Problems with Control Cable Shield Grounding
In recent years, integrated automation technology has been widely adopted in substations. For microcomputer-based secondary equipment to operate safely and reliably in such high-intensity electromagnetic fields and strong electromagnetic interference environments, two conditions must be met: first, these secondary equipment must have a certain level of electromagnetic interference tolerance; second, the electromagnetic interference level entering the equipment must be lower than the equipment's own tolerance level. This requires minimizing interference intrusion from control cables and reducing the level of interference signals, while selecting appropriate shielding and grounding methods. Improving the anti-interference protection of secondary cables requires a correct understanding of the role of cable shields and how to properly ground them. This article discusses the grounding methods for control cable shields, taking into account the main interference pathways, principles, and shielding functions in substations, and proposes corresponding improvement measures.
1. Problem Identification and Cause Analysis
The Shenghua Thermal Power Plant substation is a newly constructed 35 kV substation built in 2005. Nanjing Lidao microcomputer protection devices are used throughout the station. The substation is located in Zhongguan Town, a thunderstorm-prone area with an average annual number of 34 days of high intensity thunderstorms. Shortly after commissioning, the system suffered lightning damage, burning out its microcomputer protection device. Information from: Power Transmission and Distribution Equipment Network
Lightning, a strong atmospheric overvoltage, can damage equipment in two ways. Direct lightning strikes are rare, while the majority of damage is inductive, indirect, and harmful, affecting equipment through coupling to secondary circuits and other channels. Cable ports connecting wires to equipment are the primary transmission pathway for electromagnetic interference, transmitted through power lines, grounding wires, and signal lines. Inspection revealed the presence of low-pass filter capacitors in the power lines, the high-frequency switching used in the power modules, and the intact metal grounding wires and protective grounding wires. Initial suspicion is that the signal control lines introduced the interference. Further on-site inspection revealed that the cable trench was not laid in multiple layers. Due to site constraints, numerous control cables were densely packed within the trench. Furthermore, the control cables were closely attached to the grounding wire and the steel bars securing the cables, and the shielded control cables were not grounded. Measurements by on-site operators showed that the shield voltage of the control cables reached 200 V after the lightning strike. Therefore, it is concluded that there are many microelectronic devices in the Shenghua Thermal Power Plant, and various lines and cables are intricate and most of them are laid in the cable trench and close to the ground wire. When the control cable and the ground wire are arranged in the same cable trench, the ground wire will be struck by lightning and a strong electromagnetic field will be generated around it, causing induced overvoltage between the control cable cores and between the cores and the ground, thereby causing false signals and false operations, and even damaging the microcomputer protection equipment in serious cases.
2 Main interference propagation paths of substations
Electromagnetic interference (EMI) paths of substations are divided into two categories according to the medium: conducted interference and radiated interference. Conducted interference refers to interference transmitted through power lines, ground wires and signal lines; radiated interference refers to interference transmitted through space. According to the nature, it can be divided into capacitive coupling and inductive coupling [1]. Electromagnetic interference exists in the form of electromagnetic fields, and mainly affects signal transmission lines and equipment signals through electric fields, magnetic fields, electromagnetic fields and other channels.
2.1 Capacitive coupling
Due to the distributed capacitance between electrical equipment, the voltage on the high-voltage busbar and equipment of the substation generates interference voltage in the control cable system through the distributed capacitance. The higher the voltage, the stronger the capacitive coupling. The closer the high-voltage component is to the secondary equipment, the stronger the capacitive coupling.
2.2 Inductive Coupling
Alternating current flowing through primary equipment such as the substation's high-voltage busbar generates an alternating magnetic field within the space where the control cables are laid. This change in magnetic field induces a voltage in the control cables. The magnitude of the interference voltage is determined by the mutual inductance and the relative spatial position of the primary equipment and the secondary cables.
In actual production, the coupling between various interference sources and the secondary circuit is extremely complex. The same interference source often affects the secondary circuit in multiple ways. Based on the different interference sources, appropriate anti-interference measures are implemented, and by summarizing experience, the electromagnetic compatibility requirements of the substation are gradually met.
3 The Function of Shielded Cables and Comparison of Shield Grounding Methods
Currently, most substations use shielded cables to protect against electromagnetic interference. Control and signal cables are often braided with fine copper wires with a coating, and the shielding layer generally provides 90% coverage. To combat interference from substation primary equipment on secondary control cables, the primary interference mitigation method currently employed is grounding the cable shield. There are two methods: grounding the cable shield at one end or grounding the cable shield at both ends. The characteristics and applicable conditions of these two interference mitigation methods are discussed below.
3.1 Preventing Capacitive Coupling
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 couples directly to ground.
The metal shield of a shielded cable provides electrostatic shielding, allowing the strong primary high-voltage power lines to terminate at the metal shield, reducing the internal electric field strength to zero. This protects the core wires within the shield from interference from strong external electric fields. From an electrostatic shielding perspective, to ensure a fixed equipotential surface on the shield surface, one end of the shield should be grounded.
3.2 Preventing Inductive Coupling
Grounding the shield at both ends effectively suppresses electromagnetic induction. The induced potential generated by I1 on the cable core is E21 = jωM12I1.
The induced potential generated by I1 on the shield is Em = jωM1mI1.
When one end of the shield is grounded, an induced voltage is generated on the shield, but no loop is formed, no current flows through the shield, and the spatial magnetic field distribution is not changed.
It has no effect on the induced voltage generated by inductive coupling on the secondary cable core.
When both ends of the shield are grounded, the induced current flowing through the shield is Im = Em/(jωLm + Rm).
The induced potential generated by Im on the secondary cable core is E2m = jωM2mIm.
The induced potential generated by Im on the secondary cable core is E2 = E21 - E2m.
The induced current flowing in the shield is generated by the external electromagnetic field, and its actual function is to offset external electromagnetic interference. Therefore, grounding both ends of the cable shield can effectively suppress electromagnetic induction. 4. Implementing Appropriate Technical Improvements
Based on the above discussion, a proposed technical improvement measure for the control cable shielding at the Shenghua Thermal Power Plant is to ground both ends of the control cable shielding. The shielding layer's ability to reduce induced overvoltages is primarily due to the magnetic field generated by the current in the shielding layer canceling out the magnetic field generated by the interference current. Grounding both ends of the shielding layer is recommended because the shielding layer is less likely to burn out when short-circuit currents or lightning currents pass through it, as these currents are short-lived. If the shielding layer is grounded at one end, there is no current loop, but its overvoltage protection and interference immunity are significantly reduced, rendering it ineffective. Corrective measures include: First, control cables are shielded and grounded simultaneously in the switchyard and control room. The shielding layers of communication cables should also be properly and reliably connected to the ground. Second, dedicated grounding busbars should be installed for secondary equipment and cables to minimize interference from ground potential differences. Third, dedicated optoelectronic isolation should be implemented for all switching input and output contacts in the substation. After the transformation, after nearly a year of operation, the secondary equipment has not had any lightning-related failures so far. The system operation has been greatly improved, and the power supply reliability has been greatly improved. As the plant substation of Shenghua Thermal Power Plant, it has reduced power outage time and generated huge economic benefits.
