Overview Overvoltage protection

Transcription

1 A P P L I C AT I O N N OT E 1.0 Overview Overvoltage protection

2 The APPLICATION NOTES (AN) are intended to be used in conjunction with the APPLICATION GUIDELINES Overvoltage protection Metal-oxide surge arresters in medium-voltage systems. Each APPLICATION NOTE gives in a concentrated form additional and more detailed information for the selection and application of MO surge arresters in general or for a specific equipment. First published October 2018

3 OVERVOLTAGE PROTECTION 3 Protection of equipment For economic reasons the insulation of the equipment cannot be designed to withstand all possible system conditions. Therefore, overvoltages that occur can endanger the electrical equipment. 1 Introduction A reliable, economical and safe electrical system needs protection of the equipment against unacceptable overvoltage stresses. For the following reasons protection of insulation of equipment and installations is in a high focus of manufacturers and utilities: Reduction of the voltage withstand capability of the insulation of equipment in new designs. Safety margins are taken out for cost reasons. A lot of old equipment is still installed in the system (30 to 40 years old) and the insulation may be aged, which may lead to increased failures due to overvoltages. This is a question of replacement policy. Upgrading of system voltage. To increase the power delivered one possibility is to increase the system voltage by some percent. This leads to a higher stress of the insulation in general and may be risky for aged insulation. Policy to increase life time of equipment. Increasing number of new alternative energy sources (e.g. wind power parks). 2 Types of overvoltages We have to distinguish between two types of overvoltages: Temporary overvoltages (TOV) with power frequency. They occur, for example, during load rejection or because of earth faults in a system. They are generally not critical for the insulation, but they are decisive for the selection of the continuous operating voltage U c of the metaloxide surge arresters (MO surge arresters). Further they are used for test purposes of the arrester housings, and for thermal stability tests of the MO surge arresters. Transient overvoltages Here we have to distinguish between: Slow-front (SF) overvoltages (e.g. switching overvoltages), which may occur during switching actions. They have to be considered mainly in transmission systems. Fast-front (FF) overvoltages (e.g. lightning overvoltages), which originate in atmospheric discharges. Lightning overvoltages can reach extremely high values and are dangerous for equipment in distribution systems. Very-fast-front (VFF) overvoltages occur in SF 6 -gas insulated substations (GIS) and with vacuum circuit breakers and endanger the equipment.

4 4 APPLICATION NOTE OVERVIEW 3 What can happen due to overvoltages (and endangers the equipment)? Breakdown of insulation Air-insulated substations (AIS) or installations, e.g. overhead lines, have a self-restoring insulation. The magnitude (peak value) of the overvoltages may lead to a short interruption (re-closing) of power supply, or a permanent outage if the fault cannot be cleared. This is typical a flashover of an insulator. In SF 6 gas-insulated substations (GIS) and equipment with liquid or solid insulation, e.g. transformers or cables, a breakdown of the insulation leads to a complete failure of the equipment. In such cases the equipment must be replaced, which leads to long outages. Ageing of insulation (pre-damaging) The steepness of the overvoltage and repetition rate (frequency) of the overvoltage have both a strong influence on the ageing (reduction of voltage withstand capability) of liquid and mainly solid insulating material, e.g. cable insulation. The consequence is a reduced life time of the equipment, and the risk that pre-damaged insulation fails suddenly during an overvoltage stress. Malfunction Overvoltages with high rates of rise, i.e. fastfront (FF) and very-fast-front (VFF) overvoltages can lead to EMC problems with electronics, computers etc. 4 Which equipment needs overvoltage protection (and why)? Transformers: breakdown of the transformer insulation or the bushing of the transformer leads to a complete outage and needs replacement of the failed transformer. Cables: breakdown of the insulation or cable bushing. Replacement of a cable or cable section is a worst case scenario. Further, repeated overvoltage stresses lead to a weakness (ageing) of the insulation because of the growth of water treeing and electrical treeing, which will lead at the end to a complete breakdown. Cable sheath: The outer insulation between the metallic sheath and the earth is relatively thin and overvoltages will lead to a perforation of the outer insulation. This leads with time to an ingress of humidity into the cable and consequently to water treeing and electrical treeing. This is again a reduction of the expected lifetime of a cable. It may also lead to a sudden breakdown of the insulation of the cable in case of an overvoltage. Rotating machines (generators and motors): breakdown of the winding insulation. Additional, stator windings of generators may need protection against gliding discharges. This can be an application of microvaristors for appropriate field grading (semi-conducting tapes, varnish or compounds). Capacitors: breakdown of insulation. Filters/line traps: avoiding flashovers in case of lightning. Insulators: insulators need protection against flashovers due to pollution or in areas with high tower footing resistance. In such cases MO surge arresters (line arresters) are installed in parallel to the insulators. Mobile phone stations: transmitter units for mobile phone stations are often installed in existing high voltage towers. In such cases a strict separation of the earthing systems is essential for safety reasons. MO surge arresters are necessary to protect the safety transformer against lightning. Wind Power Parks: in wind power parks the transformers and cable connections need protection, which is a standard application.

5 OVERVOLTAGE PROTECTION 5 5 Measures against dangerous overvoltages Overvoltage protection can be basically achieved in two ways: avoid lightning overvoltage at the point of origin, or limit the overvoltage near the electric equipment. Avoiding overvoltages: Earthed shielding wires: in medium voltage systems shielding wires are used only in special cases, and only in combination with MO surge arresters. Franklin rods (lightning rods): are used to protect substations and buildings against direct lightning. Limiting overvoltages: MO surge arresters: a MO surge arrester is a device having a strong non-linear voltage-current characteristic. It is designed to limit voltage surges by passing the surge discharge current to earth and automatically limiting the flow of the power system follow current. RC protection (snubber): an RC protection is a combination of a capacitor in series with a resistor, and used in addition to MO surge arresters. RC protection is used mainly for the protection of inductive loads being switched frequently (motors). It is used especially to protect against high rate-of-rise of voltage transients, which is especially critical for windings. Filter chokes: this is a series impedance installed upstream the device to be protected. It is a very specific application and should be used all the time in combination with MO surge arresters. As for the RC protection it is used to protect against overvoltage transients with high rate-of-rise. Microvaristors: microvaristors are used in compounds, varnish or tapes for field grading purposes or to avoid gliding discharges. An application may also be the protection against electrostatic discharges (ESD). 6 Economic considerations To reach an appropriate overvoltage protection in medium voltage systems, it is necessary to find the best compromise between the costs and the benefits of the protection devices to be used. An optimized technical-economical balance is to be striven for. The overvoltage protection, which is accurately applied, reduces: Outages of lines and substations Interruptions of critical manufacturing processes, which demand good voltage stability Costs due to interruptions in the energy supply Costs for the replacement and repair of electrical equipment Ageing of the insulation (e.g. cables) Maintenance work Etc. The aim of overvoltage protection is to guarantee an uninterrupted supply of electrical energy with good voltage stability to the greatest degree possible. Therefore, the costs for a set of surge arresters are not the most important consideration, but the cost that may arise on a long-term basis if adequate overvoltage protection is not used. 7 Final note The most effective protection against overvoltages in mediumvoltage systems is the use of MO surge arresters in the vicinity of the equipment. They protect against transient overvoltages travelling along the line in direction to a substation, transformer, cable, etc. They cannot protect against voltage oscillations (resonances) in the equipment (typically transformers, rotating machines) that are triggered by high frequencies or transients.

6 ABB Switzerland Ltd. PGHV Surge Arresters Jurastrasse 45 CH-5430 Wettingen/Switzerland Tel Fax sales.sa@ch.abb.com abb.com/arrestersonline Additional information We reserve the right to make technical changes or modify the content of this document without prior notice. With regard to purchase orders, the agreed particulars shall prevail. ABB AG does not accept any responsibility whatsoever for potential errors or possible lack of information in this document. We reserve all rights in this document and in the subject matter and illustrations contained therein. Any reproduction, disclosure to third parties or utilization of its contents in whole or in parts is forbidden without prior written consent of ABB AG. Copyright 2018 ABB. All rights reserved Specifications subject to change without notice 1HC EN AB