Transients generate substantial radiated energy, electric and magnetic fields, and transient currents within the substation grounds. Any of these phenomena can couple into poorly executed control wiring systems, but none of them can couple to an appreciable degree into well-executed control wiring systems.
The obvious and correct approach to GIS control wiring is to enclose the entire control system in a Faraday cage, i.e., within a metal enclosure. This is much simpler than it sounds, as will be described below.
A Faraday cage is a metal enclosure that fully surrounds the system, offering protection from EMI. In the case of control wiring, the system is typically a sensor (e.g., a gas density relay), the attached control wiring, the local control wiring cabinet in which the control wiring is terminated, the control wiring from the local cabinet to the substation control room, and the relay or computer racks in the control room.
Each of these elements is usually well shielded. The sensor is usually housed in a metal case that sits on the GIS. The control wiring is usually shielded by a solid copper shield or several layers of braid.
The local control cabinet is metal and well shielded, as are the computers or relay racks in the substation control room. The problem, therefore, is not to shield the individual elements, which are all usually well shielded, but to ensure the continuity of the shield from one element to the next. To this end, it is necessary to understand something about the flow of high-frequency currents in metals.
From 1 MHz to 100 MHz, the skin depth of current in copper varies from about 70 mm to 7 mm, respectively, so that almost no current flows in the conductor more than 0.25 mm below its surface. Since the copper cable shield, sensor enclosure, and local cabinet are all thick compared to the skin depth in this frequency range, independent currents can flow on the inner and outer surfaces of the cable shield.
A large switching-induced transient current could be flowing on the outside surface of the shield with negligible current flowing on the inside surface. No coupling will occur to the sensor or control wiring withinthe shield so long as the current is not allowed to cross over from the outside of the shield to the inside.
The key to proper control wiring practice for GIS is effecting connections between shielding elements that provide shield continuity and avoid such crossover. When a cable enters a control cabinet, the cable shield should be terminated immediately on the control cabinet enclosure as the cable conductors enter the cabinet.
A long pigtail termination of the cable shield after the cable has entered the cabinet is poor practice, as this brings the transient on the control cable shield within the control cabinet where it can couple to all of the conductors therein.
Coaxial termination of the cable shield on the cabinet forces the shield currents to flow on the outside of the metal cabinet, which shields the conductors within from the shield currents. A range of connectors and cables is suitable for coaxial termination of cable shields.
Cable with a solid copper shield offers best performance, and such cables do not necessarily cost more than cable with a less effective braided shield. Some components in GIS require careful design to avoid the coupling of transients into the control wiring system.
Voltage transformers (VT) are of special concern, as they effect a connection between the high-voltage conductor and the control wiring system. The interwinding capacitance in a magnetic VT can result in unacceptable coupling of transients from the GIS conductor to the low side of the VT unless an electrostatic shield is employed between the windings.
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