VOLTAGE DROOP CONTROL IN MICRO

GRID BY DISTRIBUTED GENERATORS

ABSTRACT:

For the operation of autonomous microgrids, an important task is to share the load

demand using multiple distributed generation (DG) units. In order to realize satisfied

power sharing without the communication between DG units, the voltage droop control

and its different variations have been reported in the literature.

However, in a low-voltage microgrid, due to the effects of nontrivial feeder impedance,

the conventional droop control is subject to the real and reactive power coupling and

steady-state reactive power sharing errors. Furthermore, complex microgrid

configurations (looped or mesh networks) often make the reactive power

sharing more challenging. To improve the reactive power sharing accuracy, this paper

proposes an enhanced control strategy that estimates the reactive power control error

through injecting small real power disturbances, which is activated by the low-bandwidth

synchronization signals from the central controller. At the same time, a slow integration

term for reactive power sharing error elimination is added to the conventional reactive

power droop control. The proposed compensation method achieves accurate reactive

power sharing at the steady state, just like the performance of real power sharing through

frequency droop control. Simulation and experimental results validate the feasibility of

the proposed method.

OBJECTIVE:

In our project a low-voltage microgrid, due to the effects of nontrivial feeder impedance,

the conventional droop control is subject to the real and reactive power coupling and

steady-state reactive power sharing errors.

The reactive power control error to be controlled through injecting small real power

disturbances, which is activated by the low-bandwidth synchronization signals from the

central controller.

SCOPE:

A networked microgrid model has been established using MATLAB/Simulink.

An improved microgrid reactive power sharing strategy to be proposed.

Our project is especially suitable for a complex mesh or networked microgrid.

MICROGRID:

A microgrid is a localized grouping of electricity generation, energy storage, and

loads that normally operates connected to a traditional centralized grid (macrogrid). This

single point of common coupling with the macrogrid can be disconnected. The microgrid

can then function autonomously. Generation and loads in a microgrid are usually

interconnected at low voltage. From the point of view of the grid operator, a connected

microgrid can be controlled as if it was one entity. Microgrid generation resources can

include fuel cells, wind, solar, or other energy sources. The multiple dispersed generation

sources and ability to isolate the microgrid from a larger network would provide highly

reliable electric power. Byproduct heat from generation sources such as microturbines

could be used for local process heating or space heating, allowing flexible trade off

between the needs for heat and electric power.

DISTRIBUTED GENERATION:

Distributed generation, also called on-site generation, dispersed generation,

embedded generation, decentralized generation, decentralized energy or distributed

energy, generates electricity from many small energy sources. Most countries generate

electricity in large centralized facilities, such as fossil fuel (coal, gas powered), nuclear,

large solar power plants or hydropower plants. These plants have excellent economies of

scale, but usually transmit electricity long distances and negatively affect the

environment. Distributed generation allows collection of energy from many sources and

may give lower environmental impacts and improved security of supply.

For reasons of reliability, distributed generation resources would be

interconnected to the same transmission grid as central stations. Various technical and

economic issues occur in the integration of these resources into a grid. Technical

problems arise in the areas of power quality, voltage stability, harmonics, reliability,

protection, and control. Behavior of protective devices on the grid must be examined for

all combinations of distributed and central station generation. A large scale deployment

of distributed generation may affect grid-wide functions such as frequency control and

allocation of reserve.

STATIC TRANSFER SWITCH: (STS)

A transfer switch is an electrical switch that switches a load between two sources. Some

transfer switches are manual, in that an operator effects the transfer by throwing a switch,

while others are automatic and switch when they sense one of the sources has lost or

gained power.

A static transfer switch uses power semiconductors such as Silicon-controlled

rectifiers (SCRs) to transfer a load between two sources. Because there are no mechanical

moving parts, the transfer can be completed rapidly, perhaps within a quarter-cycle of the

power frequency. Static transfer switches can be used where a reliable and independent

second source of power is available and it is necessary to protect the load from even a

few power frequency cycles interruption time, or from any surges or sags in the prime

power source.

CENTRAL CONTROLLER:

To initialize the compensation, the proposed method adopts a low-bandwidth

communication link to connect the secondary central controller with DG local controllers

. The commutation link sends out the synchronized compensation flag signals from the

central controller to each DG unit, so that all the DG

units can start the compensation at the same time. This communication link is also

responsible for sending the power reference for dispatchable DG units during the

microgrid grid-tied operation.

The power control strategy is realized through the following two stages.

1) Stage 1: Initial Power Sharing Using Conventional Droop Method:

Before receiving the compensation flag signal, the conventional droop controllers

and are adopted for initial load power sharing. Meanwhile, the DG local controller

monitors the status of the compensation flag dispatched from the microgrid central

controller. During this stage, the steady-state averaged real power (PAVE) shall also be

measured for use in Stage 2.

The first-order LPFs have already been used in measuring the real and reactive

powers (P and Q) for the conventional droop controller in and , the cutoff frequency of

LPFs cannot be made very low to get the ripplefree averaged real power (PAVE) due to

the consideration of system stability . Therefore, a moving average filter is used here to

further filter out the power ripples . The measured average real power (PAVE) is also

saved in this stage, so that when the synchronization signal flag changes, the last saved

value can be used for a reactive power sharing accuracy improvement control in Stage 2.

It is important to point out that although the averaged real power (PAVE) is measured at

this stage, the real and reactive powers used in droop controller are still conditioned by

only first-order LPFs as shown in .

2) Stage 2: Power Sharing Improvement Through Synchronized Compensation:

In Stage 2, the reactive power sharing error is compensated by introducing a real-

reactive power coupling transient and using an integral voltage magnitude control term.

As this compensation is based on the transient coupling power control, it shall be carried

out in all DG units in a synchronized manner. Once a compensation starting signal (sent

from the central controller) is received by the DG unit local controller, the averaged real

power calculation stops updating, and the last calculated PAVE is saved and used as an

input of the compensation scheme. During the compensation process, the combination of

both real and reactive powers is used in the voltage droop.

MULTILEVEL INVERTER:

A power inverter, or inverter, is an electrical power converter that changes direct

current (DC) to alternating current (AC); the converted AC can be at any required voltage

and frequency with the use of appropriate transformers, switching, and control circuits.

Solid-state inverters have no moving parts and are used in a wide range of

applications, from small switching power supplies in computers, to large electric utility

high-voltage direct current applications that transport bulk power. Inverters are

commonly used to supply AC power from DC sources such as solar panels or batteries.

A multilevel inverter synthesizes a desired voltage from several levels of direct

current voltage as inputs. The advantages of using multilevel topology include reduction

of power ratings of power devices and lower cost. There are three topologies - diode

clamped inverter, flying capacitor inverter and cascaded inverter.

MOSFET:

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS

FET) is a transistor used for amplifying or switching electronic signals. Although the

MOSFET is a four-terminal device with source (S), gate (G), drain (D), and body (B)

terminals, the body (or substrate) of the MOSFET often is connected to the source

terminal, making it a three-terminal device like other field-effect transistors. Because

these two terminals are normally connected to each other (short-circuited) internally, only

three terminals appear in electrical diagrams. The MOSFET is by far the most common

transistor in both digital and analog circuits, though the bipolar junction transistor was at

one time much more common.

PI CONTROLLER:

PI control - definition

The definition af proportional feed back control is still

where

e = is the "error"

KP = Proportional gain

The definition of the integral feed back is

where KI is the integration gain factor.

In the PI controller we have a combination of P and I control, ie.:

where

τI = "Integration time" [s]

τN = "Reset time" [s]

 NB: There is - in the real life - some confusion in the use of these two definitions.