Kostas Gouramanis

Ampacity of Low-Voltage Power Cables Under Nonsinusoidal Currents

This paper investigates the ac resistance in the presence of harmonics and proposes ampacity derating factors for cables made according to CENELEC Standard HD603. These cables are widely used in low-voltage industrial and building installations. Four-conductor cables of small, medium, and large conductor cross sections are considered. The fourth conductor is used as the neutral conductor. The cables are modeled using finite-element analysis software. The ac/dc resistance ratio is shown to increase with the frequency of the current and the cross section of the conductor, the increase being much larger when zero-sequence harmonics are present. A derating factor is defined and calculated for five typical nonsinusoidal current loads, for example, computer equipment. The derating of the cables ampacity is shown to be very large when zero-sequence harmonics are present. The cross section of the neutral conductor is shown to be significant only when zero-sequence harmonics are present. The validity of the method is verified by comparison with data given in IEEE Standard 519-1992 and with measurements conducted on a cable feeding a large nonlinear load

Sheath voltage calculations in long medium voltage power cables

This paper investigates the operation of single core underground medium voltage cables connected in parallel. The examination is based on an existing power cable arrangement connecting a 38 MW wind farm with the transmission grid. The cable arrangement consists of nine single core cables connected in parallel to form a triple 3-phase system. Several connection scenarios such as the earthing of the cable sheaths at one or both ends and the application of sheath cross-bondings are examined. Various simulation parameters are also investigated such as the grounding resistance of the cable sheaths and the number of the cable sheath transpositions. The respective voltages and currents induced on the cable sheaths are calculated under steady-state and short circuit conditions.

Natural canceling of current harmonics in office loads and its effect upon the transmission capacity of distribution cables

Office loads and especially computer-type ones, draw significant zero-sequence harmonic currents, which in a TN-S type network, add up in the neutral conductor. In this paper, measurements have shown that fluorescent lighting loads that employ magnetic ballasts draw a 3rd harmonic current that is almost in opposition with the 3rd harmonic current drawn by a computer. This leads to a natural reduction of the 3rd harmonic current in the neutral conductor when mixing these load types and thus to an increase of the transmission capacity of low-voltage power cables. This increase is also verified mathematically by a significant increase of the ampacity derating factor of these cables when they feed mixed computer and fluorescent lighting loads.

Voltage Multiple-Zero-Crossings at Buses Feeding Large Triac-Controlled Loads

Triac-controlled loads operating in three-phase four-wire electric networks produce large zero-sequence current harmonics that cause large currents in the neutral conductor. Under certain conditions, the phase-to-neutral voltage (PTNV) at the bus feeding a large triac-controlled load is distorted to such a degree that multiple zero crossings (MZC) appear in the voltage waveform. This situation may cause malfunction of the triac control systems or any other control systems that are based on the detection of voltage zero crossings. The influence of various parameters of the triac-controlled load and of the cable feeding it, on the appearance of voltage MZC, is investigated. A simulation model is built with alternative transients program version of Electromagnetic Transients Program and is validated by comparison with measurements taken at a real theatrical lighting installation controlled by triacs. The occurrence of MZC in the voltage waveform is also explained by an analytical approximation. It is shown that certain cable configurations should be avoided when feeding large triac-controlled loads, or otherwise, MZC will appear in the PTNV.

Harmonics-induced problems in theatrical-lighting installation: Real-case measurements and proposed solutions

Professional light-dimming equipment used in theaters, usually employees triacs to control the rms value of voltage on the light bulbs. This paper presents a real-case problem where, voltage harmonic distortion, induced by the high harmonic currents of the light-dimming equipment, led to dimming-control malfunction. It is shown that control systems based on the detection of voltage zero-crossing may not function properly when high triplen (3rd, 9th, 15th, 21st, etc) current harmonics -such as those produced by triac operation- flow in a circuit. Measurements of current and voltage harmonic distortion at the Royal Theatre of Thessaloniki, Greece are presented. Computer simulation shows that the arrangement of the cable feeding the triacs, plays a significant role on the harmonic voltage distortion, whereas the cable cross-section has a smaller effect.

Cable Overheating in an Industrial Substation Feeder Due to Untransposed Power Cables - Measurement and Simulation

This paper presents measurements of asymmetrical current distribution among parallel connected cables on the secondary side of 20/0.4kV transformers. Overloading of some cables is shown to lead to premature cable ageing. Simulation of the situation by EMTP shows that simple and affordable cable layout - other than cable transposition - can significantly deteriorate the current asymmetries. The increase in cable temperature and losses as well as the deterioration of cable useful life is calculated for various cable configurations

Voltage Harmonic Distortion at Buses Feeding Office Loads

The total harmonic distortion of the phase-to-neutral voltage at buses feeding office load is investigated for various cable configurations, lengths and cross-sections. The investigation is carried out with a software model which was validated with approximate analytical expressions and measurements taken at a real installation. It is shown that the selection of a cable should based not only on its ampacity and allowable voltage drop but its configuration should be taken into account in order to avoid excessive voltage harmonic distortion. Configurations where the neutral conductor is part of a four-core cable cause relatively small harmonic distortion while cable configurations where the neutral conductor runs separately cause high voltage harmonic distortion.