IEEE Power Electronics Magazine - March 2018 - 54

Expert View

by Magnus Callavik

grid resilience by Power Electronics
Use subtransmission HVDC interties for novel
emergency power control of split networks

odern society is becoming
increasingly dependent on
reliable electric grids. For
years, many critical functions have
used backup systems to maintain
normal or basic functionality during
disturbances. However, it is not possible to back up the entire grid,
much less all individual grid assets.
In recent years, several devastating
natural disasters have struck with
great force, causing blackouts on a
large scale and putting people and
civil functions at risk. Therefore,
new methods of improving grid
resilience, limiting the spread of
blackouts, and providing provisions
for the rapid recovery of power grids
are needed.

Emergency Network Splitting
During Severe Events
Controlled network splitting is the
last line of defense to protect the
power systems from incidents of
widespread outages during severe disturbances. With this type of emergency
control, the electric grid is split into
designed subnetworks, typically by
load areas or load zones (Figure 1).
Emergency network splitting strategies have been studied by regional
system operators and large urban grid
utilities. One critical concern for
implementing such control strategies
is the likelihood of large active and
Digital Object Identifier 10.1109/MPEL.2017.2782440
Date of publication: 23 February 2018

IEEE PowEr ElEctronIcs MagazInE

reactive power imbalances in some
split subnetworks. A swift control of
generation and load shedding may
reduce the impact, but the risk of frequency or voltage instability due to a
dramatic drop in the frequency or
voltage still exists. Additionally, since
power system behavior during emergency conditions highly depends on
the installed control and protective
schemes, risky power oscillations
may occur during the course of network splitting.

Stable Operation of Split
Networks with HVDC Interties
Voltage-sourced converter high-voltage direct current (VSC-HVDC) is a
method of transmitting electric power
long range without significant losses.
The dc power flow is controlled by
high-power electronic semiconductors, i.e., gate-controlled transistors.
These links have the capacity to move
power from one dedicated point in the
grid to another point far away, without
being exposed to parallel ac network
power-flow problems. These stations
are also able to support the grid voltage and frequency, thus enabling the
operation of very weak grids that may
collapse under normal ac conditions.
Recent developments in compact VSC
technologies open up possibilities for
HVDC applications in subtransmission networks, particularly for urban
power grid modernization.
Improving grid resiliency through
the use of controlled network split-

z March 2018

ting ca n be greatly enha nced by
installing subtransmission HVDC
interties, implemented as a point-topoint link or a back-to-back arrangement between the designed subnetworks. The core emergency power
control functions of the HVDC interties include fast frequency response,
power oscillation damping, and dynamic voltage support to the target
subnetwork, provided that the subnetwork connected to the other
converter station is not severely influenced by the disturbance.
The emergency power controller
of HVDC interties receives real-time
measurements from the converter
terminal stations and performs continuous assessments to detect and/or
predict existing and/or emerging
problems. Fast dc power control and
dynamic voltage support can be initiated i n respon se to developi ng
system conditions to minimize the
impact of disturbances, to stabilize
the frequency and voltage of split network s, a nd t o su r v ive ex t reme
events. The subtransmission HVDC
intertie solution for improving grid
resiliency may be extend to a multiterminal configuration for more flexibility in the planning of subnetworks
and provisions of emergency support
t o t he t a r get subnet work f rom
healthy subnetworks. Recent technological developments have demonstrated the probability of realizing
low-loss, fast mechanical dc breakers
at subtransmission voltage ranges.

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - March 2018