IEEE Power & Energy Magazine - July/August 2019 - 61

functionality of the HVdc grid controller depends on the
design philosophy of the particular grid, the structure of
the ac grid, the regulatory framework in place, and network codes.
HVdc converters that are part of HVdc grids should
have plug-and-play characteristics to guarantee effective
operation in a multivendor environment. This requires the
converters to handle a loss of communication by relying
on local measurements via autonomous converter control.
These control perspectives have a direct consequence for
the control hierarchy in the system and the communicationsystem requirements.

Intelligent Electronic Devices for Protection
Relay technologies have evolved from early electromechanical to static, digital, and numerical relays. Modern
numerical relays, or so-called intelligent electronic devices
(IEDs), use one or more digital signal processors optimized
for real-time signal processing and capable of running a
wide range of protection functions. Numerical IED performance greatly surpasses that of other technologies in terms
of accuracy, range of parameter settings, built-in communications, and multiple functionalities that include monitoring
and self-diagnostics.
The main differences between ac and HVdc IEDs encompass the protection algorithm, operational speed, and the

input/output requirements. AC IEDs are typically interfaced
only with their associated ac circuit breakers and coordinated among local and remote terminals. However, HVdc
IEDs might be required to carry out more complex functions to coordinate protection and restoration, such as sending fault detection and fault-clearing signals to the converter
or pole rebalancing equipment. In addition, HVdc circuit
breakers are likely to have more intelligent functions, such
as self-diagnostics, leading to potential coordinated protection through the communication of breaker failures to
nearby IEDs.
In Figure 2, a decentralized protection configuration is
given in which each IED is responsible for protecting one
component. A centralized protection configuration is also an
option, where all protection functions are implemented physically on one computer. The benefit of the former is higher
reliability because the failure of one IED most likely leads to
a backup IED operation. The latter likely provides simplified
design and protection coordination; however, failure of the
central protection might lead to unacceptable consequences.

Communication
Since existing HVdc links were built as turnkey projects by a
single company, communication protocols for control and protection are, to a large extent, vendor specific. For the development of large meshed HVdc grids, standardization is essential

Digital HVdc Substation (Based on IEC 61850 Architecture)

Substation
Level

Communication
Channel for HVdc Grid
and Station Controls

Station Control and
Monitoring

HVdc Grid Control

Station Bus (e.g., 10/100/1,000-MB Ethernet)
Bay Level

Converter
Control

Converter
Protection

Line/Busbar
Protection

IED

IED
Teleprotection
Channel Between
Substations

Process Bus (e.g., 0.1/1/10-GB Ethernet)
Trip
Signal

Sampled Values

Process
Level

figure 2. An example of digital dc substation protection and control architecture based on IEC 61850.
july/august 2019

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IEEE Power & Energy Magazine - July/August 2019

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