IEEE Power Electronics Magazine - March 2016 - 35

z	IEEE PowEr ElEctronIcs MagazInE

FIG 3 The field data from a 5-MW, 12-mi feeder in the southeastern United States compare the performance of the conventional centralized volt-var control with grid-edge
volt-var control. (Figure courtesy of Varentec.)

11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00
Time of Day

220

Pmax 3.39 MW
225

Pmax 3.61 MW

230

215
11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00
Time of Day

No ANSI Violations
Varentec Solution: On

5% Voltage Reduction
ENGOs On, SP = 235 V
LTC Set at 235 V (0.98 p.u.)

220

Pmax 3.95 MW

235

225
225

240

230
230

245

235
235

250

240
240

No Voltage Reduction

ENGOs On, SP = 240 V
LTC Set at 240 V (1 p.u.)

245
245

Minimal ANSI Violations
Varentec Solution: Off
March 2016

250
245
3% Voltage Reduction
240
235
No ANSI Violations 230
Varentec Solution: On 225
220
215

250
250

Pmax 3.69 MW

220
215
215
11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00
11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00
Time of Day
Time of Day

ANSI Violations
Varentec Solution: Off

3% Voltage Reduction
ENGOs Off
LTC Set at 240 V (1 p.u.)
Substation Voltage On 240-V Base
Secondary Voltage at 24 Locations
ANSI Low Limit
ENGOs Off
LTC Set at 247 V (1.03 p.u.)

renewables to substantially increase
the mix of clean energy sources with
the standard grid in the next ten-plus
years. During the presentation,
Prof. De Doncker argued that comparing the standard ac grid versus the dc
grid is like comparing beer and wine.
Unlike a glass of beer, which has significant amount foam at the top, a glass of
wine is flat at the top. A dc grid is like
wine, with very minimal disturbance
on the line.
In addition, as medium-voltage
(MV) dc grids gain popularity, electronic transformers, also known as
solid-state transformers (SSTs), will
gain momentum. This, in turn, will simplify the use of energy-storage systems.
Also, in-house dc distribution grids will
further improve cost savings and reliability with less maintenance.
Subsequently, Prof. Ron Hui of
the University of Hong Kong and
Imperial College London presented
his paper, "Use of Power Electronics Technology for Power System
Stability." He noted, "While the trend
toward renewable energy sources is
gaining momentum worldwide for
backup and clean energy, its intermittent behavior is initiating destabilization of electric grids, causing
potential blackouts." Consequently,
Prof. Hui added, "The grand challenge is how to increase wind/solar
power substantially and simultaneously while achieving power balance
and voltage and frequency stability."
His solution to stabilize the power
system is to use grid-connected power inverters as a stabilizing force. In
other words, Prof. Hui is proposing
grid-connected inverters mimicking
electric springs (ESs) for demand
response. By his definition, an ES is
a power electronics system that can
be used like an active suspension
device, distributed over the power
grid to stabilize the mains voltage
in the presence of a large percentage of intermittent renewable power
generation. Consequently, he continued, many small but distributed ESs
should provide a collectively robust
stabilizing effect on the future smart
grid (Figure 4).

35



Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - March 2016

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