IEEE Power Electronics Magazine - June 2019 - 21

development to cater to the needs of customers, and
quency components, to ensure that these variables track
they have presented most successful MG controllers.
their set points. Primary control does not need communicaFor example, ABB designed an MG plus control systion because it is performed locally, working on measured
tem as a networked control system
signals. This control is responsible for
that is responsible for coordinating
system reliability and stability,
the operation of different generaimproving the system performance,
The increasing intertion sources and loads. The goal is
and ensuring a proper power sharing
est in smoothly inteto ensure the successful integration
[22]. Figure 5 shows how the three
of renewables a nd storage techcontrol levels are linked together for
grating renewable
nologies into both new and existing
different MGs.
energy into the power
MG installations. The MGC600 is
an ABB MG controller specifically
Inverter Output Control
grid is one of the
designed for the integration of genThe controllers of voltage source
main reasons for the
eration devices into isolated power
inverters (VSIs) are used for regulatsupply systems (mostly MGs). The
ing the distributed-generation output
development of the
currents and voltages. Each inverter
Ovation control system with an in MG
concept.
controller is composed of two stages:
tegrated MG controller designed by
the inverter output control and the
ABB manages the MG's distributed
power-sharing control.
energy to maintain stable operating
conditions with minimal operational staffing. Optimal
Power Solutions and Princeton Power Systems designed
Proportional-Integral Controllers
different MG controllers that come into play when an
Proportional-integral (PI) controllers with additional feedMG islands from the main grid that optimizes and inteforward compensation are commonly used because they
grates its various supply resources. Siemens RTU prodimprove the dynamic response of the MGs through improvucts for MG control were developed and tested in the
ing the current regulator performance, which in turn
IREN2 project in Wildpoldsried, Germany.
achieves the system stability, even in the presence of nonlinAll of these types of controllers aim to optimize the
ear loads [23].
operation of the MG during interconnected operation,
that is, maximize its value by optimizing the production
Proportional-Resonant Controllers
of the local distributed generation and power exchanges
Proportional-resonant (PR) controllers are used to improve
with the main distribution grid. The hierarchical control
system performance with a robust inner current controller.
scheme is divided into three control levels, primary, secA PR controller can be implemented by using
ondary, and tertiary, which together are used to provide
s
a coordination strategy for DERs. A detailed comparison
(1)
C PR (S ) = K p + K i a 2
k,
s - ~2
of the three levels of control is presented in Table 1 [21].
Figure 4 shows the schematic diagram of the hierarchical
where ~ is the resonant frequency, K p is the proportional
control levels of the MG.
gain, and K i is the integral gain of the controller. The controller performance is based on achieving a very high gain
around the resonance frequency [24]. This allows eliminatPrimary Control Level
ing the steady-state error between the controlled signal and
The primary control, or field control, is the first level. This
its reference.
level works on system variables, such as voltage and fre-

Types of
Control

Objectives

Table 1. The hierarchical control scheme of an MG.
Primary Control

Secondary Control

Tertiary Control

Inner control
Used to meet the voltage and frequency
requirement of DERs
Islanding detection control that checks
the operation mode for interconnection
to the utility grid
Controls applied at the source and load sides

Restoration control
Tracking the desired voltage and
frequency of the DER
Interaction control through
optimization methods
Minimize area control error

Market operation
Power-sharing control to the grid based on
distribution network operator interface
Directing the operation of medium
and low voltage
Reference generating for frequency and voltage values of the secondary control level

Decentralized control
Primary control that depends on the droop
control method regarding the various
sources connected to DERs

Can be centralized or decentralized
Frequency control, amplitude
regulation, power-quality
improvements

Can be centralized or decentralized
Decentralized as a MAS, which is preferred
Interconnection of MGs and utility grids

June 2019

z IEEE POWER ELECTRONICS MAGAZINE

IEEE Power Electronics Magazine - June 2019

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - June 2019