IEEE Power & Energy Magazine - November/December 2019 - 44

Anyone who generates, transfers, controls, delivers,
or utilizes electric energy should commit to these reliability
efforts as we move toward an integrated energy system.

9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0

Load Resource Stack
1,519 MW Solar

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Load (MW)

connected, with sizes ranging from 20 to 80 MW. Very little
solar PV is colocated with customer loads in North Carolina.
As such, D-PVs in the state is modeled as a stand-alone generating resource at the distribution level. Location is critically important, and some areas of the transmission grid are
saturated with generation and need upgrades to accommodate additional generation of any type. DER back-feeding
onto the transmission grid is common, but even without it,
DERs are having an impact.
Traditionally, utilities have understood and been able to
accurately predict the daily and seasonal fluctuations of customer demand. Unplanned generation or transmission outages have long been incorporated into planning and operating practices. The significant growth of solar PV generation
has added a new and independent dimension to planning
and operating the BPS. Solar output does not follow load
and is generally not dispatchable. Figure 10 presents an
example of a daily load curve for generation and customer
load in a DEP on a mild winter day. The top orange line
represents a typical winter customer load shape, with dual
peaks in the morning and evening. Generation resources are
separated into baseload nuclear, regulating resources such as
gas and coal generation, and solar PV. Solar is further split
into distribution- and transmission-connected categories. As
the figure shows, the customer load is at a minimum when
solar generation is at its maximum on a winter day. Regulating resources that need to be online for the peaks have a

Hours
Nuclear
Solar Distribution
Regulating Resources

Excess Generation
Solar Transmission
System Load

figure 10. An example of a mild-winter load curve and
resource stack in DEP.
44

ieee power & energy magazine

minimum output represented by the red line and are potentially forced off at midday due to solar PV generation. However, many regulating resources have start-up and shut-down
times of hours or days. The only solutions on some days are
to sell energy to neighboring balancing areas at a low price,
buy peak energy from neighbors at a high cost, and curtail
solar PV generation.
Determining the planning conditions for the BPS requires
understanding how the grid operates, which, as discussed,
has become more complicated. Transmission planners
examine the worst realistic conditions; in North Carolina,
that has been the summer and winter peak loads and the
valley load. With the addition of DERs, new operating hours
are becoming the most limiting or worst case. As shown in
Figure 10, generation output can vary significantly throughout the day. While the annual minimum customer load may
occur at night, light load conditions on a mild Sunday afternoon during spring can closely correlate with the maximum
solar PV output, resulting in the minimum net customer
load. BPS voltages can be high, and power-flow patterns may
occur that have never been observed in operating practice.
During summer, while the customer load may peak at 5 p.m.,
high solar PV output earlier in the day may increase flows on
the transmission system.
Worst-case conditions can occur during nontraditional
planning hours, and DEP is frequently reviewing the actual
operating conditions to ensure that planners are focusing on
the worst realistic conditions for the BPS. DEP is currently
planning for various combinations of customer load and
solar output, focusing on the following situations of stressed
system conditions:
✔ 100% summer peak load, 50% solar output (summer
peak hour)
✔ 90% summer peak load, 100% solar output (1 p.m. on
a summer peak day)
✔ 90% summer peak load, 0% solar output (sunset on a
summer peak day)
✔ 100% winter peak load, 0% solar output (winter
peak hour)
✔ 35% load, 0% solar output (a mild night during spring)
✔ 40% load, 100% solar output (noontime on a mild spring
Sunday).

NERC System Planning Impacts
of DER Working Group
NERC, in coordination with its stakeholders, has been
studying the effect of increasing DER penetration on the
november/december 2019

IEEE Power & Energy Magazine - November/December 2019

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