IEEE Power & Energy Magazine - May/June 2019 - 40

systems for many critical functions (measurement, control
implementation, communication, and so on), and these
schemes are frequently responsible for the transmission of
substantial quantities of power across large distances. as a
consequence, hvdc systems play important roles in overall
power system stability in many systems worldwide. thus,
both the security of this critical infrastructure and the role it
can play in increasing resilience are of utmost importance.

VSC and Line-Commutated Converter
Control and Communication
Requirements
at the most fundamental level, a line-commutated converter
hvdc station requires minimal control effort. Firing pulses
for thyristors, which are usually delivered through magnetic
or optical isolation, is a common control method. converter
transformer tap changing is often employed for voltage control as well. a vSc hvdc station adds additional flexibility
through the use of insulated-gate bipolar transistors, allowing
for the controlled turn on/turn off of switches. this additional
control variable allows vScs to simultaneously control two
quantities, such as real power transfer and ac reactive power
injection. little to no communication with the outside system
is necessary for this particular action.
however, external communication is required to implement the hierarchical control methods employed to ensure
proper voltage levels and obtain desired power flows. current, voltage, and power feedback loops responsible for controlling individual thyristor stacks and power flows from
converter stations are typically controlled by a network of
Scada systems. these systems collect data that reside well
outside of the physical confines of the converter station.
nearly all hvdc-based applications are supported by oneor two-way communication technologies between hvdc stations and control centers, neighboring substations, and other
control facilities. For example, the set points of hvdc stations
are usually controlled by remote control centers, while the realtime measurements collected at the hvdc converter stations
are communicated to the control center through Scada. in
addition, many hvdc applications also rely on real-time measurements collected from phasor measurement units (pMUs)
through wide-area measurement, protection, and control
(WaMpac) platforms. Because hvdc-based applications are
increasingly developed and deployed in modern power systems,
the hvdc station and controller data will be accessible to multiple control entities. in this sense, cybersecurity of Scada and
WaMpac is an increasingly critical building block for efficient
and secure operation of hvdc systems.

Considerations for SCADA Cybersecurity
From the it perspective, the objective of the security function is to protect business information from loss or alteration
that results in risk to the organization. in systems reliant on
the secure use of Scada, the purpose of security is to provide for continued safe operation, even when under attack or
40

ieee power & energy magazine

during degraded operational conditions. Both of these are
risk management efforts, but the sources of the risk and the
capabilities of the systems are different, resulting in a need
to tailor the security systems differently.
the susceptibility of Scada systems to false data and
command attacks is exacerbated by the fact that much of the
Scada network traffic is unauthenticated, in part because
of the constraints of computation and communication and
the difficulty of managing cryptographic authentication on a
large number of devices. in particular, legacy devices cannot
support modern authentication techniques.
although traditional it security practices do not always
transition to Scada systems and, in particular, a digitally
controlled ac transmission system, these systems present
opportunities to leverage physics to enhance resilience and
cybersecurity in ways not possible in enterprise it systems.

WAMPAC Communication
Requirements for Cybersecurity
the cybersecurity of measurement and control signals transmitted over WaMpac platforms is crucial to ensure secure
and reliable operation, protection, and control of bulk power
systems. a typical WaMpac platform, illustrated in Figure 1,
consists of modern and legacy network devices, sensors, actuators, and digitized controllers. this design provides a suite of
solutions including intelligent electronic devices (ieds), phasor data concentrators (pdcs), and various communications
tools to meet wide-area application requirements.
pMUs (i.e., synchrophasors) are strategically installed
on ieds in substations to provide high-speed, high-accuracy synchrophasor measurements at 30-120 time-tagged
samples/s. Synchrophasors sample the magnitude and phase
angle of voltage and current quantities using a common time
source for synchronization, usually provided by a highprecision GpS. this allows pMUs to access the coordinated
universal time and so time-stamp data collected from a wide
geographical area. Within a single substation, there can be
multiple pMUs collecting real-time data from current and
voltage transformers as well as breaker status in the field
through a local area network (lan). as the raw analog data
are sampled, digital positive-sequence measurements are
generated by the analog-to-digital converter and the phaselocked loop module as well as a frequency correction algorithm. additional data validation and interpolation modules
are also employed to improve data quality.
the phasor data are received by pdcs, which process and
concentrate information from multiple pMUs. an important functionality of data concentration is time alignment,
because multiple pMU data streams must be aligned according to the standard time reference provided by the GpS
clock. another functionality the pdc performs during data
concentration is error check and data interpolation. outlier
data are identified and discarded, and missing data slots are
labeled using syntax specified in the pMU communication
protocols (e.g., ieee c37.118 and ieee c37.244). in some
may/june 2019

IEEE Power & Energy Magazine - May/June 2019

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2019