scada – rtu interface

BY S . GANESH BABU

• A formal definition of SCADA system, as recommended by IEEE, is:

• “A collection of equipment that will provide an operator at a remote location with sufficient information to determine the status of a particular equipment or a process and cause actions to take place regarding that equipment or process without being physically present”.

05/03/23 2s ganesh babu

• A power system consists of devices that generate, transmit, and distribute power.

• Power system automation is the act of automatically controlling the power system via automated processes within computers and intelligent I&C devices.

• It consists of three major processes:• data acquisition, • power system supervision,• and power system control, all, working in a

coordinated automatic fashion.


• Data acquisition refers to;• Collecting data in the form of measured analog

current or voltage values,• The open or closed status of contact points.• Control refers to sending command messages

to a device to operate the I&C (A collection of devices that monitor, control and protect the system is referred as instrumentation and control(I&C) system)and power system devices


Measurement and feeding of various parameters such as : Voltage, Current, power, circuit breaker status, and alarms

needs a supporting sub system. The parameters are fed into the substation telemetry

equipment called RTU ( Remote telemetry unit or remote terminal unit).

Analogs: Bus voltage: source : bus potential transformer. and currents in all feeders and transformers: source –

current transformers.


Power in MW, KVAR, Power factor from LV breaker.

Analogue measurements carried out by Industrial standard transducers with an output of 4 – 20mA and 1 to 5v D.C.


Breaker “ON / OFF” status. Alarms for oil, buch’olz, oil and winding

temperature. OLTC positions for transformers. Capacitor banks. All status inputs calls for voltage free

contact terminals.


Central control room monitors a series of mixture of analog and status inputs the frequency of which is decided by the priorities and fleeting events.

Scanning rate depends on nature of substation parameters.

Voltage rating and type of substation. Non fleeting status and analog would require a

scanning rate 0f 2 to 3 minutes. Fleeting and high priority inputs require a

scanning rate of 5 to 10 secs and is decided based on the following factors.


Importance of the substation to the grid. Voltage rating : Higher the voltage rating, the

more the importance, Cost factor, which decides the type of

communication and the choice of the scan rate. Ex: High Speed TDMA link between central

control center and the substation and its exclusivity.


Automatic Switching Emergency Load shedding Re-routing services for station maintenance Automatic transfer schemes Load sectionalizing Custom, automatic reclosing schemes Automatic service restoration Circuit breaker control and interlocking Feeder automation and fault recovery


Protection and Control• Circuit breaker lockout

• Protective relay interface/interaction

• Dynamic protective relay setting for dynamic station topology

Voltage Regulation Management• Load Tap Changer (LTC) control

• Voltage regulator control

• Capacitor control


Transformer Management• Parameter monitoring and alarming

• Real-time modeling

• Interface to existing transformer monitors Automatic System Diagnostics

• Power apparatus health monitoring

• PLC and communications self monitoring

• Report and alarm on IED self diagnostics


Maintenance and Safety

Kirk Key interlock management

Maintenance ‘Lock-out/Tag-out management

Automatic circuit isolation control


Station MMIs – Graphical User Interface (GUI)

Interface real-time single-line displays Interactive real-time breaker and switch

control display On-line operation and maintenance logs Sequence of events recording IED detail displays Parameter trending displays


Increased reliability, lower costs. Forecasting accurate demand supply

management Faster restoration of power in case of a break

down Better active and reactive power management Reduced maintenance cost, conditioning

monitoring Reduce human influence and errors Assists operator for faster decision making Automated meter reading Easy fault diagnosis


Availability of information Analysis of information Decision making Optimized system operation (competitive

environment) Equipment condition monitoring (ECM) Equipment parameters are automatically

tracked to detect abnormalities. Timely action, Extended life ECM IEDs available Sequence Of Events (SOE) recording


A SCADA system generally consists of a master station and a number of geographically dispersed Remote Terminal Units (RTUs), all interconnected to master station via a variety of communication channels.


RTU is the eyes, ears and hands of the Master Station.

In most applications the RTU is the slave of the Master Station, but in some applications the RTU is equipped with internal computational and/or optimizing capabilities.


AVAILABILITY: The availability of the RTU shall

be 0.999 over the 12 month period of maintenance exclusive of communication channel and other purchaser caused down time.

Availability = MTBF / ( MTBF + MTTR)

MTBF = Mean Time between failure

MTTR = Mean Time to repair


RTU consists of:

The protocol processor module Communication module Data processing Module I/O modules for analog inputs Digital input and output RTU signals from transducers, relays, breaker

auxiliary switches, and TVMs. Data transmitted to DCC through

communication media (TDMA and MAR)


Functions of RTU include

Input scanning

Alarm,

And change of state detection.

With high degree of reliability, security and integrity.


The I/O channels are defined based on the need of each substation, and the RTU is configured accordingly.

These modules interface through memory mapping to the micro processor.

The RTU scans analogue and digital data non stop, and stores the analogue information after converting it in its memory.

The RTU’s at each substation are accessed and addressed independently.

The electrical and communication format to the mail port is usually RS 232.


Typical diagnostics and indications provided in the RTU are : Power LED:

Indicates power applied to the base board when switched on. RUN LED:

Indicates healthy operating system. LPAT LED:

Indicates too low voltage on the input side. Watch Dog time out LED. Status LED:

Gives a binary coded output if the system fails a soft ware confidence test.


Diagnostic capability at the I/O module level.

Hardware read back at telecom and output circuit for verification of data at the field connection point.

Verification of memory and certain peripheral chips in operating system.

Communication diagnostics to indicate activity on the receiver and transmitter lines for both main and local ports.


The communication subsystem is the interface between the communication network and the RTU internal logic.

This subsystem receives the messages from the master, interprets the message, initiates action within the RTU, and sends an appropriate response to the master.


PRIMARY COMMUNICATION 64 kbps TDM/TDMA (time division multiple

access operating in 2.3/2.5 GHZ band and the out stations extend to EHV stations.

SECONDARY COMMUNICATION: Is 9.6 KBPS MAR (multiple access

Radio) system operating in 849/943 MHZ band.

Mar remotes are located in all 33/11 kv substations.


The logic subsystem consists of the main microprocessor and the database.

It handles all major processing, time keeping, and control sensing.

It also handles analog-to digital conversions and computation /optimization, if required.


The termination subsystem provides the interface between the RTU logic and external equipment such as the communications lines, primary power source and the substation devices.

These assemblies must be designed to protect the RTU logic from the harsh environment of the station.


The power supply, like the termination subsystem, must meet the various voltage surge standards.

The power supply convenes primary power, usually from the substation battery, to the supply requirements of the other RTUs subsystem.

If a station battery is not available, the unit is normally ordered for ac operation with built-in battery back-up to cover several hours of operation.

Care should be exercised in selecting an ac supply circuit to assure a reliable and stable, noise-fee source.


In addition, occurrences of noise on the communication channel seeks reinforcements in the form of additional master station and RTU hardware and software design to ensure distortion free signal transfer.

Communication channels limit the speed at which data acquisition and control can be performed, thus affecting the master station man-machine interface and applications software design.


The configurations of communication systems are dictated by the following:

Number of RTUs Number of points at RTUs and required

update rates Location of RTUs Communication facilities available Communication equipment and techniques

available


Block diagram for a one-on-one SCADA system configuration, with a master unit for each remote unit

M R 1

M R2


Block diagram for a star or hub SCADA system configuration, with one master unit for several remotes, but with only one remote on each communication channel.

M

R2

R1 R3R4


Block diagram for a party-line SCADA system configuration, with several remote units on a single communication channel.

M

R1 R2 R3 R4


Block diagram for a SCADA system configuration, combining the characteristics of the star and part-line configurations, with one master and several remotes on each of various communication channels

M

R2

R1

R3

R4

R7

R8

R6

R5


Consists of two marshalling boxes accommodating: One, Interfaces the substation equipments with the

RTU. Accommodates transducers, TVM’s, contactors etc., The other panel holds all other radio and

communication accessories along with RTU. The interfacing work at the substation

includes: Erection of SCADA marshalling panel, either

designed and custom made, or fabricated. Fixing of components and accessories, Wiring and dressing of the wires to be connected to

the components.


Transducer

Input 5 A/1A

Output 4-20 mA


Digital data (on/off) may have alarms attached to one state or the other.


Pulse data (e.g counting revolutions of a meter) is normally accumulated or counted.


Laying of 4 core and 10 core cables from the field/ control room to the panel including formation of cable duct where ever required.

Termination of the leads at appropriate points including crimping of lugs and provision of ferrules.

Providing of Earth connection to the marshalling box from the equi-potential bar.


Ensuring vermin proof cable entry points in the marshalling box with the provision of suitable cable glands.

The components that are housed in the SCADA marshalling box are:

AC current transducer to every feeder to measure Y phase current.

Secure make electronic trivector meter for measurement of TVM parameters at power transformer LV breakers.

Voltage transducers to measure HV and LV voltages.


Position transducers to indicate the tap position.

DC voltage transducers to measure D .C. voltage.

48V DC contactors to initiate close and trip pulse to the breaker.

48V contactors to raise and lower taps on the OLTC.

220/24 V contactors for annunciating buch’olz and temperature alarms for each transformer.


Signal cables shall be of a type and installed in a manner so to reduce common-mode and induced noise to a minimum.

Metallic shields shall be employed on all cables. All grounding of shields and signal circuits shall be at one point only to prevent ground loops, except where double shielded cable is used, in which case the outer shield may be grounded at both ends.

05/03/23 s ganesh babu 56

All terminals and terminal blocks shall be clearly identified.

All connectors, cable harnesses and inter – rack wiring shall be clearly identified so that no ambiguity can occur.

Connector pin numbering shall be identical at each and of the cable.

05/03/23 s ganesh babu 57

Supplies DC voltage to RTU and the contactors which activates the operation of OLTC and breaker operations.


Thank you all for a

patient listening

!05/03/23 59s ganesh babu

scada – rtu interface

Documents