IEEE Power & Energy Magazine - November/December 2017 - 67

In a recent project completed by the Power Systems Engineering
Research Center, the viability of a zero-inertia power system
has been systematically examined and analyzed.
storage or headroom will depend on the speed of the controls;
small-sized devices could be sufficient.)

Protecting the Converter Against High Current
Voltage-controlled converters are prone to over-current during
grid transients. In case of a grid transient, if the voltage angle
shifts, it leads to a shift between active and reactive power, but
the total current remains constant and limited to the maximum
value. When converters are operated in the voltage-source
mode, the active and reactive power that flows out is guided
by the network voltage amplitude and phase before the action
of the controls. The apparent power of the converter has very
limited impact on the over-current, and network modification,
due to topology changes can lead to high over-current, as illustrated in the "Near-Term Solution: Power Electronics Must
Be Able to Provide Ancillary Services" section. Constraining
transients for a grid-forming converter can be tie-line opening
and closing or "large" load connection/disconnection close to
"small" converters.

Synchronization of Converters
and Virtual Synchronous Machines
The synchronous torque inherently synchronizes rotating
machines, but this is not obvious for converter-based generation. The control must achieve synchronization of every
unit with changing conditions and, most importantly, without
measuring frequency-for at least some of them (as previously stated, some converters need to form the grid).
This synchronization can be achieved with two main
varieties of grid-forming converter-control algorithms. The
first variety emulates a synchronous machine and is commonly referred to as a virtual synchronous machine (VSM).
The conventional grid-following converter current-control
loop architecture has a control bandwidth significantly in
excess of 50/60 Hz and aims to source sinusoidal balanced
currents synchronized with the existing grid voltages using
a phase-locked loop. It is possible to completely replace this
control architecture with one that, instead, mimics a physical synchronous machine rotor. Only a generic representation is required, so parasitic effects such as saliency and nonlinearity are not crucial. (See "Grid-Forming Converters: A
20-Year View from an Island.")
The critical model parameters are rotor inertia, rotor
electrical damping, and the transient impedance X´. In
practice, X´ is equivalent to (and can be defined) by the
converter filter inductance, which behaves almost exactly
as a real machine X´. The simulated rotor dynamics form
november/december 2017

a second-order transfer function with a resonant frequency
(normally) around 2 Hz for reasonable values of inertia H (in
seconds), damping, and X´, just as in a real machine. It can be
shown that, to provide a response closely approaching true
inertia from a converter, it is necessary to accept that there
will be a damped resonance. It may be possible to configure
higher levels of damping than a real machine provides, even
up to critical damping, because in the converter, damping is
implemented mathematically and is not constrained by physical damper-winding design or efficiency losses. However,
more research is needed to fully understand the ramifications of high damping levels.
The VSM needs to have a suitable automatic voltage regulator (AVR) control loop applied, which provides voltage and
reactive power control just as in a real machine. A governor is
also required and can take many forms, either following activepower set points, controlling frequency, providing droop
response, or any combination of these. Optionally, "slow"
prime-mover responses can be simulated, i.e., steam turbine
responses. Determining exactly which governor and primemover models to use for a particular scenario requires more
research, especially in the context of renewables. The energy
and power flow against time, as well as the requirement for
short-term or long-term energy stores to ride through dynamic
events, needs to be examined.

Grid-Forming Converters:
A 20-Year View from an Island
In the latter years of the 20th century, the Native Alaskan
community of Metlakatla faced a difficult technical and economic power system riddle: how to manage the extreme
active power swings associated with the biggest load on their
island grid. To replace an expensive, dirty diesel generator
needed for frequency regulation, a 1 MW-class utility-scale
battery-converter system was built. That system uses 1990s
vintage self-commutating converters, with an early version
of grid-forming virtual synchronous machine controls. The
system has been used to set frequency and voltage, blackstart the grid, and manage a variety of difficult grid dynamics.
After 20 years, the system has saved the community millions
in diesel fuel costs, while maintaining a cleaner and more reliable grid.

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2017