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Copperinfo Harmonics

Alternating current tends to flow on the outer surface of a conductor. This is known as skin effect and is more pronounced at high frequencies. Skin effect is normally ignored because it has very little effect at power supply frequencies but above about 350 Hz, i.e. the seventh harmonic and above, skin effect will become significant, causing additional loss and heating. Where harmonic currents are present, designers should take skin effect into account and de-rate cables accordingly. Multiple cable cores or laminated busbars can be used to help overcome this problem. Note also that the mounting systems of busbars must be designed to avoid mechanical resonance at harmonic frequencies. Design guidance on both these issues is given in CDA Publication 22, ‘Copper for Busbars’. Problems caused by harmonic voltages Because the supply has source impedance, harmonic load currents give rise to harmonic voltage distortion on the voltage waveform (this is the origin of ‘flat topping’). There are two elements to the impedance: that of the internal cabling from the point of common coupling (PCC), and that inherent in the supply at the PCC, e.g. the local supply transformer. The former is illustrated in Figure 15. The distorted load current drawn by the non-linear load causes a distorted voltage drop in the cable impedance. The resultant distorted voltage waveform is applied to all other loads connected to the same circuit, causing harmonic currents to flow in them – even if they are linear loads. The solution is to separate circuits supplying harmonic generating loads from those supplying loads which are sensitive to harmonics, as shown in Figure 16. Here separate circuits feed the linear and non-linear loads from the point of common coupling, so that the voltage distortion caused by the non-linear load does not affect the linear load. When considering the magnitude of harmonic voltage distortion it should be remembered that, when the load is transferred to a UPS or standby generator during a power failure, the source impedance and the resulting voltage distortion will be much higher. Where local transformers are installed, they should be selected to have sufficiently low output impedance and to have sufficient capacity to withstand the additional heating, in other words, by selecting an appropriately oversized transformer. Note that it is not appropriate to select a transformer design in which the increase in capacity is achieved simply by forced cooling – such a unit will run at higher internal temperatures and have a reduced service life. Forced cooling should be reserved for emergency use only and never relied upon for normal running. Induction Motors Harmonic voltage distortion causes increased eddy current losses in motors in the same way as in transformers. However, additional losses arise due to the generation of harmonic fields in the stator, each of which is trying to rotate the motor at a different speed either forwards or backwards. High frequency currents induced in the rotor further increase losses. Where harmonic voltage distortion is present motors should be de-rated to take account of the additional losses. Zero-crossing noise Many electronic controllers detect the point at which the supply voltage crosses zero volts to determine when loads should be turned on. This is done because switching reactive loads at zero voltage does not generate transients, so reducing electromagnetic interference (EMI) and stress on the semiconductor switching devices. When harmonics or transients are present on the supply the rate of change of voltage at the crossing becomes faster and more difficult to identify, leading to erratic operation. There may in fact be several zero-crossings per half cycle. Harmonic problems affecting the supply When a harmonic current is drawn from the supply it gives rise to a harmonic voltage drop proportional to the source impedance at the point of common coupling (PCC) and the current. Since the supply network is generally inductive, the source impedance is higher at higher frequencies. Of course, the voltage at the PCC is already distorted by the harmonic currents drawn by other consumers and by the distortion inherent in transformers, and each consumer makes an additional contribution. Clearly, customers cannot be allowed to add pollution to the system to the detriment of other users, so in most countries the electrical supply industry has established regulations limiting the magnitude of harmonic current that can be drawn. Many of these codes are based on the UK Electricity Association’s G5/3 issued in 1975, recently replaced by G5/4 (2001). This standard is discussed in detail elsewhere in this Guide. Harmonic mitigation measures The measures available to control the magnitude of harmonic current drawn are discussed in detail in later sections of this Guide. In this section a brief overview is given in generic terms. Mitigation methods fall broadly into three groups; passive filters, isolation and harmonic reduction transformers and active solutions. Each approach has advantages and disadvantages, so there is no single best solution. It is very easy to spend a great deal of money on an inappropriate and ineffective solution; the moral is to carry out a thorough survey – tools suitable for this purpose are described elsewhere in this Guide.