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A Study on Electromagnetic disturbance and Immunity of Wireless Sensor Unit in Substation
AN Bo1, ZHANG Weidong1, MA Haijie1, CUI Xiang1, JIN Yang2, MENG Fanbo3
1State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (North China Electric Power
University), Beijing 102206, China, [email protected]
2China Electric Power Research Institute, Beijing 100000, China, [email protected]
3Liaoning Province Power CO.LTD.State Grid Liaoning Electric Power CO.LTD.,Shenyang,China,[email protected]
Abstract Wireless sensor unit is important part of the Internet of Things, which is responsible for signal acquisition and
transmission. In order to ensure reliability and security of the Internet of Things, electromagnetic disturbance issue in substation where wireless sensors are usually located is studied. This paper focuses on the electromagnetic disturbance to wireless sensor unit generated by power system and immunity of wireless sensor unit. Research on characteristics of steady-state electromagnetic disturbance in substations of different voltage levels and of different insulation types is performed, through field measurements. Statistical characteristics of electromagnetic disturbance in frequency domain are obtained, based on extensive field measurement data. Simulation is applied to reveal influence of electromagnetic disturbance to wireless sensor unit. Immunity of wireless sensor unit in the Internet of Things in substation is studied. Experiment indicates that wireless sensor unit is influenced by electromagnetic disturbance and immunity test of wireless sensor unit are performed.
1. Introduction State Grid Co., Ltd. Proposes a “Internet of Things” program that is featured by “Sense, Transmission, awareness” in
“Smart Grid”, in order to enhance management level of grid operation. Smart monitoring system using the Internet of Things in substation is a vital part of Smart Grid and secures grid operation[1].
Original data that consists of impulse noise is obtained through field measurement in a 400kV GIS substation performed by British power system Co., Ltd.[2] Impulse noise is generated by partial discharge, periodic process of power electronics and switch operation. Research also shows that SF6 gap breakdown and vacuum gap breakdown lead to obvious negative influence on Bluetooth wireless communication[3].
Research testifies that the rise time of current impulse produced by partial discharge in strong dielectric such as SF6 may reach to 50ps. That is to say, the impulse may include frequency components up to 3GHz[2], which may generate electromagnetic disturbance to wireless sensor unit. Scotland power system Co., Ltd. in UK researches on influence to wireless sensor unit caused by electromagnetic disturbance of partial discharge and already gains some results[4].
This paper mainly researches on electromagnetic compatibility of wireless sensor unit in which ZigBee technique is employed. ZigBee adopts DSSS OQPSK modulation and communicates in the band of 2.4GHz~2.5GHz. Frequency band of ZigBee communication is fixed, and therefore, it is easy to be influenced by electromagnetic disturbance[5]. Wireless sensor unit system may lose information packet or may even stop working, under severe electromagnetic disturbance.
Electromagnetic disturbance in complex electromagnetic environment in substation influences wireless sensor unit in the Internet of Things. By field measurement, this paper studies on characteristics of electromagnetic disturbance, which may yield influence on wireless sensor unit, produced by electric field of power frequency, magnetic field at power frequency, corona discharge, gap breakdown and partial discharge in substations of different voltage levels and of different insulation types. Measurement data of electromagnetic disturbance in various substations in frequency domain, which consists of steady-state data, is obtained by wide-band antenna, digital oscilloscope and receiver. Simulation and immunity test of wireless sensor unit are performed in order to reveal its sensitivity at specific frequency and field strength of disturbance.
2. Measurement and Statistical Spectrum Analysis of Electromagnetic Disturbance in Substation
Electromagnetic disturbance on network of wireless sensor unit that is caused by complex electromagnetic environment cannot be negligible, along with wide application of the Internet of Things. Research on electromagnetic compatibility of wireless sensor unit in substation is performed in this paper. Through field measurement, electromagnetic disturbance characteristics in substations of different voltage levels and different insulation types are obtained. Statistic properties of electromagnetic disturbance are gained.
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Figure 1 illustrates steady-state spectrum around a circuit breaker in a 500kV GIS substation measured by receiver. The dominant frequencies are 208MHz, 532MHz, 675MHz, 871MHz, 936MHz, 1.84GHz and 2.43GHz, which cover the communication frequency band of wireless sensor unit. Packet loss rate of wireless sensor unit is about 5.6% in a performance test in this GIS substation.
Owing to difference of disturbance spectrum of each single measurement, data of single measurement is not representative for overall level of disturbance. Figure 2 illustrates spectrum envelope diagrams of 100%, 80% and 50% measured at 4 measurement points in 500kV open air and GIS substations. The 100% spectrum envelope diagram represents the maximum value at each frequency in spectrums of all the measurement data. The 80% spectrum envelope diagram denotes 80% of the maximum value at each frequency in spectrums of all the measurement data. The 50% spectrum envelope diagram means half of the maximum value at each frequency in spectrums of all the measurement data. The dominant frequencies are 0.64MHz, 150MHz, 900MHz, 1.8GHz, 30MHz~300MHz and 2.2GHz~2.4GHz, which may influence communication quality of wireless sensor unit.
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Frequency [Hz]
E [
dBµ
V/m
]
Figure 1. Steady-state spectrum electric field
around a circuit breaker in a 500kV GIS substation
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Frequency [Hz]
E [
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Spectrum Envelope Diagrams
100%
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Figure 2. Spectrum envelope diagrams of
100%, 80% and 50% at 4 measurement points in open air and GIS substations at the voltage level of
500kV
Figure 3. Photo and model of wireless sensor
unit
3. Simulation A. Simulation of Electromagnetic Disturbance to Wireless Sensor Unit Field measurement and experiment show that wireless sensor unit is influenced by electromagnetic disturbance in substation.
Analysis of the measurement data indicates that frequency range of electromagnetic disturbance includes the communication frequency of wireless sensor unit. In order to reveal the distribution of induced voltage in wireless sensor unit, model of wireless sensor unit is established, using simulation software.
The photo and model in an ideal open space field of wireless sensor unit are illustrated in Figure 3. Set plane wave as an excitation source and assign electric field strength 10V/m according to the IEC Standard 61000-4-3 definition for the third class test level. Set a disturbance signal (a partial discharge signal) as an excitation source, as shown in Figure 4. There are 2 voltage probes in the model to observe the sensitivity of wireless sensor unit to electromagnetic disturbance. One probe is on the antenna that is oriented in the z-direction, while the other one is on PCB inside shield.
Simulation is performed and influence of electromagnetic disturbance that is obtained by experiment is illustrated on the antenna of wireless sensor unit in Figure 5.
2 4 6 8 10
x 10-9
-0.01
-0.005
0
0.005
0.01
t(s)
U(V
)
.Figure 4 A disturbance signal as an excitation source
Plane wave travels along the x-axis, while electric field polarization is in the z-direction that is the same polarization direction of the antenna in wireless sensor unit. Electric field strength is 10V/m. Figure 5 illustrates spectrum of induced voltage on the antenna, the dominant frequencies of which are 1.2GHz and 2.3GHz. The maximum value of induced voltage is 1.04V, which is high enough to lead to communication packet loss of wireless sensor unit. After all, the 1.2GHz signal component can easily generate 2.4GHz signal by frequency multiplication, which is communication band of wireless sensor unit, and leads to same frequency interference. The 2.3GHz signal component produces neighboring interference. The frequency components at 1.2GHz and 2.3GHz disturb communication of wireless sensor unit.
B. Simulation of Electromagnetic Environment Where Wireless Sensor Unit is Applied When wireless sensor unit is applied in substations, the electromagnetic disturbance from electric equipment is needed to be
studied in order to reveal the immunity of wireless sensor unit. Therefore, simulation model, which employs uniform theory of diffraction at high frequency range (2.4GHz) to solve large size problem, is established to obtain the magnitude of electromagnetic field at the point where wireless sensor unit is placed, when the substation works. Different placement location
Figure 5. Spectrum of induced voltage signal on the antenna(GHz)
of wireless sensor unit results in different effect of electromagnetic disturbance, because of the distance of disturbance source and layout of obstruction in substations.
Figure 6 illustrates the layout of a substation through simulation, inside which red points stand for location of wireless sensor unit. Wireless sensor units locate around transformers, switchgears, and in the main control room, communication room.
Figure 6. Layout of a substation where wireless
sensor unit is applied
Table 1. Received Signal Voltage of Wireless Sensors
Wireless Sensors Location
Received Signal Field Strenth
(Quasi-Peak Value) Transformers 8.72 V/m Switchgears 6.51 V/m
Main Control Room
0.92 V/m
Communication Room
1.03 V/m
Figure 7. A transformer model Through simulation, it is demonstrated that when substation works normally, different location results in different magnitude
of electromagnetic disturbance, due to distance and obstruction. Disturbance sources are modeled using dipole transmitting antenna on transformers and switchgears according to the measurement data, while wireless sensor unit is modeled as dipole receiving antenna, and the simulation result is shown in Table 1. Since the measurement data is different in different substations and around different equipment, the electromagnetic disturbance voltage is normalized to 10V/m. Through simulation, the received signal is obtained so that the inducted voltage inside wireless sensor can be evaluated, which may influence communication quality.
Because of the large-size equipment in substations, a transformer model is established in order to reveal the obstruction of communication between wireless sensors, as shown in Figure 7.
By simulation, it is illustrated that obstruction is an important factor to influence communication quality. Different location of wireless sensors results in different communication quality around the transformer. Field experiment verifies this conclusion.
4. Immunity Test of Wireless Sensor Unit In order to obtain performance of wireless sensor unit under electromagnetic disturbance and to describe variation of work
state with variation of electric field strength of electromagnetic disturbance, immunity test on wireless sensor unit is conducted at an open area test site. Layout of the open area test site is illustrated in Figure 8, in which the wireless sensors act as transmitter and receiver under strong electromagnetic disturbance produced by signal generator, power amplifier and transmitting antenna. A high-frequency electric field probe is placed close to the transmitter to gain electric strength of electromagnetic disturbance.
The electromagnetic disturbance, which varies in the frequency band range of 800MHz to 2.4GHz, is amplitude modulated by sine wave of 10kHz. Electric field strength produced by transmitting antenna is regulated by power amplifier. Critical electric field strength of normal-operation/packet-loss work state and of packet-loss/halt work state is obtained by test around frequency of 2.4GHz. “Critical electric field strength of normal-operation/packet-loss work state” means that under this electric field strength, work state of wireless sensor unit alters from normal operation to packet loss. “Critical electric field strength of packet-loss/halt work state” means that under this electric field strength, work state of wireless sensor unit alters from packet loss to halt.
Antennas of the transmitter and the receiver are all in horizon polarization direction that is in the same polarization direction of the transmitting antenna. The frequency of the immunity test ranges from 100MHz to 2.5GHz. The most sensitive frequency ranges are around 2.4GHz and 1.2GHz. Critical electric field strength of normal/packet-loss work state and of packet-loss/halt work state is acquired, as shown in Figure 9 and 11 at the frequency of 2.4GHz and 1.2GHz, respectively.
Figure 8. Layout of the open area test site
2 . 4 0 0 2 . 4 0 2 2 . 4 0 4 2 . 4 0 6 2 . 4 0 8 2 . 4 1 0
0
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1 2 Cr i t i c a l E l e c t r i c F i e l d S t r e ng t h
o f No r ma l / P a c k e t - l o s s Wo r k S t a t e Cr i t i c a l E l e c t r i c F i e l d S t r e ng t h
o f P a c k e t - l o s s / Ha l t Wo r k S t a t e
Crit
ical
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ctri
c Fi
eld
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ngth
(V/
m)
F r e q u e n c y ( GHz )
Figure 9. Critical electric field strength at 2.4GHz
As shown in Figure 9, at electric field strength of 0.80V/m that is almost at the noise level at the frequency of 2.405GHz,
wireless sensor unit stops working. At the frequencies that are higher or lower than 2.405GHz in the frequency range of
2.4GHz~2.409GHz, the critical electric field strength is higher than 0.80V/m. 2.405GHz is actually the current communication frequency of wireless sensor unit. The same frequency interference area is, therefore, around 2.405GHz, and the influence of electromagnetic disturbance at this frequency is outstanding. At the frequency of 2.409GHz, taking as an example, electric field strength that makes wireless sensor unit start to lose packet is 9.6V/m and when it rises to 11.6V/m, wireless sensor unit stops working. With ascension of electric field strength at each frequency, wireless sensor unit experiences a process from normal operation to packet loss, and then to halt, its packet loss rate increasing from 0 to 100%, at the frequency of 2.409GHz demonstrated in Figure 10.
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et L
oss
Rat
e (%
)
Electric Field Strength (V/m)
Packet Loss Rate
Figure 10. Packet loss rate variation at the frequency of 2.409GHz at the
frequency of 2.409GHz
1. 201 1. 202 1. 203 1. 204
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Cr i t i cal El ec t r i c Fi el d St r engt h of Nor mal / Pac ket - l os s Wor k St at e
Cr i t i cal El ec t r i c Fi el d St r engt h of Pac ket - l os s / Hal t Wor k St at e
Crit
ical
Ele
ctri
c Fi
eld
Stre
ngth
(V/
m)
Fr equenc y ( GHz )
Figure 11 Critical electric field strength at 1.2GHz
In the frequency range from 1.2GHz to 1.2035GHz, there is also a distinguished sensitive area as described in Figure 11. The
most sensitive frequency in this range is 1.203GHz, which is about half of communication frequency of wireless sensor unit, 2.405GHz. However, the whole electric field strength level is much higher than that in the frequency range of 2.4GHz. Wireless sensor unit halts working at electric field strength of 2.45V/m at 1.203GHz, but changes work state from normal operation directly to halt at 1.2035GHz at electric field strength of 36.93V/m, while it does not stop working at 1.2GHz even at the maximum value of electric field strength in this open field test site.
The immunity test reveals that wireless sensor unit may be influenced and lose packet at 2.4GHz and 1.2GHz. Detailed plots in Figure 9, 10 and 11 describe the sensitivity of wireless sensor unit at specific frequency and field strength of disturbance.
5. Conclusion The measurement data of steady-state operation in substations of different voltage levels and of different insulation types are
studied and statistically analyzed. The frequency range of electromagnetic disturbance in substation contains communication frequency of wireless sensor unit. Experiment and simulation indicate that wireless sensor unit is influenced by electromagnetic disturbance. Wireless sensor unit in substation loses communication packet and even stops working under electromagnetic disturbance. Simulation and immunity test indicate that wireless sensor unit is sensitive to disturbance at the frequency around 2.4GHz and 1.2GHz in different degree. It is of great significance to research on electromagnetic disturbance and immunity of wireless sensor unit of the Internet of Things in substation.
6. Acknowledgments The author is grateful for the support of “Study of Communication Equipments’ Electromagnetic Compatibility and Protection
Technology in Substations” (No.TX71-13-038) Project by the State Grid Corporation of China.
7. References [1] F. Cleveland, Member IEEE. "Use of Wireless Data Communications in Power System Operations" [J]. 2006 IEEE PES Power Systems Conference and Exposition, pp. 631-640, 2006. [2] Qingshan Shan, Ian A. Glover, Robert C Atkinson, Shahzad A Bhatti, Iliana E Portugues, Philip J Moore, Richard Rutherford, Maria Fatima Q Vieira, Antonio Marcus N. Lima and Benemar Alencar de Souza, "Estimation of Impulsive Noise Measured in an Electricity Substation", Transactions on Electromagnetic Compatibility, 2011 , pp. 653 - 663. [3] Zhaoxi Liu, Yulong Huang, Liangzhong Yao, Youjun Gong, Min Han, Rong Zeng, Zhiqiu Li, "Experiment on the application of Bluetooth in vacuum switch cabinet", Electric Utility Deregulation and Restructuring and Power Technologies, 2008. DRPT 2008. Third International Conference on, Nanjing, China, April 2008. [4] Qingshan Shan, I A Glover, P J Moore, I E Portugues, R J Watson, R Rutherford, R Atkinson, Shahzad Bhatti, "Laboratory Assessment of WLAN Performance Degradation in the Presence of Impulsive Noise", Wireless Communications and Mobile Computing Conference, 2008. IWCMC '08. International, pp. 859 – 863, 2008. [5] Axel Sikora, “Coexistence of IEEE802.15.4 (ZigBee) with IEEE802.11 (WLAN), Bluetooth, and Microwave Ovens in 2.4GHz ISM Band”, Revised Version V0.3: 12th September 2004.
