IEEE Power & Energy Magazine - July/August 2020 - 35

The shift from large centralized synchronous generating units to small
distributed generators, many of which are powered by intermittent
renewable resources, introduces a new dimension in grid cooperation.
operation, especially in terms of the observability of DG
units, since available information is collected and concentrated by the DSOs' SCADA systems.
Aggregated Distributed Energy Resources
and Microgrids

Aggregated distributed energy resources (DERs) and microgrid concepts are potential solutions to the visibility and
control of DG. Aggregated DERs can be a virtual power
plant capable of forming an island when matched with suitably sized loads. Microgrids are islands of balanced generation and loads. In the future they may be used by TSOs and
DSOs to facilitate the restoration process.

Operational Tasks in Restoration
Changing Operational Focus
Transmission system states can be classified into normal, alert,
emergency, blackout, or restorative (see Knight in the "For
Further Reading" section). A transmission system is considered to be in a "blackout state" if more than 50% of demand
is lost and the formal restoration process must be initiated.
In the restorative state, the focus changes to restoring supply to customers, maintaining voltage and frequency control,
and unplanned decisions and actions. Unplanned decisions
and actions are those that are in response to events as the restoration process unfolds. They are not anticipated and written in procedures. Additionally, the coordination of multiple
control center actions is crucial. Control centers for transmission systems, the underlying distribution systems, and
generating units must interact in a coordinated way.
The coordination of tasks among control centers should
be part of individual restoration plans. The ENTSO-E operational policy considers restoration an ancillary service, a
responsibility of the TSO. The TSO must require that all DSOconnected and conventional synchronous generators have
restoration plans available. TSOs are responsible to provide
black start capabilities for the transmission system.
The critical tasks in restoration are as follows:
✔ initialization
✔ frequency control
✔ voltage control
✔ control of conventional synchronous generation (centralized)
✔ control of DG
✔ control of power flows and interchanges
✔ customer supply.
july/august 2020

The main objective of system operation is to always keep
the critical system values within operational limits. Common strategies for restoration must always maintain safety
and security, independent of the level of DG penetration in
the system.

Phases in Restoration
After a blackout, the TSO coordinates and executes the
restoration process in its control area. In the analysis and
coordination phase, the scale of the outage is identified.
All parties involved, such as neighboring TSOs, DSOs, dispatch centers, and conventional generation, are informed of
the blackout. Each party makes its own preparations, which
are usually based on predefined plans. Communication is
established. All market-driven activities are suspended.
The TSO collects cross-border information for restoration
strategy development. The TSO and DSO initially prepare
the grids for restoration by splitting them both horizontally
and vertically. Also, in many restoration scenarios, all
circuit breakers are opened in the blacked-out network
to protect loads and generators. Other switching actions
such as the preparation of black start paths depend on the
chosen strategy.
Generation units have additional preparation tasks. These
include the following:
✔ securing generation units' house loads
✔ preparation of black start units
✔ preparation of tripped units for start up
✔ selection of control modes for restoration (switching of steam units to bypass control, activation of
speed control or power control mode, voltage control mode)
✔ observing safety regulations while switching: before
any switching can be done, a network operator must
make sure that no one is working on that part of the
network, lines are not down, and that there is enough
interrupt capability when circuit breakers close into a
short circuit so as not to damage equipment
✔ ensuring synchronization can take place before closing two energized portions of the system.
A neighboring TSO can sometimes provide support over
ties, which today can be provided via either high-voltage
ac or high-voltage dc links. The partner provides voltage
support and controls the frequency. Both TSOs must agree
on a band of exchange power for the control purpose. This
enables the supporting TSO to organize sufficient control
power for frequency control with restoration of the grid
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
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
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