Medium-voltage cables, duct banks and system grounding

Lesson 3 described the medium-voltage (MV) switchgear and transformers that distribute power around a campus. Between them run the cables, usually underground in concrete-encased duct banks, and beneath every part of the system lies a decision about how it is connected to earth. Both are easy to overlook on a one-line diagram and both cause long, expensive outages when they go wrong: an MV cable fault can take days to locate and repair, and the system grounding method decides whether a single ground fault trips a whole building or merely sounds an alarm. This lesson covers MV cable construction and ratings, how heat limits what a duct bank can carry, testing and failure, and the grounding of MV and low-voltage systems.

Medium-voltage cable construction

An MV power cable is a carefully layered structure designed to keep a strong electric field evenly contained:

• Conductor: stranded copper or aluminum.
• Conductor shield: a thin semiconducting layer that smooths the surface of the strands so the electric field is uniform.
• Insulation: usually cross-linked polyethylene (XLPE) or ethylene propylene rubber (EPR), several millimeters thick, rated for a continuous conductor temperature of 90 °C (105 °C for some EPR), 130 °C in emergency overload and 250 °C during short circuits.
• Insulation shield: another semiconducting layer that confines the field inside the insulation.
• Metallic shield: copper tape or wires, grounded, which carries charging current and fault current and keeps the outside of the cable at ground potential.
• Jacket: an outer protective layer of PVC or polyethylene.

Because the insulation is highly stressed, any defect, such as a void, a sharp protrusion, water ingress or a poorly prepared joint, concentrates the field and starts partial discharge, tiny sparks that slowly erode the insulation until it fails. Most MV cable failures occur at terminations and splices, where the factory-made layers are cut and rebuilt by hand in the field, which is why only trained, certified cable splicers should do this work and why manufacturers' kits and instructions must be followed exactly.

Insulation levels

MV cable is specified by its voltage class and insulation level. A 100% insulation level suits systems where ground faults are cleared within about one minute, which is the case for solidly and low-resistance grounded systems. A 133% level, with thicker insulation, is used where faults may persist for up to an hour, as on ungrounded or high-resistance grounded systems, because during a ground fault on such systems the unfaulted phases rise to full line-to-line voltage above ground.

Ampacity: heat is the limit

A cable's ampacity is the current it can carry continuously without its conductor exceeding its rated temperature. Heat generated by \(I^{2}R\) losses must flow out through the insulation, the duct, the concrete and the surrounding soil to the ground surface. Every layer adds thermal resistance, so ampacity depends far more on the installation than on the cable itself:

• Soil thermal resistivity: moist soil conducts heat well (around 0.9 K·m/W); dry sand can be over 2.5 K·m/W, cutting ampacity substantially. Engineers measure soil thermal resistivity on site, and specify a thermal backfill with known properties around duct banks.
• Burial depth and spacing: deeper or closer cables run hotter.
• Mutual heating: in a duct bank of many circuits, each cable is heated by its neighbors, and the cables in the middle of the bank run hottest.
• Load factor: cyclic loads let the soil cool between peaks; data center loads are nearly constant, so little benefit is available.

Ampacities for standard installations are tabulated in the electrical codes; for real duct banks, engineers calculate them with the Neher–McGrath method (IEEE 835 tables) or IEC 60287, usually in software.

Voltage drop and losses on MV feeders

Because MV currents are modest, voltage drop is rarely a problem over the distances within a campus, but it should be checked, together with the losses, for long feeders. For a three-phase feeder of length \(L\) with resistance \(r\) and reactance \(x\) per unit length, carrying current \(I\) at power factor angle \(\phi\):

Testing and failure

New MV cables are tested after installation and before energization, and many operators test them periodically. Common tests (IEEE 400 series) include:

• Insulation resistance with a high-voltage DC tester: a quick check for gross defects.
• Very low frequency (VLF) withstand: applying an AC voltage at about 0.1 Hz at several times normal voltage for a set time; good insulation withstands it, while weak spots fail during the test rather than in service.
• Tan delta (dissipation factor): measuring how lossy the insulation is, which rises as it ages or absorbs water.
• Partial discharge testing: detecting and locating the tiny discharges that signal defects, especially in terminations and splices.

When an underground cable fails, the fault must be located before it can be repaired, using techniques such as time-domain reflectometry and "thumping" (applying high-voltage pulses and listening for the discharge at the fault). Repairs can take days. That is why critical facilities use ring or dual-radial MV arrangements (Lesson 3) so that any single cable can be lost without losing load, and keep spare cable, splice kits and contracted repair crews on hand.

System grounding

System grounding means the intentional connection of a power system's neutral (or one conductor) to earth. It is different from equipment grounding and bonding, which connect the metal enclosures of equipment together and to earth for safety (Lesson 9). The choice of system grounding determines what happens during the most common fault: a single phase touching ground.

Resistance grounding inserts a neutral grounding resistor (NGR) between the transformer neutral and earth, limiting the fault current to a value set by Ohm's law. A high-resistance grounding resistor must allow more current than the system's capacitive charging current, or transient overvoltages can build up during arcing faults.

Data centers typically use solidly grounded systems for the IT distribution (so that single-phase loads can be served and ground faults trip promptly), low-resistance grounding on the MV system, and sometimes high-resistance grounding on 480 V or 600 V mechanical systems, where riding through a first ground fault keeps chillers and pumps running.

In the US, the National Electrical Code permits high-impedance grounded neutral systems at 480 V to 1,000 V only under conditions such as qualified maintenance and ground detection (250.36), and covers systems over 1,000 V in the articles for higher voltages.