outline a theoretical approach for a real-time wireless power quality monitoring system based on sensing current and voltage phasors at the facility. Index Terms ...
Proceedings of the IEEE International Conference on Mechatronics & Automation Niagara Falls, Canada • July 2005
Real-Time Power Quality Monitoring Using MEMS Based Wireless Current Sensors S. R. I. Gabran, G. S. A. Shaker, E. F. El-Saadany, M. M. A. Salama Department of electrical and computer Engineering University of Waterloo Waterloo, Ontario N2L 3G1, Canada {salam & georgeshaker}@ieee.org {ehab & msalama }@hivolt.uwaterloo.ca
Abstract - Real time monitoring of current phasors is fundamental for power quality analysis. In this paper, we outline a theoretical approach for a real-time wireless power quality monitoring system based on sensing current and voltage phasors at the facility.
Permanent systems using computers provide more powerful analysis capabilities. Some units can be connected via modems or have Ethernet LAN communication capability for remote monitoring and connection to workstations. Our proposed architecture characterizes a system optimized for cost, flexibility and portability. Two aspects contribute to the low cost of system; first of all, the integration of sensors and additional circuits and antennas on a single chip. Secondly, the wireless network of sensors and signal relaying hubs which also render the system flexible and portable. Such a system can be used for monitoring and evaluating the PQ on temporary basis with minimal cost. The inputs of a power quality analysis system are voltage and current phasors. Diverse techniques are developed for power quality analysis; these include machine inductive learning implemented using the C4.5 algorithm [4], modified discrete wavelet transform [5] and electromagnetic transients programs [6]. Sensing the electric and magnetic fields is employed in measuring the accompanying currents and voltages. MEMS sensors are proposed for the system due to their CMOS compatibility and low mass production cost. Some sensors are based on Poynting’s theorem [7] [8] and comprise a coil that detects the Poynting vector which is related to the power per unit area of an electromagnetic field. These types of sensors are bigger in size compared to newer technologies based on MEMS. Magnetic field sensors utilize the Hall Effect in magnetic field detection [9]. Hall Effect sensors must have compensation circuits to treat non-linearity and temperature dependence. N-channel MAGFETs can be also used in measuring magnetic fields [10]. MAGFETs are characterized by small area and compatibility with CMOS circuits [10]. Magnetic fields can cause changes in the electrical resistance of some materials; this property is called magnetoresistance and is also used in magnetic field detection [11]. Capacitive and inductive probes are conventionally used for voltage measurement. New technologies in electric field and voltage sensing employ the Pockels electro-optic effect in high voltage sensing [12] [13] [2].
Index Terms - Power quality, current sensing, harmonics, magnetic field sensor, electric field sensors. I. INTRODUCTION Monitoring power quality is a growing concern in electric power production and distribution which is driven by the effects of power quality on distribution system economics and quality of service [1] as well as the deregulation of electric power industry [2]. For example, in a sub-distribution system; the cost of harmonic losses can reach up to 15% of fundamental losses cost [1]. Thus improving power quality (PQ) provides a leap in increasing distribution system efficiency. Moreover; PQ monitoring serves in preventive maintenance of power devices and prediction of malfunctions. Disturbances and variations of PQ arise in diverse forms, e.g. voltage sags, swells, harmonics, etc… [3]. Consequently, the first step in PQ monitoring is keeping track of the current and voltage phasors followed by implementing digital signal processing algorithms for feature extraction, classification and identification. In order to achieve the maximum protection and productivity, the system has to run in real-time which introduces constraints in system design. This includes redundancy of sensors and signal pick-up locations for more reliability and decreasing the latency and processing times in the system. PQ monitoring systems are available from various suppliers in different types offering a range of accuracy, flexibility and performance depending on the system architecture and the implemented algorithms. A simple system allows monitoring the current and voltage waveforms to track and record power quality issues whereas a complicated one executes a based real-time analysis algorithms on a network. Power quality meters can be handheld for portable troubleshooting applications. As well as permanently installed systems for continuous power quality process improvement.
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II. POWER QUALITY PRIMER A. Introduction Power quality analysis aims to optimize the reliability and safety of supplying electric power and improve the distribution system economics [15]. B. Power quality issues Main power quality issues can be identified as [16] [17]: Harmonics Sags, dips, and swells Transient over-voltages Flicker Frequency variations High frequency noise Mains signalling Unbalance On three-phase systems C. Harmonics - an example of power quality issues Harmonics are integer multiples of a fundamental frequency generated by non-linear electrical and electronic equipment. They are superimposed on the fundamental frequency -which is the required component- and the resultant is a distorted waveform. Effects of harmonics on system performance are adverse and have to be considered and render the power system prone to failure. They can be summarized in: Metering errors Reactive power increase [17] Reducing current capacity of wiring system Overheating of electrical lines and equipment Distorting waveforms and changing r.m.s. and average values of voltage levels increase dielectric stresses on capacitor banks [1] Finally, this deteriorates system efficiency and revenues. D. Measuring R.M.S. values of phasors Measuring the r.m.s. of non-sinusoidal voltages and currents is not possible using conventional analog meters due to their poor response to higher frequency components. Some techniques used in digital meters evaluate r.m.s. values are not valid in a harmonic-rich environment.
requirements and ambient environment in terms of high field intensities and high voltages. 2. Circuit design phase: This phase involves antenna and circuit design including filters, amplifiers and transmitter circuits. Recent research in planar antenna designs produced CMOS compatible planar antenna which make it possible to integrate the antenna together with sensor and circuits on the same chip. 3. Communication layer phase: There are two wireless links each is characterized by data rate, number of channels and range of transmission, starting at the sensors circuits to the hubs, and finally to central processor. This phase involves choosing the modulation technique for each link and involves building the protocol and signal encoding which is required for data compression, noise immunity and interference suppression. 4. Software phase: The system executes 2 main software blocks: 1. electric and magnetic field waveforms conversion to current and voltage waveforms, 2. feature extraction, classification and identification. Sensor design phase
Parameter selection Design constraints
Circuit design phase
Antenna design Filters, amplifier and transmitter design
Modulation technique Protocol Encoding
Communication layer phase
Software phase
Current and voltage waveform conversion Feature extraction, classification and identification
III. SYSTEM DESIGN Figure 1: System design phases
A. System phases System design and development include three distinct phases as shown in figure 1 where each phase can be optimized to improve the overall system performance: 1. Sensor design phase: The first step is choosing the parameters of interest for the processing, according to the PQ analysis algorithms’ requirements; the current and voltage waveforms are chosen. The induced electric and magnetic fields emitted from the lines are converted to current a voltage phasors. Constraints on sensor design are dictated by system
B. System block diagram and description Figure 2 illustrates the system blocks starting at the electric and magnetic field sensors to capture the induced fields accompanying the overhead unshielded power lines. Then, the acquired signals are pre-processed by the signal conditioning circuits prior to modulation and transmission. Following to sensor signal reception by the data hubs from signal pick-up locations in the vicinity, signals are retransmitted to the central processor. This approach is preferred than installing high-power long-range transmitters in each
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sensor with the required complicated communication protocols. In the end, the central processor collects all the signals from the sensors and elicits current and voltage waveforms followed by executing the PQ analysis algorithms.
created new generations of sensors with smaller area, diminished power consumption and reduced cost besides to compatibility with CMOS circuits. Sensors based on Poynting vector concept use a coil [7] [8] or a miniature antenna to capture the electromagnetic field. The very low frequencies detected require coils with large number of turns that can not be integrated on a single-chip with other CMOS circuits. The sensor proposed to our system is a MEMS magnetic field sensor based on a magnetic resonator introduced in [19]. It suits low frequency applications where setting the resonant frequency is controlled by changing the cavity geometry and material properties. Moreover, its CMOS compatibility allows its integration with other circuits on a single chip.
Signal conditioning and pre-processing
C. Application and constraints The system is targeted for overhead unshielded power lines at the facility (known as the distribution circuits) just before the transformer feeding the loads. The destined environment is characterized by intense fields due to the high voltages and currents carried by the lines. Normally, the phase voltage is 13.8KV and the current flowing is 200 amperes. This dictates constraints on the sensors theory and design.
Magnetic field sensor Electric field sensor
Data hub and communication relay
Wireless data link
Central processor
Figure 2: System block diagram IV. SENSORS A. Current measurement concept Current measurement is based on sensing the magnetic field. Current phasor is extracted from the magnetic field induced by the current in wires. Referring to fundamental electromagnetics and Biot-Savart law (figure 3); the magnetic field lines intercepted by the sensor indicate the variation in the current flowing in the power line.
Iˆ (A' )d' x a j (A' ) Bˆ j = μ o . 2 A 4 π r j (A' )
³
dA' r j (A')
" (1)
Figure 3: Biot-Savart Law Formulation Bj
Where: μo is the permeability of free space
r j (A')
is a vector from the source point to the field point
aj (A')
r (A') is a unit vector in the direction of j
I(A')
B. Optimizing sensor placement Selecting the monitoring locations depends on the facility design, critical loads, power conditioning equipment, and the specific objectives of the monitoring. Basically, monitoring should include the utility supply locations, outputs of power conditioning equipment, and backup generators. More extensive monitoring includes critical air conditioning loads and individual loads within the facility, such as communications equipment and individual load buses. Monitoring within the facility can help characterize load interaction issues. An example of an ideal situation is one circuit feeding load with 200 amps at 13.8kV without any other circuits nearby, and at a high altitude from ground, the magnetic field lines follow simply Ampere’s right hand rule, while the
dA' is a differential element at A' in the direction of the current reference A' is a measure of distance along the current path
B. Magnetic field sensor Diverse physical phenomena are utilized in sensing the magnetic fields. New technologies in MEMS fabrication
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magnetic intensity variations are proportional to the variations in the current flowing in the line. This means that measuring the field intensity requires that the measuring devices, is aligned with the direction of field lines in order to capture the variations of the field intensity. The measurement of the amplitude and direction of magnetic fields is feasible with such equipment, as shown in figures 4-6; a magnetic field sensor is introduced. This sensor can be of a huge potential in power monitoring applications. Once the sensor is positioned in alignment with the magnetic field lines, the readings of sensor would vary with the intensity of the field at its location. The complexity comes from the fact that the sensor now is affected by the fields from different lines. A solution would be adding three sensors around the lines, which offers three different sets of magnetic field readings. Once those readings are achieved, equation (1) can be used to extract the current waveforms in each line.
Figure 5: Magnetic field intensity distribution in a single isolated line
Figure 4: Simulation for 3 powerlines and their magnetic field vectors. Figure 6: Simulation for Electric Field vectors of 3 overhead lines.
D. Practical concerns However, a practical situation holds 3 overhead lines for a 3 phase distribution circuit above a considerable distance from ground. This implies that the field lines around them would suffer from the near field effects, along with the effect of the earth, which according to the image theory (figure 7), introduces an exact replica of these lines at the same distance from the earth surface. These factors introduce huge variations in the field lines directions and intensity.
E. Voltage measurement concept The Pockels electro-optic effect is defined as the dependence of the refractive indices versus an electric field [12]. This effect on materials like LiNbO3 can be used in measuring the electric fields in a crystal [13] as well as in high-voltage measurements [14]. An integrated optical sensor for high-voltages is presented in [14]. These sensors are immune to electromagnetic interference and isolation makes them competent for the high voltage environment. An appropriate sensor design for use in high voltage environments and power systems is described in [2].
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L 3
indeed the simplest available to date. Throughout an FDMA system, the receiver will discriminate between the signals by tuning to the particular frequency channel that carries the desired signal. It is important to mention at this point that other schemes may be used as TDMA or CDMA. However, such schemes require more system complexity, which is not needed for simple power monitoring. An FM link is a suitable solution for the sensor-to-hub link as their transmitter circuits are relatively simple and can be integrated along with other sensor circuits. In contrast, longer ranges and more sophisticated protocols require a DSP processor and higher power transmitter circuits ending with more expensive circuits on a bigger area.
L
1
2 o
d Region 0 (air) Region 1 (earth) 1
3’ 1’
2’
Perfect image Facility 1
Figure 7: Image theory of 3 overhead power lines
Simple RF link using FM Central processor
Data hub 1 V. WIRELESS DATA LINK AND COMMUNICATIONS Facility 2
A. Why wireless Although the power grid offers high connectivity and can which can be useful for data transmission over the power lines, but wireless channels are more preferable than transmission over power lines to avoid the limitations and distortions inherent in power line transmission [18]. Furthermore, advances in planar antenna design and fabrication introduced a huge leap in reducing antenna sizes and rendering them CMOS compatible.
Data hub 2 Complicated RF link using FDMA/TDMA or CDMA Figure 8: Communication links
VI. CONCLUSION
B. Wireless Data channel and communication protocols A wireless link allows ease and flexibility of installation, in the same time; a communication protocol has to be used to manage and control data transfer. To make use of the evolution in communication protocols, FDMA, CDMA or communication cells (like the cell-phone networks) can be used to provide flexibility and robustness for the system. As the complexity of the communication protocol increases, a DSP processor is required to execute the codec which raises the price tag of the system as well as the power requirements of each sensor circuit. A simple way out is by using hierarchy of layers of links as shown in figure 8. The first link connects a cluster of sensors to a central data hub implementing simple FM channel. This link conveys data from a group of sensors to a central data hub using low power and simple transmitters provided that the transmission range is short (within few kilometers). Data hubs retransmit the data to the central processor over FDMA links using a higher power and long range links. Frequency Division Multiple Access (FDMA) is one of the three main multiplexing techniques that enable users to share the radio spectrum using an analogue transmission technique in which the frequency band allocated to a network is divided into sub-bands or channels. This scheme was among the first incorporated schemes in wireless systems and is
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