IEEE Power & Energy Magazine - September/October 2017 - 65

september/october 2017

Limiter
±TRegD

RegD
Signal

RegA
Signal
Limiter
±TRegA

∑

lp Filter

Payback Calculation to
Determine Required Payback Cum. Sum
Gain (Conditional Neutrality)

+
∑

figure 7. An overview of a conditional neutrality controller.

Controller
Gain

PID
Filtered
(i.e., Control)
ACE

-1

Switch

lp Filter

Energy Bal.
Area Control
Signal (ACS)

∑
Gain

becoming more difficult to control area control error (ACE)
during ramping hours and, in response, implemented limits
on the total amount of RegD allowed in the clearing during
ramping periods of the day. Lower returns have disrupted the
growth in energy storage investment in PJM, which has caused
significant market seller concern. PJM is currently engaged in
a stakeholder task force dedicated to improving its regulation
market design to accurately model battery resources and their
characteristics and address system operation needs.
In January 2017, PJM updated its energy management system controller logic to a new system called "conditional neutrality." While the RegD signal was good at correcting shortterm frequency deviations within the ACE, it was ultimately
a poor design for controlling longer-term ACE excursions.
PJM observed that it was sending signals to the energy storage
resources to perform considerable movement and was compensating them at higher rates than traditional resources because of
this movement, but this movement was not producing effective
control. Additionally, the payment structure incentivized maximizing offered power, not energy storage capacity. The batteries were often not able to hold more than 10 min of sustained
deviation, compared to traditional resources that could provide
deviation for the entire hour if asked.
The conditional neutrality controller (Figure 7) is a modified proportional-integrator design, with two output signals.
Every 2 s, the area generation control calculates an optimum
control signal that would balance ACE to zero. The controller output is sent to a low-pass filter and then out to the RegA
resources. The residual between the optimum signal and
the RegA is filtered and then sent to the RegD resources.
Operational experience shows that advanced energy storage devices perform to their signals almost perfectly within
10 s of receipt, so the generated signal is a good proxy for
resource response. The RegD signal is then accumulated to
simulate the net effect of charging and discharging a 30-min
battery and is transformed into a neutrality offset added back
to the RegA signal. The net effect of this second loop is to
"oversteer" the RegA resources in a direction that reduces
the accumulation on the RegD resources. This allows the
two signals to work together to provide both energy storage
neutrality and ACE control within the same controller.
All storage devices are eligible to participate in the reserve
markets. Traditional storage resources, such as pumped storage
hydroelectric, participate in the reserve markets by increasing
output or even by curtailing pumping load if requested. The
energy-limited storage resources, however, due to their limited
capability to sustain for 30 min, currently only participate in
the regulation market.
There is also an economic factor in choosing which ancillary service to provide. Today, the price of regulation service
is higher than the price of reserves. In 2016, the weighted
average regulation market clearing price was US$15.72 per
effective megawatt, compared to a weighted average synchronized reserve clearing price of US$4.88/MW. As the regulation market rules evolve over the next year, we predict that

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