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ADVANCES IN POWER SYSTEM MANAGEMENT
Volker Lohmann
ABB Power Automation Ltd, Baden/Switzerland
ABSTRACT
In view of the global deregulation process in the electric power industry, utilities are examining the application of
information technology (IT) as an option to support corporate business strategies that focus on improving service
and power quality as well as reducing cost of operation and maintenance. Key issues for the improvement of the
power system performance to achieve overall higher productivity are more and better information concerning the
dynamic behaviour of the entire power system and reliable automatic control concepts to maintain power system
integrity in case of multi-contingencies. Monitoring of the service condition of physical assets, e.g. circuit breakers
and power transformers, allow lower safety margin for operation as well as cost efficient maintenance and assetmanagement.
Wide area protection systems are intended to complement existing protection and control systems and provide state
of the art solutions for counteracting system instabilities. They are designed to detect abnormal system conditions
early enough to initiate predetermined counter actions secure reliable system performance.
Intelligent electronic devices (IED) for protection, measurement, monitoring and control tasks in substations as
substitutes of electro-mechanical or static devices provide an infrastructure to collect, to process and to transmit data
and information, which are utilised for advanced power system management.
The integration of the various SA systems in a high performance communication network allows system wide
adaptive protection and real time automatic power restoration procedures
Keywords:
Wide area protection, substation automation, condition monitoring, dynamic load shedding, communication
networks, reliability centred maintenance
INTRODUCTION
In the increasingly competitive arena there is significant pressure on power providers for greater system reliability
and improvement of customer satisfaction, while similar emphasis is placed on cost reduction. These cost reductions
focus on reducing operating and maintenance expenses, and minimizing investments in new plants and equipment.
If plant investments are to be made only for that which is absolutely necessary the existing system equipment must
be pushed to greater limits in order to defer capital investments.
Utility executives, on the other hand, are examining automation solution alternatives to support corporate business
strategies that focus on improving power quality and reducing cost of operation and maintenance.
The areas where advanced information technology (IT) applications can contribute significant benefits in terms of
better power system performance and reduction of operating and maintenance costs concern power system
management, substation automation and on-line condition monitoring.
The prerequisite for implementing advances electronic systems is a efficient communication network not only for
supervisory control and data acquisition (SCADA) and energy management systems (EMS) but also for providing
the protection, maintenance and planning departments with direct access from remote to information from the
substation primary and secondary equipment.
As new and higher levels of digital technologies have made its way into substations in terms of numerical protection
devices and control systems, protection engineers are suffering today from data overload. They have more data than
can be processed and assimilated in the time available. Therefore, today the challenge is to automatically convert
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data to information to predict maintenance, which frees manpower to implement condition based or preventive
maintenance.
WORKING PLANTS HARDER
Enhancing the management and performance of plant and power systems is being discussed widely at many
international conferences. The overall conclusion perceived is that there are a lot of new technologies available,
which will help planners and operators to find new solutions to maximise the use of the power systems and adapt to
the fast changing environment.
There are three area where advanced information technology (IT) applications can contribute significant benefits in
terms of better power system performance and reduction of operating and maintenance costs: (Figure 1)
1. Advanced power system management, which results in higher reliability of power supply
2. Intelligent substation automation which assures higher availability.
3. On-line power system monitoring which allows to work assets harder and to save maintenance costs
On-line Condition MonitoringConditionrelated data
Disturbance records
Fault history & analysis
Early indication of faults
Asset management support
Substation Automation (SA)
Fibre optic broadband
communication
Advanced Power System Management
Voltage and current phasor measurements Voltage instability prediction
Intelligent load shedding
Automated islanding
Automated power restoration
Figure 1: IT applications for advanced power system management
The prerequisite is a efficient communication network not only for supervisory control and data acquisition
(SCADA) and energy management systems (EMS) but also for providing the protection, maintenance and planning
departments with direct access from remote to information from the substation primary and secondary equipment.
(Figure 2)
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Protection /
Engineering
Department
Planning / Asset
Management
Department
Operation /
Maintenance
Department
Non-Real TimeIntranet
WAN
Station 1 Stat ion 2 Station 3 Station n
Real timeCommunication
Network
EMS / SCADA
Centre 1
EMS / SCADA
Centre 2
Non Real time data
Parameter
Disturbance records
Detailed protection signals
Protection measurement values
Non-urgent alarms
Monitoring data
Real time data
Position status
Commands
Interlocking
Automatics
Alarms
Process values
Figure 2: Corporate communication network for efficient data exchange
It is suggested to split the communication system into two partial networks. One for real time data exchange,
controls and fast automatic interactions between the various substations and energy management systems (EMS).
The second Intranet wide area network (WAN) for non-real time data e.g. parameters, disturbance records,
measurements and monitoring data.
ADVANCED POWER SYSTEM MANGEMENT
In view of the fact that power utilities are forced to increase the performance and the profitability of their power
systems, the transmission and distribution networks have to be operated harder to their limits in order to satisfy the
ever-increasing demand for electric power. This, however, increases the risks for outages due to the presently
insufficient assessment of power systems stability limits.
Since the beginning of the electrification area primary equipment protection has been very important, in order to
prevent destruction of objects in case of faults. In these days the power supply is so important to the entire society,
that large efforts have to be made to maintain power system integrity and mitigate the consequences of faults. This
situation will make power utilities increasingly dependent on modern information technologies that provide wide
area protection to counteract wide area disturbances and minimise power outages.
In response to these new needs ABB has created PsGuard, Wide Area Protection System, which complements
existing protection and control systems and provides state of the art solutions for counteracting system instabilities.
It is designed to detect abnormal system conditions early enough to initiate predetermined counter actions secure
reliable system performance.
New Methods for Instability Recognition
In order to obtain accurate and actual real time information from the power system stability conditions, phasor
measurement units (PMU) need to be installed at critical points throughout the transmission network for sampling
voltages and currents phasors i.e. instantaneous values of both magnitudes and relative rotor angles. They are
synchronised by GPS satellites for taking simultaneously snapshots of phasors. Further processing of these data
delivers accurate values of the safety margin S to voltage instability at the various locations as well as for the
entire network in the system protection centre. The objective is to detect incipient problems early enough to initiate
preventive actions. (Figure 3)
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PMU
Im
U2
I2
U1
U3
I1
I3
Re
Im
U2
I2
U1
U3
I1
I3
Re
PMU PMUPMU
Im
U2
I2
U1
U3
I1
I3
Re
Im
U2
I2
U1
U3
I1
I3
Re
Im
U2
I2
U1
U3
I1
I3
Re
System
Protection
Centre
Transmission Network
Figure 3: PSGuard Wide Area Protection (WAP) scheme
Time Frame Related to Power System Phenomena
The time frame for wide area protection applications related to power system phenomena ranges between typical
responses times of protection devices and the time, which is consumed until operators in the network control centres
are in the position to interfere manually. (Figure 4) In view of the fast response needed to counteract power system
instabilities, operators have very limited chances to act fast enough to maintain system integrity. Therefore, they
need either support by automated control systems or an indication of incipient problems early enough that they can
take preventive actions in time.
0.001 0.01 0.1 1.0 10 100 1000Time [sec]
Electromagneticswitching transients
Transient stability(angle & voltage)
Small signal
stability
Power systemoperation
Long term stability
Long term voltage stability
Equipment protection Automatic actions Manual operation
Automatic shunt switching Gas turbine
Start upGenerator
rejection
Tap changer
blocking
Underfrequency load
shedding
Actions on AGC
Undervoltage load shedding
Controlled islanding
Remote load
sheddingTimerang
eofWAPApplications
Powersystem
phenomina
Response
Range
Figure 4:WAP time frame related to power system phenomena
The following classification of power system instabilities in relation to time scale and dominating /critical system
components has been agreed within IEE.
Dominating/Critical System Components
Time Scale Generators Loads
Fast
Angular Instability
Transient Steady State
Fast Voltage Instability
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Slow Frequency Instability Slow Voltage Instability
Response to Power System Instabilities
In the first stage of implementation, PsGuard should be used as a monitoring system only in order to assess the
dynamic behaviour of the power system. In the second stage, analytical system studies need to be conducted to
establish a defence plan that defines the actions, which are required for maintaining the power system integrity. It is
recommended that this work should be a joint effort between ABB and the utility operating the power system. Thereason is that the experiences the utility has made with multiple contingencies have to be taken into account, as well
as the utilitys operating policy, the load restrictions, and options of network topology and power generation. In the
final stage, PSGuard is used to prevent instabilities by initiation of the most appropriate actions according the
defence plan.
Of vital importance to the reliability of transmission networks is the co-ordination of wide area protection functions
with the legacy control and protection systems.
The extensive system wide PMU measurements can further be used to investigate at which locations in the network
the installation of FACTS (flexible AC transmission system) would be feasible to optimise the power flow.
The following typical sequence of actions would be initiated by PSGuard, if the power system approached
instabilities:
1. Alerting the system operator by indication of the remaining safety margin S and by providing on-line
guidance to counteract a critical situation. In addition, corresponding information is produced for the energy
management system (EMS).
2. Control actions are initiated if the safety margin S reaches a pre-set critical level to avoid voltage instabilities
to occur, e.g.
FACTS (Flexible AC Transmission System)
Can produce or consume reactive power This action is instantaneous and efficient in case of voltage collapes It can counteract voltage instability following loss of several transmission lines
LTC (Load tap changer control)
If the load current increases LTC is supposed to raise the tap position to compensate for the voltage drop In the course of severe power system disturbances this ,however, would be a counterproductive action. Therefore, Psguard blocks LTC or changes the setpoint of the tap changer to preserve system stability
AGC (Automated generator control)
Objectives of AGC are: to regulated frequency and to maintain balance of power AGC controls the load reference setpoints of a group of generator Control is confined to an individual area
Load shedding
Underfrequency initiated to minimise the risk of system collapse Undervoltage initiated to preserve system stability
In any case to be conducted before islanding is initiated
Islanding
Last defence measure towards saving the power system
Should only be applied if specific load/generation areas can be defined Risky operation as it can cause total collapse of the sislanded individual systems Should only be conducted after load shedding is conducted to estabilsh generation spinning reserve
The block diagram in Figure 5 shows the interactions between the various applications.
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Automated
Control
VIP
VoltageInstabilityPredictor
Phasor
angle
difference
S System OperatorGuidance(Display)
S
EMS
AGCTap
Changer
Control
FACTS
Cos Adaptation
Tap ChangerBlocking
GenerationAdaptation
Load
Shedding
LoadAdaptation
S : Safety margin asproximity to voltage collapse
Automated control actions
Phasors
U
I
U
I
U
I
PhasorEvaluation
PMU
PMU
PMU
Islanding
SplitPower System
Figure 5: Responses to power system instabilities
An Example of a Defence Strategy
The second line of defence consists of the two following actions:
1. Load shedding on frequency criterion
2. Islanding of out-of-step areas
Islanding of out-of-step Areas
As soon as loss of synchronism occurs in the network, violent transients are induced on the generating units located
inside or at the border of the out-of-step area and customers have to stand with large disturbances. If the transientinstabilities last more than a few seconds, the phenomenon spreads through whole power system and the protective
relays of the generating units are put into operation: the units are tripped and the power system begins to collapse.
The strategy against transient instabilities to be chosen is to isolate out-of-step areas as fast as possible and thus save
the rest of the grid. With such a strategy PSGuard is the solution to
Detect transient instabilities
Be selective enough to disconnect the out-of-step areas only
Be the result of a compromise between rapid action to avoid the spreading of disturbance and a slower
action enabling a possible resynchronisation
The design of an emergency plan is recommended to be based on the carrying out of numerous specific fault
simulations in the network going beyond the conventional N-1 stability studies and to monitor the power system
reactions by PSGuard during the initial installation. The aim of these simulations is to define the present securitymargins of the system, to determine the behaviour and the limits of the present defence measures and to maximise
the impact of the new strategy or of better tuning of the existing equipment.
Decentralised Power System Control
In case of major disturbances, data from disturbance recorder, change of network configuration, protection relay
signals and switching procedures all these aspects have to be considered for appropriate corrective actions to be
taken. This process is very complex due to the amount of data and the restricted communication of information and
limited real-time performance, which can be managed from SCADA and handled by an operator. Decentralization
of power system control allows automated isolation of faulted sections of a substation after protection has tripped a
feeder or a busbar. The corresponding system structure below shows the allocations of functions and the
communication links. (Figure 6)
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Electrical System
Automated
Control System
Communication
Network
Power
Generation
Power
Transmission
Power
DistributionConsumer
Power Plant
Automation
Transmission
AutomationDistribution
Automation
Demand side
Automation
Control Centre Energy
Management
Transmission
Management
Distribution
Management
Demand Side
Management
National Dispatch Regional and District
System
Planning & Operation
Maintenance & Asset
ManagementBack-office
Corporate Communication Network
Figure 6: Decentralized Automated Power System Control
SUBSTATION AUTOMATION
Substation Automation for T&D
Substation Automation Systems (SA) for T&D applications provide a platform of multi-functional intelligentelectronic devices (IED) for the integration of control and protection functions as well as for condition related data
acquisition into one single system (Figure 7)
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Station Bus
Local workplace
process database
Interbay bus
Distribution
Interbay bus
Transmission
Switch yard
Coupler Coupler
Substation
Monitoring
Workplace
(SMS)
Intranet
Corporate information system (CIS)
NCC /SCADA Backoffice
X X X X XX
Cabling
X XX
X X X
Maintenance
Server
400 kV 66 kV
MODEM
Load
Shedding
Figure 7 Substation automation system with IEDs for integrated control and protection
The system architecture provides for 400 kV and for 66 kV separate subsystems, which are interconnected via thestation bus. This allows for data exchange with the local workplace as well as to the maintenance server. The data
exchange with SCADA and EMS as well as with the back office is enabled via the corporate information system.
Real time interaction between protection and control IEDs via the fibre-optic interbay bus allows automation
functions as well as adaptive protection schemes. The following examples demonstrate how modern SA concepts
can be effectively applied to improve the power system performance.
Dynamic Load Shedding
When tripping of generation occurs on a network, the variation of frequency depends of several dynamic factors in
interaction such as the quantity of spinning reserve, the limitations of the prime mover system and the speed of
governors, the inertia of the power system or the sensitivity of customer load. When drop in frequency is large, the
loads can be shed by underfrequency load shedding and finally, the generating unit may be tripped because of the
action of low frequency protective relays, leading to a general collapse. This phenomenon is particularly importanton isolated power systems where the largest generating unit represents a high proportion of the total demand. On
these kinds of power systems, many blackouts can be avoided with the aid of well-tuned load shedding plans.
The conventional load shedding approach is static, as it initiates tripping of pre-selected circuit breakers when a
certain level of under-frequency is reached, regardless of the actual load conditions. The reason is that the actual
load behind each individual circuit breaker is not taken into account.
Microprocessor based load-shedding schemes, however, are in the position of considering the actual load currents
and to dynamically select only those feeders to be opened, which are needed to regain the frequency stability.
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Distribution Network
Intelligent Load Shedding
X %
Adaptive
shed table
< f,U>df/dt
dU/dtP1...
Pn
P
Pref
Transmission
Network
U, f Automated load
shedding
I1
..In
Dynamic selective
feeder tripping
commands
Busbar
Priorities
selective
decision
Feeder Currents
Figure 8 Intelligent load shedding scheme
The load shedding function block (LFSB) of the intelligent load shedding scheme continuously monitors the load
currents of each feeder. (Figure 8) It obtains the actual measured current and voltage values either directly
hardwired from dedicated CTs and a busbar VT or via communication links from the CTs and VT's, which are
incorporated in a numerical protection/control devices.
The LSFB compares the reference power Prefwith the individual feeder load measurements P1...Pn. To each feeder
a priority index Pr is assigned for load shedding. The LSFB selects from the power inputs P1....Pn the sum of the
power which is larger than Prefthus minimising the difference between the selected and reference power. If the pre-
determined load shedding criteria (LSC) in terms of under-frequency (< f) or frequency change (> df/dt) is fulfilled,
a predefined percentage X % of total load Ptot is shed by opening selected feeders. The selection of the feeders to beopened also takes the predefined priority index Pr into account.
If the network frequency continues to drop or remains stable on an under-frequency level, the shed of the next load
class is initiated, i.e. shedding of a second predefined percentage X% of the total load Ptot (Figure 9). Otherwise, if
the network frequency starts to increase within a definable time delay, the next load class will not be enabled and
the load shedding scheme is reset as soon as the network frequency has recovered. If the network frequency has
recovered, the integrated network restoration function will be started automatically.
f (Hz)
P (MW)
Step 2
Step 1
t (s)
t (s)
Disturbance
New load
balance
Load of priority 1 X % of total Load
Load of priority 2 Y % of total Load
fLim 1
f Lim 2
fN
Figure 9: Dynamic Load Shedding
In contrast to the conventional way of load shedding, stabilisation of the frequency can often be reached in the firstshedding step. In addition, only the necessary load is tripped resulting in a minimum impact for the plant supply.
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Line tripping to isolate out-of-step area
Load shedding orders are initiated, if necessary in areas, which could be destabilised by area isolation. Orders are
received by specific circuit breakers, which simultaneously open the borderlines of out-of-step areas and by the
local station computers, which shed the supply in weakened areas.
Two redundant communications paths should ensure the communication between PMUs, central computer and
circuit breakers in the substations: preferably a satellite communication network and a microwave network. Thesetwo redundant communications means are necessary for reliability reasons.
Adaptive Line Distance Protection
The term adaptive is related to a protection philosophy, which permits automated adjustments of protection
functions and to make them more attuned to the prevailing power system conditions. This means that the
functionality of the protection scheme is enhanced by means of additional information about the network. A typical
example is adaptive distance protection:
Redundant transmission lines often run in parallel over long distances. The automatic switching of the load from
one line to the other as a corrective measure in case of one line being faulty, has to take into account that mutual
impedance exists between the parallel lines. This impedance can cause measuring failures, resulting in unnecessary
trips initiated by the associated distance relays during earth faults. In order to avoid this, the distance protection
needs to be automatically adapted to the topology of the parallel lines and to the actual service conditions (e.g.,parallel, disconnected, earthed or unearthed, both lines connected to different busbars at one side, etc.). Apart from
this, also the power carrying capability of one of the lines may have to be increased by corresponding adaptation of
the line protection.
The scheme for the corresponding exchange of information between the line bay control units and the line bay
protection units as well within the substation itself as between associated substations is shown in figure 10. It is
crucial that this communication is of very high quality with regard deterministic and real time speed behaviour. It is
therefor recommended to establish a communication link, which is dedicated for this adaptive protection task.
Line bay
controlLine distance
protection
Station 2Station 1
Line 1
Line 2Communicationwithin
thesubstation
Communicationwithin
thesubstation
Communication between substations
Communication between substations
Line bay
controlLine distance
protection
Line bay
control
Line distance
protection
Line bay
controlLine distance
protection
Figure 10: Adaptive distance protection for transmission lines
ONLINE POWER SYSTEM MONITORING
For once neglecting outages as a result of wrong human operation, there are basically three reasons for power
interruptions:
1. The breakdown of a utility asset through normal wear and ageing under working conditions.
2. The breakdown of an asset being effected by an external event (system disturbance), such as a tree falling on an
overhead line that led to a permanent abnormal working condition.
3. A temporary system disturbance where either the external influence disappears ("self-healing"), or a protective
system isolates the assets from the electric grid, and by means of network redundancy avoids a power outage at
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all, or leaves a limited area without power. With respect to the condition of assets, however, this temporary
disturbance most likely caused accelerated wear.
Condition monitoring mainly addresses the wear and ageing caused by normal or temporarily abnormal working
conditions. First, in that they support the evaluation of the actual condition of assets, and second, in that they might
explicitly support the prediction of the further evolution of a detected problem, and the probability of breakdown.
However, many of today's condition monitoring systems leave the assessment of the future to the human's
interpretation based on his conclusions drawn from the current status. Whichever, even if a utility decides, e.g.,based on risk management considerations, to let a worn out asset in operation until it breaks, the breakdown will be
a planned one, and so will the repair action be. Hence, the power interruption will most likely be rather short and the
problems posed by the interruption alleviated as good as possible.
Apart from monitoring the condition of primary equipment and thereby attempting to proactively prevent power
interruptions, an elaborate post fault analysis supported by monitoring systems is equally important. It has been
observed that a large proportion of major blackouts of electric power systems is caused by protective system
failures. These failures are generally hidden and only exposed during the rare occasion of system disturbances.
According to utility opinion derived from a questionnaire over 60% of these failures are based on wrong protection
settings, protection calibration, or protection maintenance. It is therefore important to capture as much details as
possible during a system disturbance and have access to as much protection relevant data as possible during the
entire analysis. The conceivable subsequent settings refinement phase is a measure to prevent the same interruption
from happening again, or, at least, minimise its impact on the power distribution
Data Acquisition
With computing power making its way into the primary equipment, more and more internal data from high voltage
equipment can be made available to the outside at reasonable costs. Interfaces to acquire such internal data were
previously not provided for cost reasons. Data that will be accessible includes, but is not restricted to:
Switching counters,
Thermal information,
Quality of isolation media,
Entire timing curves of switching operations,
Switching currents,
Manufacturing data,
Original value of key performance criteria.
This kind of data can be the source of valuable condition information and exploited for building condition
monitoring systems for those assets that exhibit the highest failure rates and/or cause unacceptable power
interruption impact. Without doubt the transformers and circuit breakers are the prime candidates for these kinds of
monitoring systems.
The second trend within the data acquisition falls into the category of intelligent electronic devices (IED), i.e.
secondary equipment like protection terminals. Besides their primary functions, they host more and more additional
functionality, which increase their attractiveness compared with dedicated single function units. Many of these
additional functions provide a sound foundation for basic monitoring systems, cost-efficient and perfectly suited for
medium and distribution voltage level IEDs for protection or control may comprise: (Figure 11)
Disturbance recorders
Event recorders
Statistical value recording (peak current indicators, number of starts/trips, current at tripping, etc.) Power quality analysers
General purpose programming capabilities that allow to conduct customer specific applications on the
IEDs.
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CE
Mo 12. 11. 96 GMT 17:02.43.305
Ayer Rajah & Labrador Feeder One
GPS
# Of trips
Alarm
Classes
Advanced analysis
tools Automatic printing
Summary report
Bay
CE
RF
IO
12345678910111213141516
CE
Universal Time
synchronization
User friendly
visualization
Sequence of Events
CONCISE / FAST
Distance to Fault
Indactic
650
Indactic
425
IEDs
Figure 11: Intelligent electronic device (IED) for protection
Substation Monitoring
Substation monitoring systems are often defined and understood as functional and even physical subsets of
substation automation systems, with mostly the control functionality not being included. This perception has largely
been established on the grounds of marketing reasons. This is, however, a rather narrow focus that does no justice to
the importance of the monitoring applications, and is backed by the currently growing interest in condition
monitoring applications, and the increasingly deployed commercial information technology for standalone
monitoring systems. The more general definition of monitoring is better suited to describe the modern monitoring
approach:
A station or network management technique, which exploits the regular evaluation of the actual operatingcondition, in order to minimise the combined costs of power transmission/distribution and maintenance.
ABB offers scalable solutions for substation monitoring ranging from communication kit for single IED up to
complete standalone systems with PC for decentralised data evaluation and failure analysis. The SMS PC for data
archiving, evaluation and processing to information may be located at various locations:
Locally within the substation
At any remote location as a centralised SMS allocated to a specific region
Operation and Maintenance Centre
Engineering Centre for protection and planning
Network control Centre
An SMS located within the substation archives the data, which are collected from the numerical protection devices
through an inter-bay bus. The data are presented after analysis on dedicated SMS operator display. (Figure 12)
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Interbay-Bus (IBB)
Mo 12.11.96 GMT17:02.43.305
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Event printer
GPS Hardcopy printer
Baylevel
Stationlevel
Bus Connection
Module
Modem
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Alternative
Remote link
Figure 12. Typical ABB Substation Monitoring System SMS530
Data Evaluation and Information Transmission
For centralised retrieval and transmission of data, and for transforming data into information a application package
is provided which enables the maintenance and protection system engineer easy judgement of condition of the
power system. (Figure 13)
GIS
ERP
LDB
MMS
InformationSystem
ANALYSE INTEGRATE
E_notify
E_param
E_statistic
E_history
E_history
E_history
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Process
Measurements
Sequence Of
Events
IED
Parameters
IED sDisturbanceRecorder
GIS - Geographical Information Systems
ERP - Enterprise Resource Planning
LDB - Lightning Data Base
MMS - Maintenance Management System
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Figure 13: EVEREST: Evaluation package for power system condition related data
Reliability Centred Maintenance
The new approach is to move from the traditional time-based maintenance policy to condition-based reliability
centred maintenance (RCM) policy. This calls for differentiation between the following four types of maintenance
policy: Predictive or condition based maintenance, i.e. to monitor if something is going to fail
Preventive maintenance, i.e. overhauling items or replacing components at fixed intervals
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Corrective maintenance, i.e. fixing things either when they are found to be failing or when they have failed
Detective maintenance, i.e. to detect hidden failures by means of special functional checks and diagnostics
The type of maintenance policy to select for specific equipment for transmission and distribution depends on
reliability and on economic and customers business related availability considerations, which take the
consequences of failures into account.
ENHANCING LEGACY CONTROL AND PROTECTION SYSTEMS
The application of modern IT solutions with implementing IEDs is the state-of-the-art for new substations. The
benefits of advanced power system management as outlined above can, however, only be exploited if the legacy
electro-mechanic control and protection systems in existing substations are substituted with modern IEDs and if
access is provided for data retrieval via modern communication networks.
Even if modern wide area networks (WAN) are available, which enable real-time data exchange, there remains still
the decision to be made concerning the most feasible step-by- step retrofit strategy for the substitution of the legacy
equipment. The strategy as outlined below suggests nine upgrade options, which depend on the scope of
functionality required: (Figure 14)
Remote control unit (RTU) Numerical protection
Central control system Integrated digital fault recording
Decentralised control system Data retrieval via Modem
Interaction of IEDs via inter-bay bus
Substation Automation via local PC
Monitoring of primary equipment
Local power restoration
Inter-station
Automation
Utility BackofficeUtility BackofficeSCADA / EMSSCADA / EMS
IEC870-05-103
Corporate Information System
Conventional primary equipment Legacy protection systemLegacy control system
1. The provision of a remote terminal (RTU) enables remote control of a substation from supervisory control
systems (SCADA) in network control centres and the substitution of the legacy protection system by numerical
protection offers more functionality and the acquisition of condition related data.
2. The substitution of the legacy control system by a central control system with IEDs enhances the functionalityof a RTU with regard to control and interlocking and the use ofintegrated of digital fault recording
incorporated in protection IEDs reduces the costs for finding and fixing of faults. In cases where no separate
modem connection provided for retrieval of fault data a serial link with the IEC 870-05-103 protocol can be
provided to connect the protection IED with the RTU.
3. The provision of a decentralised control system with IEDs close to the primary equipment offers significant
cost reduction for secondary cabling and the data retrieval via modem from the utility back-office allows
cost-effective maintenance and parameter adaptation from remote.
4. If the interaction of IEDs for control and protection via an inter-bus is provided, more complex control
functions improve the flexibility and availability of substations.
5. Modern substation automation via local PC enables local operation of substations, comprehensive substation
monitoring and the provision of a substation data base for condition related data processing.
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6. Monitoring of primary equipment enables to work the primary equipment harder to their thermal limits and
alter the maintenance strategy form time based maintenance to cost efficient condition based maintenance and
reliability centred maintenance.
7. Local power restoration allows fast automatic response to contingencies, reduces the impact of faults and
avoids system collapse.
8. Inter-station automation is applied for advanced energy management, load shedding and island of subsystemto maintain power supply integrity.
9. The installation of modern corporate information systems in terms of WAN and broad band technology
allows the exchange of data and information between substations, SCADA/EMS and utility back-office in order
to insure that the right information is transmitted to the right people at the right time.
CONCLUSION
There are three areas where advanced information technology (IT) applications can contribute significant benefits in
terms of better power system performance and reduction of operating and maintenance costs:
1. Advanced power system management, which results in higher reliability of power supply
2. Intelligent substation automation which assures higher availability.
3. On-line power system monitoring which allows to work assets harder and to save maintenance costs
The prerequisite is a efficient communication network not only for supervisory control and data acquisition
(SCADA) and energy management systems (EMS) but also for providing the protection, maintenance and planning
departments with direct access from remote to information from the substation primary and secondary equipment.
In response to these new needs ABB has created PsGuard, Wide Area Protection System, which complements
existing protection and control systems and provides state of the art solutions for counteracting system instabilities.
It is designed to detect abnormal system conditions early enough to initiate predetermined counter actions to secure
reliable system performance.
Substation Automation Systems (SA) for T&D applications provide a platform of multi-functional intelligent
electronic devices (IED) for the integration of control and protection functions as well as for condition related data
acquisition into one single system. On-line power system monitoring allows to work assets harder and to save
maintenance costs.
Microprocessor based load-shedding schemes are in the position of considering the actual load currents and to
dynamically select only those feeders to be opened, which are needed to regain the frequency stability. In contrast to
the conventional way of load shedding, stabilisation of the frequency can often be reached in the first shedding step.
In addition, only the necessary load is tripped resulting in a minimum impact for the plant supply.
ABB offers scalable solutions for substation monitoring ranging from communication kit for single IED up to
complete standalone systems with PC for decentralised data evaluation and failure analysis. For centralised retrieval
and transmission of data, and for transforming data into information an application package is provided, which
enables the maintenance and protection system engineer easy judgement of condition of the power system.
The full scope of benefits of advanced power system management can only be exploited if not only new systems are
equipped with state-of-the-art IED based control and protection systems but also the legacy systems control and
protection systems associated with existing conventional substations are substituted by modern IEDs. A nine stepretrofit strategy is explained to enable a cost-effective step-by-step approach to enhance legacy control and
protection systems.