Projects:2015s1-70 Design of Power Line Communication Coupler for Single-Wire Earth Return Lines

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Project Team

Team Members

Chao Yue
Minsheng Liu
Wenyi Lu (Vannie)
Xiaotong Sun (Margaret)

Supervisors

Dr C.J.Kikkert
Dr Wen Soong

Introduction

In remote region of Australia and New Zealand, electric power is provided by Single-Wire Earth Return (SWER) system. The low cost, relatively high reliability SWER line can satisfy the requirements of a stand-alone long distance farm electrification scheme. However failure of overheating the transmission lines put the customer at high risk of bushfire. A Power Line Communication (PLC) coupler is designed and implemented for information transmission and hence voltage regulation and power supply service monitoring can be achieved on-line. The deliverable of the project is to implement a prototype of toroid based inductive narrow band PLC couplers pair which provides high coupling efficiency, low insertion loss and sufficient bandwidth. The proposed PLC couplers are designed to operate at frequency ranged from 35kHz to 478kHz under both low voltage and medium voltage operating condition, which are applicable with Australian SWER Line system and are accomplished with Australian safety standards. A high-pass filter (with cut-off frequency at 9103Hz) system is also integrated into the coupling system to mitigate the effect of saturation.

Significance

PLC transfers information along power lines. PLC on SWER lines would provide an opportunity to implement remote meter reading and improving the voltage regulation at the end of the line. With an effective narrow-band PLC network, the impedance analyser can be used to allow power line impedances to be measured on-line. The MV PLC coupler will provide SmartGrid benefits for people living in remote areas who are dependent on SWER lines. One of the advantages of inductive PLC couplers is that the couplers do not directly contact the line. It is easier to install PLC couplers on the line than other conventional methods of measurement such as inductive shunts. Although inductive coupling normally works well with low impedance lines (low voltage), the system developed in this project is verified to be compatible with 33 kV voltage rating, which shows a significant step in the implementation of smart-grid applications. PLC is found useful in many other areas, including advanced metering infrastructure (smart metering), domestic appliances remote control and homeplug.[1] For an increase in demands upon energy-saving around the world, it is worthy and encouraged to develop PLC systems which may help optimise energy efficiency.

Project Objectives

- Design and construct inductive narrow-band PLC couplers.
- Bandwidth covers working frequency range of G3-PLC (36kHz-91kHz) and PRIME (42kHz-89kHz) on CENELEC A.
- 120A mains current capability and tolerance to magnetic saturation.
- 33KV medium voltage rating.
- Acceptable and competitive insertion loss for on-line impedance measurement.
- Appropriate insulation.

System Overivew

All measurements are based on the test circuits shown in Figure 1. In the preliminary test, for rapid testing and its corresponding effect analysis, SC tests are undertaken on a pair of single core. Mains current flows from a AC current source to the centre of the couplers, and then to the shunt, and finally flows back to the current source. The primary and secondary winding of the toroid are connected to the signal generator and the Picoscope respectively. The signal generator sends signals to the toroid and the Picoscope draws signals from the toroid. Ultimately signals transfer to the computer.

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Design Considerations

Effect of Saturation

Saturation is a characteristic of ferrite, magnetic materials. It is the state at which an increase in applied external magnetic field H cannot increase the magnetization of the material further.[2] Saturation occurs when the total magnetic flux density B exceeds saturation flux density. It is necessary to mitigate the effect of saturation because saturation creates a practical limit to the maximum magnetic fields achievable.
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As mains current is pushed up, the bandwidth of frequency response becomes smaller as well as the gain plot is shifted down. The difference between the maximum point of each curve and 0dB implies the amount of insertion loss. With 0A mains current, the insertion loss is roughly -6dB while with 120A mains current applied, insertion loss is increased to approximately -22 dB. Insertion losses increase dramatically when current saturates. Also, when current increases, low frequency response shifts to the right which indicates a decrease in Lm whereas high frequency response shifts to the left which indicates a rise in Ls. Effect of saturation shows a negative impact on frequency response of the system, resulting in low coupling efficiency as well as undesired frequency bandwidth.

Effect of Air Gap

It is common to implement an air-gap in magnetic circuits because air gaps can modify parameters of magnetic materials by increasing saturation current and linearising B-H curves. Air gap introduces a high reluctance in the magnetic main path, which impedes the increase of flux density. The overall permeability is then reduced, resulting in a reduction in the magnetizing inductance.
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With a larger current applied (120A), it can been seen that the gain plot lifts up as the length of air gap increases. With a 100air gap, the attenuation of gain is approximately -15dB, however, with a 700 air gap, the attenuation of gain decreases to about -9dB. Therefore, at high current, implementing air gap helps to reduce the insertion loss by providing sufficient coupling and hence increasing coupling efficiency. It also has been seen that air gap narrows the frequency bandwidth at high level of current. With a larger air gap, low frequency response shifts to the right and so does high frequency response.

Effect of Turns Ratio

The total induced voltage in the secondary winding of a transformer is determined mainly by the turns ratio, which is the ratio of the number of turns in the primary to the number of turns in the secondary (Np/Ns), and also by the amount of voltage applied to the primary winding. On the other hand, saturation is caused by excessive voltage applied per second into the primary winding.[3] Therefore, adjusting turns ratio is one of the effective ways to improve the system’s tolerance to saturation. Due to the fact that adding more turns on the primary winding is not practical on power lines, the effect of secondary turns actually represents the effect of the turns ratio by primary turns being 1 and secondary turns being N.
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By increasing the number of turns in the secondary while keeping the primary turns unchanged, frequency response is shifted to the left. Adding secondary turns number will result in an increase in both Lm and Ls and shifting of lower corner frequency and higher corner frequency towards left. However, the bandwidth and insertion loss will not be affected by change in the secondary turn. With the primary turns fixed at a certain value, the effect of the secondary windings implies the effect of the turns ratio. If the turns ratio is reduced, i.e. more turns in the secondary, the frequency response will shift to the left. But if the turns ratio is enlarged, i.e. less turns in the secondary, the frequency response will be shifted to the right.

Effect of the Number of Cores Used

In order to develop medium voltage couplers, multiple cores are glued in series in a string to enhance the amount of coupling and hence the coupling efficiency. Glued cores form a tunnel for more magnetic flux to go through. With a higher current applied under medium voltage conditions, it is necessary to have more cores in one coupler, which can mitigate saturation effect by increasing the material area to support more magnetic flux.
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The input current applied to the multiple-core couplers is larger than that applied to the single-core couplers. To ensure the induced current the same in the secondary, the number of secondary windings for the single-core coupler is greater than that for the multiple-core coupler. With controlled variables (same primary winding = 1, same current in the secondary, same frequency sweep range, same testing environment), the effect of different number of cores used can be observed. Comparing with the frequency response of using single-core couplers, it can be seen that the frequency bandwidth increases, the insertion loss significantly decreases and the frequency response shift to the right when the coupler consists of multiple cores. 5-core couplers produce a wider frequency band than 4-core couplers do. There is not much difference between insertion losses of 5-core coupling system and 4-core coupling system. Increasing the number of cores used will not improve frequency response infinitely. The number of cores used shall be determined considering improvement on system performance as well as financial economy.

Ferrite Toroid Selection

Key Considerations

- Physical dimension (to ensure power line cable can fit in)
- Initial permeability (the higher, the better)
- Frequency range (the wider, the better)
- Cost ( limited by project budget)

Selected Core Type

Nickle-zinc ferrite F48 is selected to be used in the MV testing circuit, whose properties are shown below:

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System Improvement

Highpass Filter Design

Highpass filters play an important role in the coupling system. As 120A AC current goes through the mains cable, the PLC signal injected by the singal generator is very weak and hence signal to noise ratio of the system is very low. A filter is used to remove 50Hz components from the receiving signals. In addition, the filter provides a low impedance path for the mains current to flow through so as to prevent damages due to excessive current applied to the SG and the picoscope. Filter design is based on the entire system using the Variable Tuner built-in AWR to ensure that the leakage inductance(Ls) and magnetising inductance(Lm) of the couplers are taken into account. The filter is verified to be useful in improving low frequency response as shown below:

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Insulation

Due to high voltage rating and current rating of the MV couplers, it is essential to have the coupling system well insulated to avoid any injuries. Insulation include coupler insulation, power line insulation and contact joint insulation.

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System Performance

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The system is verified to be able to work well under medium voltage and 120A mains current. The bandwidth covers from 30kHz to 330kHz. The starting and finishing points of frequency are defined the same as those of the existing commercial product developed by PREMO to allow ease of comparison. Comparing with the bandwidth proposed in the initial project objectives, which covers working frequency range of G3-PLC (36kHz-91kHz) and PRIME (42kHz-89kHz) on CENELEC A, the working frequency of this developed system is much wider, covering higher frequency till 330kHz. This is good for PLC because the wider the frequency band is, the bigger the capacity of a communication channel is. In Figure 9 on the left-hand-side, it is the commercial model developed by PREMO and its corresponding frequency characteristics. By comparing the N-PLC bands of the two systems (Figure 9a and b), the commercial product of PREMO suffers larger attenuation in gain at N-PLC band. For the same frequency band, the overall insertion loss of the system developed in this project is approximately -3dB, which is a comparatively small insertion loss. It is confident to conclude that the system in this project works better than the commercial ones to some degree.


Project Management

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References

[1] L. Berger, A. Schwager and J. Escudero-Garzás, 'Power Line Communications for Smart Grid Applications', Journal of Electrical and Computer Engineering, vol. 2013, pp. 1-16, 2013.
[2] A. Janse Van Rensburg and C. Ferreira, The Role of Magnetising and Leakage Inductance in Transformer Coupling Circuitry: Rand Afrikaans University, 2011.
[3] Ch 12. Basic Magnetics Theory, Fundamentals of Power Electronics. [Online]. Available: hhttps://eleccompengineering.files.wordpress.com/2015/01/fundamentals-of-power-electronics_2nd_erickson_full.pdf. [Accessed: 04- Oct- 2015]
[4] Neosid.com.au, 'Ferrite toroids', 2015. [Online]. Available: http://www.neosid.com.au/shop/category/ferrite-toroids


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