IEEE Power & Energy Magazine - July/August 2021 - 67

Cloud-based energy storage includes a shared pool
of distributed energy storage resources that provide
on-demand-access storage services.
equal to 80%. The purpose of the storage aggregation is for
grid flexibility services. A dispatching platform was constructed
in the Jiangsu power system operation center. It is
directly connected to the Jiangsu dispatching center and
is dispatched as an integrated storage facility. It receives
signals from the power grid dispatching center and coordinates
the distributed storage stations to meet the dispatch
signals. The dispatching platform communicates with the
battery management systems (BMSs) of each storage station
through fiberoptics.
The control of each storage station is based on two key
functions: aggregation and dispatching. The dispatching
platform has access to the battery's state information, including
charging/discharging power, the maximum charging/
discharging duration, the state of charge, and limits. The
platform synthesizes the aggregated parameters of all storage
stations, i.e., the total charging/discharging power and the
maximum duration of charging/discharging. Based on the
synthesized parameters, the dispatching platform formulates
an aggregation model of the distributed storage and updates
the relevant information. The model provides the boundary
information for the power system dispatching center so that
the distributed storage can be dispatched as one element.
When the cloud-based energy storage dispatching platform
receives signals from the power system dispatching center, the
distributed storage system dispatching can follow two control
stages: state switching and power adjustment. In the " stateswitching "
stage, the dispatch signal is first satisfied by changing
the operation state of storage, e.g., from charging to discharging
when the signal is to increase the power output. If the
state switching does not meet the dispatch signal, the dispatch
begins the power-adjustment procedure. In the " power-adjustment "
stage, the remaining dispatch power is allocated to each
controllable storage system, the output of which is based on the
state of charge. Among all controllable storage stations, one
with a higher state of charge will be dispatched to discharge
more than one that has a relatively lower state of charge, and
the discharging power is proportional to the state of charge.
Using such a two-stage power dispatch strategy, the charging
and discharging of multiple storage resources can be balanced.
The grid flexibility services that the aggregated storage
provides to the system include the following:
1) Peak load shifting: During the peak demand season
in summer, the project mainly contributes to peak
load shifting based on a day-ahead power dispatching
strategy. The battery station usually charges/discharges
once or twice a day, as demonstrated in Figure 3.
july/august 2021
From mid-July to September 2018, the project had a
total of 157 charging and 149 discharging activities.
The maximum charging/discharging power reaches
94.59 and 99.38 MW, respectively.
2) Frequency regulation: Since October 2018, besides
load shifting, the storage stations also participate in
real-time frequency regulation based on automated
generation control. The project participates in the Jiangsu
grid's regional frequency control. The storage
station power change rate is 200 times higher than
that of coal-fired units, thus clearly demonstrating fast
regulation benefits from aggregated storage. In real
operation, the automated generation control signal
tracking error is about 1%.
3) Contingency frequency control: The project also
serves as a fast frequency response resource after a
contingency, i.e., a large generator failure or ultrahighvoltage
dc line disruption. It usually takes fewer than
100 ms for the storage station to switch from charging/discharging
mode to discharging/charging. Actual
measurements show that the aggregated storage stations
participating in contingency frequency control
could respond in fewer than 350 ms, including the
computing and communication time. In this way, the
aggregated storage can significantly reduce load curtailment
after a contingency.
4) Reactive power/voltage control: The reactive power
of each storage station can be tuned to achieve voltage
control. The voltage of the bus that each storage station
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figure 3. Operation with controllable storage stations.
ieee power & energy magazine
67
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IEEE Power & Energy Magazine - July/August 2021

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