Energy and Power Engineering, 2013, 5, 958-961
doi:10.4236/epe.2013.54B184 Published Online July 2013 (http://www.scirp.org/journal/epe)
Design of Distributed Temperature Measurement System
for Switchgear
Qingsong Yang, Jianfei Yang, Huaren Wu, Long Liu
School of Electrical and Automation Engineering, Nanjing Normal University, Nanjing, China
Received April, 2013
ABSTRACT
This paper develops a novel distributed temperature measurement system based on DSP and DS18B20 digital ther-
mometer. The real-time temperature of each node in the switchgear is obtained by several DS18B20s which are con-
nected on the 1-wire bus together. RS-485 master-slave communication protocol is used to centralize monitoring tem-
peratures of several switchgear cabinets. The system also has the function of temperature alarm. The operation of simu-
lation experiment has showed that the system is able to complete monitoring real-time temperatures in high voltage
switchgear.
Keywords: High Voltage Switchgear; Distributed Temperature Measurement; 1-wire Bus; RS-485 Master-slave
Communication
1. Introduction
With the advancement of technology, temperature moni-
toring has broad application in electrical equipment. As
one of the most important electrical equipment in substa-
tions and power plants, high voltage switchgear is under
long-term operation. Switch contacts, busbar connections
and some other parts in the switchgear produce heat eas-
ily due to aging or excessive contact resistance [1]. If
these heating spots could not be monitored effectively
and abnormal temperature rise could not be handled on
time, the electrical equipment would be damaged. To
make matters worse, it would cause fault and power out-
ages [2]. The automatic temperature monitoring and alarm
is very necessary to avoid the above-mentioned acci-
dents.
The traditional temperature measurement methods in-
clude temperature-indicating wax label, pt resistance ther-
mometry, infrared thermometry and so on[3]. Tempera-
ture-indicating wax label has difficulty in achieving on-
line temperature monitoring. Pt resistance thermometry
requires high insulation properties and pt resistance can
not be attached to high voltage equipment directly. In-
frared thermometry needs regular inspection of the staff
and the measurement result is susceptible to interference
by the surrounding electromagnetic environment [4]. Ow-
ing to the above reasons, a multipoint real-time monitor-
ing and alarm system based on DSP and DS18B20 is
proposed in this paper. The system can not only complete
real-time monitoring of the heating spots in the switch-
gear but also send an alarm signal to the user according
to the preset threshold. At the same time, the user can
obtain functions such as historical inquiry and graphical
display of temperature data and recording of temperature
changes when alarming by the host computer.
2. Operating Principle of System
The system is made up of monitoring computer, data
processing unit and temperature collecting unit. The
structure of the system is shown in Figure 1.
*Project 51177074 supported by National Natural Science Foundation
of China. Figure 1. Distributed temperature measurement system.

Q. S. YANG ET AL. 959
Four to eight temperature measurement nodes are in-
stalled in each switchgear to monitor temperatures.
Various temperature measurement nodes are fixed in the
switchgear and connected to a MCU by twisted pair ca-
bles, which makes up a temperature measurement unit.
MCU sends temperature data of the temperature acquisi-
tion unit to the data processing unit DSP through RS-485
bus. DSP is used to converse and store the temperature
data and make an alarm signal when the temperature is
higher than the preset threshold. An additional GPS tim-
ing module is mounted on RS-485 bus in order to adjust
system time at a certain interval. The alarming time can
be recorded accurately for the analysis of the fault condi-
tion. DSP can not only communicate with PC through
RS-232 serial port but also attach the system to other
microcomputer protective devices to form network.
Monitoring computer displays real-time temperature data
of the switchgear by software. The user can inquiry his-
torical temperature data and find where and when the
alarm occurs with the graphic user interface (GUI) on the
computer.
3. Implementation of Hardware Circuit
3.1. Temperature Collecting Circuit
DS18B20 is a 1-wire digital thermometer produced by
American DALLAS Company. It can produce a digital
signal recognized by microprocessor and send the tem-
perature data to microprocessor without any external
component[5,6]. The DS18B20 digital thermometer pro -
vides 9-bit to 12-bit Celsius temperature measurements.
It has an operating temperature range of -55℃ to +125
℃and is accurate to ±0.5℃ over the range of -10°C to
+85°C. The temperature of the switchgear can be meas-
ured directly by temperature sensors intalled in the
switchgear. Considering its advantages such as small size,
low cost, good output linearity, DS18B20 is suitable for
the temperature monitoring in the switchgear.
Each DS18B20 has a unique 64-bit serial code, which
allows multiple DS18B20s to function on the same
1-Wire bus. Thus, it is simple to use a MCU to control
many DS18B20s (generally less than eight) distributed in
the switchgear. The connection between DS18B20s and
MCU is shown in Figure 2.
P1.6 port of 89C52 MCU is connected to 1-wire bus so
that the microcontroller can achieve signal acquisition
and output control of multiple DS18B20s. A 4.7K re-
sistance pulled up to the power supply is connected to the
port.DS18B20 adopts the method of external power sup-
ply in order to convert temperature accurately. Its VDD
pin is connected to +5V power supply of the microcon-
troller. A 0.1µF bypass capacitor is connected between
VDD and GND of DS18B20 for suppressing voltage
surge of power supply. The temperature of each node in
the switchgear is transmitted by 1-wire bus, which
greatly simplifies system wiring and improved the stabil-
ity and interference immunity of the system.
3.2. Data Communication Circuit
In view of the fact that the switchgear is under compli-
cated electromagnetic environment, data processing unit
and temperature collecting unit communicate with each
other through RS-485 bus. Advantages of RS-485 are
simple structure, low cost, long communication distance
and fast data transmission speed. Thus, it’s widely used
in industrial field[7]. ADM2483 is used to convert TTL
level to 485 level during the communication between
DSP and MCU. ADM2483 integrates magnetic coupling
isolation in the internal, which can effectively avoid the
common mode interference on the bus. Figure 3 shows
the data communication circuit between DSP and MCU.
Figure 2. Temperature collecting circuit.
R
E
R
E
R
E
R
E
Figure 3. RS-485 master-slave communication circui t.

Q. S. YANG ET AL.
960
TMS320F28335 is a floating-point digital signal
processor launched by TI Company[8]. It has three SCI
modules, namely SCIA, SCIB and SCIC. The serial port
of MCU and SCIB of DSP are connected to TXD and
RXD of ADM2483 in one-to-one correspondence. RS-
485 works in half-duplex mode. At any time, only one
node on the bus should be in sending state. GPIO52 of
DSP and P1.7 of MCU are used to enable the transmitter
or receiver. When the output of GPIO52 and P1.7 are at
high level, DSP and MCU are in receiving state. On the
contrary, they are in sending state.RS-485 network uses
shielding twisted pair as transmission medium. A 120Ω
resistance at each end of the bus is installed to match the
network. The pull-up resistance connected to A and pull-
down resistance connected to B ensure that unconnected
ADM2483s are in the idle state. In addition, communication
isolation power should be used in the converting circuit
beside DSP in order to protect DSP from strong
interference on the bus.
Monitor computer and data processing unit communi-
cate with each other through RS-232 serial port. SCIC of
DSP is connected to COM1 of monitor computer with a
MAX232 conversion chip.
4. Implementation of System Software
4.1. Temperature Collecting Circuit
Each temperature collecting unit is made up of a MCU
and several DS18B20 nodes. Each DS18B20 has a
unique ROM code, namely 64-bit serial code. Obtain the
serial code by MCU first and store it into the nonvolatile
memory of MCU before the DS18B20 is connected to
the system. The MCU distinguishes one DS18B20 node
on the bus from another by the serial code and reads the
temperature that corresponds to each node. For the pur-
pose of illustration, number the nodes sequentially from
1 to N. As there is only one data wire, the communica-
tion between DS18B20 and DSP must be accomplished
in accordance with strict timing requirements.
The program procedures that MCU of each tempera-
ture collecting unit reads multiple DS18B20s are as fol-
lows:
1) MCU resets all DS18B20 nodes;
2) MCU issues a skip ROM [CCH] command to all
nodes;
3) MCU issues a convert T [44H] command to all
nodes, which converts the resulting thermal data into
binary scale and stores the binary in the scratchpad
memory;
4) MCU resets all nodes again;
5) MCU issues a match ROM [55H] command to all
nodes;
6) MCU issues the 64-bit ROM code sequence of No.1
DS18B20 to all nodes;
7) MCU issues a read scratchpad [BEH] command to
all nodes. Only No.1 DS18B20 on 1-wire bus transmits
low byte and high byte of the temperature sequentially to
MCU. Thus, reading the temperature of No.1 DS18B20
is accomplished;
8) Repeat the procedures from 4 to 8. Issue the 64-bit
ROM code sequence of the remaining DS18B20s se-
quentially to all nodes and read the temperature one by
one. Thus, temperature measurement of a temperature
collecting unit is over.
4.2. RS-485 Bus Communication
In the distributed temperature measurement system of the
switchgear, RS-485 master-slave network is adopted for
the remote communications.All the communications on
the bus are controlled by DSP. A command / response
mode guided by DSP prevents the bus from data race. As
there are multiple MCUs on the bus,it is necessary to
assign different slave address codes to them. Figure 4
shows how DSP communicates with different MCUs for
temperature measurement.
Figure 4. Process of communication between DSP and
MCU.

Q. S. YANG ET AL.
961
Figure 5. Running interface of simulation experiment.
Data communication is always started by DSP.DSP
issues a communication command consisting of slave
address code and temperature measurement command
word to all MCUs and is converted into receiving state
from sending state soon afterwards. After receiving the
command, each MCU checks if the address code is in
accordance with native code.If there is a match, the
corresponding MCU will be converted into sending state
from receiving state and perform temperature measure-
ment command.The remaining MCUs keep the receiving
state for the next communication command.The matching
MCU reads temperature data from all DS18B20 nodes of
the unit.Then the data are transmitted to DSP in order of
slave address code, temperature measurement command
word ,high byte and low byte of each node from 1 to N.
Finishing the data transmission, the MCU is converted
into the receiving state again for the next communication
command. DSP deals with the data from MCU and store
them in the corresponding memory cell.Afterwards DSP
is converted into the sending state in preparation for
initiation of next data communication.
4.3. Host Computer Interface
The monitoring interface is compiled with C++ Builder
which is a visual integrated development tool launched
by Borland Company. C++ Builder makes use of serial
port communication control functions from Pro Pcom
library to realize the communication between PC and
DSP by way of function call.Monitoring computer reads
temperature data stored in DSP through RS-232 serial
port with function of display temperature value
online,historical data inquiry and alarm.
Figure 5 shows the running interface of simulation
experiment after the whole system is completed. The
temperatures of all nodes in several switchgear cabinets
can be displayed on the host computer. When the
temperature of one node in the system surpasses the
reset value, DSP will make a warning sound and send
the alarm signal to host computer. The indicating signal
light behind the node will turn from green to red. The
user can find where the alarm occurs exactly from the
computer. The user can also choose the switchgear
number, node number and specific time to query on the
historical data and view the corresponding temperature
curves.
5. Conclusions
The paper presents a master-slave communication
network on basis of RS-485 bus to monitor real-time
temperatures of the switchgear.RS-485 is suitable for a
lot of unfavourable conditions such as harsh
electromagnetic environment,long distance between
temperature site and control room, dispersed wiring, etc.
What’s more,the wiring in the switchgear is reduced
significantly via 1-wire bus technology.Thus, the system
has the advantage of high measurement accuracy, low
cost and strong anti-jamming capability. Online
monitoring temperature makes the switchgear operate
safely. The operation of simulation test shows its good
feasibility and application value.
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