Typical Power Quality Problems
Computer lockups
Earth current originating in equipment, results in a voltage drop between the equipment and earth. Although small, this noise voltage may be significant when 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.
Flickering lights (P28)
Flicker is caused by load switching within electronic apparatus and is commonly produced by devices such as arc-furnaces, rolling-mills and multiple welder loads, electronic ballasts, light dimmer switches. When the supply cannot fulfil the current demand, the ac voltage will temporarily dip and the effect on a 60W incandescent light bulb connected to the same supply point would be a temporary reduction in light, which if repeated would constitute flicker. The amplitude and frequency of these deviations can cause incandescent lamps to flicker. This is not only potentially annoying, but it can trigger seizures, especially in people with epilepsy and is responsible for the so called ‘sick building syndrome’.
Overheating of transformers
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 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 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.
Skin effect
Individual conductors (or grouped conductors) carrying alternating current will be surrounded by an alternating magnetic field, which will induce opposing eddy currents within the conductor itself. Near the axis of the conductor those currents will tend to reduce the main current, but near the surface they will increase it. The result is a non-uniform distribution of the current across the conductor with more current at the surface than the middle. For a.c. the induced e.m.f.’s increase with frequency until the current in flowing in a thin layer only (or skin). The effect is more pronounced with higher order harmonic currents, but a load with a 3rd harmonic current will increase the effect. 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.
PLC problems
Severe harmonic distortion can create additional zero-crossings within a cycle of the sine wave. Microprocessors in process control equipment use the zero crossing as a reference point for synchronisation and therefore 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? For an unexplained reason, the network locks up, e-mail services fail, it is no longer possible to print...
Misapplied 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. This problem can be easily overcome by applying the correct type of power factor correction equipment through the use of tuned reactors connected in series with the capacitors. The above voltage distortion levels show a marked reduction when switching the PFC on and off!
In the last 2-3 years the majority of customer requests to attend site for power quality related issues have been traced to incorrectly applied power factor correction equipment.
Voltage flat topping
SMPS as found in IT equipment only draw current at the peak of the voltage waveform. A site with a significant amount of single-phase SMPS will produce voltage distortion in the form of flat-topping of the supply Voltage. Since current is consumed only at the peak of the voltage waveform (to charge the smoothing capacitor), voltage drop due to system impedance will also occur only at the peak of the voltage waveform. This reduced peak voltage will translate to a lower DC bus voltage in the SMPS. Input current to the SMPS will increase because the computer or other electronic load still requires the same amount of power. These increased I2R losses in the SMPS will not only accelerate the aging of its components they will in effect increase the kWh consumption. Also of concern is that the power disturbance ride-through capability will also be reduced since the reduced peak voltage means the large filter capacitor on the DC bus of the SMPS will not be able to store as much energy.
Overloaded Neutrals
In a 3-phase circuit, there are 3 active conductors, and a return conductor, which carries the unbalance between the 3 phases. However, with the triple-n harmonics summating, significant currents can flow in the neutral conductor. As many neutral conductors have been, in the past, half-sized, this situation can become critical, even when the phase conductors are operating well below full load. In some instances the current flowing in the neutral can actually exceed the current flowing in the individual phases.