How are Microgrid Networks Optimized to Preven Voltage and Frequency Droop

Short Answer

With the advent of multiple small
generators and energy storage devices in the modern power network, innovative
solutions to the unique problems and opportunities of these new systems must be
found.  One of these problems (or
opportunities) is the issue of how to correct voltage and frequency droop in
distributed Microgrids.  This paper
describes some of the methods that can be used to help mitigate this issue.

Smartgrids are collections of
various power producing, storage, and distribution systems that are controlled
in a manner to optimize the system in a number of different ways.  Micorgrids are smaller potentially
self-contained systems that may consist of Distributed Generators (DGs),
storage units, and local distribution lines.
Microgrids interface with the Maingrid through a Point of Common
Connection (PCC).  The DGs within the
Microgrid may consist of Photovoltaic (PV) cells, wind generators, small fuel
burning units, or other power producing units.
The energy storage units may consist of Uninterruptable Power Supplies
(UPSs), capacitor banks, batteries, or electrical cars connected to the power
network.  Within Microgrids, the
distribution system is usually transmitted at a relatively low voltage in
comparison to the voltages that can be transmitted throughout the Maingrid.

In the case that there is a fault or
generation failure in the Maingrid, Microgrids have the capability to isolate
themselves from the Maingrid and perhaps maintain power within them by a
process referred to as Islanding.

Loads within the system can come
online or go offline and when this occurs, it will cause fluctuations within
the power system.  This is seen as
changes in the voltage or frequency levels.
These changes are referred to as droop.
In order to counteract the droop introduced by the changing power
requirements, a control system that has the ability to adjust the power
provided by DGs and storage units may be implemented.

The power in a power system consists
of two types of power, Real power (P) that is consumed by resistive loads, and
Reactive power (Q) which is consumed by inductive or capacitive loads.  The sum of these two powers is called the
Apparent Power (S).

(1)

If power is transmitted across a
lossless line (one without a resistive load) the real and reactive power can be
related to the voltages on either side of the line and to the power angle by
(2) and (3).

(2)

(3)

Since the power angle will be small,
we can say that
and
.  This allows (2) and
(3) to be rewritten as (4) and (5).

(4)

(5)

A number of Distributed Generators
(DGs), which may contain various power producing units such as Photo Voltaics
(PVs), batteries, wind generators, fuel cells, or fossil fuel burning
generators are connected within microgrids.
These in turn are connected to the main power grid.  Two main objectives for control and
optimization of the power system are required.
First, the control of the various DGs is necessary to provide stability
by keeping the voltage and frequency profile within an acceptable range.  Second, the optimization of the cost/profit
to the main grid and the microgrid operators will be determined.

To insure that the voltage/frequency
droop control is kept within the nominal range, the DGs will work cooperatively
together utilizing intermittent, asynchronous, low bandwidth links.  By sensing the local power conditions and by
communication with other DGs, the local DG can determine its individual output
requirements.

To determine the cost/profit between
the microgrid operators and the main grid power provider, an optimization
problem is presented.  Both the microgrid
and the main grid operators will want to maximizes their profit (minimize their
cost), but this must be accomplished within the requirement of grid
stability.  In other words the DGs must
supply power at peak times and store power at lulls.  To model this optimization, different game
theory methods with be used to find optimal bidding strategies.  Specifically, the Nash game for interaction
between the DGs and the Stackelberg game for the interaction between the
microgrids and the main grid will be used.