Post on 12-Mar-2015
description
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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)
4
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
6
08h: Loop-back Test
10h: Store multiple Set-points
Flow Diagram for PQM
CT
PT
PQM
MVI94 IPU
Backplane
Flow Diagram
RS485
220kV Line
7
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
11
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.