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IMPROVING THE MANAGEMENT OF ]PROTECTION AND CONTROL SYSTEM
ASSETS BY USING A PLATFORM CONCEPT
D.M. Peck, B. Nygaard, H. Meier
ABB Network Partner Ltd, Switzerland
ABSTRACT
Throughout the history of power system protection, improvements
have continuously been made to all1 aspects of protection
equipment. The most important advances were made possible with the
introduction of static technology, and more recently numerical
technology. Microprocessor technology has been utilised in power
system protection, control and measurement equipment for some years
now. Multiple- function numerical protection relays are current1 y
available from many protection equipment manufacturers, providing
user-configurablc inputs and outputs, substation control system
jnterhces, disturbance recording, etc.
The most recent developments e:mploy a common software and
hardware platform foir various protection and control products.
This paper outlines the principles upon which such a platform is
designed and considers the factors that will influence future
equipment development. Such platforms are characterised by a
modular hardware architecture, running modular application software
from a library of functions.
The rapid development of microelectronics technology has had a
great influence on the life-cycle of systems in which it is
employed and thereby on the management of Protection and control
equipment assets. When updating e.g. processor module hardware and
software, a platform approach allows an increase in processing
power respectively protection and control functionality and an
extension to system life-times.
A platform approach also allows appreciable long-temi economic
and technical benefits for both manufacturers and users alike. For
example, users profit from the fact that, for a complete range of
products, the number of spare parts is practically reduced to that
of one equipment. The amount of training for testing, maintenance,
and operation of the man-machine interface etc. is also
correspondingly reduced.
I. INTRODUCTION
Protective relaying equipment can be divided into three general
categories or generations, electromechanical, static and numeric.
The protection functions anti characteristics of electromechanical
equipment are determined in the main by the physical geometry of
this magnetic circuits, iron cores and moving parts (e.g. induction
discs) and the winding arrangements. Their fixed functionality
together with the fairly limited
parameter setting possibilities, at least compared with todays
expectations, are but two of the obvious disadvantages of this
generation of equipment.
Static or electronic relays, enabled the performance,
flexibility and reliability of protection equipment to be improved.
Flexibility was increased since relays were built using common
hardware modules. Thus the difference between, for example, a line
distance relay and a transformer differential current relay was
basically in the measuring module; interposing input transformers,
tripping output units, power supplies etc. were part of a family of
modules. Newer technology, especially the use of digital
components, brought further advantages, reducing size and auxiliary
power supply requirements, as well as allowing more of the
manufacturing to be made automatically.
The newest generation of equipment is known as numeric or
digital. Analogue input currents and voltages, having been
transposed to electronic level voltage signals, are digitised
immediately in an analogue-digital converter. All protection
functions are then implemented as numerical algorithms, that is to
say, software programs running on general purpose microprocessors.
The protection functions are determined by the software alone, and
for a given hardware, virtually any protection function can be
realised. Some of the early microprocessor based protection relays,
e.g. simple overcurrent and frequency relays, were a combination of
static and numerical technology and as such not strictly numeric
since analogue signal processing circuitry preceded the
analogue-digital converter and microprocessor circuitry.
A numeric protection relay can be compared to a personal
computer. Whereas a PC is used to run a multitude of application
programs stored on a hard-disk (commercial programs, complex
mathematical packages, etc.), a numeric relay executes protection
and control application programs stored in non-volatile flash
memory or EPROM. A PC is however obsolete within a few years,
mainly because new application programs (and operating systems)
require faster hardware and more memory. On the other hand, more
powerful hardware opens up opportunities for more complex software.
This leapfrog behaviour is also evident in the area of numeric
protection and control equipment. The expectations and demands made
upon them are forever increasing, user-friendly man-machine
interfaces (MMI), time stamped event lists, integrated
Developments in Power System Protection, 25-27th March 1997,
Conference Publication No. 434, 0 IEE, 1997
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disturbance recordings, connections to station control systems
with a choice of protocols, etc. are demanded. The processing power
of todays microprocessors is orders of magnitudes greater than
those of earlier devices employed in the first microprocessor based
relays, and the trend continues with ever increasing performance,
as with PCs.
When considering the life-cycle of the protection equipment it
can be seen that this has been reduced through the generations.
Many electromechanical relays are still in service after 40 years
or more and are still manufactured in some countries. Static relays
having been introduced in the mid-60s are making place for numeric
equipment, which is expected to have a life-cycle in the order of
20-25 years, and spare parts to be available for a period of
approximately 10 years following phase out.
2. PROTECTION AND CONTROL EQUIPMENT AS ASSETS
Every part of a power system is a power utilities asset, this
includes protection and control equipment as well as primary plant
equipment. In the era of supply industry deregulation and cost
cutting, the procurement of protection and control equipment is an
investment in which cost aspects play an increasingly important
role. One indication of this trend is shown by the fact that many
power system and protection engineers are seen as being, and are
indeed called, asset managers. In this respect, their task is to
manage secondary systems and equipment, including refurbishment,
such that long term operating costs are as low as possible whilst
assuring the quality of the electricity supply. Trained personnel
as well as technical know-how should also be considered as assets,
which should be employed efficiently.
When calculating costs the complete equipment life- cycle must
be considered, including installation, commissioning, staff
training, maintenance and support. If a utility has many types of
equipment from various manufacturers installed in its power system,
costs resulting from training and maintenance will be unnecessarily
high. If on the other hand, they have equipment from at the most
two manufacturers, these costs can be reduced to an acceptable
level, especially if a number of products are part of the same
family and based on a common software/hardware platform.
Utilities often have one or two examples of exotic protection
equipment requiring proportionally more time to maintain than more
standard equipment. Each time an engineer perfoms maintenance tests
or changes settings, he must again familiarise himself with the
equipment. If all the numeric equipment shares the same MM1
concept, using a windows-based PC program, the differences between
relays is reduced to the protection functions and their setting
parameters. Clear, user friendly MMI programs with transparent
setting parameters (e.g. using engineering units directly) as well
as on-line documentation save time
and costs. Engineers can familiarise themselves with an MMI
program off-line and directly load settings into the equipment once
in the station. This reduces the risk of incorrect settings and
increases system security.
Equipment spare parts represents unproductive capital investment
which utilities obviously wish to keep to a minimum. This is an
important aspect of a platform comprising common hardware
components. The number of spare parts for a complete range of
products comprising for example line, transformer and generator
protection as well as bay control, is practically reduced to that
of one equipment. System availability is improved and
mean-time-to-repair reduced.
Given the shortening life-cycles of electronic components and
the equipment in which they are used, long-term strategies are more
important than ever. This raises the question of how to take full
advantage of the continuous progress being achieved in
microelectronic technology without requiring investment in new
equipment every few years. The solution to this lies in the concept
of a modular hardware and software platform, as discussed in more
detail below. By upgrading intelligent processing modules,
equipment continuity is guaranteed and its life-cycle extended.
Numeric equipment permits copper wiring to be replaced by serial
communication using fibre-optic cables, thus reducing installation,
engineering and maintenance costs. Transmission reliability and
insensitivity to electromagnetic disturbances are also improved.
Marshalling wiring can be replaced by multi-core cables using
standard patch-panels.
Other aspects of numeric equipment resulting in an overall cost
reduction can be summarised as follows: self-supervision
considerably improves equipment availability and maintenance can be
performed on demand. Disturbance recording information can improve
understanding of the power system and its weaknesses, leading to
improved protection strategies and to a more secure power system.
Furthermore, acquired data can be used for monitoring primary plant
equipment, optimising maintenance.
3. PLATFORM FOR PROTECTION AND CONTROL PRODUCTS
Hardware
Applying modular hardware design to protection and control
equipment is nothing new, since this was one of the central
features of most static equipment, enabling relays or control
equipment of different complexity and size to be built using common
components. A truly modular construction allows individual
components to be replaced or upgraded easily, without having to
change additional units. Considering a protection relay for
example, the major part of the hardware is for the purpose of
interfacing to the primary process (intermediate current and
voltage transformers, tripping relays), and conventional signalling
circuits. The technology employed in these circuits is not
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changing as rapidly, indeed it is virtually the same as was used
for static equipment, even though the size of some components has
been reduced and their reliability improved. The major part of a
numeric relays functionality is usually concentrated in processing
modules. It is here that the large technological advances can be
made, and these are the units that have to be replaced if a relays
performance is to Ik upgraded. Peripheral units have a longer
life-cycle and do not have to be replaced.
The architecture of modern numeric protection and control
equipment is usually based on a bus structure. Some manufacturers
have developled equipment with a high bit-rate serial bus
arrangement, others have used a parallel bus either based on their
own design or on existing international standards e.g. VME bus.
Such a bus is central to the design of an equipment platform, since
this determines to what extent equipment can Ix upgraded. If the
equipments processing power is concentrated in one module and the
amount of data transferred between modules is niinimal (e.g. binary
input and output signals) then the bus arrangement is uncritical.
If however, numerical processing is distributed over a number of
modules such that data transfer rates are high, great care has to
be taken during the design phase to ensure ithat the bus does not
turn out to be a bottleneck when the processing capability is
enhanced.
An example of such a hardware platform is shown schematically in
Fig. 1.
,%rial hus
Binary in-loutputs \
Serial hus IEc1375
Power supply
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Fig. 1 Platform hardware structure
Features which support equipment continuity are described in the
following:
A small number of universal casings including a standard
parallel bus to intlerconnect modules. Customer specific
functionality can be fulfilled l y equipping a casing with analogue
input and binary inputloutputs modules as required.
A fast deterministic serial bus is incorporated (IEC- 1375),
enabling the platform to be used as part of a station automation or
distributed protection system 114.
Modules are designed for the following functions: - analogue
current and voltage inputs - binary inputs/outputs - remote binary
inputloutputs - signal processing - isolating amplifiers - serial
communication - star coupler connections - auxiliary power
supplies.
The number of module variants has been dramatically reduced
compared to previous designs. This leads to simplified order
handling and reduced costs for spare- parts. For example, the
analogue input module combines 1 and 5 A rated currents
respectively. 1OOV and 200V rated voltages. The binary input module
and auxiliary power supply covers a voltage range of 48 - 250 V DC.
Binary outputs with heavy- duty contacts can be connected directly
to the breaker tripping coil.
All settings are performed using the MMI program running on a
PC, which means jumpers and switches on modules have been
eliminated.
0 A number of Application Specific Integrated Circuits (ASIC)
were developed for some of the previously mentioned modules, in
order to reduce component space requirements and costs.
Furthermore, future procurement of these devices is secured, since
the manufacturer has ownership of the device design.
Serial interfaces for substation automation have an impact on
hardware. PC card (PCMCIA) slots on a processing module allow
protocol specific PC cards to be inserted and enables easy
connection to substation automation systems.
Software
Since all protection and control functions are defined by
software, it is extremely important that the software platform is
designed and implemented according to a modular concept, both at
the application level (protection and control functions and
communication) and at lower levels, which are invisible to the
user. Application software must be independent of the hardware on
which is to run, and of the software interfacing to this hardware
as well as from the run- time operating system. This is a
prerequisite for the portability of the software and in turn for
the long-term continuity of the platform.
Functions are held in a library and can be selected once, or a
number of times with different settings or inputs and outputs.
Three function types can be identified, for protection control and
monitoring applications, as illustrated in Fig. 2.
Control and interlocking applications are realised using a
graphical function-plan language as covered by the IEC 1131-3
standard. An editor and code compiler are used to generate data,
which is loaded into the equipment via the MMI. This software
allows signals
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from all functions to be interconnected, including those defined
in function-plan blocks. This is one of the main benefits of
combining protection and control applications in one equipment.
Protection functions are implemented using a high- level
programming language as opposed to function- plan language because
the performance requirements for protection functions are higher
than for control, and their functionality is more or less
standardised rather than customer specific.
I I
Fig. 2 Platform software structure
Analogue and binary input signals from the process have only to
be configured once for the complete equipment, and can thereafter
be processed by any of the selected functions. On the other hand,
trip signals from any protection function can be combined and
assigned to a common output [ 2 ] .
Prolongation of the equipment life-cycle is guaranteed through
portable software tools and compatible microprocessor successors
delivered from stable manufacturers giving superior product
support. Attention must be paid to the selection of software
development tools, since they are closely linked with the choice of
programming language and microprocessor family.
4. CONCEPTS FOR PROTECTION AND CONTROL
Traditionally, separate equipment has been employed for bay
protection, bay control and station protection. Redundancy can be
realised by duplicating equipment in main I and main I1 according
to voltage level and importance, Fig. 3a and Fig. 3c. Numerical
technology allows protection and control functions to be combined
in one unit avoiding unnecessary duplication of hardware, Fig. 3b.
Redundancy would be provided Raving main protection and back-up
control in one equipment, respectively main control and back-up
protection in the other, as shown in Fig. 3d,
During recent years it has been possible to connect bay
equipment to the station computer in a master-slave configuration,
using an interbay bus (IBB). Today an IBB allows peer-peer
communication directly between
bay units, thus eliminating conventional hard wiring, e.g. for
interlocking. Depending on the type of serial bus (e.g. IEC-1375),
this can be independent of the station computer. Furthermore, the
IBB can be implemented redundantly, such that all bay units are
connected to both IBBs.
Transmission
, IRB
Fig. 3 Basic bay equipment arrangements
5. PARTNERSHIP
The rapid technological changes and growing complexity of
products and systems, demands an increasingly close partnership
between component suppliers, software suppliers and manufacturers,
and manufacturers and power utilities.
The use of new technology also leads to quicker, flexibler
development and engineering of new functions. For example, a number
of utilities worked together with this manufacturer in developing a
customer specific autoreclosure scheme to be used in conjunction
with main 1 and main 2 line protections, including the
co-ordination between them. This had previously been implemented
using static units, requiring an engineering effort of six months.
The scheme, as well as additional enhancements, was realised in two
weeks using function blocks. The complete functionality is divided
between two function blocks, now part of the function library, the
first for fast and the second for slow autoreclosure.
6. FUTURE DEVELOPMENTS
The future will bring yet more protection and control
functionality being integrated into one equipment, for which new
redundancy and back-up concepts will be developed.
Some of the intelligence currently available at bay level will
migrate down to the process level, e.g. in remote input/output
units, non-conventional transducers, actuators, sensors. These
could include condition monitoring functions to improve the
management of primary plant equipment. Process level units must
also be an integral part of the platform concept. They require
fast, secure serial communication facilities to replace the
hard-wiring still remaining in the station (e.g. to C.T.s. V.T.s
and circuit breakers). Optical
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communication lends itself to this, since the environment is
harsher than at bay level. It will be possible to use the
information made available from such units more widely than
presently, either via a bay unit or directly through a high-speed
communication network, such that the extent of vertical and
horizontal integration in the station will increase. As a
consequence, the amount of data available at any one point in the
system will also increase. This could be used for protection
techniques which could not be so easily implemented up to now, e.g.
multi-ended line differential protection. Process level signal
processing may create a demand for new signal-processing algorithms
still in their infancy, e.g. using artificial neural networks.
On-line access to plant condition monitoring information or
protection apparatus performance, would be of great benefit to an
asset manager. As far as utilities are concerned, a consequence of
a numeric platform should be a move in the direction of an "open
system".
7. REFERENCES
1. D.M. Peck, B. Nygaard, K. Waddius, 1993 "A new numerical
busbar protection system with bay- oriented structure", Fifth
International Conference on Develo~ments in Power System
Protection, IEE Publication No. 368, 228-23 1.
2. W. Wimmer, W. Fromm, P. Muller, F. Ilar, 1996 "Fundamental
considerations on user-configurable multifunctional numerical
protection", CIGRE Session, Paris 1996, Publicaition 34/202
