This article will introduce how to select control cables in power systems and how to ensure proper operation and interference reduction during use.
I. Overview of Control Cables
Control cables are cables that transmit signals or control operational functions from the control center to various systems. Early control cables had relatively simple functions, including indicator lights, instrument indications, relay and switchgear operation, and alarm interlock systems. In recent years, the widespread use of weak current and computer networks has necessitated new functions and higher requirements for the selection and application of control cables. This article discusses some new issues that have arisen in the selection and use of control cables in recent years for research reference.
II. Main Series and Varieties of Control Cables
Today's main control cable products fall into three categories: polyvinyl chloride (PVC) insulated control cables, natural styrene-butadiene rubber (SBR) insulated control cables, and polyethylene (PE) insulated control cables. Cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) insulated control cables are also available. Oil-impregnated paper-insulated, lead-sheathed control cables, once produced, have been phased out.
The rated voltage of a control cable is expressed as U0/U. my country's national standard, issued in 1998, specifies a rated voltage of 450/750V for plastic-insulated control cables. Some countries, including Germany, have proposed standardizing 600/1000V control cable products. Currently, my country can also produce 600/1000V plastic-insulated control cables. The rated voltage for rubber-insulated control cables is 300/500V. Control cables have a copper core with a nominal cross-section of 2.5mm² or less, with 2 to 61 cores; 4 to 6mm² with 2 to 14 cores; and 10mm² with 2 to 10 cores. The operating temperature of control cables is 65°C for rubber insulation and 70°C and 105°C for PVC insulation. Control cables used in computer systems generally use PVC, polyethylene, cross-linked polyethylene, or fluoroplastic insulation.
3. Measures to ensure the normal operation of control cables and prevent interference
To ensure that the scope of influence is reduced when insulation breakdown, mechanical damage or fire occurs in control cables, the national standard GB50217-91 "Design Specifications for Power Engineering Cables" stipulates that two systems that require enhanced reliability, such as dual protection current, voltage, DC power supply and tripping control circuit, should use independent control cables.
After the control cable is put into operation, there will be electrical interference between different cores of the same cable and between cables laid in parallel. The main causes of electrical interference are: (1) electrostatic interference generated by the capacitive coupling between the cores due to the external voltage; (2) electromagnetic induction interference generated by the current. In general, when there is a high voltage and high current interference source nearby, the electrical interference is more serious. Since the distance between the cores of the same cable is small, the interference level is much greater than that of adjacent cables laid in parallel.
For example, the control circuit for a phase-operated circuit breaker at a certain ultra-high voltage substation shared a single cable for all three phases. This caused an incident where the pulses from the phase-operated circuit breaker triggered the thyristors in other phases, mistakenly causing the three phases to malfunction. After switching to separate cables, the misoperation incident has stopped. Another example is a computer monitoring system at a power plant that shared a four-core cable with the analog low-level signal line and the transmitter power line. This caused a 70V interference voltage to be generated on the signal line. This clearly affects the normal operation of low-level signal circuits measured in millivolts.
There are three main measures to prevent or mitigate electrical interference.
1. Grounding a spare core of the control cable
Practice has shown that grounding a spare core in the control cable can reduce the amplitude of the interference voltage to 25%-50%. This is simple to implement and adds minimal cost to the cable.
2. For circuits where electrical interference could have serious consequences, do not share a single control cable.
These include:
(1) Weak current signal control circuit and strong current signal control circuit;
(2) Low level signal and high level signal circuit;
(3) The weak current control circuits of each phase of the AC circuit breaker for phase-by-phase operation should not use the same control cable. However, if each pair of return conductors in the weak current circuit belongs to different control cables, a ring arrangement may be formed during the laying process. Under the cross-linking of electromagnetic wires of similar power sources, an induced potential will be generated, and its value may have a significant impact on the interference of the low-level parameters of the weak current circuit. Therefore, it is advisable to use a single control cable for the return conductors.
3. Metal shielding and shielding layer grounding
Metal shielding is an important measure to reduce and prevent electrical interference, including total shielding, sub-shielding and double-layer total shielding of the core. The selection of the metal shielding type of the control cable should be based on the strength of the possible electrical interference and take into account the comprehensive interference suppression measures to meet the requirements of reducing interference and overvoltage. The higher the requirement for anti-interference effect, the greater the corresponding investment. When steel tape armor and steel wire braided total shielding are used, the price of the cable increases by about 10% to 20%.
