IEEE Power & Energy Magazine - July/August 2021 - 57

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Fixed
Limit
(a)
figure 5. A comparison between a fixed limit and an operating envelope.
(i.e., purchase a battery system and engage with a DER aggregator).
These active customers are denoted with an asterisk
(*) in Figure 4. The considered battery system is rated at
7 kW/13.5 kWh, which is based on the options currently
available in Australia. The effects on network voltages due to
additional power exports by the aggregator (when providing
local and/or system-level services) are analyzed for two different
scenarios:
1) adopting the existing fixed limit of 5 kW
2) adopting operating envelopes.
The comparison between these two scenarios is illustrated
using Figure 5: the distribution of customer voltages
at noon (during peak photovoltaic generation hours) is illustrated
in Figure 5(a), along with the statutory limit (red dotted
line); the resulting operating envelope for the entire day
is shown in Figure 5(b). Figure 5(a) shows that the existing
export limit of 5 kW is unable to keep customer voltages
within the statutory limit, while network integrity is ensured
by adopting operating envelopes.
The maximum allowed export at noon is 3.5 kW. The
necessary restrictions to ensure network integrity during
peak photovoltaic generation hours are further illustrated
in Figure 5(b). During solar hours, the operating envelope
is below the 5-kW export limit. The figure also shows that
the operating envelope can unlock further flexibility outside
these hours for active customers and their aggregator (i.e.,
the operating envelope is above the 5-kW export limit).
Since the previous example considers the operating
envelopes to be the same for all active customers, the
corresponding objective embeds elements of fairness. However,
different objectives can have different outcomes. For
instance, consider an alternative objective: maximize the
sum of power exports from active customers, disregarding
fairness. Using this objective, Figure 6 shows the resulting
operating envelopes. When the objective is to maximize the
sum of power exports, it can result in a large disparity among
active customers.
july/august 2021
An active customer located at the far end of the feeder
cannot export any power between noon and 3 p.m. However,
an analysis shows that the sum of power exports is
larger than that obtained with the previous objective. For
instance, at noon, the sum of exports is close to 56 kW,
compared with only 28 kW (3.5 kW times eight) when the
objective is to allocate the same operating envelope for all
active customers.
Flexibility and Reactive Power Modeling
Active and Reactive Power Interaction
in DER-Rich Distribution Networks
Reactive power plays a key role in the voltage profile management
of distribution networks, which is currently managed
through a combination of reactive power compensation
devices and on-load tap changer-fitted transformers. In the
wake of increasing DERs, voltage-rise issues are the main
cause of concern for the DSO. They are the chief drivers for
the establishment of the 5-kW static export limits in Australia.
However, an inverter-based DER has the potential
to inject/absorb reactive power and thus enable the
DSO to dynamically adjust the network voltage profile to
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Time (hh:mm)
figure 6. The resulting operating envelopes to achieve
maximum aggregated power exports. Colored lines: operating
envelopes of individual active customers. Dashed gray
line: fixed limit (5 kW).
ieee power & energy magazine
57
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Operating
Envelope
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Fixed Limit (5 kW)
Operating Envelope
6:00
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Time (hh:mm)
(b)
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Voltage (pu)
Maximum Export (kW)
Maximum Export (kW)
IEEE Power & Energy Magazine - July/August 2021

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