IEEE Electrification - June 2021 - 44

behaviors that both support and
enhance grid reliability. Today's
PV and energy storage inverters
can be deployed individually and
in a mixed design, affording plant
designers options for energy capture
and grid support. The following
topics are as general as possible
with some unavoidable linkage to
SMA inverters.
The Current Inverter Design
Inverters today are classified by both
market and design. From a market
perspective, the utility market is
largely served by central inverters,
which will be the focus of this article. The commercial and
industrial market is largely served by three-phase string,
or minicentral, inverters. The residential market is still
dominated by single-phase string inverters and modulelevel
microinverters. Central inverters, which have a much
higher capacity compared to string inverters, are multimegawatt
generators designed for a utility-scale deployment.
String inverters can range from a few kilowatts to
hundreds of kilowatts.
From a design perspective, utility-scale central
Inverter grid
supporting functions,
along with voltage
and frequency ridethrough,
provide key
behaviors that both
support and enhance
grid reliability.
medium-voltage (MV) level, as displayed
in Figure 2.
Table 1 provides general guidance
regarding the grid behavior of these
design categories. Generator owners
and utilities should use a mix of these
designs to provide the required behavior
on the grid.
The Inverter Circuit
The basic central inverter circuit
(Figure 3) can be described as follows
(IGBT is defined as insulated
gate bipolar transistor and MVT as
medium-voltage transformer):
dc input ➝ dc bus ➝ dc switch
➝ IGBT circuit ➝ LC filter ➝ ac breaker ➝ MVT.
x DC input: Provides the source of dc energy to the
inverter. This will be either PV, battery strings, or both.
DC input circuits typically are protected by fuses. Current
monitoring on each individual input is available
and is useful for monitoring PV array performance at
a high level.
inverters are built to be the main generator in the following
three distinct design categories: PV only, PV
plus dc-coupled energy storage, and ac-coupled energy
storage. The reference to coupling is the point at
which the energy storage is introduced to the system.
For dc coupling, the battery is connected to the PV
inverter dc bus, as shown in Figure 1. For ac coupling,
the battery is connected to the system at the ac
x DC bus: Provides a common point at which the dc
energy is aggregated. For central inverters, this bus is
designed for the available current of the PV and/or battery
source circuits connected to it. The bus often
employs surge protection and insulation monitoring.
For grounded PV arrays, the negative bus pole is
grounded through a fuse, breaker, or energy device
designed to open the connection should a PV ground
fault occur.
x DC switch: Provides an automatic disconnection of
both dc poles to isolate the dc from the IGBTs. The dc
dc
ac
Modbus TCP
SMA Scope
PV Module
MV Power Station Storage Solution
Sunny Central
MV Transformer
Overlaid
External
Controller and
Monitoring
System (SCADA)
Plant
Controller
Battery Strings
With Master BMS
SMA
dc-dc Converter
Grid Measurement
Figure 1. DC coupling: PV plus storage. BMS: battery management system; MV: medium voltage; SCADA: supervisory control and data
acquisition.
44
IEEE Electrification Magazine / JUNE 2021
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IEEE Electrification - June 2021

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