Electrical Cable Protection: Overcurrent, Overvoltage & Fault Detection
Protection Against Overcurrent and Overvoltage
The goal of protecting cables and limiting surge currents is to prevent them from exceeding their maximum allowable temperatures and to prevent potential fires.
Overloads
A facility or part of it is considered to be subject to an overload when, for some time, it is traversed by a current greater than the normal or assigned current, without any malfunction or failure in the installation. In short, the surge produces a reduction in the life of conductors. We must keep in mind that underground cables cannot have their sections changed; new lines must be designed.
Shorts
Short circuit faults will be analyzed as direct contacts. Contacts are defined as parts of the plant at different potentials with a fault impedance of zero or negligible. Considering the impedance of zero defects, shorts tend to lead to large current surges, which cause a temperature rise in conductors (thermal effect). Conductors may not exceed the maximum temperature, called CBT.
Common wire and insulation temperatures are:
- Crosslinked polyethylene (XLPE) …… 250 º C
- Ethylene propylene (EPR) …………………… 250 º C
- Impregnated paper …………………. 220 º C
The most common causes of short circuits are:
- Defects in the connected loads
- Insulation faults
For the network to be protected against short circuits, the following must be met:
- The breaking capacity of the protection elements must be greater than or equal to the maximum short circuit current intensity.
- Conductors must carry the maximum short circuit current for the duration.
The choice of the section and quality of insulation of the leads is very important to ensure a secure supply quality. As suppliers, companies choose to use high cable sections such as 150, 240, and 400 mm2 to cover all cases.
High voltage surges, caused by lightning or activities in high voltage networks, can pierce the insulation of wires, machine windings, etc.
The protection of installations and equipment against surges (transients) is achieved by using surge limiting devices, also called lightning arresters or surge arresters, connected between the active parts of the element to be protected and the ground.
Signals and Faults
For troubleshooting, we will have equipment capable of providing, as closely as possible, the location where the damage has occurred. This troubleshooting equipment is suitable for laboratory use.
Troubleshooting methods involve tracing the route of the line, equipped with special sensors (sound level meters) to capture the current pulse injected into the cable. These pulses will be captured at the damage site and will coincide with the readings given by the machine, depending on the cable length from the source to the fault location. It is very important to have trace levels of the underground cable route, which is saved with the Directorate’s work.
We can distinguish two main types of faults:
- Interruption of line continuity
- Insulation damage
Pipe, Permits, and Authorizations
The pipe is usually installed in the public domain, such as land and sidewalks. Pipelines under roads are not accepted except in crosswalks or with municipal authority or requirement. Highway crossings shall be perpendicular to the curb.
The protection of cables on the sidewalk will consist of a sand bed of about 20 cm, over hard plastic plates joined together (like a toy race track) that are 20 cm wide. These plates are used to detect the presence of underground electric cables during any subsequent trenching for the installation of other services. Above the trench, it is filled with clean stone, compacted in 20 cm layers. Before the final replacement, a plastic tape will be placed, also signaling the presence of the underground electrical cable. The trench replacement is done with the same materials that were originally used for the sidewalk.
In roadways, the cables are protected with cement or thermoplastic pipes. The pipe is placed on a concrete bed and will be covered with up to 30 cm of concrete. The trench replacement will be done with the material it was originally made of, usually asphalt.
Radial and Non-Radial Field Leads
In unipolar cable lines, the electrostatic field strength has a radial shape, with the insulation only supporting the electrical stress of the field established between the conductor and the outside of the cable.
In tripolar cables and power lines, the field lines do not follow radial trajectories because the potentials existing in the space between the conductors and the outside are not simultaneously equal. The field lines can be broken down into two components: one perpendicular and one tangential to the insulating layer.
The perpendicular component stresses the insulating layer, which it can withstand as it was designed to do. However, the tangential component stresses the filler mass between the cores, which has a puncture resistance ten times lower. Therefore, non-radial field cables are only applicable up to 15 kV voltage when the insulation is impregnated paper. In dry insulated cables, the limits are 6 and 10 kV, depending on the specific insulation used. Above these voltages, cables must have a radial field.