A novel strategy, for control of the power flow for a smart micro-grid is proposed. The
utility grid power is dynamically controlled by a Power Electronic Transformer (PET). A
60 Hz, step-down transformer is generally used at the point of common coupling (PCC),
to connect the micro-grid to the power system grid. Substitution of the conventional 60Hz
transformer, by a PET, results in enhanced micro-grid power management system, during
grid-connected operation. The smart micro-grid is a set of controllable loads and
distributed energy resources (DER); both renewable and non-renewable; that supply
demand of a group of customers. The proposed dynamic power limiter (also referred to as
PET) is a high-frequency, isolated power-converter system, comprised of a highfrequency
step-down transformer and three-phase to single-phase matrix converters. The
matrix converters are modulated with a novel pulse width modulation (PWM) strategy for
a bi-directional power flow control. The output of the matrix converter generates a high
frequency (few kHz) pulsating single phase AC at the primary and secondary of the transformer, which are phase shifted for active power control. The PET also allows
voltage regulation by control of reactive power. The entire system; represented as two,
three-phase AC systems with an intermediate high-frequency transformer; is simulated
using Matlab/Simulink. The equivalent system has utility grid at the input side and a
micro-grid on the output side. The micro-grid is modeled as an interconnected system
consisting of set of DERs and smart loads. The simulation analyzes the change in micro grid’s power generation and consumption in response to the change in its local grid
frequency, upon limiting the utility grid power. The PET hence restores the system
frequency by adjusting supply and demand at the PCC. The micro-grid can now
participate in frequency regulation for the main grid. The simulation results are obtained
to verify the operation and claims of the dynamic power limiter as stated below:
1. Restricted active power flow to the micro-grid, at a desired value determined by
the main utility grid.
2. Utilization of the change in local grid frequency, to dynamically control the active
power generation or consumption within the micro-grid.
3. Decentralized control of the DERs as well as the controllable loads, which operate synchronously, to supply the demand within the micro-grid.
4. Bi-directional active-power flow capability at the PCC.
5. Voltage regulation by control of reactive power.
6. Contribution of the micro-grid components in frequency regulation of the main
7. Smooth transition from islanding to grid-connected mode of the micro-grid,
without the need of grid synchronization.
8. Extra degree of freedom due to the presence of active-power controller in a
possible deregulation and market strategy within the micro-grid.
University of Minnesota Ph.D. dissertation. May 2011. Major: Electrical Engineering. Advisors: Ned Mohan, Bruce F. Wollenberg. 1 computer file (PDF) xii, 70? pages.
Shah, Jalpa Kaushil.
Dynamic power flow control for a smart micro-grid by a power electronic transformer..
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