Why is a DC link capacitor required in a grid integrated renewable energy system?
Suppose we consider a single stage system with solar PV array feeding the grid than why do we require a DC link capacitor? A MPPT technique will ensure that maximum power and required reference voltage is made available at inverter terminals.Why is an energy saving device required at the inverter input terminals? Is it working as a filter? Is it that the PV array will transfer its entire energy to DC link capacitor which will further transfer it to the grid? If yes, why can't it be done without capacitor?
In steady state do we need to use DC link capacitor or is it just required for dynamic conditions?
Dear Maulik.
The DC link capacitor is a coupling element. When you connect an inverter to the grid, indirectly an ondulation is induced at the DC side as a product of the ondulatory component of the instantaneous power. This capacitor allows to delimite the amplitude of that ripple component. This aspect can be really important in order to avoid an unexpectated behavior of the MPPT because this ondulation is directly perceived in the voltage of the PV module. In fact, some researchers have proposed to use this ripple to search the maximum power point. Then, the capactitor has a decoupling function in stationary state.
On the other hand, the output power of the PV module can change as a function of the irradiance and hence many disturbances will produce in this variable. The DC link capacitor play a damping role in this aspect contributing to mantain the stability of the MPPT and the grid-connected inverter during these transitory behaviours.
we need a DC link capacitor due to the fluctuation natural of the renewable energy sources. Therefore to keep the DC link voltage constant and decreases the ripple of the DC link voltage, we must uses an capacitor.
DC link capacitors are used to stabilize the DC link voltage of grid-connected inverter. by increasing the drawn energy from PV, the injected voltage to grid increase. so the voltage of DC link can be remained constant.
Due to temperature induced variation to the total resistance within PV cells leading to a nonlinear output efficiency, is why capacitance is needed to allow electronic control methods to maintain maximum output power. In some cases, with switch mode power conversion from DC to AC, there can be a minimum load requirement to maintain stability of the switch mode power converter, but as large gulps of current can be demanded by the load, the additional capacitance covers that peak demand without causing the power converter input voltage to drop below the minimum required to maintain power transfer.
One other aspect, often missed when using MOSFETs as power switches, even when applying low voltage loss GaN MOSFETs is the absolute maximum Drain Source Voltage. This is why, when switching inductive loads, a good example being a flyback switch mode power converter is to safely capture the flyback inductive discharge, storing the charge safely rather than dissipating any excess to cause power loss, so that the MOSFET Drain Source Voltage is not stressed to the extent of causing the MOSFET to be destroyed. By using applicable design methods with multiple MOSFETs flyback charge can be captured and redirected using diodes to provide the correct voltage polarity to an applicable storage capacitor. In some instances, with a 'dead time' needed between MOSFET switching cycles, and the slow speed MOSFET body diode, a Schottky or similar high speed power diode is used in parallel with the MOSFET Drain Source Body Diode to recover as much flyback inductive discharge as possible, whilst protecting the MOSFET. It is similar in principle to synchronous power rectification using MOSFETs to overcome the forward voltage loss associated with rectifier diodes, provided that the power required to drive the MOSFETs does not offset the rectification benefits in terms of reduced rectification losses. GaN MOSFET, when economically viable may well provide dramatic improvements in terms of reduced power loss for rectification.
The calculation method takes the value of inductance, the charge transfer required and the duration of that charge transfer. It is then a method of calculating the value of capacitance required for the inductive charge capture, so that the capacitor voltage is sufficient to avoid an excessive voltage. This is critical when using MOSFETs within the switching method, as you must not exceed the MOSFET Drain Source Voltage.
It is very similar to the method applied to a switch mode power converter, where inductance and capacitance values are matched to the charge transfer per clock cycle where, in some systems based upon an AC supply, this is also used to produce a form of active power factor correction. In other words, to the AC power line, the load behaves as if it is a pure resistive load. This dramatically reduces the system power loss, and helps the system provide a greater power output that would otherwise be possible.
I'd like to find a book section in order to lear it, if you know one, i'd appreciate it.
I shall see if I can find a suitable book, which I may have at home, as there is a formal method of calculating capacitance against inductance and, knowing the power line or switching frequency, a precise method to provide electronic control. By using electronic control, allows charge transfer to track the power line frequency, or rather rise and fall of the power line voltage, so that the current is electronically controlled to follow the voltage profile as would happen if the load was a pure resistor. This provides a method of active power factor correction, where the electronic control is independent of the power line frequency, so that it works in any country without a need to modify the circuit design.
Dear Maulik.
The DC link capacitor is a coupling element. When you connect an inverter to the grid, indirectly an ondulation is induced at the DC side as a product of the ondulatory component of the instantaneous power. This capacitor allows to delimite the amplitude of that ripple component. This aspect can be really important in order to avoid an unexpectated behavior of the MPPT because this ondulation is directly perceived in the voltage of the PV module. In fact, some researchers have proposed to use this ripple to search the maximum power point. Then, the capactitor has a decoupling function in stationary state.
On the other hand, the output power of the PV module can change as a function of the irradiance and hence many disturbances will produce in this variable. The DC link capacitor play a damping role in this aspect contributing to mantain the stability of the MPPT and the grid-connected inverter during these transitory behaviours.
The dc-link capacitor eliminates the ripple voltage from the rectifier output
Apart from serving as filter and other purposes as noted by others the large DC link capacitor serves as a local source of reactive energy and power and plays a very important role. Reactive energy is required to both establish and maintain the em fields and which PV devices lack. This is important for standalone systems which unlike synchronous generators, which are a natural source of reactive power, will require reactive energy to both sustain and maintain the em fields and voltage in the network to prevent voltage collapse in a transmission and distribution system.
Dc link capacitor is connected after rectifier stage in shunt primarily to provide smooth dc input as far as possible to inverter located at grid side in a renewable power generation. Here capacitor acts to suppress the ac ripples content in rectifier output and helps to improve the performance and efficiency of overall power system or grid.
Dear Maulik
the DC link capacitor is primordial in DC power systems, because it maintains a low voltage ripple as well as in case of using an inverter on its output, you will have less losses which means more efficient system.
Good luck
Dear colleagues, I think the importance of this pasive component is capital for the well behaviour of a solar PV systems. I have seen how it produce a more reliable output signal, additionally, this steadiness make more efficient the operation of the MPPT algorithm, these are my conclusions after few years of experimentation.
I do hope this migth help you to overcome whatever doubts you still have.
Best regards,
Maulik,
welcome,
The function of the capacitor is energy storage as potential energy. So, it can supply the the load when the source is reduced and it will store the surplus power when in case of Generator excess. So it serves as regulator and stabilizer between the source and the load. As it increases, it will store more energy and can fix the voltage for longer time.
Best wishes
Theoretically (and provided you sport a 3-phase interter to an ideal grid and making some more ideal assunptions), you could do without such a capacitor.
In the real world, you need it for short-term intermediate energy storage:
* the output stages are switching, thus they do not draw energy continuously.
* a single-phase inverter is even worse: grid energy injection is not constant but following the grid's sine.
* your control elements are not reacting infinitely fast.
* etc. pp.
Overall: in the real world, you can't do without a link capacitor of reasonable capacitance.
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