Advances in Power System Management

7/30/2019 Advances in Power System Management

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ADVANCES IN POWER SYSTEM MANAGEMENT

Volker Lohmann

ABB Power Automation Ltd, Baden/Switzerland

[email protected]

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


protection

Line bay

control

Line distance

protection

Line bay


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)

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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

E_web

E_gis

E_erp

E_com

E_navigation

E_ldb

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

E_wineve

E_database

E_mms

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.

Advances in Power System Management

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