IEEE Power & Energy Magazine - May/June 2021 - 38

systems (i.e., the portion of the system that operates at the
microgrid's primary voltage).
Single-Phase Inverter Topology

A basic inverter circuit topology called an H-bridge is shown
in Figure 1. The inverter itself comprises four switches,
S 1-S 4, and an inductor-capacitor (LC) low-pass filter. This
inverter is serving a resistive load R and, thus, is shown in
an off-grid mode, in which case it would be operated as a
grid-forming inverter.
Three-Phase Inverter Topology

A three-phase H-bridge inverter connected to a larger utility is
shown in Figure 2. The inverter now has six switches, S 1-S 6 .
The dc voltage Vb is required to be larger than the peak of the
ac voltage on the inverter output terminals. When there is a
utility voltage source on the right, this inverter does not regulate the magnitude and frequency of its output voltage, as
those are controlled by the utility grid. Instead, the inverter is
operated in the so-called grid-following mode. Most inverters using the topology shown in Figure 2 require that the
transformer between the IBR and grid be ungrounded on the
IBR side. This can have a significant impact on protection

+
S1

L
+

Vdc
S4

Vac

figure 1. A diagram of a single-phase H-bridge inverter.

+
Vdc
-

because an ungrounded transformer blocks grid-side zerosequence voltages from reaching the inverter, and, thus, the
inverter's built-in protection cannot " see " these. The transformer itself will impact the zero-sequence current available
during asymmetrical faults.
Many commercial inverters today use a more complex
circuit topology called a neutral-point-clamped (NPC) converter. NPC converters have differences in their faulted
behaviors from the standard H-bridge configuration. For
example, an NPC converter does not require that the transformer between the IBR and grid be ungrounded on the
IBR side, so a transformer can be used through which the
inverter can " see " and respond to zero-sequence voltages
from the grid side. Details of the types and operation of
NPC converters can be found in the " For Further Reading "
section of this article.

Types of Inverter Controls
Inverter controls can be broadly grouped into two main categories of grid-forming and grid-following inverters. These
are described in Table 1, and block diagrams of each type are
shown in Figure 3.
Grid-forming inverters regulate voltage and frequency
at their point of interconnection. When there are multiple
sources sharing responsibility for frequency regulation, frequency droops are commonly used, which provide a slope
of the frequency setpoint versus the power output from the
generator. The frequency droop ensures the frequency stability of the microgrid while sharing active power among
IBRs. Grid-forming inverters are also equipped with voltage
droops to accommodate a coordinated voltage regulation in
the microgrid while sharing reactive power among IBRs.
Grid-following inverters are equipped only with power and
current control loops, which facilitate their operation as
controllable sources of active and
reactive power.

Generator Step-Up
Transformer

+ vL -

Devices and Materials

Cdc
S2

Grid

figure 2. A three-phase H-bridge inverter connected to a utility grid.
table 1. The types of inverter controls.

IEEE Power & Energy Magazine - May/June 2021

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