What is a 33kV cable?

A 33kV cable is a type of electrical cable designed to carry high-voltage electricity, specifically at a nominal voltage of 33 kilovolts (kV). These cables are often used in power distribution systems to transmit electricity over longer distances. Here’s a breakdown of key characteristics:

• Voltage Rating: 33kV is considered medium voltage (MV) and falls within the range typically used for power distribution networks.
• Construction: They usually consist of a central conductor made of aluminium or copper, surrounded by layers of insulation, shielding, and protective sheathing.
• Types: Common types include:
  • XLPE (Cross-Linked Polyethylene): This is a popular insulation material for underground cables due to its good electrical properties and moisture resistance.
  • PVC (Polyvinyl Chloride): This is often used for lower voltage applications, but can be used in 33kV cables as well.
  • PILC (Paper Insulated Lead Covered): This is a traditional type for high-voltage cables but is less common now.
• Applications: 33kV cables are frequently used in:
  • Power distribution: They connect substations and feed electricity to industrial areas, commercial buildings, and residential neighbourhoods.
  • Underground mining: They are used for powering mining equipment and operations.
  • Infrastructure projects: They are crucial for large construction projects, railways, and other infrastructure developments.

In summary, a 33kV cable is a specialised type of electrical cable designed for high-voltage power distribution, providing reliable and efficient electricity transmission over considerable distances.

33kV cables are a type of medium voltage (MV) cable. They are typically made with one central 33kV XLPE cable with an aluminum wire shield (AWA) or three 33kV underground cables with a steel wire reinforcement (SWA) layer. The layers provide excellent mechanical protection when installed. These cables are designed for power distribution and are suitable for direct burial. 33kV underground cables are well-suited for large projects like building construction, underground mining, power substations, and railways. 3-core 33kV underground cables with flame-resistant (LSZH) insulation are also available.

There is no standard size for 33kV underground cables. The inner core typically consists of a solid aluminum or stranded copper conductor, covered by a semi-conducting conductor screen, XLPE insulation, a semi-conducting insulation screen, water-swellable tape, and an MDPE sheath. This is then covered by a copper wire screen wound with copper.

33kV is a voltage rating listed in IEC 60038 (IEC Standard Voltages) and is included in several British Standards:

• BS 6622 (Electric cables – Armoured with thermosetting insulation for rated voltages from 3.8/6.6 kV to 19/33 kV – Requirements and test methods.)
• BS 7835 (Electric cables – Armoured cables with thermosetting insulation for rated voltages from 3.8/6.6 kV to 19/33 kV having low emission of smoke and corrosive gases when affected by fire – Requirements and test methods)
• Part 4.10 of BS 7870 (Specification for distribution cables with extruded insulation of rated voltages of 11 kV to 33 kV – single-core 11 kV to 33 kV cables).

11kV lines are used in residential areas to feed local transformers, which then distribute power to buildings. 33kV lines operate at higher voltages and are used to distribute power from one small substation to another.

33kV lines typically resemble towers with 4-6 large wires separated by arms. They are used to transmit power between stations and have no wires connected to them. They often use 5 or 6 disc insulators or a post insulator made from a stack of 12 smaller discs. The height of the powerline is usually between 10 to 20 meters. The number of conductors is usually 3 bare active wires.

H poles, joist poles, and PSC poles are commonly used for 33kV lines. The height of the powerline is typically between 10 to 20 meters. However, there is no single standard pole for all 33kV lines in Australia. Factors like voltage, line configuration, environmental conditions, and local standards affect the choice of pole type. Common pole materials used in Australia for 33kV lines include:

• Steel Poles: Durable, often used for higher voltage lines and challenging environments.
• Concrete Poles: Popular for lower voltage lines, but can be used for 33kV.
• Timber Poles: Used for lower voltage lines, but treated timber might be used in certain 33kV situations.
• Composite Poles: Made from a combination of fibreglass and resin, offering high strength and corrosion resistance.

Voltage variations in 33kV and 11kV feeders should not exceed these limits under peak load conditions and normal system operation:

• Above 33kV: (-) 12.5% to (+) 10%
• Up to 33kV: (-) 9.0% to (+) 6.0%
• Low voltage: (-) 6.0% to (+) 6.0%

For cables with copper conductors, the approximate diameter over the conductor is 12.8mm. However, stating a specific conductor size for a 33kV overhead power line in Australia is difficult. The number of conductors will usually be 3 bare wires. The conductor size is influenced by:

• Line Length: Longer lines need larger conductors to minimise voltage drop.
• Load Current: Higher current requires larger conductors to prevent excessive heating.
• Environmental Conditions: Factors like temperature, wind, and ice loading affect conductor selection.
• Specific Line Design: Each overhead line project has its own unique design constraints.

There is no single standard size for 33kV lines in Australia. Australian standards and regulations provide guidelines, but the project engineer ultimately determines the specific conductor size.

There is no definitive answer to this question. The ampacity of a conductor depends on several factors. The maximum load on a single 33kV feeder should not exceed 45 MVA.

Larger conductors can handle more amps before reaching their thermal limits, which can vary depending on the ambient temperature. For a 33kV line, the combined capacity for all three phases is expected to be between 5 to 20 MW. However, sometimes legacy voltages are pushed into higher amperages to avoid changing transformers along the feeder.