Cressall Earthing Resistors n! Al nglig - Alltid ö pp tillgä e tl id en Alw ! www.chscontrols.se ay op s av ailable - Always Cressall earthing resistors are installed all over the world. They can be found indoors and outdoors. The climate can vary from arctic cold to the heat and humidity of the Middle East. The earthing resistors are built around a standard design with a high level of flexibility for each installation. The design meets all major standards like IEC, ANSI, NEMA and IEEE. Long life with a minimum of maintenance is expected from an earthing resistor and is an important part of the design. Cressall is an ISO 9001 registered company. Cressall Earthing Resistors - Cost-efficent in Design - Reliable in Operation Earthing System The main purpose of earthing is to protect life and property in the event of an earth fault. There are basically three ways of neutral earthing in a star connected three phase system • Un-earthed neutral, fig 1 • Direct earthed neutral, fig 2 • Impedance earthed neutral, resistive or inductive, fig 3 In case of an earth fault, systems with unearthed neutral exhibit a low fault current, but high over-voltages will arise on the other two phases and the fault may remain undetected. By comparison, direct earthed neutral will exhibit lower over-voltages on the other two phases, but the earth fault current will be very high. Neither of these charac- teristics is desirable. Therefore, depending on local regulations and practice, system voltage and the overall design of the system, the compromise solution of impedance earthed neutral, either by inductance or by resistance is usually selected. Furthermore, a resistive earthing impedance - earthing resistor - is in many cases the most economical solution. It is also worth noting that the presence of an earthing resistor tends to reduce the magnitude of switching surges on the system by damping the oscillations resulting from cabling capacitance and transformer reactance. The choice of earthing method is depending on local regulations and practice, voltage level and finally the size and design of the distribution network. Impedance earthing by resistance, in form of an earthing resistor with suitable accessories, is in many cases the most common and economical solution. Fig 1 Fig 2 Fig 3 Earthing systems Cressall Air Cooled Earthing Resistors Earthing resistors are used for protecting transformers, generators and distribution networks. The earthing resistors are designed in accordance with IEEE 32-1972. The resistor elements will be made from high temperature stainless steel or nickel-chrome alloy which allows the earthing resistor to safely operate up to 1100°C. Higher operating temperatures require less active mass, resulting in more compact and economical designs. Modern materials guarantee long and reliable service even if the temperature exceeds 760°C as specified in IEEE32-1972. The earthing resistors can be supplied enclosed, ingress protection IP 23 to IP 55 or mounted in an open frame, IP 00. Enclosures for outdoor usage are made of stainless steel. Enclosures rated up to 72,5 kV are constructed as a fully earthed main frame. The earthing resistors are designed to meet all major international standards as IEC, ANSI, NEMA and IEEE. Resistor element GP Coil Plain wire-wound resistors on a solid ceramic former, using nickel-chrome resistance material. Mainly for very high impedance. CC Coil Spiral wire-wound resistors on a ceramic former, using nickel-chrome resistance material. Mainly for fault currents #50 A. CC Coil GP Coil RP Coil Oval edge-wound resistors on ridged ceramic insulators and supported on a central stainless steel strap, using high grade stainless steel resistance material. Mainly used for fault currents for 50+ A. RP Coil Connections The earthing resistor is designed to permit ease of access with various options for main neutral connection like • • • • External air insulated connection box External bushing External plug contact Direct on resistor element, IP 00 versions Overtemperature protection System earthing connection is separated from the enclosure/framework earth. Auxiliary circuit terminals are located in an external terminal box. For some applications, overtemperature protection may be required. The short timerating of the earthing resistor demands that a fast-acting protection device be used. Cressall’s Minotaur protection system detects the infrared radiation from the resistor elements, thereby providing the necessary speed of response. Complete Earthing Systems Earthing resistors are frequently used in conjunction with auxiliary devices such as • • • • • Manual or motorized knife switches Vacuum contactors Current transformers Voltage transformers Lightning arresters Traditionally, such auxiliary devices have been considered to be part of the switchgear, but considerable space and cost advantages can be achieved by combining them with the earthing resistor. The earthing resistor and its associated devices then arrive on Site as one integrated unit, with considerable savings in cabling requirements and installation times. RP Coil open style earthing resistor Example on earthing systems consisting of three vacuum contactors, three current transformers and one earthing resistor. The apparatus inside the dashed lines are installed in the same enclosure. General Data Ambient temperature Cooling method Resistor elements Current range -40 - +50°C and according to customer specification Air cooled, self convection GP Coil, CC, Coil, RP Coil # 4000 A Connection time, typical 5 s, 10 s, 30 s and continuous duty Resistor Element Data GP Coil Resistor material Mean temperature coefficient Temperature rise Ni-Cr alloy 0,00014% per °C #300°C CC Coil Resistor material Mean temperature coefficient Temperature rise Ni-Cr alloy 0,00014% per °C #760°C RP Coil Resistor material Mean temperature coefficient Temperature rise Stainless steel Fe-Cr-Al 0,0355% per °C #1000°C Enclosure Data Insulation voltage, IEC 71 Enclosure/frame material Surface treatment Ingress protection, IEC 529 0,8; 3,6; 7,2; 12; 17,5; 24; 36; 52 and 72,5 kV Galvanised steel frame, stainless steel enclosure Mild steel: galvanised or painted Stainless steel: none or painted IP 00, IP 23, IP 54 and IP 55 Standard IEEE 32-1972 and customers specification n! Al nglig - Alltid ö pp tillgä e tl id en ! www.chscontrols.se Alw CHS Controls AB Florettgatan 33 SE-254 67 Helsingborg, Sweden Phone +46 42 386100, fax + 46 42 386129 chs@chscontrols.se www.chscontrols.se ay op s av ailable - Always Leaflet PM4036E, May 2000 Technical Data for Earthing Resistors