Electric Energy T&D - Index

Electric Energy T&D - EE Magazine March / April - Index

Protection and Time
Synchronization
IEC 61850 was viewed by the utility as important
for the future, but not required for immediate
implementation. Although it was decided
that protection functions would continue to be
accomplished through traditional hard-wiring to
relays initially, it was also decided that a network
architecture capable of easy migration to
IEC 61850 in the future was required.
Time synchronization was also considered
important. IEDs are synchronized using the
most accurate method supported by each
IED: IRIG-B, NTP/SNTP, and DNP. All
three time synchronization methods were
implemented in the system, depending
upon how each IED vendor supports time
synchronization. Some IEDs did not support
any of these standards, however.
IRIG-B calculations were performed during
the initial design to ensure proper signal
levels. It was discovered that the load from
the backup relays and the distributed I/O
devices required the IRIG-B network to be
split up, and a high-drive output was needed
on the satellite clock. The high-drive output
requirement made it impossible, however, to
include a timeserver on the satellite clock.
The substation computer was connected
to IRIG-B and configured as a network
timeserver that is ± 50 milliseconds, relative
to the IRIG-B source. All data received in the
HMI on the substation computer is either
time stamped at the IEDs or at the data
concentrator to ± 1 millisecond accuracy.
Bringing IRIG-B to the substation yard using
copper cables impacts performance and may
not be possible because the copper cable runs
are too long for reliable operation. Distributed
I/O devices located in the substation yard wer
e connected using a fiber optic port achieving
much better isolation and eliminating the
distance constraints of copper cables. The same
fiber optic cable used for the LAN connection to
the IEDs in the substation yard was also used
for distributing IRIG-B to the switchyard.
Programmed Logic and Ancillary
Services
Programmed logic is the basis for substation automation.
Supporting system-wide programmed
logic functionality requires a high-speed peerto-peer
communication network. While IEC
61850 supports this requirement using highspeed
messages with guaranteed performance
requirements, it is also possible to use DNP3
over Ethernet for non-protective functions.
This is accomplished by IEDs broadcasting
data to multiple masters or multiple masters
polling the same slaves. In this application,
this type of DNP3 functionality was a
differentiator in the selection of distributed
I/O devices and the RTU/data concentrator.
How programmed logic is implemented in an
IED can also vary. While most data concentrator
vendors support an embedded IEC 61131-
3 PLC engine, others only support a text-based
or object-based programming language.
IED Selection
IED selection is always a challenging and
time-consuming process because there are
many IEDs available with various issues
impacting the final selection. As with many
utilities, this utility had already selected most
of the IEDs before system design even began.
Most importantly, they did not want any risk
of “vaporware” in unproven products.
The utility was planning on using their
standard SCADA RTU, but significant
performance drawbacks were identified with
this RTU related to the integrated system. This
resulted in considering other RTUs and data
concentrators from various suppliers. A data
concentrator was ultimately selected based on
how well the IED met the evaluation criteria.
Because a traditional RTU was not provided
and the I/O capability of the relays was not
being used, a distributed I/O device was
required in the design. A distributed I/O
device was selected based upon superior
flexibility. However, a media converter was
required for the Ethernet port to make the
device fully compatible with the broader
design approach.
The utility initially wanted to use their SCADA
master software in the substation as the HMI.
One reason was that the utility wanted the
tags placed at either end replicated at the
other end. After some detailed technical
review with the vendor, the utility was not able
to use the existing SCADA master software as
54 I March-April 2008 Issue
the substation HMI. Thus, an evaluation of
several HMI packages from various industries
was undertaken to find a suitable alternative.
Software from a well-established source was
eventually selected that offered a number of
significant performance advantages.
Substation Computer
The utility knew that they wanted a substationhardened
computer that met IEEE Standard
1613. This was a contributing factor to the
SCADA master HMI not being used in the
substation because of memory and operating
system requirements. One useful feature was
that the substation computer would easily
accept an IRIG-B input and could act as an
NTP/SNTP time-server.
Security
Like many utilities, this utility needed to
address NERC CIP (Critical Infrastructure
Protection) requirements that went beyond
the new substation but were not addressed by
this project. Even so, the physical perimeter is
monitored. Cyber security issues are addressed
by providing substation LAN equipment that
includes features that help address NERC CIP
requirements and implement best practices
that provide a “defense-in-depth.” The general
approach was to physically separate the substation
network from the corporate network. The
substation data concentrator could also be used
for access control to the substation IEDs and
monitoring and logging all IED connections.
Architecture Selection
The architecture selection was based on
Ethernet for four primary reasons. First, the
utility had already extended their private
corporate WAN and SCADA WAN to the
substations, and they wanted to better utilize
these networks. Second, several Ethernetbased
relays were installed previously, and the
utility wanted to continue using them due to
their ease of use and network support. Third,
the client wanted more than just SCADA data
out of their investment in IEDs, including
remote access and non-operational data.
All of this data and functionality has value,
but it requires a medium that can support
significant bandwidth requirements and
multiple simultaneous connections. Finally, a
system architecture that could easily support
IEC 61850 was desired.