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Copperinfo Power Quality and Utilisation

Earth current originating in the equipment results in a voltage drop between the equipment and true earth. Although small, this noise voltage may be significant compared with the signal voltages (of a few volts) on which IT equipment operates. PC hardware is designed to minimise sensitivity to this kind of disturbance but it cannot be eliminated entirely, especially as the noise frequency rises. Modern communications protocols have error detection and correction algorithms built in, requiring retransmission of erroneously received data – and consequently reducing the data throughput. As a result, PCs will often slow down or lock-up, a frequent phenomenon in today’s office environments. In a TN-C network, the combined neutral-earth conductor actively carries current, creating voltage drops. The earth reference plane of different computers on different floors is no longer at the same potential. Currents will flow, for example along the shields of data cables, connected to earth at both ends for EMC compliance. Flickering screens Triple-n harmonic currents sum in the neutral conductor. In a TN-C configuration the neutral and protective conductor are combined and connected in many places to the structure of the building. As a result, neutral return currents can flow anywhere in the metal structure of the building and create uncontrolled and uncontrollable magnetic fields. In extreme cases, these fields can result in flicker of computer screens. Neutral current always needs to be returned to the point of common coupling using a separate conductor as in the TN-S and TN-C-S systems. In fact, the discipline of having one and only one neutral-earth connection point in the installation improves safety and EMC. Flickering lights Short duration voltage changes, resulting from switching, short-circuits and load changing can result in light flicker. The permissible magnitude of light flicker is regulated by International Standards, based on perception criteria. Excessive flicker can cause migraine and is responsible for some instances of the socalled ‘sick building syndrome’. Overheating of transformers at moderate load Harmonics cause additional losses in the transformer. When the transformer is close to maximum load, these losses can lead to early failure due to overheating and hot spots in the winding. With the current trend to push equipment harder to its limits, and the increasing harmonic pollution in low-voltage networks, this problem is occurring ever more frequently. Losses in transformers are due to stray magnetic losses in the core, and eddy current and resistive losses in the windings. Of these, eddy current losses are of most concern when harmonics are present, because they increase approximately with the square of the frequency. In a typical mixed load building the transformer eddy current losses will be about 9 times higher than would be expected, approximately doubling the total load losses. Before the excess losses can be determined, the harmonic spectrum of the load current must be known. Induction motors Voltage harmonics cause extra losses in direct line-connected induction motors. The 5th harmonic creates a counter-rotating field, whereas the 7th harmonic creates a rotating field beyond the motor’s synchronous speed. The resulting torque pulsing causes wear and tear on couplings and bearings. Since the speed is 2 Power Quality Self-assessment Guide fixed, the energy contained in these harmonics is dissipated as extra heat, resulting in premature ageing. Harmonic currents are also induced into the rotor causing further excess heating. The additional heat reduces the rotor/stator air gap, reducing efficiency even further. Variable speed devices cause their own range of problems. They tend to be sensitive to dips, causing disruption of synchronised manufacturing lines. They are often installed some distance from the motor, and cause voltage spikes due to the sharp voltage rise times. Special care has to be taken at start-up of motors after a voltage dip when the motor is normally operating at close to full load. The extra heat from the inrush current at start-up may cause the motor to fail. Optimum sizing of motors should take into account: ‹ that the motor has been designed to run at maximum efficiency at about 70 % load ‹ frequency of voltage dips, and time one can afford to wait to resume motor operation. Overheating of conductors due to skin effect All harmonics cause additional losses in the phase conductors. The skin effect, which is negligible at 50 Hz, starts to play a role from 350 Hz (7th harmonic) and upwards. For example, a conductor with 20 mm diameter has 60 % more apparent resistance at 350 Hz than its dc-resistance. The increased resistance, and even more, the increased reactance (due to higher frequency), will result in an increased voltage drop and an increased voltage distortion. Correct functioning of process control equipment Severe harmonic distortion can create additional zero-crossings within a cycle of the sine wave, affecting sensitive measurement equipment. Synchronisation of process control equipment in continuous manufacturing may be disturbed and PLC devices may lock up. Data network congestion Earth leakage currents cause small voltage drops along the earthing conductor. In a TN-C network, the combined earth-neutral conductor will constantly carry significant current, dominated by triple-n harmonics. Due to the increasing use of low-voltage signals in IT equipment, bit error rate increases, up to the point that the entire network locks up. How many large and small, privately owned networks enjoy this phenomenon almost on a weekly basis? For an unexplained reason, the network locks up, e-mail services fail, it is no longer possible to print … Problems with power factor correction equipment Harmonic frequencies may coincide with resonant frequencies of the combined stray inductance and power factor correction (PFC) equipment, creating excessive voltage or current and leading to premature failure. Moreover, as a general problem, measurement devices may not correctly measure the loading of the PFC, as they incorrectly measure the harmonic content in the current (see Section 3.2.2 of this Guide). Problems with specific (long) lines or when switching heavy loads Long lines mean higher impedance, resulting in higher voltage disturbances from inrush currents, for example when a heavy motor starts up, or when switching on computers. Harmonic currents generated by variable speed drives, or switch-mode power supplies, located at the end of long lines, result in higher harmonic voltage distortion. Therefore, upsize long power lines for low voltage drop. As a side benefit, upsized power lines will have lower losses. When loaded more than 3,000 hours, the economic payback will be very short.