The rated voltage of control cables must not be lower than the operating voltage of the circuit in question, and it must satisfy the requirements regarding transient and power-frequency overvoltages to which it may be subjected. To ensure that the scope of impact is minimized in the event of insulation breakdown, mechanical damage, or fire, the national standard GB50217-91, *Code for Design of Cables in Electric Power Projects*, stipulates that two sets of systems requiring enhanced reliability-such as those for redundant protection (current and voltage circuits), DC power supplies, and trip control circuits-must utilize separate, independent control cables.
Once control cables are put into operation, issues regarding electrical interference arise both between different cores within the same cable and between cables laid in close parallel proximity. The primary causes of such electrical interference include:
(1) Electrostatic interference resulting from applied voltages acting through capacitive coupling between cable cores;
(2) Electromagnetic induction interference generated by current flow. Generally speaking, electrical interference is more severe when high-voltage or high-current interference sources are present nearby; furthermore, due to the minimal spacing between cores within a single cable, the degree of interference among these cores is significantly greater than that occurring between closely laid parallel cables. For instance, in the control circuit of a phase-separated circuit breaker at a certain ultra-high-voltage substation-where a single cable was shared by all three phases-an incident occurred in which the control pulse intended for a specific phase inadvertently triggered the thyristors of the other phases, resulting in an erroneous simultaneous tripping of all three phases. After subsequently switching to separate, independent cables for each phase, no further instances of erroneous operation occurred. Similarly, in the computer monitoring system of a certain power plant, routing low-level analog signal lines and the power supply lines for transmitters through the same four-core cable resulted in the generation of a 70V interference voltage on the signal lines; for low-level signal circuits operating in the millivolt range, such interference obviously impairs normal operation.
Measures to prevent or mitigate electrical interference primarily fall into the following three categories:
1. Grounding one spare core of the control cable.
Practical experience has demonstrated that grounding one spare core within a control cable can reduce the amplitude of interference voltage to between 25% and 50% of its original level. This measure is simple to implement and adds only a negligible amount to the overall cost of the cable.
2. Circuits where electrical interference could lead to severe consequences should not share the same control cable.
This includes: (1) low-voltage signal control loops and high-voltage signal control loops; (2) low-level signal loops and high-level signal loops; and (3) the individual low-voltage control loops for the phase-separated operation of AC circuit breakers-none of which should utilize the same control cable. However, if the outgoing and return conductors for a single pair within a low-voltage loop are routed through different control cables, their physical layout during installation may inadvertently form a closed loop. Under the influence of electromagnetic flux linkage from nearby power sources, an induced electromotive force (EMF) may be generated within this loop. The magnitude of this induced EMF could significantly interfere with the low-level parameters of the low-voltage circuit; therefore, it is generally advisable to route the outgoing and return conductors of such loops together within a single control cable.
3. Metal Shielding and Shield Grounding
Metal shielding is a crucial measure for attenuating and preventing electrical interference. Shielding methods include overall shielding (covering all cores), individual shielding (covering specific cores), and double-layer overall shielding. The selection of the appropriate metal shielding type for a control cable should be based on the anticipated intensity of potential electrical interference, while also taking into account comprehensive interference suppression strategies, in order to satisfy requirements for reducing both interference and overvoltages. Generally, the more stringent the requirements for interference suppression, the higher the corresponding investment cost; for instance, when utilizing steel tape armor or overall steel wire braiding for shielding, the cost of the cable typically increases by approximately 10% to 20%.
Regarding interference within high-voltage power control loops-given the inherent strength of their signals-control cables without metal shielding are generally acceptable, except in specific scenarios such as installation within ultra-high-voltage distribution switchgear or when routed in close, parallel proximity to high-voltage power cables over extended distances. For control cables utilized in low-voltage signal control loops, if the installation environment is subject to interference and effective external anti-interference measures are lacking, it is advisable to select control cables equipped with metal shielding. This precaution serves to prevent electrical interference from causing malfunctions within the low-level signal circuits or leading to insulation breakdown. Alternatively, if the control cables for low-voltage loops can be physically separated from power cables by a sufficient distance, or if they are routed through steel conduits, external electrical interference may be effectively reduced to within permissible limits. The principles for selecting the shielding type for control cables within the signal loops of a computer monitoring system are as follows:
(1) For discrete (digital) signals, overall shielding may be used.
(2) For high-level analog signals, overall shielding covering the twisted pairs is recommended; where necessary, individual shielding for each twisted pair may also be employed.
(3) For low-level analog signals or pulse signals, individual shielding for each twisted pair is recommended; where necessary, a composite overall shield-incorporating individual shielding for the twisted pairs-may also be employed.
Regarding the grounding methods for shielding layers, the following points should be observed:
(1) The shielding layers of control cables within the analog signal loops of a computer monitoring and control system should preferably utilize centralized single-point grounding. The rationale for this lies in the requirement to ensure the proper operation of the system, as an interference voltage of even approximately 1 V could lead to errors in logical decision-making; centralized single-point grounding effectively prevents the occurrence of ground loops.
(2) With the exception of the shielding layers for computer monitoring and control system cables-which are strictly limited to centralized single-point grounding-the shielding layers of other control cables should generally adopt two-point grounding when electromagnetic induction interference is significant, whereas single-point grounding should be adopted when electrostatic induction interference is significant.
(3) For cables featuring double shielding or composite overall shielding, the inner shielding layer should preferably utilize single-point grounding, while the outer shielding layer may utilize two-point grounding.
(4) When selecting two-point grounding, consideration must also be given to ensuring that the shielding layer will not be damaged or burned out under the influence of transient currents.
