IEEE Power & Energy Magazine - July/August 2020 - 37

the level of DER penetration. This section examines different restoration cases with various levels of DER penetration.

Case Definitions
Due to the large number of possible variations in task allocation, as a first step, the following boundary conditions should
apply in the restoration process after a complete blackout in
the TSO system:
✔ The TSO performs a black start at the transmission
level. For the DSO(s), a top-down strategy is performed. Consequently, the operational task "frequency control" always must be performed by the TSO.
✔ In the TSO system, centralized generation is available. These generation units are coordinated by the
TSO and can be used for voltage control in the TSO
system.
✔ Pumped storage is available at the TSO.
✔ In the DSO system, no centralized generation is available (by definition).
✔ In the TSO system, there is no DG.
✔ All customers are connected to the DSO systems,
which comprise voltage levels from 110 kV to 150 kV,
and medium voltage. Often, these voltage levels are
controlled by different control centers.
The following limitations summarize changes in restoration issues for various levels of DG at the DSO:
✔ No DG, passive system (passive DSO): This case is
similar to traditional distribution systems that serve
load without local generation.
✔ DG not controllable, low penetration: That means the
system load can be supplied with centralized generation (low DG: not controllable).
✔ DG not controllable, high penetration: That means
the system load cannot be supplied with conventional
generation (high DG: not controllable).
✔ DG partly controllable, high penetration: That
means the system load cannot be supplied with conventional generation. Controllability of DG comprises limitation of active power (known as generation management) (high DG: partly controllable
active power).
✔ DG partly controllable, high penetration: That means
that the system load cannot be supplied with conventional generation. Controllability of DG comprises
both limitation of active power (generation management) and voltage support (high DG: active and reactive power partly controllable).
The cases "all DG controllable" and "no conventional
generation" do not exist in the integrated power systems
of Europe.
One common principle that applies to all levels of DG penetration is the compliance to security rules while performing
the switching operations. In a passive DSO, the lower voltage level will not be live, as the power flow is strictly from
higher voltage levels to lower voltage levels. The possibility
july/august 2020

that lower voltage levels may already be energized increases
with higher levels of DG penetration. This is especially true
where observability and controllability of DG is low from a
TSO perspective.

Passive DSO
The DSO without any DG or conventional generation is
termed a passive DSO. The main task for a passive DSO is to
connect load, usually on demand from the TSO. In this case,
all restoration strategies will end up in a top-down scenario
with the main task being voltage control.
The restoration process is completely controlled by the
TSO. The DSO can support the TSO through
✔ the observation of active power delivery from the TSO
to the DSO
✔ keeping both amount and step-size within agreedupon limits while reconnecting customers
✔ the observation of reactive power exchange.
In case of any limit violation, the DSO must contact the
TSO. Disconnection of load is the only operational measure
that can be performed by the DSO on an emergency basis
without coordination with, or approval by, the TSO.

Low Uncontrollable DG
The DSOs with low DG penetration may be treated as passive DSOs from an operational perspective. At the DSO level
there is DG, but it may not be observable or controllable. In
the restoration preparation phase, all DG should be disconnected from the system, insofar as this is possible. In some
cases, all DG cannot be completely disconnected, resulting
in the remaining DG automatically resynchronizing and
supplying reactive power. Here cases arise where the TSO
must consider DG infeeds in the restoration strategy. After
the TSO provides reactive power (and hence voltage support
to the DSO system and connecting load), DG may automatically synchronize and feed in maximum power that may
reverse power flows from the DSO to the TSO system. Due
to the low level of DG, this usually does not create overloads in the DSO systems or on coupling transformers. The
fluctuating load must be balanced; therefore, the demand for
reserve power increases. In the restoration process, the TSO
can control the frequency by
✔ observing (reversed) DSO power exchanges
✔ issuing coordinated set points to conventional
generation
✔ allowing the DSO to connect additional load.
In contrast to the previous case, the coordination work is
more demanding. Here again, in case of any limit violation,
the DSO must contact the TSO.

High Uncontrollable DG
In this case, the DSO is still passive, operationally speaking, but the DG can be observed. From the DSO point of
view, this case is critical. DG is needed for restoration but
cannot be controlled. The reversed power flows may lead to
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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
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