IEEE Power & Energy Magazine - November/December 2017 - 62

80
5,000
4,000
3,000
2,000
1,000
2010
2012
2014
2016
2018
2020
2022
2024
2026
2028
2030
2032
2034
2036
2038
2040
2042
2044
2046
2048
2050

Imports Solar Wind
Geothermal and Biofuel
Open-Cycle Gas

Hydro Coal
Combined-Cycle Gas
Nuclear

Wind Penetration (% of Load)

Generation (TWh)

6,000

Year
Month
Week
Day
Hour

70
60
50
40
30
20
10
0

40
60
(% Time)

100

figure 1. Renewable energy projections for the United
States. (Source: NREL, 2017).

figure 2. Wind penetration duration curves for a nominally 30% annual energy system.

penetration is illustrated in Figure 2. This figure considers
a near-future scenario with enough wind generation to meet
30% of a study system's annual energy needs. The first dotted line is at 27% and represents the actual annual energy
from wind as a fraction of the load energy consumed-for
the year. The available wind energy was 30%, but this system
experienced curtailment of wind during some operating hours
because of dynamic security limitations.
Moving to shorter time frames, the blue trace shows that
some months are relatively windier than others, with the
highest monthly penetration reaching approximately 32%

and the lowest reaching approximately 22%. Likewise, some
weeks are windier than others (green trace), some days are
windier (orange), and some hours are windier (purple). This
relentless trend is common to all systems with wind and
solar. We can see that this particular 27% annual wind energy
system exceeds 70% hourly penetration at times-a level of
wind, or wind plus solar, that is well within near-term reach.
For this system, increasing the annual wind energy to about
40% would result in many hours during which the desired
operation of the system is at 100% (or greater, if export is
possible). Some power systems in the world already find

Current Shares of Nonsynchronous Sources

Many small power systems (up to 10-MW peak demand) al-

it can be operated at certain times without any synchronous

ready achieve very high instantaneous nonsynchronous gen-

generator; i.e., a 100% instantaneous NSG penetration level

eration (NSG) penetration levels of 60 to 80%. Instantaneous

could be achieved.

NSG is thereby defined as generation from power converters

Also, larger power systems already reach significant penetra-

[plus high-voltage dc (HVdc) imports, if applicable], divided by

tion levels. Tasmania is an island electrically connected to main-

demand plus exports.

land Australia by a monopolar HVdc interconnector (Basslink)

The power system on the island of St. Eustatius, for in-

rated at 500 MW but offering a dynamic rating of 630/480 MW (ex-

stance, is using 1.9 MWp of photovoltaic (PV) and 1 MW of

port/import). Tasmanian demand ranges from 900 to 1,700 MW,

batteries together with three diesel generators, with a total

and the generation sources include hydro at 2,250 MW, gas at

capacity of 4 MW. PV and battery instantaneous penetration

380 MW, wind generation at 300 MW, PV at 96 MW, and 478 MW

levels as high as 89% have been observed, and the instanta-

HVdc import. The record NSG penetration level reached 78%,

neous penetration level will increase further as the PV system

considering wind, PV, and HVdc import.

is extended. Currently, even at high NSG penetration levels, a

In Ireland (5-GW peak demand and approximately 3-GW

synchronous machine (diesel generator) operates at low out-

wind power installed), the maximum NSG penetration level

put power, acting as a synchronous compensator to provide a

is currently limited by the system operator to 60%, but it is

solid voltage source, while the converters provide the power.

expected that the limit will be increased incrementally in the

Plans are under way to upgrade the St. Eustatius grid so that

near future.

ieee power & energy magazine

november/december 2017

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