sible
economic returns from their existing assets.
The second common theme is that the
management and supervisory systems set
up to operate and control the power
systems are struggling to cope with the
increasingly complexity of the grid
interconnections demanded by the
consequences of deregulation of the various
energy markets around the world.
The NERC/US-Canada Power System
Outage Task Force report on the North
American blackout on August 14, 2003
notes that, in many cases, data collected
from sub-station equipment was not time-
stamped at all, and in other cases, the time
stamps recorded were not synchronized
across the network.
TIME SYNCHRONIZATION
One of the primary recommendations of
the report is that power utilities should take
steps to ensure that power plants and
substations control and supervisory data
recorders are synchronized by signals from
the Global Positioning System (GPS).
Time synchronization simplifies the task
of fault analysis in the aftermath of a fault
situation even between networks. Fault
records in the form of oscillographic
waveforms and histogram records showing
the sequence of protection device
operation, provide essential information
that can lead to an understanding of just
how protection devices operated.
The fault record has become a tool that
allows the protection engineer to perform
a cross check of the operation of the device
against the appropriateness of the settings
that were applied.
Time synchronization also increases the
accuracy of control decisions by automatic
control and protection equipment in the
power network, therefore allowing optimal
utilization of network assets.
PROTECTION AND CONTROL
While the synchronization of the AC power
grid has always been of primary importance
to grid operators, somewhat less attention
has been paid to the need for
synchronization of the protection, control
and supervisory equipment that is an
essential part of a modern power utility
substation.
Historically, primary protection
equipment was designed to trip the
incoming supply to a substation based on
local operating conditions exceeding a set
of predefined criteria. Protection relays
were largely electromechanical devices.
Automatic recording of data from such
devices was simply not available nor was
it seen as particularly important, as the
supply grid was a relatively simple network
with minimal interconnect paths.
Growth in demand for electricity,
together with privatization and increasing
de-regulation have led to vastly more
complex grid structures in which power can
be switched to flow over multiple different
paths on a second by second basis. The
factors influencing power flow paths within
the grid are no longer related solely to
technical issues of demand, generation, and
optimized grid use, but also to external
market issues such as the spot price of power
generation offered from competing
generating companies and to constraints
placed on the grid operators by
environmental considerations. Consequently,
the need for closer monitoring and control
of power utility network assets over a wide
area arises, and continues to grow.
F
TWO COMMON THEMES HAVE EMERGED FOLLOWING THE BLACKOUT EVENTS OF 2003.
40
PT&D
December 2005/January 2006
www.powertrans.com.au CURRENT AND EMERGING PRACTICE
While the basic function of a protection
relay remains the same today as it has
always been, modern microprocessor based
protection relays and other IED (Intelligent
Electronic Devices) installed in substations
offer a host of monitoring and control
functions that can generate large amounts
of real time and historic data about the
operating state of the power system.
In addition to providing real time
measurement of voltage, current and
frequency, recording of sequence
components, phasor measurements,
transients and other parameters relating
to power quality is now accepted practice.
In addition to the time stamping of fault
records and sequence of events data
increasingly manufacturers of protection
relays are using time synchronization to
maximise the robustness for example of
line differential protection techniques and
distance protection. The advent of relatively
cheap GPS-controlled clocks means that it
is now economically viable to deploy a time
source that effectively offers close to atomic
clock performance in each substation, thus
making possible network wide, continent
or even world-wide synchronization.
The investment in an accurate GPS-
controlled clock in each substation is best
utilised by the installation of a dedicated
time synchronization bus system delivering
time signals directly to all front-line
equipment such as protection relays and
IED equipment. With such a system in
place, time stamping is done at the precise
point in time and space that an event is
first detected by a protection relay or other
IED, and the timestamp becomes an
integrated part of the data associated with
the event.
TIME SYNC IMPLEMENTATION
Most modern Protection Relays and IEDs
come fitted with a port to accept a time
synchronization signal the most common
being the IRIG-B time code.
A time signal bus can be realised by using
a single-pair copper cable carrying the time
code signal from the GPS-controlled clock
output to all of the equipment that requires
synchronization on a multi-drop basis.
Purpose built isolation devices can be used
to maintain isolation along the bus.
Alternatively optical fibre can be used for the
connection between the clock and the IEDs,
thus overcoming isolation and noise issues.
FUTURE TRENDS
In the latest generation of IEDs designed
to operate under the new IEC 61850
regime, time synchronization is possible
using an NTS Network Time server clock
which is accessed over the Ethernet IP
based Station Bus network that connects
devices locally in the substation.
The above article has been supplied
by Geoff Vaughan from HV Power, the
NZ Distributor for Tekron International
and was derived from a white paper
entitled Shedding Light on a Black
Art GPS Time Synchronization
authored by Brian Smellie from Tekron
International. The paper discusses the
various methods of time
synchronization employed by utilities,
highlighting many of the practical
considerations of implementation and
it also provides technical details of
various time signal formats.
For further information contact:
information@tekroninternational.com
PROTECTION
& CONTROL
UTP
SNMP
Copper
AM-IRIG
ST Fibr
e
DCF77
IRIG-B
or IP
GPS Time Synchronization clock
Isolation
Devices
GPS Antenna
NAVSTAR Satellites
Bay Controllers
Protection Relays
IEDs
Serial
RTU /
Server
December 2005/January 2006
PT&D
41
www.powertrans.com.au