IEEE Power & Energy Magazine - July/August 2020 - 34

(secondary control power). Activated secondary control
power is released by tertiary control power, mostly manually activated by the TSO.
However, neither the TSO nor DSO can control all conventional generation directly. With increasing levels of DG,
the DSO is taking over more "system tasks" such as the provision of power for system balancing. Thus, DSOs are starting to provide contributions to ancillary services, including
black start.

Congestion Management
Congestion management is independent of voltage level.
Loop flows make this more challenging in highly meshed
systems (such as those operated by TSOs), than in radial
systems (medium voltage). An example of congestion that
affects power restoration is the coupling of transformer loop
flows between TSOs and DSOs, which must be respected in
restoration plans. In the case of an overload, caused by a DG
infeed, the infeed needs to be reduced. Here, observability
and controllability of DG in the DSO systems can be crucial. The DSO must reduce infeeds in a deliberate and transparent way by 1) analyzing the locations where it is possible
to reduce infeeds and the impact of the reduction of flows
on the critical circuits, 2) determining optimal reductions
of flows, and 3) identifying the impact of reductions on
transformer flows.

In the case of congestion, DG usually cannot just be reduced.
Redispatch typically takes place at the same time conventional generation is ramped up to maintain system balance.
Redispatch of the bulk system is a TSO responsibility.
Voltage control is usually done in cooperation with conventional generation units. The TSO and DSO communicate
directly and in a collaborative way. Clear communication
instructions must be in place that direct generator operators
and all power electronics-based devices, such as static var compensators. Voltage control with DG becomes more important
as conventional synchronous machine generation is replaced.
Due to these complexities in the operation of the power
system, the system control and data acquisition (SCADA)
system and, where appropriate, the energy management system (EMS) of the DSO system must provide support to its
own operators and to those of the TSO.

Controllability and Observability of DG
One main difference between conventional centralized
generation and decentralized generation is the absence of
dedicated operators for decentralized production. Wind
turbines and photovoltaic cells usually operate automatically and are optimized for operation under normal conditions. They perform tasks such as automatic startup
(synchronized) and ramping to maximum available power.
DSO control centers (Figure 2) play a crucial role in DG

figure 2. The Swissgrid Control Room. (Source: Swissgrid; used with permission.)
34

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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
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IEEE Power & Energy Magazine - July/August 2020 - Cover3
IEEE Power & Energy Magazine - July/August 2020 - Cover4
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