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PQM Communication and Data Formats

Prepared by: SARMAD RIAZ

Technical Guidance:

Mr. NAVEED AHMED QUERESHI Mr. HAFIZ SHAHZAD

Submitted to:

Mr. ALI NAWAZ

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TABLE OF CONTENTS

SR #

TOPIC Page

1 Power Quality Meter

3

2 Communication

4

3 Data Formats

7

4 Unsigned Long Integer

7

5 32-Bit Floating Point Hexadecimal Number

8

6 Configuration of MVI94

9

7 Comparison b/w PQM and ISTAT 12

TABLE OF FIGURES

SR# TOPIC Page

1 RS-485 Specifications 4

2 Daisy Chain Configuration 5

3 Flow Diagram for PQM 6

4 Data Conditioning Logic for Unsigned Long Integers 7

5 Bit Distribution 8

6 Data Conditioning for IEEE 754 32-Bit Floating Point Numbers

9

7 Snapshot of Configuration Table 11

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POWER QUALITY METER

The GE Multilin Power Quality Meter is used for continuous monitoring of a single or

three phase system. It provides metering for current, voltage, real power etc.

Previously on each line 8 different transducer cards were installed to measure different

quantities such as

Phase Voltages

Line Voltages

Current

Power

The different lines that are being metered by the ISTAT transducers are

1. 220 kV Line-1

2. 220kV Line-2

3. Bay-1

4. Bay-2

5. GSU-Secondary Side (220 kV

6. SUT-Primary Side (220 k)

7. UAT-Primary Side (24kV)

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COMMUNICATION

Previously the multiple transducers were sending their signals as analog signals, thus

consuming eight different analog inputs. The PQM transmits its data via RS-485 serial

communication protocol. It connects with the Modbus interface module (MVI94)

installed in the IPU. 4 analog outputs are also available for direct interface with the PLC.

RS485

Standard EIA RS-485

Physical Media Twisted Pair

Network Topology Point-to-point, Multi-dropped, Multi-point

Maximum Devices 32 (32 drivers & 32 receivers)

Maximum Distance

1200 meters (4000 feet)

Mode of Operation Differential

Maximum Baud Rate

100Kbps - 10Mbps

Voltage Levels -7V to +12V (max), +/-6V (Commonly used)

Mark(1) Negative Voltages

Space(0) Positive voltages

Available Signals Tx+, Tx-, Rx+, Rx- (Full Duplex)

Connector types Not specified, Commonly Screw terminals

Modbus is an application layer (Layer) messaging protocol. It provides client/server

communication between devices connected on different types of buses and networks. The

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PQM 2 is always a Modbus Slave. The PROSOFT MVI94-MCM Module will be used as

the Modbus Master module. PQM 2 supports the Remote Terminal Unit (RTU) version

of Modbus.

2-wire RS-485 link, data flow is bi-directional and half duplex. Termination is done by

120 Ù resistor and 1nF capacitor. One data frame of an asynchronous transmission to or

from a PQM-2 consists of 1 start bit, 8 data bits and 1 stop bit resulting in a 10-bit

data frame. If multiple PQM-2 devices are to be connected then they will be connected

in a daisy chain configuration.

Modbus Functions Supported by PQM-2:

The following Modbus functions are supported by our PQM, but during normal operation

we only use 03h (Read).

03h: Read Set-points and Actual Values

04h: Read Set-points and Actual Values

05h: Execute Operation

06h: Store Single Set point

07h: Read Device Status

Prosoft MVI94-MCM

Modbus Master

PQM-2 Modbus Slave

PQM-2 Modbus Slave

PQM-2 Modbus Slave

PQM-2 Modbus Slave

Daisy Chain Configuration Modbus Master-Slave Link

RS-485 Link

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08h: Loop-back Test

10h: Store multiple Set-points

Flow Diagram for PQM

CT

PT

PQM

MVI94 IPU

Backplane

Flow Diagram

RS485

220kV Line

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DATA FORMATS

Unsigned Long Integer

Data Sent (from PQM) Data Received (by DCS) Data Displayed (by DCS)

High Word Low Word High Word Low Word

220000 (35B60h) 3 (0003h) 23392 (5B60h) 3 23392

221689 (361F9h) 3 (0003h) 25081 (61F9h) 3 25081

216589 (34E0Dh) 3 (0003h) 19981 (4E0Dh) 3 19981

195642 (2FC3Ah) 2 (0002h) 64570 (FC3Ah) 2 -966

The PQM sends data in form of different data formats, some of which are

F1 Unsigned Integer (2 Bytes [FFFF])

F3 Unsigned Long Integer (4 Bytes [FFFFFFFF])

F4 Signed Long Integer (4 Bytes [FFFFFFFF])

In case of an unsigned long integer that is 4 bytes long, the DCS interprets is at two

different signed 2-byte words. The table shown above depicts the data as it is sent and as

it is displayed.

Data Conditioning Logic for Unsigned Long Integers

High Word

Low Word

× 65536

< 0

No

Yes Low Word

Low Word

+

+

65536

Actual Value

8

To extract the original data out of the displayed data we multiply the High word by

65536. If the Low word is less than 0 then we add 65536 to the Low word otherwise we

do not alter the Low word. We add the result of Low and High word processing and

display the calculated value.

32-Bit Hexadecimal Floating Point Number (used by PAC)

The PAC Module provided by Siemens sent its data in the 32-Bit Hexadecimal Floating

Point Format. The IEEE 754 standard specifies a 32 Bit Binary Floating Point Number as

having:

Sign bit: 1 bit

Exponent width: 8 bits

Significand precision: 24 (23 explicitly stored)

The true significand includes an implicit leading bit with value 1 unless the exponent is

stored with all zeros. Thus only 23 bits of the significand appear in the memory format

but the total precision is 24 bits (equivalent to log10(224) ≈ 7.225 decimal digits). The

bits are laid out as follows:

IEEE-754 Bit Distribution of 32-Bit Floating Point Number

(-1)Sign × 2Exp

× Decimal

Exponent Fraction

Sign

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Data Sent

(From PAC)

Data Received Data Displayed

High Word Low Word High Word Low Word

139.0703125 17163(430Bh) 4608(1200h) 17163 4608

Extraction of Exponent and Decimal values from the Displayed Data for IEEE 754 32-Bit Floating

Point Number

The above shown method will remain true for all positive values, i.e. both the

High word and Low word are positive (MSB is 0). To check for negative values

we will need to make slight adjustments in the above Flow Diagram.

17163

4608

÷ 128 134.0859375

134 127 7

÷ 65536 × 0.007812262 0.000549299671875

+ 1

0.0859375 +

1.086486799671875

EXP

Decimal

High

Low

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CONFIGURATION OF MVI94 (MODBUS MASTER) MODULE

Before connecting a PQM to the MVI94 module we need to edit the configuration file of

MVI94. The configuration file is edited according to our requirements and loaded into

MVI94 (Modbus Master) module. When editing the configuration file most of the times

the only thing that we change is the configuration table. This table contains the source

and destination addresses and information about the amount of data and the action to be

performed on the data.

Parameter Address of Holding Register

of PQM (Hex)

Address of Holding Register

of PQM (Dec)

Average Current 0243h 579

Voltage Van (High Word) 0280h 640

Voltage Vbn (High Word) 0282h 642

Voltage Vcn (High Word) 0284h 644

Voltage Vab (High Word) 0288h 648

Voltage Vbc (High Word) 028Ah 650

Voltage Vca (High Word) 028Ch 652

3 Phase Real Power (High Word) 02F0h 752

3 Phase Reactive Power (High Word) 02F2h 754

3 Phase Power Factor 02F6h 758

Frequency 0440h 1088

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Snapshot of Configuration Table

Translating into simple English the highlighted command line states the following

�The command is enabled. Modbus Master will Read the value stored in

one register starting at address 644 [0284h] (Voltage Vcn) of the device at

Node 1 and place it in internal address 58 of the MVI94 Modbus Master

module.�

The internal addresses are configured during the installation of MVI94 module into the

DCS Backplane. They can be changed or reconfigured from the System window in

ORCA-View.

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COMPARISON b/w PQM and ISTAT

The basic reason for changing the ISTAT transducers is that they are obsolete.

COST:

We can connect 32 different PQMs on one RS-485 link with one MVI94 module. If we

use ISTAT transducers we would require 256 Analog Inputs to obtain equivalent amount

of data. The PQM would drastically reduce our system cost and simplify our system

design.

RELIABILITY:

Compared to the ISTAT transducers the reliability of PQM is less because in case of a

faulty communication link or a fault in the MVI module we would lose all our

parameters, whereas in the case of ISTAT as all the parameters are being transmitted

separately as analog signals so the probability of all the parameters being lost is very

remote.