Relevant Aspects for the Proper Selection and Application of Transmission Line Arresters for Improvement of the Transmission Lines Lightning Performance Lightning Activity and Effects on Transmission Lines • In many countries lightning has been reported as the major cause of non-scheduled outages on overhead sub-transmission and transmission lines with rated voltage up to 230 kV. • Lightning is the most frequent cause of transmission outage and service interruption in the United States, accounting for about 30% of all power outages, and resulting in economic losses approaching $1 billion annually. http://www.epri.com Power supply utilities and industrial consumers have been working on lightning analysis aiming lightning performance improvement for lines. Lightning Activity and Effects on Transmission Lines Lightning phenomena on overhead lines have important consequences in many safety and technical aspects. The problem has special importance in countries or in regions with higher electrical activities and / or higher soil resistivity: Load losses on power systems caused by voltage sags due to lightning transitory activities on transmission lines. Permanent and transitory disturbances on important lines / whole system due to outages. Critical for all modern industries now so reliant on sophisticated electronic equipment and especially production process very sensitive to momentary disturbances in the system. Lightning Activity and Effects on Transmission Lines Three main aspects are involved in the lightning performance of transmission lines: 1. 2. 3. Discharge current characteristics: peak value, time-to-crest and rate of rise. The attachment process between lightning channel and the transmission line components. The electromagnetic response of the overhead line struck by the lightning. Transmission lines may present several different configurations for the towers, overhead conductors and tower-footings. Different configurations establish different transient responses under lightning stresses. Unshielded Lines: • Basically all lightning strikes on structures or on the phase conductors will produce flashovers along the insulator strings. Shielded Lines: • Possibility of back flashover occurring across insulator strings. • Transmission line lightning performance depends strongly on the transient grounding system behavior. Shielding failure: • Flashover occurrence as in case of the unshielded lines. Induced voltages: • Critical for systems with rated voltage up to 45 kV Procedures to Improve Transmission Line Lightning Performance Which Method Should Be Used? INPUT OUTPUT Current outage number Desired outage number Lightning studies and analysis using computational tools in which for the desired transmission line configuration is estimated in number of transmission line outages per 100 km/year. • Current configuration and other configurations considering different procedures to improve the line performance Procedures to Improve Transmission Line Lightning Performance Which Method Should Be Used? INPUT OUTPUT Current outage number Desired outage number Starting from the results obtained in the theoretical studies and knowing the target outage index, methods and procedures to improve the line lightning performance have been evaluated taking into account the cost – benefit balance. • From the methods currently available for lightning improvement, in most of the cases Transmission Line Arresters (TLA) sometimes associated with the improvement of transient grounding behavior are usually considered the most effective. Transmission Line Arresters - TLA General information: • TLA have been used primarily for lightning protection but can also be applied to control switching overvoltages. • Installed along critical sections of transmission lines with poorer lightning performance to reduce unscheduled outages due to lightning. reduce system interruption increase system reliability • NGLA or EGLA configuration Transmission Line Arrester – TLA Which TLA design? Non-Gapped Line Arresters (NGLA) + Indication of overloaded NGLA by disconnector + Reasonable current/energy sharing for kA currents + Can be used for all applications. • May be longer and heavier than EGLA • Mechanical stress on disconnector lead Externally Gapped Line Arresters (EGLA) + No continuous voltage stress on ZnO unit • Can only be used for tripping protection on shielded lines • Tricky coordination with insulator withstand levels to avoid spark-over for slow front overvoltages or TOV events • Minimal current sharing and no indication of overloaded EGLA Transmission Line Arrester – TLA Externally Gapped Line Arresters (EGLA) Source: Supressor de sobretensiones por descargas atmosféricas (SSDA) para líneas de transmisión – CFE - 2004 Transmission Line Arrester – TLA Non-Gapped Line Arresters (NGLA) Transmission Line Arrester – TLA Which Application? • Minimize tripping of unshielded and shielded lines due to lightning o New transmission lines with high theoretical outage indexes due to lightning o Virtually eliminating interrupted power supply for sensitive industrial processes o To protect double circuit lines, virtually eliminating double-circuit flashover • Compact line with reduced insulation withstand levels o Building a new line or upgrading an existing line. • Control of switching overvoltages profile along the line o Substitute for pre-insertion resistors (works well together with controlled switching) • Extended substation protection o Reducing both amplitude and steepness severity of the incoming overvoltages entering at air and GIS substations. o Eliminating flashover risks near substations. • Temporary line arresters for hot line work Transmission Line Arrester – TLA Non-Gapped Line Arresters (NGLA) Transmission Line Arrester – TLA Non-Gapped Line Arresters (NGLA) Source: various transmission line arrester applications, installations, and designs – MSA / ABB – CIGRÉ tutorial – Rio de Janeiro – 2005 Transmission Line Arrester – TLA Non-Gapped Line Arresters (NGLA) Source: limitation of switching overvoltages by line arresters in combination with controlled switching - Carl E. Solver - CIGRÉ 2006 - Q1.12 – Paris Transmission Line Arrester – TLA Non-Gapped Line Arresters (NGLA) Proper Selection for NGLA The proper selection for NGLA in terms of electrical and mechanical requirements, as well as the proper design are important to guarantee the long term performance especially for critical environmental conditions. • Electrical Requirements • Mechanical Requirements • Automatic disconnector device – Proper coordination between the NGLA and the disconnector characteristics. • Proper design – constructive characteristics Proper Selection for NGLA - Electrical Requirements Totally different application than substation arresters: • Used to protect self-restoring equipment • Short protective distances • Its only purpose is to prevent flashover of the insulators strings Rated voltage and MCOV: • Highest voltage of the system & maximum temporary overvoltages and their duration. Housing withstand voltages for lightning / power frequency (switching). Maximum protective levels required. Energy requirements for lightning & switching surges. TLA short-circuit capability. Proper Selection for NGLA – Electrical Requirements Which energy requirements shall be considered? Substation arresters - energy requirements based on: Transmission line discharges during close and reclose of the lines Capacitor bank discharges and sometimes TOV requirements Transmission Line Arresters – first need to know the application Protection against lightning strikes only or also protection against switching events? In case of protection against lightning only: • • Outage rate acceptable for the specific line – TLA number and location Acceptable failure rate for TLA due to excessive lightning energies – this defines the minimum energy requirements. Proper Selection for NGLA – Electrical Requirements Which energy requirements shall be considered? Factors that affect the energy requirement for a NGLA In case of protection against lightning only: • If the overhead transmission line is unshielded or shielded: o Unshielded lines - Energy equivalent to previous IEC LDC 3 or IEC LDC 4, depending on the Ground Flash Density of the region o Shielded lines - Energy equivalent to previous IEC LDC 1 – 3, depending on the Ground Flash Density and the transient ground system behavior • The transient grounding system behaviour o Shielded lines - Higher grounding transient impedance usually means higher energy absorbed by TLA • Numbers of TLA installed on the protected tower & along the line Proper Selection for NGLA – Electrical Requirements Which energy requirements shall be considered? Factors that affect the energy requirement for a NGLA Protection against switching events: • The energy absorbed by TLA during switching surge on the transmission lines depends on the length of the line as well as the number of TLA installed along the sections of the line. • TLA installed along the sections of the transmission lines share the total energy among them reducing the energy absorbed by each TLA. • For switching surges, lower energy discharge class can be chosen when many TLA are installed along the line. Proper Selection for NGLA - Mechanical Requirements Installation issues I. Types of installation: usually chosen by the customer • • • • Suspended on the conductor Mounted on the insulator Suspended at the tower Supported in the tower II. Common installations issues (more critical for suspended on the conductor): • • • • • Hot line clamp x suspension clamps – degrees of freedom and vibration Break at lugs/braid – material quality, crimping and proper length Materials applied (stainless, galvanized, etc.) – proper material selection for long term performance taking into account the risks of corrosion depending on how critical is the environmental condition Chain deformation – proper plating selection avoiding corrosion and size Disconnector failure – Proper installation and enough free movement for the disconnector cable, avoiding higher mechanical stresses on the disconnector III. Preferable installations: From mechanical aspects it is usually recommended mounted on the insulator or supported in the tower Proper Selection for NGLA - Mechanical Requirements Effects of conductor vibration Suspended on the conductor Resonance conditions happens if natural pendulum frequency of the line arresters is similar to span natural frequency 1 3𝑔 𝑓𝑝𝑒𝑛𝑑𝑢𝑙𝑢𝑚 = 2𝜋 2ℎ Arrester height (m) Frequency (Hz) 0.25 1.22 1 0.61 2 0.43 3 0.35 4 0.31 𝑓𝑠𝑝𝑎𝑛 = Where: g: gravity acceleration h: arrester height T: conductor tension w: conductor weight n: nr. of standing wave loops S: span length 𝑇. 𝑔 𝑛 . 𝑤 2. 𝑆 Aeolian Galloping Sub-span Frequency (Hz) 3-100 0.1-3 0.15-10 Amplitude (conductor ∅) 0.01-1 5-300 0.5-20 Wind speed (km/h) 3-25 25-65 15-65 Proper Selection for NGLA - Mechanical Requirements Effect of the vibration - Interaction with dampers • • • • Aeolian vibration dampers are placed in specific locations to dissipate energy. When installed suspended on the conductor, TLA changes the standing waves because it adds an additional weight on the conductor. Dampers configuration need to be checked. It is recommended to install additional dampers at each side of the TLA. Install another damper outside the arrester Proper Selection for NGLA - Mechanical Requirements Disconnector failure due to mechanical issues Disconnection of some line arresters due to failures in the flexible cable and in the links connection (eye bolt) caused by the incidence of strong winds and / or vibration in the line Allow enough free movement of disconnector cable. Proper Selection for NGLA - Disconnector Important Disconnector Characteristics How fast it operates. The disconnector shall always continue its opening operation once it is triggered to operate even if the voltage is switched off. A disconnector is a simple device that typically reacts on heating from power frequency currents. It cannot distinguish between TOV currents or slow oscillating front currents passing through the NGLA or real short-circuit currents. • This shows the importance to always select a high enough rated voltage so that the NGLA do not see TOV stresses that can interfere with its disconnector operation. Proper Selection for NGLA - Disconnector Matching Disconnector and NGLA Characteristics Typical curve showing opening time versus current TIME versus CURRENT CURVE for disconnector 10000 Time (ms) 1000 100 10 1 1 10 100 Current (A) 1000 Proper Selection for NGLA - Disconnector Matching Disconnector and NGLA Characteristics Typical TOV withstand curve voltage versus time TOV withstand Curve in p.u. of Ur With Prior Duty without Prior Duty 1.2 1.15 1.1 Utov / Ur (p.u.) 1.05 1 0.95 0.9 0.85 0.8 1 10 100 Time (Seconds) 1000 10000 Proper Selection for NGLA - Disconnector Matching operation with line protection scheme Which protection scheme is used? • How fast is it? • How many reclosing may occur? • What is the short-circuit currents at installation? Electrical characteristics: • When the disconnector shall operate • When the disconnector shall not operate Proper mechanical strength: • Vibrations should be verified Proper Selection for NGLA - Disconnector Proper Coordination with NGLA and with line protection scheme 1. Disconnector operates before the line trips: depends how quick the disconnector operates plus how quickly it can quench the arc during falling out, which will be strongly weather dependent. This is then a race between the line protection scheme and the disconnector and may vary from incident to incident. No tripping occurs. 2. Disconnector operates before fast reclosing of the line: means that once triggered the disconnector shall continue to disconnect even if the power supply is switched off. This should be a repeatable operation depending on coordination of the line protection scheme including fast reclosing time and the disconnector opening time. Proper Selection for NGLA - Disconnector Proper Coordination with NGLA and with line protection scheme 3. Disconnector has not completed its operation when reclosing occurs: should not happen as this leads to a system disturbance and also leads to a second short-circuit stress on the NGLA which significantly increase the risk of complete disintegration of the arrester with larger pieces coming down. This indicates a mismatch of disconnector opening times compared to the line protection scheme of the system and may cause system disturbance every time there is a NGLA failure. 4. Disconnector operates but the arrester is not overloaded or failed: should not occur and indicates either a disconnector not matching the NGLA characteristics or a mechanically too weak disconnector design. Proper Selection for NGLA - Disconnector Proper Coordination with NGLA and with line protection scheme • Reclosing time: 500 ms • Decisive TOV of 150 kV during 1 s. substation arresters of 132 kV rating TOV currents in A TLPXX /DDX NGLA TIME/ Voltage Voltage Voltage 1 s (A) 10 s kV or p.u. kV or p.u. p.u. prior energy no energy Ur = 132 no energy no Ur = 138 energy SYSTEM TOV (kV) DDX Current (A) Voltage 40 s kV or p.u. 1.14 8 1.11 6 1.05 0.4 1.18 15 1.14 12 1.09 1.6 155 15 150 12 144 1.6 162 15 157 12 150 1.6 (A) 150 9A • Typically a higher rated voltage than the substation arresters o Limit arresters stresses due TOV and possible reduction of the grading ring Conclusions • In many countries lightning has been reported as the major cause of unscheduled outages on overhead unshielded and shielded sub-transmission and transmission lines with rated voltages up to 245 kV. This fact has been taken up by several power supply utilities and industrial consumers which have lead them to invest in the promotion of improvements along the critical sections of their overhead lines with poor lightning performance, thereby increasing their reliability. • Among the methods used for improvement of the overhead lines lightning performance, TLA installed along the critical sections of the lines with poorer lightning performance have been usually considered as the most effective for long term performance. • In order to get a good solution in the technical and economical point of view, studies shall be done to: select the TLA properly in terms of electrical and mechanical requirements as well as evaluate the proper design for longer term performance, and optimize the quantity and location of the line arresters along the line. THANK YOU www.te.com/energy
