You have seen them: tall, gangling, steel structures-known as high-tension lines-that transport hundreds of kilovolts. These rights-of-way are used to safely move transmission-level bulk power from the generators to the distribution substations. But the voltage levels do not start out or end up at those levels at the point of utilization.

Then why the need for such high-voltage levels? It is good old Ohm's and Kirchoff's laws, once again. Those same laws that explain why sags occur when large motors start also apply to happenings on the electrical provider's generation, transmission and distribution systems, known as the grid.

Most power quality related problems originate from sources within a facility, but let's review what happens outside the facility.

Most power plants produce in the range of 12kV to 15kV. Though the source of energy that turns the generators vary-coal, natural gas, water, nuclear and wind-all but wind have the common characteristic of generating a pure sine wave of voltage.

Since power is voltage multiplied by current, providing enough power for the demand of the metropolitan areas at medium voltage levels would require tremendous currents and very large diameter cables to carry that current. Instead, the voltage is stepped up at the transmission substations to 69kV to 765kV and sent on its way through smaller wires installed on the tall towers.

This results in lower current levels for the same power delivery, since the voltage drop in the wires is proportional to the current. The overall system efficiency is greater, despite whatever losses occur in the transformers.

Whereas power quality disturbances are unlikely to occur on transmission lines, contact from airplanes, hot air balloons, untended vegetation and lightning could provide paths for phase-to-phase or phase-to-ground shorts, resulting in dramatic arcs that are usually short in duration. Such systems are protected by fast-acting breakers that operate in less than six cycles with one recloser attempt about a half-second later, allowing time for air to de-ionize.

Having a half-million volts running down tree-lined neighborhood streets would not be practical or attractive, so the high-voltage transmission levels are reduced back down at distribution substations to medium-voltage levels, generally between 5kV and 35kV.

A distribution substation is fed by one or more sets of transmission lines; then the power is distributed to three to 10 distribution lines. These are the voltage levels found on the utility poles (often wood) found running along residential and industrial streets.

Most industrial facilities are fed by three-phase power reduced by a facility transformer to 480V, though larger industrial facilities may have their own substations and be fed the medium-voltage levels directly. Residential units usually have a single phase that is center-tap grounded to produce two 120V circuits.

Substation breakers protect the distribution feeders, though they have more recloser attempts than a transmission breaker. Faster-acting fuses also protect individual sections of the feeder. If a fuse blows and affects a small number of customers, it is better than interrupting a large number of customers on the feeder if the fault is not easily cleared.

Because distribution systems are closer to the ground, the following can cause transients, sags and interruptions: motor vehicles hitting poles; incidental contact with vegetation, animals and falling tree limbs; and lightning.

Take a sag resulting from voltage breakdown between a distribution line and a ground path (tree limb). This occurs only when the voltage is at the peak of the waveform, where there is enough potential to break down the tree's insulating nature, making it into a high-impedance conductor.

A sag occurs until tree branch movement in the wind increases the impedance enough to where the path breaks at the subsequent cycle's peak and the voltage is restored. Such high-impedance faults do not have enough current flow for the fuse or the distribution breaker to operate, though they can result in complaints of the lights flickering.

Having a facility with an electric-arc welder or electric-arc furnace can cause harmonic and voltage-fluctuation problems for other facilities connected to the same distribution system (though the latter is more likely connected at the subtransmission level). Transients that occur on the different parts of the grid will have different ringing frequencies, as the inductor and capacitive components are different.

For example, the ring frequencies of oscillating transients would typically be the following: medium voltage-400Hz to 2KHz, building distribution-10k to 100kHz, branch circuit-50k to 250kHz, and power cord-500k to 2MHz [IEEE Std 1159]. The further away from the source of the transient, the transient will be seen as lower in magnitude and frequency content, as the impedance of the wires acts to attenuate the waveform.

By looking at the characteristics of the disturbances and comparing it to what types of events and system responses are likely at different parts of the grid, the where and why questions are easily answered. You may not be able to directly fix the problem if it is occurring on the grid, but any information that you show the utility engineer will help him or her resolve the problem sooner. EC

BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.