Cellular communication based remote predictive maintenance system for power transformers

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International Journal of Electrical Engineering & Technology (IJEET)
Volume 6, Issue 7, Jul-Aug, 2015, pp.48-60, Article ID: 40220150607005
Available online at
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ISSN Print: 0976-6545 and ISSN Online: 0976-6553
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___________________________________________________________________________
CELLULAR COMMUNICATION BASED
REMOTE PREDICTIVE MAINTENANCE
SYSTEM FOR POWER TRANSFORMERS
Pallav Gandhi
M. Tech Student, Instrumentation and Control Department,
Nirma University, Ahmedabad, India
Dipak Adhyaru
Professor, Instrumentation and Control Department,
Nirma University, Ahmedabad, India
ABSTRACT
The Insulating Paper and Pressboards are used to insulate the windings of
transformer. These are mostly made from a Cellulose material. It must have a
high tensile and dielectric strength. Over the era the insulators have slowly
degraded due to ageing, high temperature and chemical reaction such as
oxidation, pyrolysis and hydrolysis. They are degraded to a point, and now
there is no extensively effective insulator. Mainly two parameters are affecting
on life of insulator and that is load current and ambient temperature.
This Paper proposes a system which estimates loss of life of Transformer.
The proposed system is composed of Energy Meter, Embedded ICM that is
heart of the system, and GPRS Gateway. It is installed at the power
transformer site. Here the three phase Energy meter is being used as
MODBUS slave, having electrical parameters such as voltage, current, power
factor, power, etc. All these parameters are sent to the MODBUS master via
MODBUS RS485. The Embedded ICM contains the Microcontroller with
MODBUS Master Function, Advance Insulation Ageing Algorithms, Power
Supply, and RS485 Communication Circuit. The Advanced Insulation Ageing
Algorithms computes the Top-oil Temperature, Hot-spot temperature, Ageing
Rate, and Loss of insulator life. This algorithm is based on IEC 60076-7
standard differential equation and all these things (algorithm) are
implemented in MSP430F5419A. These estimated parameters will be sent to
GPRS Gateway via RS232. GPRS Gateway will connect to remote location
using GPRS technology and Provide data to Remote Server. Remote Server
will have SCADA system so the data can be stored for multiple years and same
data will be utilized for advanced algorithm development.

Cellular Communication Based Remote Predictive Maintenance System For Power
Transformers
Keywords: Transformer, Hot-spot temperature, Insulation, GPRS Gateway,
Energy Meter, MODBUS Master, MODBUS Slave, Ageing Rate, Loss of Life
Cite this Article: Pallav Gandhi and Dipak Adhyaru, Cellular Communication
Based Remote Predictive Maintenance System for Power Transformers.
International Journal of Electrical Engineering & Technology, 6(7), 2015, pp.
48-60.
http://www.iaeme.com/IJEET/issues.asp?JTypeIJEET&VType=6&IType=7
_____________________________________________________________________
1. INTRODUCTION
The insulation paper and pressboard is made from high grade of cellulose. Cellulose is
an organic polymer [1] whose monomer is made of long chain of glucose ring. The
tensile strength of insulator is measured by the degree of polymerization value. The
average number of glucose ring per chain is called the degree of polymerization [1-2]
and it can be used to monitor the ageing of paper. The typical DP (Degree of
Polymerization) value for unused insulator is around 1000 to 1200 [1]. When the
insulator ages, the glucose long chain breaks into smaller chains; gradually the DP
value falls. If the DP value of insulator is found to be 200 or less then that [1-2] then it
is generally considered to be its end of life.
2. INSULATION LIFE
The insulator is continuously suffering from the electrical, mechanical, thermal, and
chemical stresses [3] during its operation. In this paper, the insulator life prediction is
mainly focused on thermal degradation of paper insulation. According to the IEEE
C57.91 the normal life of the insulator is 20 to 21 years [4]. The life of insulator will
be defined for the given temperature of the insulator. The total life between the initial
state for which the insulation is considered new and final state for which electrical
failure, dielectric stress and mechanical [1] movement occurring in normal life have
been considered. So it is essential to select good cellulose paper having good
mechanical strength. The electrical and mechanical strength of insulator is mainly
dependent on the ambient temperature, operating load, hot-spot temperature and
different physical and chemical process of the transformer [4].
3. AGEING FACTOR
The dielectric strength of insulator mainly depends on the moisture content of the
insulator, temperature, and the content of the oxygen and acid. The temperature of the
insulator is main factor for ageing agents [5-6]. The maximum temperature of the
insulator is used for calculation of ageing of insulator. The insulation ageing is very
complex process. Due to rapid increase in load leading to increase in hot-spot
temperature causing the thermal decomposition of insulator [7]. There are mainly
three mechanisms which contribute to cellulose degradation i.e. hydrolysis (water),
oxidation (oxygen), and pyrolysis [1, 8-10] (thermal degradation).
3.1. Hydrolysis
The ingress of water from the atmosphere or ageing of insulation paper causes the
breaking of the glucose chain [9], i.e. The oxygen bridge between glucose rings is
affected by water molecules, causing a break in the cellulose chain and creates two –

Pallav Gandhi and Dipak Adhyaru
OH groups, each attached to its ring. The Result is degradation in DP value and
weakening of the insulator [9].
This also leads to decrease in the mechanical life of the insulator by half for every
two molecule of water [1]. Therefore, the thermal deterioration of insulator is
proportional to the water particles. For example, reducing the water particles of the
insulator from 1% to 0.5% doubles the life of the insulator. According to Lundgaard
study from SINTEF energy research shows that if the insulator’s normal life is
defined as ageing under well dry and oxygen free conditions, for 1% water contents in
insulator the life of the insulator decreases t to 60% of the normal life. If the water
content increases to 3% - 4% then the life assessment will fall to 25 % of normal life
[10].
3.2. Oxidation
The main source of oxygen molecules comes from either atmosphere or from the
thermal degradation of insulator. Oxygen molecule attacks [9] the carbon atom in the
glucose monomer and form aldehydes, carboxylic acids, and ketones. The bonds
between rings are weakened which causes degradation of DP [10].
3.3. Pyrolysis (Effect of Heat)
The main source of heat generation in the transformer is due to winding temperature
and this is termed as hot-spot temperature. This heat will cause the breakdown of
individual monomer of the long chain [9]. The insulator decomposes rapidly if its
temperature is above 140 . Sometimes due to the high temperature there is reduction
in DP value of insulator and the insulator becomes brittle; the result is solid residue
and gases are formed. The formations mainly consist of carbon dioxide, hydrogen,
water vapour and carbon monoxide. Further glucose rings decomposes to other
compounds called furans [10].
4. INSULATOR AGEING RATE
The ageing of Insulator is a time function depending on the temperature, oxygen
content, moisture content, and acid content. The temperature distribution in the
insulator is not uniform, [6, 9-10] making it a complex process. The part which is
operating at the highest temperature undergoes [8] the highest deterioration.
Therefore ageing rate is the rate at which the deterioration of insulator for a hot-
spot temperature is accelerated compared with the degradation rate at the reference
hot-spot temperature. The relative ageing rate V is defined by equation (1) for non-
thermally upgraded paper and equation (2) for thermally upgraded paper [11].
(1)
(2)
Where,
is a hot-spot temperature in
110 is rated reference hot-spot temperature for thermally upgraded paper
98 is rated reference hot-spot temperature for non-thermally upgraded paper

Transformers
5. INSULATION LOSS OF LIFE
The loss of life over a period is equivalent to life consumed by the insulation in hours
or days during that period [9]. Mathematically it is calculated by integrating the
relative ageing rate over certain period of time.
The loss of life (L) over period of time is equal to [11]
(3)
And in discrete-time form over a number of time intervals,
(4)
Where,
is relative ageing rate during interval n
is nth time interval
n is number of each time interval
N is total number of interval during the period
6. MODBUS RTU PROTOCOL
MODBUS is used for serial communication protocol. It is derived from the
master/slave architecture. The master may communicate to one or more slave devices
on different slave address. Always only a master device can initiate the
communication to slave devices on MODBUS network. Slave device can only
respond to the request data from the master. MODBUS RTU (Remote Terminal Unit)
is the MODBUS protocol used on a serial line communication with RS-232 and RS-
485 port as the interface.
To initiate communicate with slave device, a master device sends a frame that
contain following data as also shown in Fig.1:
Figure 1 Modbus RTU Frame Structure
 Slave device address
The device address can be in a range of 0 to 247 addresses. Addresses 1 to 247 are
applicable for specific slave devices and Address number 0 is used for broadcasting
frame received by all the slave devices.
 Function code
The function codes defines the command that tells the slave device what kind of
action takes place, such as read and write data.
 Data
The data defines addresses in the device’s memory map for reading functions.It
contains data values to be written into the device’s memory.

 Error Check
The error check is a 16-bit numeric value representing the cycling redundancy
check (CRC). The CRC is generated by the master device and being checked by the
receiving device. If the CRC code does not match with code of the receiving devices
then the receiving device asks for a retransmission.
7. NOVEL IDEA TOWARDS TRANSFORMER CONDITIONING
MONITORING SYSTEM
This paper presents a new system that employs the present technologies, coupled with
wireless GPRS Gateway, to provide data acquisition to the end user and then analyses
the received data from the GPRS module. The main aim for developing this system is
to provide real time data to the end user.
The main objective of this system is to monitor the insulator ageing rate and hot-
spot temperature. The ageing of insulator is mainly affected by changes in
temperature and electrical operation characteristics of transformer. The failure of
insulator when in use is mainly due to the temperature rise, ageing, over load, and
improper installation and maintenance. Out of these factors temperature rise and
ageing rate of insulator needs continues monitoring to save the insulator life.
8. EXPERIMENTAL
The Main Objective of this system is used to monitor the condition of the transformer
insulation system. The wireless GPRS based device being used works for transformer
insulation conditioning monitoring system. The transformer insulation conditioning
monitoring system will monitor following parameters:
 Top-Oil Temperature
 Hot-spot temperature
 Ageing rate of insulation
 Insulation loss of life
The above parameter are calculated as per the IEC 60076-7 standards. Fig.2
contains the complete block diagram of the transformer insulation monitoring system,
consisting of Energy Meter, embedded ICM (Insulation Conditioning Monitoring),
GPRS Gateway, and SCADA for ICM (Insulation Conditioning Monitoring) System.
Figure 2 System Block Diagram

Transformers
8.1. Energy Meter
The 3-phase Energy meter is used to measure electrical parameters in power
transformer. For this system Schneider Conserve EM 1200 series power meter is used
[12]. It has an integrated serial RS-485 Modbus RTU interface. This facility allows
direct reading of all applicable parameter such as voltage, current, power, power
factor, etc... Due to this advantage the meter can measure the main electrical
parameter and make them available via COM port. This COM port allows connecting
meter remotely. The meter has unique address from 1 to 247 (up to threedigits). This
will allow communication from one port to master device. The meter never initiates
communication it can only respond.
Table 1Parameter of Master and Slave Station
When master device wants the electrical parameter from the meter, the master
device sends messages that contain the meter address and the parameter it wants and
checks the sum for error detection.
In order to implement the MODBUS Protocol communication between Energy
meter and Master device, first of all we should configure both MASTER Device and
Energy meter for the same communication mode and same BAUDRTE that is 9600
BPS (bits per second) [13]. Configuration is shown in table 1.
8.2. Embedded ICM
Embedded ICM (Insulation Condition Monitoring) consist of MSP430F5419A
microcontroller [14] with MODBUS master and Advanced Insulation Ageing
Algorithms, Power supply and RS485 Communication Circuit. As discussed earlier
the electrical and mechanical strength of transformer insulation is mainly dependent
on the ambient temperature and operating load of the transformer.
Here, MODBUS master is not device but it is a Function that is implemented in
the Microcontroller. The MODBUS Master function will gather data from MODBUS
RTU (RS 485) based Energy Meter. MODBUS Master will poll the collected data for
Voltage, Current, and Power Factor, Power etc. for the calculation of Hot-spot
Temperature, Ageing Rate, and Insulation loss of life, the Load (Current) is the most
important parameter. So MODBUS Master Function will provide these data to the
Microcontroller on UART (Universal Asynchronous Trans receiver).
For ambient Temperature measurement the microcontroller having in built
temperature monitoring sensor will be used for ambient temperature monitoring. Due
to this advantage system doesn’t require extra sensors to measure ambient
temperature and it will make the system cost effective.
Parameter Type Settings
Communication Mode RS485 Modbus
Baud Rate 9600
Parity None
Delay (ms) 100
Slave Address 1~247
Select Literacy Read
Stop bit 1
Read Number (Byte) 255

In addition to this for the calculation of Hot-spot temperature some other
parameters also required that is based on the power transformer specification sheet.
These parameters will be provided by user input from the specification sheet in Table
2.
Advanced Insulation Ageing Algorithms consist of the IEC 60076-7 standards
based differential equation [11] that is implemented in MSP430F5419A
microcontroller.
There are two methodologies to deal with hot-spot temperature. Either to measure
it or to calculate it. But measuring of hot-spot temperature can prove to be costly to
the system. For this reason, several models [4, 11] are used for prediction of hot-spot
temperature. For prediction of Hot-spot temperature IEC 60076-7 [11] Standards is
used in this paper.
Based on Ambient Temperature, operating Load of Power transformer and power
transformer specifications the Top-oil temperature and Hot-spot temperature will be
calculated. This is as per the IEC 60076-7 Standards. Once Hot-spot Temperature is
derived; based on that one can easily calculate Relative ageing Rate as Per the IEC
60076-7 Standards [11]. If integrated value with defined sample time interval of
Ageing Rate one can calculate insulation loss of life as per IEC 60076-7 standards
[11].
Once all these parameters such as operating load of power transformer, Ambient
Temperature, Top-oil Temperature, Hot-spot Temperature, Relative Ageing Rate, and
Insulator Loss of life are derived; then that will be transmitted to remote server via
GPRS Gateway. The remote server is located at particular central zone. Remote server
is having a facility for further analysis and load shedding as well as predictive
Maintenance, all these will be taken care for the transformer.
Table 2 Power Transformer Specification Sheet [11, 15]
Parameter Type Description
Rated Power 20/27 MVA
Voltage (High voltage/ low voltage) 66/22 kV
Cooling ONAN-ONAF
System Frequency 50 Hz
Oil time constant (τ0) 150 min.
Winding Time Constant (τw) 7 min.
Winding Exponent (y) 1.3
Oil Exponent (x) 0.8
Loss Ratio (R) 8
Thermal Time Constant (K11) 0.5
Thermal Time Constant (K21) 2
Thermal Time Constant (K22) 2
Top-oil temperature rise above ambient (∆θor) 45
Hot-spot temperature rise above top-oil temperature (∆θhr) 35
Power supply unit will provide power supply to the microcontroller, GPRS
Gateway, and RS485 Communication Circuit. The RS485 Communication circuit will
connect UART port of the microcontroller and RS485 port of the energy meter. I.e. it
will provide interface between UART port and RS485 port.

Transformers
8.3. GSM/GPRS Gateway
GSM/GPRS Gateway will connect remote location using GPRS technology and
provide data to the Remote Server. For this system here HK GPRS Gateway device is
used. This GPRS Gateway will provide interface between wireless output and
MSP430F5419A Microcontroller. The main role of the GPRS Gateway in this system
is to transmit all the parameters to the remote server.
8.4. Remote Server
Remote Server will store operating load of transformer, Ambient Temperature, Top-
oil Temperature, Ageing Rate, and Insulator loss of life data. For this system HK
SCADA System is used. These data will be used for analysis purpose. Remote server
has SCADA system so that data can be stored for many years and the same data will
also be applicable for the development of advanced algorithm and for further analysis.
HK SCADA system performs the critical function of data collection, storage, and
processing, display and report generation. It gathers the plant floor data collected by
distributed GPRS, ZIGBEE, Wi-Fi, RF or an in-plant Ethernet / RS - 485 networks,
and stores it in a central database.
9. EXPERIMENTAL IMPLEMENTATION
As earlier discussed the time-varying load factor (K) and Ambient temperature (θA)
are used for finding the Hot-spot Temperature [4]. Fig.3, inputs are load factor K, and
Ambient Temperature θA on the left side. The desired output is Hot-spot Temperature
θh on the right side. The Laplace variable s is essentially the derivative operator d/dt.
Figure 3 Block Diagram that represents the differential equations

The second block of the uppermost part represents the Hot-spot Temperature
dynamics. The first term of second block is k21 that represent the fundamental Hot-
spot Temperature rise. And second term of that block is k21-1 that represents changing
the rate of oil flow. The combined effect of these two terms will sudden rise in high
peak in Hot-spot Temperature [11]. The values for k11, k21,k22, are described in the
above table 2. If you have measured Top-oil Temperature then that will be directly
fed through the switch and that will be shifted to the right side; the Top-oil calculation
is not required.
The differential equation for finding Top-oil Temperature θo is mention below
[11]. For this equation inputs parameters are K, and θA and output Parameter is θo.
(5)
The differential equation for finding Hot-spot Temperature rise ∆θh is sum of two
differential equations [11], for this equation input parameter is K and output
parameter is ∆θh.
Where,
(6)
The two equations are
(7)
And
(8)
The final equation for the finding the Hot-Spot Temperature θh [11] is sum of the Top-
oil Temperature and Hot-spot Temperature rise.
(9)
For finding Relative Ageing Rate and the Insulation Loss of life both these equations
are broadly described in the above section.
These above equations are used for finding Top-oil Temperature, Hot-spot
Temperature, Ageing Rate, and Insulation Loss of Life. All these are implemented in
algorithm form in the MSP430F5419A Microcontroller and the output results of that
particular parameter is shown Fig.4 in the SCADA system.
10. RESULTS
The Transformer Real time load current data is provided by the Energy Meter via
MODBUS RTU. The estimated top oil temperature is used to determine the hot-spot
temperature parameter
Hot-spot parameter is used to determine Ageing Rate and Loss of Life of
transformer. At the end of the all these, the parameters will be transmitted to the
SCADA remote server via GPRS Gateway.
Fig.4 shows the different real time indicators in the main page of the SCADA.
There is a difference in a Rated loss of life and actual loss of life of the transformer.
The rated working hour loss of the power transformer is 1 hour but in the Actual, loss
of life of transformer is 2 hour during this working hour

Transformers
Figure 4 SCADA Main Page that Show Transformer Real time Indicators
Figure.5 SCADA Trend Page for Ambient Temperature and Hot-spot Temperature
The effect of the increase in the load profile of the Power transformer and ambient
temperature of the environment can be seen in the Fig.5 and Fig.6 As previously
discussed in the section 9, the load current and ambient temperature must have effect
on the hot-spot temperature of the power transformer. Fig.7 shows the increase in load
current and ambient temperature increases the hot-spot temperature of the
transformer.

Figure.6 SCADA Trend Page for Rated Ageing and Actual Ageing
Figure.7 SCADA Trend Page for Rated Current and Actual Current
From the Fig.5 it is concluded that when there is an increase in the hot-spot
temperature (shown in Fig.5) the ageing of the transformer also increases. Because of
the ageing and hot-spot temperature are linearly proportional to each other.
HK SCADA has a facility to show real time parameter in web client site. So, Fig.8
shows all estimated parameters on SCADA web client site. Similarly Fig.9, 10 and 11
shows group wise parameters on SCADA web client site

Transformers
Figure.8 SCADA Web client Page which shows all Estimated Parameters
Figure.9 SCADA Web client Page which show Particular Group wise Parameters (Group-1)

ACKNOWLEDGEMENT
I am very grateful and would like to thank all the member of NOVATRICE
Technologies (P) Ltd. And HARIKRUPA AUTOMATION (P) Ltd. for their advice
and valuable support without them it would not have been possible for me to complete
this paper.
I would like also thank to all my friends, colleague and classmates for all the
thoughtful and mind stimulating discussions we had, which prompted us to think
beyond the obvious
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Transformers: Part I - History and Development, in Electrical Insulation
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Transformers: P art II – Maintaining Insulation Integrity and Life, IEEE
Electrical Insulation Magazine, 22(2), 2006, pp. 5-14.
[3] Yuan Li, Ming-jie Tang, Jun-bo Deng, Guan-jun Zhang, Shu-hong Wang, An
Approach to Aging Assessment of Power Transformer Based on Multi-
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[4] IEEE Standard, (C57.91-1995) Guide for Loading Mineral-Oil-Immersed
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[10] Mohad Taufiq Ishak, Simulation Study on Influencing Parameters of Thermal
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[15] Technical Specification for 220/132 kV, 150 & 100 MVA; 132/66 kV, 50 & 100
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