IEEE Power & Energy Magazine - November/December 2017 - 52

much DPVs and load are within a
UFLS block.
60.2
Power systems-especially is-
Unit Trip
60.0
landed ones making the transition
59.7
from high penetrations of conven-
59.5
tional, synchronous generators to
Legacy PV Trip (59.3)
59.2
inverter-based generators-are
59.0
Underfrequency (58.9) Load-Shed Block 1
managing changes for maintaining
Underfrequency (58.7) Load-Shed Block 2
58.7
reliability. Conventional systems
58.5
Underfrequency (58.4) Load-Shed Block 3
have high levels of system inertia.
58.2
When a large unit trips, PFR may
Underfrequency (58.1) Load-Shed Block 4
58.0
take several seconds to respond;
Underfrequency (57.8) Load-Shed Block 5
57.7
but, because the high inertia leads
-5
0
5
10
15
20
25
to slower ROCOF, this relatively
Time (s)
slow PFR is adequate to restore
frequency. With increasing wind/
figure 1. The actual frequency response to a generating unit trip on Oahu.
PV generation, there are low levels
of system inertia, and the trip of a
trip of a 180-MW coal unit, the largest conventional genera- large unit results in a fast frequency decline. Fast frequency
tor on the island. When a large generator trip or system fault response (FFR) is then required to act quickly (on the order
occurs during a peak PV period, several characteristics of of 1 s or less) to avoid UFLS.
DPV impact reliability.
HECO is examining the use of battery energy storage
✔ DPV generation displaces conventional thermal gen- systems (BESSs) to provide FFR. Simulations of frequency
eration. While some thermal units are dispatched down, response to the loss of a 180-MW coal unit show that, with
some are decommitted and therefore do not provide in- a 130-MW BESS, the contingency avoids the trip of the
ertia to the system. This results in a faster rate of change "legacy" PVs and avoids UFLS. HECO also plans to lever-
of frequency (ROCOF), depicted by the slope of the age customer-sited demand-response resources to provide
trace after the unit trip in Figure 1. This means the sys- FFR to supplement BESSs, which will help optimize the
tem has less time to respond before UFLS acts.
size of the FFR BESS. HECO also must address over-
✔ During this frequency decline, PFR from those con- frequency response, because the overfrequency trip set-
ventional units still online act to restore frequency, but tings cannot be retrofitted and 300 MW of DPVs is set to
it takes several seconds for this governor response to trip at 60.5 Hz.
increase output.
High penetrations of PVs (either DPVs or utility-scale PVs)
✔ At 59.3 Hz, the "legacy" DPV systems (interconnect- can create balancing issues, because PV generation is con-
ed in accordance with the IEEE 1547-2003 standard) centrated within several hours of the day. Figure 2 shows the
trip offline. Hawaii's Rule 14H now requires DPVs to impact of DPVs on the worst-day forecast on Oahu for 2018.
ride through low-frequency excursions down to 57 Hz. The red trace shows the total gross load the utility would need
However, 60 MW of "legacy" DPVs remain that can- to serve were it not for DPVs and utility-scale renewables. The
not be easily (or inexpensively) retrofitted to ride- purple trace shows the net load if there were no DPVs (gross
through frequency or voltage excursions.
load minus controllable utility-scale renewables). This suggests
✔ At 58.9 Hz, the first block of UFLS is disconnected, considerable room for uncontrollable DPVs before imbalances
which disengages both the load and DPV generation occur. The gray trace shows the net load using current (2015)
on certain feeders. A block of UFLS is therefore less levels of DPVs (gross load minus utility-scale renewables
effective than in the case prior to DPV installation (or minus DPVs). On this particular day, at noon, the net load at
at night when there is no DPV output). This reduced current DPV levels (gray trace) is slightly above the minimum
effectiveness means that one or two blocks of UFLS level of must-run conventional generation (dotted horizontal
are no longer sufficient to restore frequency; rather, line) required online for reliability, balancing (reserve mar-
three, four, or even five blocks of UFLS are now re- gins), and contractual reasons.
quired. In this particular event, 76,000 customers
Further increases in DPV levels (the yellow trace indi-
were disconnected.
cates net load when DPVs in the queue is interconnected,
Hawaii, which is at the forefront of these issues, is find- while the blue trace indicates net load if DPVs were twice
ing ways to manage them. Due to increasing installations the current level) may lead to system imbalances, as the
of DPVs on the system, HECO is continually changing the total generation (the sum of DPVs and minimum conven-
feeders to be used for UFLS. HECO establishes differ- tional generation) exceeds system demand. Expected DPV
ent UFLS schemes depending on time of day and on how growth (i.e., 200% over current levels) may lead to system
(Hz)

Actual Frequency Response to a Generating Unit Trip-Oahu

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