In 3 phase converters (Invertor) We usually notice frequency is not dependent on load and voltage is also Relatively Stiff from Voltage control point of view
and It Give Circulating Currents in Parallel combinations of Converters?Can any one Givng Some Appropriate Reason for it
I believe that you are trying to compare inverter with conventional synchronous generator-type of power source? In synchronous generator, the frequency generated is dependent on the rotor rotational speed. Wherever there is a significant change in load, there is a loss of balance between the mechanical power (from the prime mover of the generator, eg, gas turbine) and the electrical power (due to change in electrical load). As a result, the rotor will accelerate/decelerate, causing a corresponding change in frequency.
On the other hand, inverter is a "static generator", ie with no moving part. The voltage magnitude and frequency of the inverter is purely depending on the reference signal (modulating signal) given by the inverter controller (in voltage control mode). Provided the the power capacity of the inverter is not exceed, it will be able to maintain its voltage and frequency regardless of the load.
With regard to the circulating current, it is due to the current controller design for the inverter. In normal three-phase inverter, the controller is designed to control only two currents (id and iq) since in a normal 3-wire-3-phase inverter, there are only two degrees of freedom (i.e only two of the currents are controllable). However, when two inverters are connected in parallel, there exist a path for common mode current to flow (i.e the circulating current). This is something not anticipated by the normal controller, hence the inverter will not be able to control it. Nevertheless, by taking into the common mode current into consideration and having an additional current controller, the inverters can easily eliminate zero sequence current without any modifications to its hardware structure.
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