Abstract—The channel characteristics of low voltage power-line as telecommunication line have important influence on communication quality, which include impedance characteristics, attenuation characteristics, noise characteristics and so on[1]. The measurement of input impedances is crucial for power-line communication (PLC) because it’s an essential parameter for transmission characteristic. According to inductive analysis of scientific information at home and abroad, four methods of measuring power-line impedances are summarized in the paper which consist of current probe, resonance, voltammery and ratio. We compare the performance of the four methods in terms of experiment equipment, circuits, steps and frequency range. the result after analyzing show that the Two-current-probe method is a effective method for measurement of power-line impedance, and we make some measurements in the laboratory The result of measurement reveal that the impedance change with frequency,which will contribute to further study for PLC.

I. INTRODUCTION OWER line carrier communication is cheap and con- venient because of universality of its trans-mission medium. The design purpose of PLC media which is

extraordinary unstable, is the transmission of power but not date. The noise effect and signal fading of PLC channel are very serious, so it is unsuited for transmitting date. It is a multipath channel due to time variances which is caused by reflection of input impedances mismatching on layup of power-line. The variation intensity of input impedances depends on signal frequency and location. [2],[3]Input impedances depend on characteristic impedance of power-line, topological structure of network and electronic load. The coupling mismatch caused by variances of input impedances in PLC network greatly increases transmission loss, then the power of signal is hard to transmit to power-line, which results in short distance of transmission.

September 1,2012. Project Supported by National Natural Science Foundation of China(NSFC)(60972004). Natural Science Foundation of Beijing City(4122073) Y Zhou, school of electronic and electrical engineering north china electric power university Beijing,China,phone:15811470750, email:zhouyang19880308@163.com, M Zhai , school of electronic and electrical engineering north china electric power university Beijing,China G Xing, school of electronic and electrical engineering north china electric power university Beijing,China.

The impedance of power load in power grid is an essential parameter in modeling and simulation of electric power system and configuration of power device. It is very hard to simulate and forecast the situation of harmonic voltage, reactive power and so on when there is nothing we can know about power grid structure and impedance parameters. Especially, the parameters of load impedance became more important when load and structure in power grid are changed. The input impedances of low voltage power-line directly affect the efficiency of signal coupling and are key parameters of transmission characteristics. The research of input impedances is necessary to improve efficiency of transmitter and increase input power of network.[4]

II. INTRODUCTION OF MEASUREMENT METHODS

A. Two Current Probes Method Two-current-probe measurement system consists of two

current probes and a network analyzer in Fig. 1. The coupling circuit composing of current probe and decoupling capacitor could avoid direct connection between 220V power and network analyzer. By injecting current probeport 1, the signal produced by network analyzer injects into power network, and current signal inducted by monitor probe (bi-directional current probe) is coupled with port 2 of network analyzer. Equivalent impedance Z of power-line could be obtained by calculating scattering parameter which is measured by network analyzer. [5]

Lz

Fig. 1. Two-current-probe measurement system

Equivalent circuit formed by inject current probe and power-line is given in Fig.2. Here, PZ is the input impedance

of current probes, pV is the terminal voltage of current

probes, lI is the current which is injected into a wire by

current probes, pL , lL ,M are equivalent primary self-inductance of the probe, self-inductance of power-line

Measurement Methods of Low Voltage Power-line Communication Channel Impedances

Yang Zhou, Mingyue Zhai and Guilan Xing

P

2012 IEEE fifth International Conference on Advanced Computational Intelligence(ICACI)October 18-20, 2012 Nanjing, Jiangsu, China

978-1-4673-1744-3/12/$31.00 ©2012 IEEE 1004

and the mutual inductance between probe and power-line, respectively.

PV

PZ

PI

PL lL

lI

lV

Fig. 2. Equivalent circuit for a current probe According to volt-ampere relation of coupling coil, we

have ( )p p P P lV Z j L I j MIω ω= + − (1)

l P l lV j MI j L Iω ω= − + (2)

Combining (1) and (2), we have

1 1l M l MV Z I V= − (3)

Where

1

2( )M l

p P

MZ j LZ j L

ωωω

= ++

(4)

1M p

p P

j MV VZ j L

ωω

=+

(5)

According to (3), the circuit for a current probe could be replaced in reference plane a by an equivalent circuit as shown in Fig.3.

From (5), let

1MR

p p P

V j MKV Z j L

ωω

= =+

(6)

Assuming that the current in the loop is evenly distributed, which means that current at the reference plane b is equal to the current at the position of monitor probe 'b .So the equivalent circuit of measuring system with d=0 is as shown in Fig.3.

'b

1MZ

1MV

2MZ

LZ

inZ

Fig. 3. Equivalent circuit for the system Assuming that the current in the loop is evenly distributed, which means that current at the reference plane b is equal to the current at the position of monitor probe 'b .So the

equivalent circuit of measuring system with d=0 is as shown in Fig.3.

Thus, let inZ be the input impedance at the reference plane b looking toward the current probes. Then

1( )M in L lV Z Z I= + (7)

Combining (7) and (5) and modifying, we get

1

1 2

2

( )( )pL R T in

p

VZ K Z Z

V= − (8)

Where, 2 2

/T p lZ V I= .1pV ,

2pV are terminal voltages of the injector probe and monitor probe. These voltages are related with scattering coefficient measured by network analyzer, here

1 1 11(1 )pV V S= + and2 1 21pV V S= .

Hence,

1

2

21

111p

p

V SV S

=+

(9)

Where 11S is the reflection coefficient of injector probe when the monitor matches with a 50� load. Since the product

1 2R TK Z has nothing to do with the value of LZ , we could

calibrate the circuit using a standard resistance L stdZ Z= =R , and compute the product

1 2

1

2

( )( )

in stdR T

pL std

p

Z ZK ZV

Z ZV

+=

= (10)

Getting the values of 1 2R TK Z and inZ which are not

relevant to LZ , the unknown impedance could be computed

using (8) with the value 1

2

( )p

p

VV

obtained by network

analyzer. In general, the measurement procedures are as follows: a)

measure the impedance inZ in the loop using the impedance measuring function of network analyzer; b) connect injector probes and monitor probes to port 1 and port 2 of the network analyzer, respectively. And connect a known load(usually a resistor R) and measure scattering parameter 11S ,then

evaluate the product 1 2R TK Z by (9) and (10); c) connect LZ

which is unknown to the measuring system; and d) measuring scattering parameters 11S and 21S ,we can obtain the

unknown value LZ using (8) and (9). The current in the loop is not uniformly distributed if the frequencies are higher (above approximately 30MHz), which

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causes large errors in impedance measurements and could be computed with d�0. Usually, d is actual Physical distance. According to transmission line theory, when the frequencies of transmission signal are extraordinary high, the loss could be ignored because of 0 0L Rω >> , 0 0C Gω >> , which

means 0 0R = , 0 0G = .And there is little error.

From transmission line theory of lossless and Fig.3, the current pI could be written as

0

cos( ) sin( )Lp L

VI I d j dZ

β β= + (11)

According to L L LV Z I= , we have

0

[cos( ) sin( )]Lp L

ZI I d j dZ

β β= + (12)

Because pI is proportional to2pV , from (7)

1

1 2 1

1 2

1 2 1

1 2

1

1 2

2

1

1 2

2 0

( )

( )

( ) ( ) c o s ( )

s i n ( )1 ( ) ( )

ML i n

L

M p pi n

p L p

M p ppi n

p p L p

pR T i n

p

pR T

p

VZ Z

IV V V

ZV I V

V V VIZ

V I I V

VK Z d Z

VV dj K ZV Z

β

β

= −

= ⋅ ⋅ −

= ⋅ ⋅ ⋅ −

−=

−

(13)

Where 1 2R TK Z is equal to

1 2R TK Z in (8) multiplied by

0

cos( ) sin( )stdZd j dZ

β β+ .

B. Resonance Method The impedance of low voltage power-line channel is measured using resonant operation principles of LC circuit in this section[6].The schematic diagram of impedance measure-ment is as shown in Fig.4

1V

2V

Fig. 4. The diagram of impedance testing method

The voltages on capacitance and inductance in LC series circuit have the same value and opposite direction in resonance condition, so the total voltage becomes zero. The voltage of second member for transformer is equal to the voltage of first member because the proportion of transformer

T is 1:1. In the resonance condition, total voltage on capacitance and inductance which are cascaded is zero, thus the voltage value is the voltage between hot wire and null line.

The resonance frequency in resonant circuit should

satisfy 1/ 2f LCπ= . So the voltage source needs to produce a voltage in frequency f. The current of right loop for transformer can be obtained by 2 /I V R= , so the impedance value between hot wire and null line is

1 1 2 1 2/ /( / ) /Z V I V V R V R V= = = ⋅ (14)

C. Voltammetry 1) The measuring principle of vector voltammetry

When we know the current vector and the voltage vector on an unknown impedance, the unknown impedance could be computed by ratio, which is vector voltammetry. The unknown impedance

XZ can be obtained by measuring .

SU and .

XU when the standard impedance SZ and

unknown impedance XZ are connected in series as shown in

Fig.5.[7] .

.X

X S

S

UZ ZU

= × (15)

Fig. 5. Schematic diagram of voltammetry approach

Obviously, in order to realize this method, we need to do vector division. Usually, these methods include fixed axis method and free-axis method. Free-axis method is as shown in Fig.6.

XU

SU

1U 3U

2U

4U

Fig. 6. A free axis X-Yplot 2) System and measuring principle

RLC measuring system with free-axis method based on digital phasing, is consist of sine wave source, front measuring circuit, phase-sensitive detector, A/D converter,

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microprocessor, reference phase generator, keyboard, display circuit and so on, [8]which is as shown in Fig.7.

oU

0R XZ

SRXU

SU

S

0I

Fig. 7. Schematic diagram From the above-mentioned coordinate axis, we have

1 2 1 2XU U jU eN jeN= + = + (16)

3 4 3 4SU U jU eN jeN= + = + (17)

Where iN is the digital value of iU , e is the calibration factor of A/D converter that is the voltage value for each number.

Combing (16) with (17), we have .

1 2.

3 4

X

S

U N jNN jNU

+=+

1 3 2 4 2 3 1 42 2 2 23 4 3 4

N N N N N N N NjN N N N

+ −= ++ +

(18)

We could finish vector division by direct operation

D. Ratio Method Input impedance for low voltage power-line from 100

KHz to 2MHz could be measured with ratio method. The measurement principle is as shown in Fig.8. Sine test signals with appropriate amplitude and frequency which are generated by signal source, couple into distribution network by isolation transformer of unit ratio. Input impedance Z can be obtained by voltage 1V (corresponding with current)

measured on constant resistor and voltage 2V measured on socket.[9]

2

1

10VZV

= (19)

1:1

Fμ1.0

Ω10

1V

2VZ

Fig. 8. Circuit used for measuring the input impedance

III. COMPARATIVE ANALYSIS According to the four above-mentioned measurement

principles, comparisons have been made.

For resonance method, the instrument is complicated. In order to meet measuring requirement, first, resonance frequency of coupling circuit should be measured. Then, circuits should be set up including sine signal generator, operational amplifier, power amplifier and signal coupler. And, the measured frequency range is narrow (3~500Hz). We need to change inductance and capacitance for couple circuit. Also absolute value without phase for impedance can just be measured, when the frequency of input signal is near the resonance frequency.

For voltammetry, the vector expression of impedance can be directly gotten, but its circuit is complex. That’s because we need sine wave source, amplifier, phase-sensitive detector, A/D converter, microprocessor, reference phase generator and so on. The range of measured frequency is only from 80Hz to 500 KHz.

For two current probes method, there are some advantages, such as less measurement instrument, simply circuit and wide-range frequency spectrum. Also, the error is about 5% when the range of frequency is below 100MHz, and the error is about 20% when the range of frequency is from 100MHz to 500MHz. Also the formula should be changed because of asymmetry current in the circuit when the frequency is above 30MHz.The effect on input impedance from noise source can not be ignored until the output voltage levels are bigger than the noise level for network analyzer. Otherwise, the measuring result will be inaccurate, and the resolution of spectrum is not high.

The advantage of ratio method is that we can easily get the value of impedance by simple operation. But only the module value of the impedance without phase information in power-line can be gotten.

IV. MEASUREMENT AND ANALYSIS OF IMPEDANCE We measure the impedance in the laboratory of the north

china electric power university by using the two current probe method.

Fig. 9. Power line impedance in the frequency range 30kHZ ~500 kHZ.

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The figure 9 and 10 show that within the frequency band

from 30kHz to 500kHz the impedance of low voltage power network is constantly changing and in general it increases with the increment of frequency.Within the frequency band from 500kHz to 20MHz the impedance changes intensively. When frequency is higher than 1 MHz,the impedance trends upward to a fixed value, so we can improve the reliability of data transmission if we select that band by impedance matching.

For phase we can get trend from the real and imaginary of impedance we measurement. The two figure show that phase of impedance also change with frequency ,especially in broadband the change is severe.

Fig. 10. Power line impedance in the frequency range 500kHZ~20 MHZ

ACKNOWLEDGMENT 1.Project Supported by National Natural Science Foundation of China(NSFC)(60972004);

2.Natural Science Foundation of Beijing City(4122073).

REFERENCES [1] Radford D. Spread-spectrum data leap through AC power writing.

IEEE Spectrum,1996 [2] Justinian Anatory, Nelson Theethayi, Rajeev Thottappillil, M. M

Kissaka, N. H. Mvungi, “The In�uence of Load Impedance, Line Length,and Branches on Underground Cable Power-Line Communications (PLC) Systems. ”IEEE Trans On Power Deliver Vol. 23, No. 1, January 2008.

[3] Fabio Gianaroli, Alessandro Barbieri, Fabrizio Pancaldi, Andrea Mazzanti, Giorgio Matteo Vitetta. “A Novel Approach to Power-Line Channel Modeling. ”IEEE Trans ON Power Delivery, Vol. 25, No. 1, January 2010

[4] VINES R M TRUSSEL H J GALES L J et al “Impedance of the residential power distribution circuit J . ” IEEE Trans onEMC 1985( 27) :6-12.

[5] Peter J.Kwasniok, Man D.Bui,A.James Kozlowski,and Stanislaw S.Stuchly, “Technique for Measurement of Powerline Impedances in the Frequency Range from 500 kHz to 500 MHz,” IEEE Trans. On EMC, Vol.35,No.I, pp.87-90, February 1993.

[6] J. Li, D. Liu and X. Sun, “Impedance measurement and matching for low voltage power-line channel,” Modern electronics technique.vol. 7 No. 34, 2011.

[7] H. Tian R. Yuan F. Li Z.Huang S. Wang S. Li K. Zhong“Measurement on Narrow Band Power Line Communication Channel Impedance of Distribution Network.” International Conference on Consumer Electronics, Communications and Networks. p 454-7, April 2011.

[8] L. Huang and R. He, “Free-axis RLC measurement based on digital phasing [J],” Modern electronics technique. vol. 15 No. 32, pp. 132-137, 2009.

[9] Y. Zhang, S. Cheng, H. He, L. Xiong and J.Nguimbis, “Modeling research of high-frequency carrier communication channel of low voltage power-line,” Automatic control on electrical power system, vol. 26 period 23, pp. 62–66,December 10 2002.

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