This article provides a detailed explanation of the differences between power cables and control cables for industry colleagues to refer to.
Power cables are used to transmit electrical energy. This means the voltage and current flowing through the cables are both high. They typically have a higher insulation grade and a larger conductor cross-section, with a single cable containing up to five cores. Control cables, on the other hand, primarily transmit low-current electrical signals, such as those used to drive and control contactors and relays, or to transmit audio and video signals. The power load carried by the cables is lower, and the voltage level is lower. Control cables contain more cores and may have separate shielding. The specific differences are discussed below.
A Brief Discussion on the Differences Between Power and Control Cables
Power cables are 0.6/1kV, such as VV and YJV.
Control cables are 450/750V, such as KVV and KVVP.
Power cables typically have fewer than five cores and are typically used for high-voltage applications. Control cables have more cores, but customer requirements vary. Typically, they have up to 37 cores, but some cables have more.
Commonly used cables are classified into five categories:
(1) Wires and bare inner conductor products: These refer to products with only conductors, without insulation or other structures. Examples include copper, aluminum, and various composite metal single wires; various structures of overhead transmission stranded wire, flexible wiring, shaped wire, and profiles.
(2) Electromagnetic wire: Wires that, in the form of windings, cut magnetic lines in a magnetic field to induce current, or that generate a magnetic field through current are called electromagnetic wires. Examples include enameled wire, fiber-wrapped wire, and inorganically insulated wire.
(3) Power cables: Wires used to transmit and distribute high-power electrical energy in the main lines of power systems are called power cables. Examples include plastic cables of various voltage levels, oil-impregnated paper cables, and non-drip cables.
(4) Communication cables: Cables used to transmit audio and various telecommunications information above audio. Examples include local telephone cables, long-distance symmetrical communication cables, coaxial communication cables, telephone equipment cables, and data cables.
(5) Electrical Equipment Wire and Cable: These are wires and cables used to transmit electrical energy directly from the power system's distribution point to the power connections of various electrical equipment and appliances. Examples include wires and cables used for electrical connections and control signals in various industrial and agricultural equipment. These products are widely used and come in a wide variety. Their structure and performance are often determined by the characteristics of the equipment and the environmental conditions in which they are used. Therefore, in addition to a large number of general-purpose products, there are also many specialized and specialized products. Examples include general-purpose insulated wire, installation wire, rubber-sheathed flexible cable, control cable, signal cable, cables for aircraft, automobiles, and tractors, lead wires for motors and electrical appliances, cables for rolling stock, wires for radio equipment, mining cables, marine cables, agricultural cables, cables for oil and gas exploration, cables for field work, and wires and flexible cables for various high-voltage DC equipment.
Power cables are used to transmit and distribute high-power electrical energy within the power system's main trunk lines. Control cables transmit electrical energy directly from the power system's distribution point to the power connections of various electrical equipment and appliances. Power cables are generally rated for voltages of 0.6/1kV and above, while control cables are primarily rated for 450/750V. When manufacturing power cables and control cables of the same specifications, the insulation and sheath thickness of the power cables are thicker than those of the control cables.
First, control cables are cables for electrical equipment and, along with power cables, are two of the five major cable categories.
The standard for control cables is 9330.
The standard for power cables is GB12706.
The insulation core of control cables is generally black with white lettering, and low-voltage power cables are typically color-coded.
The cross-section of control cables generally does not exceed 10 square meters. Power cables, primarily used for power transmission, generally have larger cross-sections.
Due to the reasons mentioned above, power cables can generally be larger, up to 500 square meters (the range of conventional manufacturers). Manufacturers with cross-sections larger than these are relatively rare, while control cables generally have smaller cross-sections, typically not exceeding 10 square meters.
In terms of cable core count, power cables generally have a maximum of five cores, depending on grid requirements. Control cables, used to transmit control signals, have more cores, with some standardly up to 61 cores, but these can also be manufactured to meet user requirements.
Shield Grounding for Electrical and Thermal Control Cables
There are two methods for grounding cable shields: two-point grounding and single-point grounding.
To prevent transient overvoltages, two-point grounding is preferred. Electromagnetic induction generates a longitudinal current in the shield. This current creates a secondary field opposite to the primary interference field, counteracting the primary interference field and reducing the interference voltage.
However, two-point grounding presents two problems:
First, when a short-circuit current or lightning current flows through the grounding network, the potential difference between the two points on the cable shield causes current to flow through the shield, potentially burning it.
Secondly, when current flows through the shield, it generates interference signals on each core. For relay protection and automatic devices, electromagnetic induction interference is a major concern, as both their input and output terminals are exposed to the high-voltage or ultra-high-voltage environment of the switchyard. Furthermore, the cable cores are located in high-voltage circuits, so interference signals generated by shield currents are less impactful. Therefore, regulations for relay protection and automatic devices stipulate that the shield should be grounded at both ends. For thermal cables, electromagnetic induction interference is less of a concern, but the interference generated by shield currents from two-point grounding on the cores can potentially cause device malfunction, so single-point grounding is preferred. Therefore, the two-point grounding requirement for relay protection and automatic devices is consistent with the single-point grounding requirement for thermal cables.
For control cable shields, the shield's ability to reduce induced overvoltages is primarily based on the magnetic field generated by the shield current offsetting the magnetic field generated by the interference current. Grounding the shield at both ends is recommended because the shield is less likely to burn out during short-circuit or lightning currents, as the high currents are short-lived. If the shield is grounded at one end, there's no current loop, but its overvoltage protection and interference immunity are both very low, making it ineffective. Corrective measures: First, control cables should be shielded and grounded simultaneously in the switchyard and control room. The shields of communication cables should also be properly and reliably connected to the ground. Second, install dedicated grounding busbars for secondary equipment and cables to minimize ground potential interference. Third, all switching input and output contacts in the substation should utilize dedicated optical isolation.
The induced current flowing in the shield is generated by external electromagnetic fields, and its actual function is to offset external electromagnetic interference. Therefore, grounding both ends of the cable shield can effectively suppress electromagnetic induction.
An ungrounded shield has no shielding effect against electric field interference, while a shield grounded at one end or at both ends provides the same shielding effect. If the shield is properly grounded, the electric field terminates at the shield and is directly coupled to ground.
The metal shield of a shielded cable acts as an electrostatic shield, terminating the primary high-voltage power supply's strong electric field at the metal shield. This reduces the internal electric field to zero, protecting the core wire within the shield from interference from strong external electric fields. To ensure a fixed equipotential surface, one end of the shield should be grounded.
