Assessment And Control Of Harmonic Resonance And Voltage Sag In Microgrid

Microgrid is an electric power generation and distribution network, which is consisted by generators, energy storage units, control facilities and consumers'loads. Microgrid is also a low voltage active power providing network with excellent capability of accommodating smart operation modernization and certain ability of isolated operation. In the aspect of technology evolution, microgrid is a new stage of the way of distributed generation penetrating into the distribution network, and critical step to establishing the Smart Grid, and solving the critical problem of how to provide increasing load with enough electric power and low environment harming effects. Therefore, it is very important technical problem needed to be solved that microgrid connection would bring no unacceptable power quality compromising into distribution network. And the problem is the precondition of take fully advantage of microgrid in accommodation of renewable energy power generation and new energy power generation. The two important power quality problems, harmonic resonance and voltage sag, are investigated in this dissertation, from the angles of both formation mechanism and harm mode. The investigation is based on the conclusions of investigations in this two power quality problems in conventional power systems, and conducted with fully awareness of the changes and new features brought by microgrid connection. According to the investigation results, the new methods of harmonic resonance assessment and voltage sag compensation in microgrid are proposed, and the main contents and related conclusions are outlined as follows:The harmonic resonance modal assessment methods for both series and parallel harmonic resonance happening in microgrid and distribution network are proposed. The proposed assessment methods are based on the modal analysis results of node admittance matrix and fundamental circuit impedance matrix. According to the results, assessment indices can be produced and than harmonic resonance frequency, fierce degrees of inspired harmonic resonance, and the places of most fierce resonance can be inspired would be all discovered. Contrast to conventional methods, the ability of assessing series and parallel harmonic resonance simultaneously and of the special resonance effects inverters makes in microgrid is the big advantage of implementing proposed method in microgrid harmonic resonance assessment.The existence of connection between the output power of series and shunt voltage sag compensation is discovered, as well as linearity between the power reduction of series compensator and power generation of shunt compensator. Therefore, the combined compensation method for voltage sag in microgrid is proposed. In contrast to the conventional methods, the advantages of new method includes making fully use of distributed generation units and other components with shunt voltage compensating capability, compensating voltage sag simultaneously in series and shunt ways, lesser constrain on microgrid voltage sag compensating capability improvement and more independence on active power output capacity of series compensator, lesser constrain on location of series and shunt compensator and far more available compensating components to be chosen by eliminating the connection line between the DC bus of the compensators.The mechanism of excessively high output power phenomena of shunt voltage sag compensation in microgrid is discovered, and the reason that relatively too small quantity of connection line between microgrid and distribution network contrasting too large quantity of line voltage makes large compensation current and power is confirmed. According to the discovered reason, the current limiter based shunt voltage compensation method is proposed. In contrast to conventional voltage sag compensation method, the proposed method has the advantages on acceptable output power of shunt compensator that compensating voltage sag independently, only with the assistance from current limiter by maintaining voltage and producing no output power. The new method can exploit the maximum voltage sag compensating capability in microgrid by all distributed generation units and energy storage device be able to mitigate voltage sag directly.