IEEE Power & Energy Magazine - March/April 2020 - 78

This is tested by commanding the ESS to charge/discharge
according to the variation of area control error signal in
the utility balancing area [the ESS power and SoC profiles
corresponding to this operation are shown in Figure 9(b)].
The demand response function is illustrated in Figure 9(c)
where the ESS is discharged at 155 kW from 3:55 p.m.
for approximately 2 h. Once the schedule is completed,
the ESS goes into charging mode to maintain SoC at the
required level. Performing demand response requires the
ability to discharge at a desired power for a desired period.
Other use cases that need similar capability (e.g., contingency reserve) could benefit from the analysis of the ESS's
demand response. The volt-var function needs the ESS inverter
to generate or consume reactive power to either hold control
voltage at a given set point or maintain a voltage limit. The
volt-var function built into the SSPC control system during
the early-stage smart grid demonstration project was refined
and integrated with additional functions during the control
capability expansion project. A var control demonstration for
controlling the 12-kV voltage at SSPC is depicted in Figure 9(d), where the actual voltage, voltage set points, and the
kvar required to control voltage are shown. The maximum
deviation between the set point and measured voltage was
found to be 0.65%. Figure 9(e) is an example of how setting
up the priority ranking allows the ESS to coordinate multiple
use cases. In this figure, both PFR and regulation are enabled
over the same time period, while the regulation function was
initially active. However, with the detection of a need to provide PFR, the ESS exits the regulation function and enters the
PFR function, which is the highest priority. Once the operation under PFR is finished, the regulation function is resumed
because it is still enabled and the ESS is ready to perform.

A Day in the Life of the SSPC ESS
A feature is included in the control system that will summarize the operation of the ESS over the last 24 h by indicating

for which use cases the ESS was ready (green), active (red),
and acknowledged but not capable (yellow). An illustration
of this feature, the ESS's operation on 27 March 2019, is
-provided in Figure 10. Ready means the service has been
scheduled, and the system can provide this service when it is
called upon. For example, PFR is normally green, meaning
that it can respond anytime an event occurs. Active means
the service is currently being dispatched. For example, PFR
has been triggered due to a low-frequency event and the
system is now discharging. Acknowledged but not capable
indicates the inability of the ESS to provide a requested
service. For instance, assume an operator has scheduled a
service, i.e., Reg Down. But because Reg Down requires
the battery to charge and current SoC is at high limit, the
system acknowledges the operator's schedule but could not
react if called upon. Currently, PFR is kept enabled for the
whole day. An analysis of historical daily operation profiles
can provide useful insight on frequency, length, and the
interruption statistics of various use cases and can be used
to enhance rule-based control strategies, build new conditions for better coordination, and so on.

Financial Value Tracker
Recognizing the importance of understanding the value generation performance of an ESS, a feature is combined with
the SSPC ESS to track the value generated under various use
cases. Although the actual revenue may differ because of
numerous reasons (e.g., dynamic price change and financial
settlement), this feature could at least provide some indication of the benefit an ESS has generated at a given point in
time. Currently, the SSPC control system is configured to
estimate value considering two aspects: one is how long it
has been available for a given service and the other is the
actual energy exchanged to perform the service. An illustration of the idea is presented in an SSPC control system
screenshot shown in Figure 11.

figure 10. A day in the life of the SSPC ESS.
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