IEEE Power & Energy Magazine - July/August 2020 - 62

typically a HV- or MV-connected gas or hydro power plant
or might, in the future, be a renewable energy plant with gridforming capability.
To achieve restoration, the DSO can select some feeders,
ideally with rather predictable load, and successively connect them to the islanded system. Leaving sufficient reserves
for stable operation of the islanded system is a high priority.
This case is designated as "with islanding" in Figure 8.
Often, and increasingly, distribution systems also include
several smaller DERs such as wind, PV, and biogas generators. If such generators are available and the DSO has at least
approximate knowledge of their location and expected active
power in feed, they can increase the overall available active
power and thus allow for supply of more load. This is shown
in Figure 8 as the "islanding with 25% DGs" case.
If the DGs are also known to contribute to active power
frequency control, they increase the final amount of load
that can be supplied and also the allowable load increment
that can be connected in a single switch action. In this way,
DGs with frequency control can form an island considerably faster than would otherwise be possible. In Figure 8,
this case is labeled "islanding with DGs contributing to
frequency control."
Figure 8 shows the supplied loads, the loss of supply, and
the total amount of lost energy, respectively, for a hypothetical example of a blackout from 6:00 a.m. to 12:00 p.m. The
total amount of energy not delivered to customers can serve
as an indicator of the overall damage caused by an outage. Although damage can be reduced by islanded operation, the exact tradeoff of justified investment is far from
obvious. It should be noted that a contribution by DGs to
frequency control does not only make restoration faster but
also more reliable.

Defining a universal cost function that weighs the
costs of a prolonged outage against potential repeated outages does not seem feasible. How to assess uncertainty
regarding available generation and load demand is not
only an issue of the power system but depends on regulatory requirements and incentives to the relevant actors.
However, many requirements are dependent on scenario
assumptions and have not been mathematically formulated
by regulators. A valid, pragmatic approach maximizes the
expected value of covered load, i.e., the integral of supplied active power, over the considered time period. Figure 3
illustrates a schematic of a possible enhanced cost function, including nonlinearities.

Socioeconomic Tradeoff
Resilience is an open-ended optimization goal in the sense
that it can never be completely achieved. Therefore, the
diminishing returns of ever-increasing investment in a more
resilient power system are certain to be outweighed by the
cost of the increasing mitigation effort at some point.
Mitigation includes the following measures:
✔ islanding capacities
✔ classic physical redundancy
✔ fallback settings
✔ ICT for better observability of the system
✔ smarter inverters
✔ stability increasing ancillary services
✔ robust planning
✔ traded flexibilities
✔ load prioritization.
Expenditures occur in terms of monetary cost, land use,
social impact, and so on. Likewise, expected damage encompasses loss of welfare in general terms (including social and

1.4
1.2

Lost Power

1
0.8
0.6

Without Islanding
Islanding
Islanding With
25% DGs Available
Islanding With DGs
Contributing to
Frequency Control

Without Islanding

Islanding

Islanding With 25% DGs

0.4
0.2
0
00:00

Islanding With DGs Contributing to
Frequency Control
03:00

06:00
Time (h)
(a)

09:00

12:00

0

1

2
3
4
5
6
Lost Energy (p.u. h)
(b)

7

figure 8. The (a) lost power and (b) lost energy for different islanding scenarios. p.u.: per unit.
62

ieee power & energy magazine

july/august 2020



IEEE Power & Energy Magazine - July/August 2020

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - July/August 2020

Contents
IEEE Power & Energy Magazine - July/August 2020 - Cover1
IEEE Power & Energy Magazine - July/August 2020 - Cover2
IEEE Power & Energy Magazine - July/August 2020 - Contents
IEEE Power & Energy Magazine - July/August 2020 - 2
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IEEE Power & Energy Magazine - July/August 2020 - Cover3
IEEE Power & Energy Magazine - July/August 2020 - Cover4
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