use of film capacitors for increasing the lifetime, they could get an ideal solution for PV-systems in the lower power range, too. Especially for highvoltage PV-modules where a lot of PV-cells are connected in series in one module their use would become very reasonable. A very general solution for decreasing the capacitance which is relatively independent of the inverter topology is the parallel active filter presented in [4] and [5]. Figure 1 shows the circuit, which will also be the circuit being considered in this paper. The main principle of the circuit is that the DC-linkcapacitor is separated into two parts, both with relatively low capacity. The capacitors are connected via a bi-directional DC/DC-converter, allowing a different voltage at both of them. The DC/DC-converter is operated in a way which keeps the voltage at the DC-link constant, while the voltage of the second capacitor C store can have a high ripple. This allows using a larger part of the stored energy in the storage capacitor. The DC/DC-converter is bidirectional. When switch S1 in figure 1 is operated, the circuit becomes a buck converter and delivers energy from the storage capacitor C store to the DClink C DCl. In this case D2 is used for freewheeling. When S2 is operated, the circuit operates as a boost converter, and charges Cstore via the diode D1. The voltage at the storage capacitor will always be higher or equal the voltage at the DC-link. In [4] the decoupling circuit is operated as a controlled current source, achieving good results in eliminating the ripple current in the DC-link. But when only controlled by a current controller, the voltage in the storage capacitor C store can drop to the DC-link voltage while it is being discharged. Then the marginal PWM operation occurs in the buck operation of the circuit, leading to instability and high ripple in the DC-link voltage. [5] presents a possible solution for the problem by recharging the capacitor via a small transformer and a diode rectifier from the grid side. Another solution is to use more sophisticated control method which also provides additional advantages as presented in the next section. II. CONTROL OF THE ACTIVE POWER DECOUPLING The control of the decoupling circuit shown in figure 1 is critical, because if the DC-link capacitor is reduced to small values, the voltage has to be stabilized fast and effectively. Otherwise a huge ripple will occur; or worse, the DC-linkvoltage will drop below the peak value of the grid voltage and deform the grid current. To compare different control strategies, simulations in Matlab/Simulink with the additional software package PLECS were performed. The first control strategy, current feedforward, is taken from the description in [4]. A second simulation was done for the configuration with an additional recharge circuit like described in [5]. Then two new concepts are evaluated, leading to a virtual capacitance -control, which operates stable without any input of reference values. That means that the DC/DC-converter and the storage capacitor can be combined to a module which behaves like a capacitor with much higher capacitance than actually installed. In all simulations the PV-generator is operating close to its MPP with a power of 2.7 kw. The short circuit current is 8 A and the open circuit voltage 400 V. The generator is connected to a DC-link capacitor from which a typical load current of a following inverter stage (I inverter = I peak sin(ωt) is simulated by a controlled current source. A. Conventional DC-link For comparison an inverter with a conventional DC-link is simulated first. The capacitance of the DC-link is 1.34 mf which corresponds to 0.5 mf/kw. A block diagram is shown in figure 2. The load current I load, the current delivered from the PV-generator I P V and the capacitor current I cap are shown in figure 3. Also the DC-link-voltage is plotted; it has a ripple of 12 V pp at MPP-operation of the inverter. From the results it can be seen that I cap = I P V I load. Later on the active decoupling circuit will deliver this current to relieve the DC-link capacitor from the low frequency loadcurrent. Fig. 2. I PV I I cap load U DClink DC AC Block-diagram for simulation with conventional DC-link B. Current-feed-forward Now the DC-link capacitance is reduced to 50 µf. In addition the decoupling circuit is connected with a storage capacitor which is also 50 µf. That means the overall installed capacitance is reduced from 1.34 mf to 100 µf. ~

3 C. Current-feed-forward with additional recharge circuit [5] proposes a concept for keeping the voltage at the storage capacitor above the DC-link voltage: A combination of a transformer and a diode rectifier recharges the storage capacitor from the grid, when its voltage drops below a defined limit. The circuit is shown in figure 5. I PV I load I inverter I grid I decouple C DCl U DClink DC AC ~ Fig. 3. Results for conventional DC-link S 1 C store S 2 The decoupling circuit has a PI-controller for current control. The reference value for the decoupling current is calculated from the average PV-current and load-current (I decouple = I P V I load ). The result is shown in figure 4. The voltage ripple at the DClink is very low, approximately 3 V. The ripple at the storage capacitor is much higher, around 200 V. That means that the decoupling is working effectively. A problem is also visible: Since the average voltage of the storage capacitor is not controlled, it changes depending on the working conditions of the inverter. In the simulated case it is slowly decreasing. When it drops below the DC-link-voltage, a further stabilization of the DC-link will become impossible. Also the voltage rating of the installed capacitor should not be exceeded. Therefore an additional control loop for the voltage of the storage capacitor is needed. Fig. 4. Results active decoupling with current feed-forward Fig. 5. recharging circuit Decoupling with recharging circuit This circuit is simulated with the current-controller from the previous simulation. The turns-ratio of the transformer is chosen to deliver a peak voltage of 450 V. When the voltage of the storage capacitor drops below that value, the recharging will start. The results in figure 6 show that the voltage stabilizes above the DC-link-voltage, allowing proper operation of the decoupling circuit. But since the recharging circuit has a passive diode rectifier it genaretes a current peak when the grid voltage reaches its peak values. So the grid current as the sum of the inverter current and recharging current contains harmonics whenever the recharging takes place. Actually, the recharging circuit seems to move the problem of power decoupling from the decoupling circuit to the grid. The distortions in the grid current are a direct consequence of missing energy in the inverter. The inverter is feeding a reduced power to the grid whenever it has to deliver the peak voltage, thus delivering a non sinusoidal current. D. Virtual capacitance method To solve the problem of the uncontrolled voltage at the storage capacitor, the control structure shown in figure 7 was developed. The idea is to make the DC/DC-converter and the storage capacitor behave similarly to a conventional capacitor with a much larger capacitance. The current controller is unchanged and used as the inner control loop. The reference current is generated by an outer control loop which controls the voltage in the DC-link. Using these two loops it is possible to keep the DC-link voltage at a given reference. To get control of the voltage at the storage capacitor, a characteristic is used which couples the DC-link voltage to the storage voltage. When the storage capacitor is fully charged the reference for the DC-link voltage is set to a high value, when the storage is nearly empty (voltage close to DC-link-voltage) the reference is set to a low value.

4 Fig. 8. Voltage characteristic Fig. 6. Results with recharging circuit Ustore UDCl,ref PI Idec,ref PI PWM + DC/DC I dec DC link UDCl Fig. 7. Block-diagram: Virtual capacitance controller An example for the storage-voltage to DC-link-voltage characteristic is shown in figure 8. In the normal operating region of the storage capacitor (e.g V it is very flat, i.e. the DC-link voltage does not change much. But when these limits are exceeded, the DC-link voltage is increased or decreased more drasticly. This forces a balancing of the storage voltage. The proposed solution has several advantages: 1.) No communication is needed between the control of the inverter and the decoupling circuit. That means the the active decoupling can be added to existing concepts without changes in the control. 2.) The storage voltage is balanced in a soft way, meaning that there is no sudden marginal PWM operation. 3.) The recharging circuit is not necessary. To show the advantage of the control structure, first a simulation without the voltage characteristic was done for comparison. The reference for the DC-link-voltage is at a fixed setpoint. Figure 9 shows the result. The storage voltage is drifting down slowly, at some point dropping to the DC-link voltage. Marginal PWM occurs and the controller gets instable. Figure 10 shows the same scenario with additional voltage characteristic. The slow drop of the storage voltage is stopped by reducing the decoupling current. The voltage at the DC-link is constant in the beginning, but is reduced during the time instances when U store is leaving the aimed working range. Normally, the control of the inverter would then reduce the current fed to the grid in order to stabilize the DC-link-voltage. (This was not included in the simulation.) In the simulated example the characteristic gets very steep when the storage voltage drops below 450 V. If a smoother Fig. 9. Controlled DC-link-voltage behavior of the decoupling circuit is wanted, this could be adjusted. Fig. 10. Controlled DC-link-voltage with characteristic

5 III. EXPERIMENTS A. Dumb replacement of electrolytics by film-capacitors As a first experiment, a commercial PV-inverter is used (SMA Sunny Boy 4000TL) and the electrolyitc capacitors are replaced by a film capacitor of approximately the same capacitance. This experiment is purely academic, because the required film capacitors have a high volume and price. Anyway, it will deliver valuable information about the losses in the electrolytic capacitors. It is expected that the overall efficiency will be higher when film capacitors are used, because they have a lower ESR and smaller leakage currents compared to electrolytic caps. A picture of the setup is shown in figure 11. The original capacitors are removed except for two which are required to generate a middle point potential. The film-capacitor (2380 µf, 800 V) is placed next to the inverter and connected by copper busbars and wires. The efficiencies of the modified and the original inverter are compared in figure 12. The thick graphs correspond to the original inverter as a reference and the thin ones to the modified setup. It is visible that the efficiency is, as expected, slightly higher when filmcapacitors are used. The difference varies between 0.1 and 0.2 percent. C DCL Fig. 12. Result of efficiency comparison C store Analog scaling Gate driver Isolation OV shutdown Gate interlock Power supply gate signals analog measurements DSP 320F28027 USB PC Fig. 13. Blockdiagram of prototype Fig. 11. Photo of Sunny Boy inverter with film capacitor IV. CONCLUSION B. Prototype of active decoupling circuit As a next stage a prototype of the active decoupling circuit is buildt. It is controlled by a DSP (TI 320F28027). Figure 13 shows a block diagram of the the prototype. The inductor current and the voltages of the storage- and DC-link-capacitors are measured and fed back to the DSP. CoolMOS-FETs from Infineon (SPW47N60C3) are used as power switches. The power and control parts are electrically isolated for safety and for easy connection to a PC via a USBinterface. Two film-capacitors can be placed directly on the board and additional capacitors can be connected externally. The used inductance is 500 µh with a saturation current of 20 A. A photo of the circuit board is shown in figure 14. The described prototype will be integrated into an inverter system for studying the simulated control structures. Also the overall efficiency will be determined. Parallel power decoupling can be used for decreasing the necessary DC-link capacitance drastically. It is not sufficient to control the circuit as a current source. An additional control of the voltage at the storage capacitor is important for stable operation. One possibility for such a voltage control is the use of a voltage characteristic, leading to a behavior of the circuit like a virtual capacitance. The losses generated by the decoupling circuit will partly be compensated by lower leakage and ESR in the capacitors and (in single stage inverters) by lower voltage ripple at the input terminals which leads to better MPP-adaption. V. FUTURE WORK The proposed contol concepts have to be ported to the DSP and the mentioned experiments with the prototype have to be done.

6 Fig. 14. Photo of prototype circuit board For controlling the circuit one-cycle-control like described in [6] could be very helpful, because it can react very quickly on changes of the voltages on both sides of the converter. Therefore it could help to increase the performance of the controllers. This will also be tested in the future. REFERENCES [1] T. Shimizu, K. Wada, and N. Nakamura, Flyback-type single-phase utility interactive inverter with power pulsation decoupling on the dc input for an ac photovoltaic module system, Power Electronics, IEEE Transactions on, vol. 21, no. 5, pp , sept [2] F. Shinjo, K. Wada, and T. Shimizu, A single-phase grid-connected inverter with a power decoupling function, in Power Electronics Specialists Conference, PESC IEEE, june 2007, pp [3] J. Schönberger, A single phase multi-string pv inverter with minimal bus capacitance, in Power Electronics and Applications, EPE th European Conference on, sept. 2009, pp [4] A. Kyritsis, N. Papanicolaou, and E. Tatakis, A novel parallel active filter for current pulsation smoothing on single stage grid-connected acpv modules, in Power Electronics and Applications, 2007 European Conference on, sept. 2007, pp [5] A. Kyritsis, N. Papanikolaou, and E. Tatakis, Enhanced current pulsation smoothing parallel active filter for single stage grid-connected ac-pv modules, in Power Electronics and Motion Control Conference, EPE-PEMC th, sept. 2008, pp [6] K. Smedley, One-cycle controller for renewable energy conversion systems, in Industrial Electronics, IECON th Annual Conference of IEEE, nov. 2008, pp