IEEE Power & Energy Magazine - November/December 2017 - 63

themselves with very high shares of converter-based power
supply (see "Current Shares of Nonsynchronous Sources").

What About the Windy,
Sunny Neighborhood of the Grid?
Another reality of wind and solar power generation is that
some places are more productive than others. In the case
of wind power, location is critical to achieving high capacity factors (and, therefore, good return on capital investment).
Similar considerations drive utility-scale PV installations. If we
consider the representative system shown in Figure 3, a challenge presents itself. In this system, one corner has a considerable amount of good wind and solar generation, extremely
limited economic synchronous generation, and limited transmission to the rest of the grid. A grid-wide view of the renewable
penetration might look like Figure 1, but the corner with a good
deal of wind and solar can easily reach or exceed 100% penetration of converter-based generation. What happens if that corner
of the system disconnects from the rest of the grid? Today, the
answer is that the island can't run with 100% converters.
The point of these simple examples is that the possibility
of achieving actual operating conditions with zero synchronous inertia exists now and will grow rapidly in the immediate future. As an industry, we cannot wait until some distant
future, when total renewable energy penetrations on an annual
basis reach toward 100%. In the very near future, economic
and reliability imperatives will require much of the industry
to run with zero inertia some of the time.
In this article, we explore the challenge and options associated with this radical transformation of the grid. We start by
examining present design and practice for the power electronics
that enable wind and PVs to deliver ac power to the grid.

100% Renewables Isn't Exactly
the Same as 100% Converters
The rapidly emerging worlds of renewable energy and converter-based generation are tightly intertwined. A recent article
in the March/April 2017 issue of IEEE Power & Energy Magazine, "Achieving a 100% Renewable Grid," looked at a spectrum of issues. But while most wind generation and all PVs rely
on converters, many renewable resources deliver their power
through synchronous machines. The most obvious is hydro
power, but solar thermal, biomass, biogas, and geothermal also
use synchronous generators.
Further, we are not concerned solely with generation: it is
understood that energy storage will likely play a role. At present, the largest energy storage resource worldwide is conventional pumped storage hydro (PSH), which uses synchronous
machines. But rapidly growing energy storage technologies,
including batteries and variable-speed PSH, use converters.
Converters providing high-voltage dc interconnection, converters serving loads, converter-based static reactive compensation,
and even nonrenewable generation that interfaces through converters (such as microturbines) all contribute to the growing
trend of converter-dominant systems.
november/december 2017

Main Grid

Subgrid with
Only Load and
Inverter-Based
Generation
Tie-Line(s)

figure 3. A fictitious grid with high wind and solar extremity.

The Actual Behavior of Converters
The main limitation preventing converters from providing
the sole source of energy on a grid is the converter controls that are implemented today. Presently, grid-connected
converters (even so-called voltage-sourced converters, using
forced-commutation devices) are controlled by software to
operate as current sources that are grid following. Physically, this means that converters provide a current that is
shifted according to the correct phase angle with respect to
the grid voltage to provide the desired active and reactive
power. Practically, it means that these converters require
"appropriate support" to provide the grid with a stiff voltage,
which is presently accomplished by synchronous machines.
Eliminating all synchronous machines would mean that no
frequency reference would be available to the grid; therefore,
grid-following converters would not be viable.
Hence, it is essential for some converters to control the voltage. These converters, called grid-forming converters, have
controls that ensure the grid's voltage waveform is stable even at
a very short time scale. This type of control enables the system
to operate at a stable voltage even if loads connect/disconnect
from the grid. Figure 4 illustrates the behavior of a converter
operating as a current source when a load connects to it.

Changing the Electrical System Paradigm
Going from a system driven solely by synchronous machines
to a system with only power electronics introduces tremendous changes in the system's dynamic characteristics. Synchronous machines are controllable in the steady state and
dynamically within certain constraints set by the device's
physical limitations. Additionally, in the case of synchronous machines, the fast-transient behavior is imposed by the
physical (both electrical and mechanical) characteristics
of the machine. Therefore, regardless of the behavior of
controls (e.g., excitation and governors), the system's performance is largely predictable.
With converters, the controls are much faster, and the
behavior of such devices depends mainly on the characteristics
of the control software. On one hand, this feature of converters makes the system more controllable. On the other hand,
the system is more vulnerable and so more dependent on reliable design and functioning of the controls. Consequently, the
specification for converters will need to be more precise and
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2017