Download Structure Design of On-line Monitoring System for Power Supply Insulation

Structure Design of On-line Monitoring System for Power Supply
Insulation
ZHANG Kai, HU Shunbin
College Mechanical and Electrical Engineering, Agricultural Electrification and Automation,
Agricultural University of Hebei, P.R.China, 071001
[email protected]
Abstract Modern national economy have more and more dependence on power supplying. Therefor,
the requirement for safe operation and power supply reliability of electric power system is higher and
higher. As one of the most important measure for reliable and safe running of power system, insulation
preventive test for electric power system plays a crucial role in checking and supervising of transformer
insulation. With the continuous development of the power industry, the defects and weak points of
traditional insulation preventive test is becoming more and more obvious. So it has important realistic
meaning to develop on-line monitoring technology of insulation.
Key words On-line Monitoring, Dielectric Loss, Simultaneous Sampling, Digital Filter
1 Introduction
With the development of electric power industry, the rank of voltage and the workload of the
insulation preventive test have been increasing constantly, and the weak points of traditional test
measure is being unfolded step by step. For example: equipment need quit running before
experimentation and set up temporary line; test voltage is so lower than operation voltage that can not
reflect the equipment 's credit condition of insulation; the test cycle is long, and much manpower and
material resource are wasted. The appearance of on-line monitoring technology of insulation makes
on-line diagnosis for insulation be carried out, which makes up well the shortage of traditional measure
efficiently. At present, the commercial products of this kind of on-line monitoring system for substation
has been developed by western developed countries, and the tentative standard for condition-based
maintenance of some equipment has been summarized. Power supply companies in China attach great
importance to the development of the on-line monitoring technology. Despite late start, its development
is quite fast in our country. But at present, some important items of on-line mornitoring such as
technology standand, product designing, monitoring items, acceptance index has not been unified in
China. In this paper, on-line monitoring of insulation for high voltage equipment in large-scale
substation or power plant will be the study object and a kind of on-linemonitoring system of insulation
will be mainly interpreted.
2 General Design of The System
2.1 Monitoring Content of The System
There are a lot of varieties of electrical equipment and they are different in structure, and each of
them has different on-line monitoring items. The main equipment and their main monitoring items are:
⑴Transformer: Chromatographic analysis for gases in oil-filled and its on-line monitoring for
partial discharge. ⑵GIS(gas-insulated metal-enclosed switchgear): Differentiate the partial discharge
signal with the others by using UHF Method, then differentiate the fault types based on the spectrum
characteristics of the signal and the location on AC voltage waveform when discharging. ⑶Capacitive
Equipment(include capacitive bushing, transducer, coupled capacitor):The value of dielectric loss and
capacitance. Correct the error led by the changing of temperature, humidity and load.⑷Arrester:
Resistive current of metal oxide surge arrester without gaps(MOA). ⑸Circuit Breaker: Recording the
journey～time curve when the circuit breaker switching. Extracting the mechanical motion parameters.
Monitoring the voltage and current waveform of the operating coil. Monitoring and analyzing the
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mechanical vibration signal out of the body of the circuit breaker. Measuring the leak current of tension
and rising bar. Measuring the resistance of static loop and dynamic loop.
2.2 System Structure
The on-line monitoring system of insulation for substation can be generally divided into six
modules as extraction of signals, signal pretreatment, synchronous sampling, digital signal processing,
main controller and computer software for management. A element class is constituted with
appropriative sensor, cable for analog signal transmission and multiple gated plate, which can be
regarded as a structure designed for concrete and different object. The object can be a group of same
type equipment (the three phase of A, B, C in the same group), several group of same type equipment, or
the phase A of different type equipment. It can be designed flexibly base on the different condition of the
substation. One of the element class’s failure don’t influence on the others, that the reliability of the
system is improved. On the purpose of industrialization and reducing cost, signal is centralized sampling
and processing. So a type of cable for signal transmission is customized to ensure that signal can be
transmitted unfailingly. The system structure shows as the following figure:
CT
CT
Passive
Sensor
Signal
Pre-Processing
Wave
Amplifying
CT
Multi-Channel
Gating
MOA
MOA
Synchronous
Sampling
Circuit
Active
Sens or
Signal
Pre-Proces sing
Wave
Amplifying
Signal
Processing
and
Master
Module
MOA
Phase locked and
frequency
multiplying
Reference
Voltage
MIS
System
Amplifying
and
Re-shaping
Voltage
Sensor
Dialing
and Fiber
Communication
Computer
Serial
Communication
Alarming
Printing
Figure 1
The signal processing module and master module determine which equipment to be monitored
based on the command sent by computer, then gating the line by relay and monitoring the equipment
which has been selected. The signal which reflect the insulation status is caught by a type of
high-precision sensor. The output signal of sensor is not only weak, but seriously interfered. If sampling
it directly, the sampling frequency must be enough high to ensure that the signal isn’t distortion. But if
do so, amount of calculation of digital filter will be ncreased and the digital filter effect will be
influenced. So the signal must be pre-processed before sampling. Because the system need high
phase-precision, we designed a synchronous sampling circuit to sampling multiplex signal in the same
time. This method can eliminate the extra phase difference come from sampling channels switching. By
synchronous sampling, fixed phase difference between voltage channel and current channel can be work
out accurately, and which can eliminate the phase-shifting out from signal channels to some extent by
subtracting the value when calculating dielectric loss tangent. Do Digital filtering and spectrum
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analyzing with the sampling signal, then work out the parameters of on-line monitoring for insulation.
At the end, send the parameters to computer to be processed, analyzed and diagnosed through serial
communication module. Then the computer can make data report and change tendency curve of
equipment insulation status, give the limit alarm of parameters changing, record the data and send it to
on-line MIS system through optical fiber network(or MODEM) simultaneously for administrators’
browse and query.
3 Design of Slave Microcomputer System
Because different substation has different amount of equipment, the amount of channel on the slave
microcomputer system can’t be determined priorly. The system is designed in modular structure, which
could be devided into control module, power module, signal processing module, pre-channel module
and backplane. All the other modules are pluged in the backplane, so the backplane is regarded as the
bus plate which connect all the other modules on it. See the structure of the system below:
Power
Module
Control
Module
Signal Processing
Module
Pre-Channel
Module
sensor
Computer
Figure 2
Power module supply the system with every kind of voltage source. Switch power is used in the
power module which can run at 16 265 V AC or DC imput. There are four kind of source can be
supplied by the power module, which has the very strong adaptability of power.
Control module is the core module of the device. The MCU which is used in the device is a highly
integrated 16 bit control chip, has independent watchdog circuit. There are high speed synchronous
sampling circuit and input surge suppression circuit in this module. Moreover, the module is furnished
with basic RS232 communication interface circuit, so bidirectional communication can be achieved
between device with the host computer and maintenance engineers can do verification and maintenance
on the system by special maintenance software through the interface.
Signal processing module accomplish the function of input signal amplification, signal
filter&signal channel selection, and impedance transformation. Operational amplifier with high ability
of common-mode rejection is used by the system. Moreover, enjoying about the same parameters and
working environment, every one of this kind of amplifier has about the same drift when some
environment parameters such as operating temperature has changed. A type of hardware low-pass filter
is designed in the module to pre-processing signal, to the effect that sampling and signal filter. The
signal channel selection circuit accomplish the function of selecting out of the signal required to be
monitored among plenty of input signal.
Pre-channel module has the main function of sending the signal which out from sensor to the signal
processing module. Different substation has different amount of equipment, the amount of channel of
the system can’t be known priorly. So we only preset 32 channel on the pre-channel module, and preset
8 faucets for pre-channel module. If the substation has only small amount of equipment, no more than
two or three the module will be need.
The slave microcomputer device of the system has the complete function of software and hardware
self-checking and independent WATCHDOG circuit, so it possess the strong ability of preventing
interference; support multiplicate communication methods, offer RS232 or RS485 communication
interface, can configure computer network interface based on the user requirement too. Hardware
modularization make it be configured flexibly and expediently based on the user requirement and the
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system can be expediently maintained
4 Design of The Management Software for Computer
The software accomplish the function of on-line monitoring system of insulation for substation,
they are system register, permission management, dipatching management, data management,
communication management. See the figure below:
Entry
Software
System
Permission
Management
Dispatching
Management
Data
Management
Communication
Management
Figure 3
Permission management is to prevent that irrelevant personnel who isn’t allowed modifying the
related configuration and monitoring data. Dipatching management can set what equipment to
monitoring and the monitoring time, and offer the operation state, dispatching status of the monitored
equipment. Data management is used to record, upload, copy, list print and display the monitoring data
of each equipment and terminal. The data can be displayed in several kinds of forms such as table,
figure, and so on. So people can judge the insulating performance of the equipment by the data to
eliminate the latent danger in it’s bud. Communication management is constituted by two part. One is
the communication with the slave microcomputer which sent monitoring parameters to this computer,
and the other is the communication with the superior main control room and electric company. The
second is the most important one. So the electric company can confirm the operation status of
equipment by remote browsing the monitoring data of the substation. In addition, the system can timed
patrol the insulation status of equipment, and alarm once the monitoring data exceed the threshold value.
The software has primary failure diagnostics function, so it can do simple fuzzy logic reasoning on the
monitoring data to judge if the equipment is in failure. If so, it will alarm and offer the possible reason
of the problem.
5 Conclusion
The accuracy of the measurement is a key index which determines the device is putting into
operation or not in future, and eliminating the interference of voltage fluctuation and harmonic is a
critical problem in the capacitive sample measurement. The design of high-accuracy double shielding
passive sensor solve the problem well.
By comparing with others, we find the scheme of on-line monitoring system for insulation
mentioned in this paper has characteristic of : ⑴Self-correcting technique. It can eliminate the error of
discreteness and drift when measuring the element parameters. ⑵Multichannel synchronous sampling
technique. It can eliminate the stochastic disturbance of the system and ensure the measurement
accuracy. ⑶Tendency comparative method united with relatively comparison method to analyzing
equipment status. ⑷Quick phaselock frequency automatic tracking technique. The system can track the
changing of frequency, sampling accurately after frequency fluctuating. ⑸Using fast anti-aliasing
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Fourier transform algorithm(FAF"I') to eliminate the error led by frequency aliasing. ⑹Multi-tier
distribution structure. The system possesses characteristic of good extension, can meet the need of every
kind scale of substation of 35KV and above with different amount of equipment. ⑺ Using
modularization design method. Card can be changed on line. System composition can be configured
flexibly. We can add monitoring items and alter monitoring function at any time.
As one of the most potential power system technology, the research direction of on-line
mornitoring technology will be centralization, high intellectualization and high precision, and it will
enjoy undoubtedly broad prospects in applications as the software and hardware technology
developing at very fast spreed.
References
[1]Zhu Deheng,Tan Kexiong. Electricity Diagnostic Technique for Insulation. China Power Press, 1999
[2]Zhang Huiping. Analysis on Operation Status of On-Line Monitoring Capacitive Equipment[D]. First
Edition. Department Electric Machine of Tsinghua University, 2002
[3]Zhou Wuzhong. The Repair, Diagnosis and Preventive Test for Power Equipments. First Edition.
China Power Press, 2002
[4]Guan Gen,He Jingliang. On-Line Mornitoring of Insulation and Condition Maintenance for Power
Equipments. China Power Press, 2000
[5]Lei Guofu,Chen Zhanmei etc. Diagnostic Technique for Insulation for High Voltage Equipments.
Hydraulic and Electric Press, 1994
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