IEEE Power & Energy Magazine - May/June 2019 - 101

response to unplanned transmission
trips to avoid thermal issues and
maintain stable system operations.
controlled network fractionalization is sometimes used as
the last line of defense to protect
regional and urban power grids
from incidents of widespread outages during severe disturbances.
the fast frequency response of a
Vsc-hVdc intertie can be activated after detecting a split network condition, as shown in Figure 7(d). emergency power support
in combination with dynamic voltage support of a Vsc-hVdc intertie can mitigate the adverse impact
of disturbances and stabilize the
frequency and voltages of the split
networks. this strategy can help the
grid survive extreme events.

VSC-HVdc Solution
Readiness and
Further Technology
Development
the first commercial Vsc-hVdc
system, installed in 1999, was rated
50 mw at a voltage level of ±80 kV
with an extruded hVdc cable. multiple ±320 -kV Vsc-hVdc systems rated up to 1,000 mw and
one ±500-kV Vsc-hVdc system
have become operational worldwide in recent years for offshore
wind connections, direct infeed
into urban areas, and network interconnections. the technology is
under continuous development and
rapidly moving into higher voltages and higher power levels with
reduced losses. the new Vsc-hVdc
design concepts are based on modular products and compact structures that provide the flexibility to
accommodate different application
specifications in an aesthetic design.
there are some further technology
innovations for urban power grid
subtransmission hVdc systems:
✔ Transformerless systems:
converter transformers are
critical components of hVdc
systems, providing galvanic
separation between the ac and
may/june 2019

Q

Q

P

P

ac Bus 1

ac Bus 2
P1
Q1

P2
Q2
Terminal 1

Terminal 2
Q

Q
P

+Udc1

P

+Udc2

ac Bus 1

ac Bus 2
Q2

Q1
Terminal 1

-Udc1

-Udc2

Terminal 2

figure 6. The basic operating modes of VSC-HVdc maintain scheduled power flow
and desired terminal voltages following hourly or subhourly dispatch orders from
the system operator. Real power can flow in either direction, and voltage support is
available at both terminals.

External Power Grid

External Power Grid

Pac
Load Area
A

Pac
Load Area
B

Load Area
A

Load Area
B

Pdc
(b)

Pdc
(a)
External Power Grid

External Power Grid
X

Pac
Load Area
A

X

Load Area
B
Pdc
(c)
ac Circuit
VSC-HVdc
Intertie

Pac

X
X
X

Load Area
A

Load Area
B

Pdc
(d)
Pac: Power Flow on ac Circuit

Disturbance

Pdc: Power Flow on dc Intertie

X Circuit Outage

figure 7. The EPC of VSC-HVdc interties enabled by fast-controllable characteristics
can provide timely emergency power support between adjacent load areas and enhance urban power grid resiliency against large or extreme disturbances. (a) Normal
condition: Pdc determined by power orders from system operator, (b) ac line outage:
increased Pdc by EPC to mitigate stress of parallel ac lines, (c) emergency condition:
fast Pdc reversal by EPC to maintain stable operation of load area B, and (d) split
network: fast frequency response of HVdc intertie by EPC to stabilize frequency of
load area B.
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IEEE Power & Energy Magazine - May/June 2019

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