Quoted by AMIR KHASHAIAR :
" This current will interfere with other systems particularly communication systems ."
I agree with AMIR. Indeed we can use delta connection as a three harmonic current trap.
Telephone circuits are particularly susceptible to the influence of ground return currents. In rural areas when power or telephone circuits use a ground return, the large inductive loop can have a large influence on the noise produced in telephone circuits. Particular care should be used to minimize nonlinear loads in this case.
A voice band telephone channel is normally designed to pass frequencies between 300 and 3000 Hz. Although harmonics in this range of frequencies are very small compared with the fundamental, they still have an effect on the telephone reception.
The effect that a ‘noisy’ power line will have on a communication line may be ascertained by considering the susceptiveness of the circuit to the effects of inductive interference. Three characteristics are of importance in this respect:
(i) the relative interfering effects of different frequencies;
(ii) the balance of the communication circuit; and
(iii) shielding effects of metallic cable sheaths and other buried metallic plant.
Standard weighting curves are used to take into account the response of the telephone equipment and the sensitivity of the human ear to the harmonic frequencies. Two weighting factors are in common use:
(1) the psophometric weighting by the CCITT, extensively used in Europe;
(2) the C-message weighting by Bell Telephone Systems (BTS) and Edison Electric Institute (EEI), used in the USA and Canada.
Using these weighting factors, the total weighted transverse (or metallic) noise is obtained from the expression:
where Vm is the metallic mode weighted voltage, Vcn is the longitudinal induced voltage,
N is the maximum order of harmonic to be considered, Cn is the psophometric or the C-message weighting factor of harmonic n, Kn is the telephone circuit shielding factor at harmonic n and Bn is the telephone circuit balance at harmonic n. The last two coefficients require detailed information on the telephone systems cables and design as well as knowledge of the local earth resistivities.
Telephone Form Factor
In the psophometric system the level of interference is described in terms of a telephone form factor (TFF), which is a dimensionless value that ignores the geometrical configuration of the coupling and is expressed as
where Un is the component at harmonic n of the disturbing voltage, N is the maximum harmonic order to be considered,
U is the line to neutral total r.m.s. voltage, Fn = pn nf0/800, pn is the psophometric weighting factor and f0 is the fundamental frequency (50 Hz). The required limit of TFF is typically 1%.
The CCITT directives recommend that the total psophometric weighted noise on a telephone circuit has an e.m.f. (i.e. open circuit voltage) of less than 1 mV. When measuring the noise voltage the telephone circuit is terminated with its characteristic impedance (which is a resistance of the order of 600 ohm) and the noise voltage is measured across such resistance. Therefore, the psophometrically weighted noise across the terminating resistor must be less than 0.5 mV.
As a general rule, if the TFF is greater than 0.5 mV it is likely to cause interference to telephone services. It must be stressed that the TFF is only a guideline measurement; it is not satisfactory as the sole measure of interference to a communication line as it takes no account of coupling and exposure factors.
Telephone Influence Factor
The C-message weighting system uses the telephone influence factor (TIF) instead of the TFF of the psophometric system. Again, TIF is a dimensionless value used to describe the interference of a power transmission line on a telephone line, and is expressed as
where Un is the single frequency r.m.s. voltage at harmonic n, N is the maximum harmonic order to be considered, U is the total line to neutral voltage (r.m.s.), Cn is the C-message weighting factor, f0 is the fundamental frequency (60 Hz), and Wn is the single frequency TIF weighting at harmonic n (the relationship between the C-message and TIF weighting factors is Wn = Cn 5nf0 and the corresponding coefficients for the first 25 harmonics are shown in Table below).
The TIF weights account for the fact that mutual coupling between circuits increases linearly with frequency, while the C-message weights do not take this into consideration.
Because of this coupling relationship the TIF weights peak at about 2.6 kHz (as compared with the 1 kHz peak of the C-message weighting curve).
The TIF index models the effectiveness of induction between adjacent circuits, and is thus particularly useful to assess the interference of power distribution circuits on analogue-type telephone systems (many recent telephone circuits, however, are of digital-type design).
Instead of voltage, the TIF is more usefully expressed in terms of line current because the electromagnetic induction relates to line current amplitude. Also, the line currents are best represented by their sequence of rotation, i.e. positive, negative and zero sequences, respectively. The relationship between the phase and sequence currents is:
As the power circuit is three-phase and the audio circuit single phase, the latter cannot distinguish between positive- and negative-sequence signals, but the effect of zero sequence is very different.
Telephone circuits are more affected by zero-sequence harmonics because these are in phase in the three phases and add arithmetically. Generally, standards are less tolerant of zero-sequence harmonics to take this fact into account. When only the zero sequence signals are included in above equation, the term residual is used in the TIF (which in the case of a balanced three-phase system include only triple frequency components). The term balanced is used when the signals included in above equation are only of positive and negative sequence. The latter contribution to the induced noise is important in the immediate proximity of the transmission line, while the residual signals are the dominant ones at greater distances from the line. When the balanced signals are expected to contribute significantly to the induced noise, they must be included in the calculation of the TIF, i.e.
where the suffixes r and b indicate residual and balanced, respectively.
More modern telecommunication services such as ADSL utilize the spectrum from about 25 kHz up to 1 MHz and beyond. As they are not using the low frequencies they are proving to be tolerant of typical power line noise, but they are likely to be sensitive to electrical fast transients.