IEEE Power & Energy Magazine - July/August 2020 - 36

The main issue in restoration is that the system
operator has ultimate responsibility for system
security but cannot adequately control DG.

and dependent on size and location of conventional generation units.
In some cases, the DSO can perform black start, especially when midsize hydro pump storage units are connected
to the DSO network. In that case, a bottom-up strategy is followed, i.e., starting at the regional level and then providing
power to the TSO. Operational tasks are the same as in the
TSO black start case. Voltage control is less critical due to
the lower amount of reactive power involved in comparison
with the transmission system during a top-down strategy.
In the first restoration phase, the focus is on the
✔ resupply of house load (station service load for substations and generators)
✔ resupply of critical infrastructure
✔ creation of a stable system (both voltage and frequency).
In the second restoration phase
✔ the system is extended
✔ customers are supplied
✔ unit commitment is organized depending on primary
energy sources (e.g., coordination of pump storage
and conventional generation).
In the final restoration phase
✔ the system is meshed to avoid overloads
✔ frequency is resynchronized with partners
✔ load frequency control (LFC) is returned to operation.
In all cases, the system operator continuously monitors the
critical operational values and takes
the required corrective actions. It
should also be noted that the time
available for a number of these
tasks is limited due to factors such
Black Start on DSO Level
as battery life and standby generators' fuel reserves.
Switching (DSO Production)

controlled by the local TSO through a stepwise energization
of the grid. Here, voltage control is challenging particularly
because energizing lines without load can lead to high voltages (Ferranti effect), which must be compensated with coils
and/or generation units. Generating units connected to the
system need to be "loaded" so active and reactive power can
be provided to the underlying DSO systems, where customers can be resupplied.
The major activity in top-down cases is the coordination of exchange power, i.e., both active and reactive. The
amount of exchange power provided to each DSO (regional
system) is dependent on available exchange power from the
supporting TSO plus available conventional generation. The
restoration process is unique to each control area. TSOs need
to provide simulator-based training sessions to successfully
manage this "multilevel" task with all its partners.
If no external support is available, the TSO may rely on
contracted generators for black start in its control area. In
this case the coordination of exchange power is replaced by
frequency control. All imbalances between generation and
load directly influence the frequency. The major tasks in this
black start situation are balancing and frequency control. For
the DSOs, the strategy is still top-down. However, due to
unstable frequencies during load reconnection, the exchange
bands are smaller and "load-step" sizes must be carefully
selected. Furthermore, available reactive power is limited

Black Start - TSO + DSO Levels
Task Distribution
Black Start on TSO Level
Switching (TSO DSO Production)
Frequency Responsibility (TSO)
Centralized Unit Control (Production)
Load Supply (DSO)
Control DER → (DSO TSO)

Frequency Responsibility (DSO)
Centralized Unit Control (Production)
Load Supply (DSO)
Synchronization (TSO DSO)
Control DER → (DSO)

* Observation
* Active Power Control
* Reactive Power/Voltage Control
* Exchange Power Control
* Availability Forecasts

figure 3. The task distribution of black start at TSO and DSO levels.
36

ieee power & energy magazine

Operational Tasks for
Restoration With High
Levels of DG
Since they are based on predefined
operational criteria, system restoration tasks are generally quite similar (see Figure 3). However, their
allocation at participating control
centers can differ significantly.
This varies according to the dependency on decentralized generation,
which, in turn, varies according to
july/august 2020

IEEE Power & Energy Magazine - July/August 2020

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