IEEE Power & Energy Magazine - July/August 2020 - 61

allowed that can be covered for at least 18 h. The area under the
line "1 h," on the other hand, represents the potential for supply if that constraint is relaxed to just 1 h of uninterrupted
supply. At most times, a certain measure of supply is possible but the requirement of sustained supply over many hours
significantly reduces this option: supplying 15 MW over 3 h
is possible approximately 40% of the time over a year. However, if the requirement is continuous supply over 9 h, this
is only during less than 10% of the year. This highlights the
regulatory challenge of providing clear criteria on temporary resupply: Is it better to intermittently supply a feeder
for a single hour or to leave it disconnected until permanent
resupply is possible?
All the load cannot be covered all the time. Topological
restrictions and the assumptions of limited controllability (balancing of load and generation) is assumed to be
only possible by HV-connected units, such as 39-MW PV
plants and 72-MW wind parks. The remaining generation is connected at the LV or MV level and not considered to be reliably controllable in a restoration situation.
Even though the biomass plants are assumed to be available 24/7 with 80% of their nominal power, they could
not be utilized at all times because they are connected to
the lower levels of the distribution system and can neither
be remote-controlled in a blackout-safe manner nor can
be topologically separated from the local load. Extending
plant controllability and topological flexibility to the lower
voltage levels can improve the capabilities of load coverage significantly.
Battery storage systems might allow more load to be connected more often (more eligible starting time points for

a given level of supply), but unless they provide dedicated
reserves for power system restoration (which seems unlikely,
given the current economic incentives), there will still be
remaining unavailability because of empty storage capacity.
Storage systems that normally optimize their depth of discharge for long lifetimes might be able to provide additional
reserves during emergencies. However, there is currently no
regulation that could incentivise such operation modes of
storage systems.
Since blackouts might be the result of a natural disaster
(e.g., extreme weather conditions), power outages can coincide with the loss of some communication infrastructure. A
restoration-friendly fallback behavior of DERs potentially
contributes to fast and reliable restoration of as much load
as possible. For further information, see "The Importance of
Active Power Control in Restoration."

The Impact of Different Islanding
Strategies on Resilience
A very common strategy of DSOs to deal with outages of the
transmission system is to wait for the transmission system
to become available again. This is very simple and, at least
historically, justified because DSOs did not have generation
capacity within their power systems. This case "without
islanding" serves as a reference to assess the potential benefits of distribution system islanding.
Operating a distribution system island is an option if at
least some generation of enough size is available within
the distribution system. This generation must either have
the capability for black start or have entered house load
operation at the time of the blackout. Such generation is

The Importance of Active Power Control in Restoration
Balancing the demand and supply of active power can be

prescribing a corresponding behavior when reconnecting

especially challenging in small islanded distribution sys-

after an outage. The latter can serve as the standard behav-

tems. The overall inertia of rotating machines in the system

ior for generators without a communication connection and

is typically limited. Furthermore, the effect of cold load

as a fallback setting in case communication systems become

pickup makes the initial load upon reconnection of feeders

unavailable. The strategy also significantly increases the al-

highly uncertain.

lowable load increment for a given power frequency nadir.

Since distribution system operators often do not have

Therefore, it is possible to connect larger feeders or more

the equipment to switch individual customers at the lower

feeders at a time with a given level of confidence in stable

voltage levels, the minimal amount of load that can be con-

operation.

nected in a single step is often in the order of magnitude

Furthermore, a larger capacity to absorb power imbal-

of several megawatts. Therefore, frequency control contri-

ances can also increase the chance of intended islanding.

butions from distributed energy resources become highly

If successful, transition to an islanded subsystem can turn

desirable.

a potential blackout into a system split. However, this re-

Operating generators below their available primary en-

quires predefined separation points and is currently only

ergy provides additional active power in the event of fre-

possible for comparatively small parts of the power system

quency drops, which counters imbalances between sup-

with a single (or few) connection point(s) to the rest of the

ply and demand. This can be achieved either by providing

system (microgrid). Moreover, the condition of unintended

a reduced active-power set point via remote control or by

islanding must be avoided for safety reasons.

july/august 2020

ieee power & energy magazine

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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
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