IEEE Power & Energy Magazine - November/December 2015 - 38

Back to the Future: Rise of the Condensers
The vast majority of wind and PV solar generation being

substantial short-term overload capability, and they estab-

added to grids around the world interface with the grid

lish quite clean, balanced sinusoidal 60-Hz voltages. These

through power electronic inverters. They are not alone.

characteristics have prompted something of a renaissance in

Much, if not most, new loads use inverters, as do battery

the synchronous condenser industry.

energy storage systems, high-voltage dc transmission, and

There has been a spurt in the deployment of new, pur-

a host of other technologies. With ever-increasing amounts

pose-built synchronous condensers. They have been sized

of inverter-based resources on the grid and synchronous

and sited at strategic locations in the grid to mitigate a range

thermal generation being displaced, concerns about weak

of dynamic performance concerns. Another option is avail-

grid problems are increasing. In the long run, many experts

able that has the potential to effectively address some of the

believe that advances in inverter technology can enable the

concerns raised by displacement of synchronous generation

operation of 100% inverter systems. But that future is a ways

in key locations: conversions. In a synchronous condenser

off, and much of the existing grid infrastructure, especially

conversion, a unit of retiring thermal power plant can have

protection, and the present generation of inverter-based

the generator salvaged to function as a synchronous con-

large-scale wind and solar PV require some synchronous

denser. In most cases, this means permanent removal of the

machines to be running.

turbine, and a number of changes to supporting equipment,

Synchronous condensers, i.e., a turbine-generator with-

such as bearings, excitation systems, starting systems, etc.

out the turbine, have been used to provide grid strength,

This option provides another tool for system planners to

short circuit current, and voltage support for about a cen-

address grid performance problems that may be associated

tury. In the later days of the 20th century, right up to the

with the retirement of uneconomic thermal generation.

time of the wind and solar revolution in North America, they

A third option being exercised in some places with a

were distinctly unfashionable. Units that had been in ser-

changing mix of generation is the use of clutches on the gen-

vice for many decades were targeted for retirement. Power

erators of gas-fired thermal plants. With a clutch, there is an

electronic-based reactive compensation devices became

operational choice to run the unit as a generator, i.e., with

the technology of choice for mitigating problems where

the turbine connected, or as a synchronous condenser, i.e.,

fast, fine control of reactive power was needed. But rotat-

with the turbine disconnected. This flexibility means that

ing synchronous machines have attributes that are highly

new generation, which may be added to meet peak power

complementary to the needs of inverter-heavy ac networks:

or ramping demands and thus have relatively low capacity

they provide positive and negative sequence short circuit

factor, has the potential to provide grid support whenever

current, they dynamically support voltage, they usually have

the grid needs it. Sometimes, the best ideas are old ones.

A rather pessimistic test was performed in which all distributed PVs in the system was assumed to have very poor
low-voltage ride-through characteristics. The PV system was
modeled as tripping when the voltage at the device drops
below 88% of nominal, as allowed by old IEEE standard 1547.
The system was subjected to trip of the Pacific DC Intertie.
That event, as discussed above, results in large power swings
and accompanying large voltage swings. The voltages at a representative location are shown in Figure 7. For the reference
case where the distributed PVs had adequate low-voltage ridethrough (the blue line), the system tolerates the disturbance.
But when there is substantial common-mode tripping of the
distributed PVs due to widespread voltage swing, the loss of
the generation causes a system separation and collapse.
This is one of the risks associated with DG that motivated
the recent revision of IEEE standard 1547. There is now an
avenue open by which this risk can be mitigated, at least
to the extent of managing intentional tripping so as not to
create bulk power system reliability risks. The behavior of
38

ieee power & energy magazine

composite system loads colocated with PVs is complex. Our
understanding is improving, but it is by no means complete.
If systems continue the trend toward more reliance on highly
DG, practice will need to continue to adapt.

System Inertia: Float Like a
Butterfly, Sting Like a Bee
Much has been said about the possible impact of loss of system
inertia due to the displacement of synchronous generation by
inverter-based resources. Loss of system inertia (as illustrated
in the "Transient Stability in a Nutshell" figure by replacing
the weights with hands) primarily affects frequency stability: the ability of the grid to stay within acceptable frequency
bounds following disturbances. The article "Emergency
Response" listed under "For Further Reading" discusses frequency stability for high wind and solar systems. One aspect
of the loss of inertia that has an impact on transient stability
as well as frequency stability is the violence with which the
system moves when first hit with a disturbance.
november/december 2015

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2015