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Power line communication technology has been used widely during recent years. It is a technique using electrical networks to deliver information. Since these power grids have already been built, it is not necessary to build a new one to transmit digital signals.
The basic theory of power line communication technique is coupling the high frequency signal on to the power line and then using these cables to transmit data. At the receiver, the modem will separate the high frequency signal from the power line and transmit it into computers or telephones.
But as we all know, we must transmit strong power simultaneously and the stability of the power network is not so good, it is difficult to analyze the electric and magnetic environment around the power line in a few words. Worse still, there is no shielding outside the power line (it is impossible to shield, I think). So there must be some problems due to poor electromagnetic compatibility.
Power transmission line is designed with the purpose of low loss of power. When it is used to transmit high speed data, the frequency of the signal should between 3-30MHz. Signals transmitting at this high frequency maybe radiated as electromagnetic wave because of the leakage on the power line. In this case, power cables could be considered as a low power linear antenna. It is just like an opening wireless channel suffering from the noise, attenuation and multipath interference. Additionally, the length of the cable is similar to the wavelength of magnetic wave below the frequency of 30MHz. If it is radiated into the space, maybe it will have a bad effect on the sensitive short wave radio like amateur radio or various wireless security services operating within the frequency range from 1 to 30MHz.
The cables that carrying power line communication signals could be considered as a radiation source. The amount of radiation depends on how symmetrical the internet is when it is working on wireless frequency. Nonsymmetrical will generate an unwanted signal. Unfortunately, power line communication network is not perfect in this, so the asymmetrical of the impedance will generate radiation.
Elements in power line communication network like PLC modems, repeaters, gateways and so on can be considered as a huge electronic system. So power line communication system may suffer internal interference and external interference. On the other hand, if the transmission of power has been failed due to poor EMC, this will be a disaster for electric power provider. The loss of money is immeasurable. The unwanted change of the voltage of power may bring some unexpected things to the receiver.
2. EMC issues in power line communication system
The possible reasons for the internal interference are as follows:
When power getting through distributed capacitance and insulated resistance, there may be a leakage which will generate interference (related to the working frequency).
Some of the components inside the system may generate heat which can affect the stability of the components themselves as well as other components.
Impedance of the source and transmission line could be coupled or mutual inductance between conductors can generate interference.
The electric or magnetic field generated by some components (with high power or voltage) may be coupled and then interfere other components.
There are some other reasons for external interference:
External high voltage, source will generate interference to electrical line, equipment or system through power leakage.
Some equipment with high power may generate strong magnetic field which may interfere system or equipment through mutual coupling.
Magnetic wave which exists in the space may affect electrical line.
The radiated EMI noise becomes more and more serious with the widely using of power line communication technology. The main cause of radiated EMI noise is common mode current without taking differential mode current in to account. In power line communication system, conducted EMI noise current along the transmission line launching a free-space radiated field, thus radiated noise comes from the conducted EMI noise current .
The conducted EMI noise current consists of two component noise, common mode noise current and differential mode noise current. Here is an EMI model in power line communication system
EMI model in PLC system
ICM represents common mode noise current. It is generated in the live line noise and neutral line noise lead to ground line.
IDM represents differential mode noise current. It is generated between live line noise current and neutral line noise current .
From the above equations, we can say that the amplitude of the differential mode noise current is smaller than the amplitude of the common mode noise current. Thus, the interference of power line is mainly due to common mode noise current. In order to estimate the radiated EMI noise on power line, we need to analysis the common mode noise current in conducted EMI noise. The conducted EMI noise includes VL (voltage on live line) noise and VN (voltage on neutral line) noise. The sum of VL and VN is the total noise. It can be separated into common mode noise and different mode noise by vector sum and subtraction .
The characteristic of common mode noise separation defined as common mode insertion loss (CMIL) and different mode rejection ratio (DMRR) . , , VOC and VCM represent input and output of common mode voltage , VOD and VDM represent input and output of different mode voltage.
If the power line is in an ideal situation, that is absolutely symmetrical, VDM is zero (there is no unwanted signal on the line), DMRR is -, the maximum CMIL is large. Then the common mode noise signals will loss a lot.
But in practical, it is asymmetrical, so the value of VOC/VCM may be less than 1 (that is just assumption, I don't know the accurate value), the maximum value of CMIL is approximately -1dB. In this situation, the common mode noise signal transmission loss is very small. Again, because of the asymmetrical, the value of DMRR may between -40 to -60dB in the frequency range of 60-90MHz.
Telecommunication port asymmetrical (tested with T-network) directly relevant to CM current EMI emission limits of EN55022, 1998 listed the conducted limits:
0.15 to 0.5MHz QP 66 to 56 (dBμV)
0.5 to 5MHz QP 56
5 to 30MHz QP 60
AV is not given, but needs to be net 10dB less .
3. How to improve?
1. Make good use of the power line
The strength of radiation in power line communication system depends on how symmetrical power line (network) is. Signals should be transmitted in differential mode (the value of the differential mode current should be equal but the direction is opposite to achieve differential mode transmission as far as possible). The characteristic of high symmetrical power line is that the ratio of different mode and common mode is large, so the radiation is small .
There are three specific measures in order to achieve this. First, install the filter on the connection point close to transformer. Second, install the filter inside the modem of power line. Third, use transformer and common mode choke to reduce common mode noise.
2. Reduce the power spectrum density of high frequency signal in the system. Because the measurement of electromagnetic radiation is below the frequency of 30MHz, so reduce the power spectrum density of the signal will reduce the voltage of radiation. And it will not influence the total power.
3. Choose a reasonable modulation. Orthogonal frequency division multiplexing (OFDM) is an efficient modulation technique. The basic idea is to transmit the data stream across many sub-carriers to reduce the signal rate in order to enhance the anti-multipath fading ability.
Based on the above analyses of the EMC aspects in power line communication system, we can install an EMI filter at the receiver end to reduce the common mode current noise. Because two power lines cannot be completely overlapped, so the electromagnetic field cannot be offset by differential mode current. Differential mode noise current should be taken into account.
Here is an EMI filter circuit
EMI filter circuit
C1 and C2 are differential mode rejection capacitors, C3 and C4 are common mode rejection capacitors, and L is common mode inductor. L is twisted on the ferrite core; this can be considered as common mode chokes. Common mode chokes showing a large inductance and will reduce common mode signals, but it has little influence on different mode signals. Both common mode and differential mode current can generate interference. This filter must attenuate all these signals. High frequency signal can be easily passed through capacitance. Let high frequency noise current from live line and neutral line floating through ground line (this is common mode) or let high frequency noise current floating from live line to neutral line (this is different mode). The impedance of the inductor can reflect the high frequency noise current to the source of interference.
The purpose of using ferrite core here is that ferrite core is an inductor used as a passive low-pass filter. The geometry and electromagnetic properties of coiled wires over the ferrite core result in a high impedance (resistance) for high frequency signals, attenuating high frequency EMI electronic noise. The absorbed energy is converted to heat and dissipated by the ferrite .
Making assumption that the value of different mode capacitance C1 and C2 is 7000 pF, the value of common mode capacitance C3 and C4 is 0.015μF, L is 3.7mH. The simulation result shows that the higher the frequency of the noise signal, there will be more attenuation when the noise signal getting through the filter . The frequency of common mode noise signal is above 2MHz, it will be attenuated when passing through the filter.
Using switching power supply (the input voltage is 24V, output voltage is 12V and the power is 25W) to simulate input signal. The voltages before filtering and after filtering showing on the oscilloscope with the bandwidth of 20MHz are 50mV and 5mV respectively. Using the equation, the value is -20dB that is the attenuation of the noise signal is 20dB.
Electrical power is connected to the equipment under test (EUT) by power line after through line impedance stabilization network (LISN). The conducted EMI noise is tested by detection device when it passes through the line impedance stabilization network. The result of the testing is the total noise consists of VL noise and VN noise.
Radiated EMI noise
Measurement of EMI noise
In power line communication system, conducted EMI noise current along the transmission line may launch a free-space radiated field. The calculation of the field strength is according to this equation
Z0 is the free space wave impedance (377Ω) , l is the length of the conductor, I is the current, r is the measuring distance, λis the wavelength related to the frequency, is the angle between field point and current direction, . The length of the conductor is far longer than the wavelength with the increasing of the frequency, current along the line is no longer uniformed. The line will be divided into N sections evenly in order to solve this problem, using radio frequency current probe in the middle of each above their respective current measurement, respectively for I1, I2 ï¹’ï¹’ï¹’ 
Radio frequency current probe schematic
The overall calculation of equivalent radiation field is according to the equation below
EC is the radiation field, L is the equivalent length of each antenna, f is the standard test distance of open area test sites (OATS), H is the height of test antenna, F is the modifying factor in calculate OATS, is the initial phase of each cable .
The radiated EMI noise of power line communication is not the same in different places of the cable. It is necessary to find the place of maximum conducted EMI noise current in order to suppress the radiated EMI noise. When power line communication system is working, set the current probe in different place (for example, near EUT, middle of transmission line, near LISN) of transmission line may find out the place of maximum conducted EMI noise current. Experiment result show that near EUT, the value of conducted EMI noise current is higher than the other two places.
Then test the circuit with and without the ferrite core.
Radiated EMI noise
Experiment result shows that when access ferrite core, radiated EMI noise drop a lot near the source. So the conclusion is that accessing ferrite core can suppress the radiated EMI noise.