IEEE Power & Energy Magazine - November/December 2017 - 64

```SG
Synchronous machines create
voltage waveforms with the same
frequency.

50 Hz

P = U.I.cos(φ)
Q = U.I.sin(φ)

Converters measure the grid
frequency.

→

I

Converters provide active and
reactive power at the measured
frequency.

→

φ

→

V

SG

figure 4. The actual grid operation with grid-following converters. SG: synchronous generation.

cover aspects that have never been examined for synchronous
machines (the shape of voltage during transients, fault current
values, and other similar characteristics).

Frequency, from Analog to Digital
System electrical frequency today is related to the rotor speed
of synchronous machines. With converters, however, the
change is radical, as frequency is a function of converter control. Frequency can vary rapidly and even in a discrete fashion.
Present equipment on the grid usually assumes that frequency
is continuous and slowly varying and will not function appropriately in a fast-frequency-varying environment.

A Weak Grid?
A weak grid is characterized by the short-circuit current being
"low." This definition can be seamlessly applied to synchronous machines because their behavior during a short circuit
and in normal operation is guided by the same electrical characteristics of the machine. Different aspects need to be taken
into account when considering weak grids with converters.
For example, synchronous machines can provide extremely
high current for a short period (1-100 ms), whereas converters
can normally provide little more than their nominal current.
This would imply that the short-circuit power will decrease.
However, the issue is more complex:
the ratio between the maximum current that can be
provided by converters and the actual active power produced before a fault may not always be as low relative
to the active power being produced. As an example,
load factor), the short-circuit power that can be fed is
approximately five times the active power delivered to
the grid.
✔ With synchronous machines, the active power delivered to the grid is often close to the machines' nominal power, based on the assumption that synchronous
machines are operated at maximum power and the
64

ieee power & energy magazine

short-circuit power delivered to the grid is three to five
times the active power fed before the fault.
With regard to the stiffness of the network, the short-circuit power is a good indicator; but, for converters, voltage
regulation is the main behavior driver, and control in steady
state and in fault mode can be radically different. As an example, the voltage regulation of a grid-forming converter can
create a very stiff grid voltage; in the meantime, controls can
block the converter during fault mode, thereby providing no
fault current.
A new definition will probably be needed to address the
grid stiffness with converters. The behavior of grid-forming
converters during a fault is one of the issues that will need to
be carefully addressed, with one of the key aspects being how
to control of the voltage while maintaining current below the
maximum converter capacity.

Technology Options
Grid-forming converters, with appropriate controls both in
the active power loop and in the reactive power loop, together
with correctly designed droop characteristics provide the
ability to develop and operate inertia-less systems. The proof
of concept can be demonstrated by means of large-scale system simulation with effective models in positive sequence
time-domain simulations of large systems.
In a recent project completed by the Power Systems Engineering Research Center (PSERC), the viability of a zeroinertia power system has been systematically examined and
analyzed. This viability analysis was conducted on a largescale system using the conventional industry practice of positive-sequence time-domain simulation. A key initial step in
this study was to develop an appropriate model for converterinterfaced generation that would capture the essential characteristics of the actual device.
The study focused primarily on the possibility of controlling and operating a zero-inertia system; therefore,
the reserve margin was not studied. It was found that the
principles of power-frequency droop, coupled with the
november/december 2017

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