IEEE Electrification - September 2021 - 57

1) Microgrid-level market: The participants of the
microgrid-level market are prosumers. The microgridlevel
market enables each participating prosumer to
sell his/her excess energy to other prosumers in the
microgrid. The prosumer-to-prosumer energy transactions
in the microgrid-level market can be carried out
in either a centralized or a distributed manner. In a
centralized microgrid-level market, each participating
prosumer submits his/her energy trading parameters
to the MSn , which is programmed to behave like an
auctioneer, matching energy sellers with energy buyers
and adjusting energy prices. In a distributed microgridlevel
market, energy sellers and buyers are programmed
to adjust their trading parameters by directly
negotiating with each other. A participating prosumer
in the microgrid-level market can also choose to sell
(buy) energy to (from) the MSn
. If so, the MSn serves as
an energy aggregator, which participates in the distribution
system-level market described as follows.
2) Distribution system-level market: This market enables
energy transactions among the microgrids. The MSn
of each microgrid hosted by the distribution system
serves as a direct participant in the distribution system-level
market. Similar to the microgrid-level market,
microgrid-to-microgrid transactions can be
implemented in either a centralized or a distributed
manner. In a centralized market, the DMS serves an
auctioneer, which matches energy demands with supplies
and set trading prices for the MSs,n
participating MSsn
whereas the
distributed market. The MSsn
are responsible for pricing in the
can also trade energy
with the DMS, and the energy traded in this way can
participate in the wholesale electricity market.
Hierarchical Control of Microgrids
Built upon the physical configuration for a future distribution
system, we introduce a hierarchical control scheme
that supports the operation of the two-level P2P market in
real time. To be specific, we present the control strategies
of grid-following and grid-forming inverters. We also illustrate
system-level control, which is used to regulate the
frequency, voltage, and power flow of a microgrid.
Inverter-Level Control
In an ac microgrid, the control schemes of a grid-connected
inverter typically fall into two categories: grid-following
control and grid-forming control. Grid-following
control is widely deployed in almost all grid-tied inverters
in today's power grids. A grid-following inverter (an
inverter with grid-following control) is designed for
injecting real and reactive power to a well-established
grid, where " well-established " is used in the sense that
the inverter terminal voltage and frequency are in nominal
ranges. Figure 3 presents a grid-connected inverter
with a standard grid-following controller. A distributed
generation unit (DGU) physically connects to the PCC
through an inverter in series with an LC filter. The inverter
is controlled by a grid-following controller comprising
four building blocks: 1) several Park transformation ( " abcdq " )
blocks, 2) a phase-locked loop (PLL), 3) a current controller,
and 4) a pulsewidth modulation (PWM) block. The
Park transformation block achieves the " abc-dq " transformation
between three-phase ( " abc " ) measurements to
variables in the direct-quadrant ( " d-q " ) rotating frame. It
uses phase angle i as an input, which is given by the PLL
in the grid-following control. The PLL takes vq
as an
input and sets i for all " abc-dq " blocks in Figure 3, where
vq
to be zero. The current controller takes vd and idq
as inputs and outputs control commands udq in the d-q
frame, where vd and idq
iabc
. The control commands udq
are transis
the quadrant component of the instantaneous
three-phase voltage measurements v .abc The PLL can synchronize
the grid-following inverter with the grid by forcing
vq
are, respectively, the direct component
of vabc and the direct quadrant of the instantaneous
three-phase measurements of the current
measurements,
formed back to the " abc " space and become the modulation
index signal
m . This signal is then fed to the PWM
block, which returns the ON/OFF signals to control the
inverter. Note that there are no frequency or voltage regulations
in the grid-following inverter as the inverter is
designed to work in scenarios where the inverter terminal
voltages vabc
are well regulated by its host microgrid.
As a result, a microgrid containing only grid-following
inverters cannot operate autonomously when it is disconnected
from its host grid.
A grid-forming inverter (i.e., an inverter with a gridforming
controller) has frequency and voltage regulation
functions; thus, it has the capability of forming an autonomous
microgrid. Figure 4 shows a standard design for a
grid-forming inverter. Typically, a power controller (also
called the primary controller of the microgrid), a voltage
controller, a current controller, several Park transformation
Inverter
DGU
+
-
dc
ac
m
PWM
uabc
idq
θ
abc
dq
udq
Current
Controller
vd
Grid-Following Control
Figure 3. A grid-following inverter. DGU: distributed generation unit;
PWM: pulsewidth modulation; PLL: phase-locked loop.
IEEE Electrification Magazine / SEPTEMBER 2021
57
abc
dq
Output LC
Filter
iabc
abc
dq
vq
PLL
vabc
PCC
IEEE Electrification - September 2021

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