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Local frequency control

A cluster of microsources operating in an isolated low voltage grid brings some problems, mainly in load-following situations, due to the slow response of microgenerators to control signals and due to the reduced inertia of the system. In order to allow islanded operation, such a system requires forms of energy storage (batteries, flywheels, supercapacitors) to ensure initial energy balance. Frequency droop control for fast load tracking was tested. The control strategy adopted combined this frequency control droop modes with the storage device response and load shedding possibilities, in a cooperative way in order to ensure successful overall operation.

The simulation platform developed within the result "Analysis of Dynamic Behavior of Micro Sources" was further upgraded in order to evaluate the dynamic behaviour of several microsources operating together in a LV network under pre-specified conditions including interconnected and autonomous operation of the MicroGrid. The simulation platform based on Matlab/Simulink allows the simulation of moving to islanded operation and load following during autonomous operation mode. Since the MicroGrid is expected to be an inverter dominated grid, its control mode was derived based on two possible schemes for controlling inverters:

"PQ control: the inverter is used to supply a given active and reactive power set point;
"Voltage Source Inverter (VSI) control logic: the inverter is controlled to feed the load with pre-defined values for voltage and frequency

A critical issue for MicroGrid islanded operation is the presence of an energy storage device to provide balance between load and consumption during transients. One can fulfil this requirement by establishing one of two operation modes for a cluster of microsources connected to an electric grid:

"Single Master Operation: A VSI (or a fully controllable synchronous machine directly connected to the grid) can be used as the reference voltage when the main power supply is lost; all the other inverters can be operated in the PQ mode;
"Multi Master Operation: Two or more inverters are operated as a VSI (no synchronous machine is needed).

Controllable loads play an important role under some MicroGrid operating conditions, namely those concerning a large imbalance between load and generation (load larger than generation). In order to deal with this problem, a load-shedding scheme was implemented to aid frequency restoration to its nominal value after the islanding of the microgrid and to avoid storage devices overload during the initial transient period. A secondary load-frequency control was also implemented to avoid storage devices to keep injecting (absorbing) power whenever frequency differs from the nominal value (storage devices inject power proportionally to frequency deviation).

Simulation studies were performed in different study case low voltage networks including several micro generation technologies, in order to evaluate the quality and performance of the developed control solutions. Different types of loads and load/generation scenarios were also considered. MicroGrid islanding was tested in two different conditions: forced islanding following a short circuit in the upstream medium voltage network or intentional islanding due to maintenance needs. In both cases, the proposed control strategies proved to be feasible.

Reported by

INESC Porto, Power Systems Unit
Campus da FEUP, Rua Dr. Roberto Frias 378
4200 465 PORTO