TRANSFORMER EFFECTS OF HARMONIC CURRENTS

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A recent phenomenon, large harmonic currents, has been the cause of considerable difficulty in modern electrical installations. Without going into detail on this highly technical subject, a brief description of the problem and its causes is given here.

Conventional electrical loads such as lighting, resistive devices (heaters), motors, and the like are linear (i.e., the load impedance remains essentially constant, regardless of instantaneous voltage). This is not the case with most electronic equipment.

Computers, modems, printers, electronic lighting ballasts, variable-speed motor drives, and solid-state equipment of all types are essentially nonlinear loads. As such, they produce harmonic currents, of which the odd-order ones are additive in the power system neutral conductor. The most troublesome of these are the third harmonic and its odd multiples (9th, 15th, 21st, . . .).

These currents can become so large in a modern computerized office (especially with electronic ballasts) that instead of the neutral conductors carrying the unbalanced current in a 3-phase system (zero in a balanced system), they actually carry more current than the phase wires.

Other serious negative effects of harmonic currents are:

• Deterioration of electronic equipment performance; continuous or sporadic computer malfunctions
• Overheating of the neutral—possibly causing neutral burnout and resulting in equipment being subjected to severe voltage variations
• Overheating and premature failure of transformers—even when the transformer nameplate rating seems adequate
• Overheating of motors because of operation with a distorted voltage waveform
• Nuisance tripping of circuit breakers and adjustable- speed drives
• Telephone interference
• Capacitor fuse blowing

The problem of destructive harmonic currents becomes progressively more severe as the amount of electronic equipment in use increases (as it does continuously).

Today, at least half of the electric load in a modern office-type facility is composed of nonlinear, harmonic-
producing equipment. It follows that all such facilities, both existing and under design, must take necessary corrective measures.

In the past, these measures consisted of oversizing equipment to avoid overload burnout; adding passive harmonic filters (which act to reduce harmonic content) in the electric distribution system; using isolation transformers at sensitive loads; selecting power sources with low output impedance to minimize voltage distortion; using controls that are relatively insensitive to harmonic distortion; adding meters throughout the system that measure true rms voltage and current rather than the average values shown by conventional meters; and other expensive (and essentially passive) power line conditioning.

In view of the increasing severity of the problem, computer-controlled variable power-conditioning equipment (called active line conditioning) has become available. Such power conditioning equipment operates in a fashion similar to that described for active noise cancellation in.

The conditioner instantaneously and continuously analyzes the harmonic content of the line voltage and injects an equal but exactly out-of-phase voltage to cancel the harmonics and produce a pure sinusoidal voltage supply. The harmonic currents that are required by nonlinear loads are supplied by a digital signal generator.

In any retrofit work, the electrical designer must obtain a detailed electrical system analysis for the existing system, performed by engineers experienced in the field of power quality. Many existing systems carry as much as 70% to 80% harmonic current and constitute a major system failure waiting to happen.

A proper power quality study, performed with such instruments as true rms meters, harmonic analyzers, frequency selective voltmeters, and spectrum analyzers, will yield a true picture of an existing system and permit the electrical rehab work to be engineered with harmonic limitations as one of the important design parameters.

TERTIARY-WINDING OVERCURRENT PROTECTION BASIC INFORMATION

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The tertiary winding of an autotransformer, or three-winding transformer, is usually of much smaller kVA rating than the main windings. Therefore, fuses or overcurrent relays set to protect the main windings offer almost no protection to tertiaries.

During external system ground faults, tertiary windings may carry very heavy currents. Hence, to guard against failure of the primary protection for external ground faults, separate tertiary overcurrent protection may be desirable.

The method selected for protecting the tertiary generally depends on whether or not the tertiary is used to carry load. If the tertiary does not carry load, protection can be provided by a single overcurrent relay connected to a CT in series with one winding of the Δ.

This relay will sense system grounds as well as phase faults in the tertiary or in its leads. When tertiary windings are connected by cables, the overcurrent protection provided to the tertiary winding should account for the thermal withstand of the cables.

Alarming and tripping as a result of a prolonged unbalance condition or load tap changer malfunction should prevent damage to cables. If the tertiary is used to carry load, partial protection can be provided by a single overcurrent relay supplied by three CTs, one in each winding of the Δ and connected in parallel to the relay.

This connection provides only zero sequence overload protection and does not protect for positive and negative sequence overload current. In this case, the relay will operate for system ground but will not operate for phase faults in the tertiary or its leads.

Where deemed necessary, separate relaying such as differential type should be provided for protection for phase faults in the tertiary or its leads. The setting of the tertiary overcurrent relay can normally be based on considerations similar to those in line time overcurrent.

However, if the tertiary does not carry load, or if load is to be carried and the three CT, zero sequence connection is used, the associated overcurrent relay can be set below the rating of the tertiary winding. This relay should still be set to coordinate with other system relays.
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