OverviewThe purpose of the investigation oftransient voltage loads and insulationcoordination is to protect operatingequipment from excessive voltageloads and to select the electrical insu-lation resistance. Overvoltage (surges)can be caused by lightning strikes,switching operations or temporaryovervoltage (e.g. ground faults). Sie-mens Power Technologies Internation-al (Siemens PTI) supports you with:

· Consulting in topics of insulationcoordination, arrester design, ar-rester positioning, protectionagainst transient surges, and selec-tion of insulation levels.

· Simulation of transient surges inplanned, existing or extended facili-ties.

· Studies of transients in breakerloads (e.g. TRV (Transient RecoveryVoltage) and "skipped current zerocrossings“) or for analysis of elec-tromagnetic transient compensationprocesses as a result of switchingoperations.

· Development of overvoltage protec-tion concepts (e.g. type and instal-lation location of arresters) withconsideration of the system config-uration, as well as economic as-pects.

The Challenge:A typical medium or high-voltage gridconsists of a large number of re-

sources, such as cables, transformersand switchgear. The acquisition costsfor these resources are significant andoften exceed the costs of a transientsimulation study by several orders ofmagnitude. The lack of a study canhave significant effects, as excessivetransient voltage loads can damageequipment. In addition to the repaircosts, there may also be additionaleconomic losses, such as penalty pay-ments (e.g. missed deliveries).

Transient voltage loads and insulationcoordination for a plant should there-fore be included in the planning phaseand also simulated in detail if this isreasonable. A reduced number of solu-tion possibilities may result at latertimes in a project (e.g. because the in-stallation of arresters is not possibledue to a lack of space).

Our solutionThere are several typical tasks regard-ing transient voltage loads that mustbe given special attention in the plan-ning phase. These include:

· Positioning and dimensioning of thesurge arresters in high-voltageswitchgear, as well as sufficient pro-tection levels in the equipment toprotect against lightning surges.

· Investigation of unaffected switch-ing surges after switching opera-tions caused by faults or by theswitching of long cables, or over-

head lines and transformers, for ex-ample.

· Determination of overvoltage pro-tection for Siemens medium-voltageswitchgear in special applications,such as vacuum breakers for arcfurnaces or compensation chokes.

· Determination of the electrical loadson breakers (e.g. TRV, arcing time)in order to enable the selection of asuitable switching device for a de-fined task.

Consulting based on experienceAlthough overvoltages are mostlycaused by complex physical processes,comprehensive studies are not alwaysnecessary, as some typical scenarioscan be defined. In this case, SiemensPTI offers support in basic questionsabout overvoltage protection and insu-lation coordination, saving time andmoney.

Comprehensive studiesIf comprehensive studies are neces-sary, appropriate simulation modelsare prepared according to the frequen-cy range to be investigated. This isusually done with the PSS®NETOMACsimulation program.

A specific number of simulation casesare specified depending on the gridtopology. Depending on the type ofstudy, these may include strike scenar-ios for lightning strikes in overheadlines (strike location, lightning cur-rents), switching states of switchgear,elements to be switched (cables, trans-formers, etc.), fault locations, etc.

The maximum voltages resulting forthe simulations are taken as the basisfor insulation coordination, for exam-ple.

Investigation of transientvoltage loads and insula-tion coordination

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Insulation coordinationInsulation coordination procedures areused to determine the necessary insu-lation level of the equipment accordingto the applied standard (IEC 60071).This yields the corresponding insula-tion levels:

· Design lightning impulse withstandvoltage LIWV per IEC standards.

· Design switching impulse withstandvoltage SIWV per IEC Standards.

Because of long lead times, the insula-tion level of the equipment is oftenalready specified. In this case, thespecified insulation levels are com-pared with the insulation levels to befulfilled, which are calculated from therepresentative overvoltages from thesimulations and additional factors. Ifthe specified insulation levels are low-er than the levels to be fulfilled,measures such as the installation ofarresters or their low-inductive con-nection are recommended.

Dimensioning and/or verification ofsurge arrestersSurge arresters are designed to preventthe anticipated violation of predefinedinsulation levels. If the insulation levelsare specified by the study, the electri-cal insulation resistance should not betoo high due to economic require-ments. Furthermore, the arrestershould not be compromised undernormal conditions (e.g. overloading ofthe arrester in the event of a groundfault).

Implementation example I:Lightning overvoltage studyThe results of a lightning overvoltagestudy are explained below. Figure 1shows a GIS supplied by two trans-formers and two cables connected tooverhead lines after 1.5 km.

Figure 1: Overview of example

Three different types of lightningstrikes are taken into account:

· Direct strike on conductor in the ar-ea of the last masts.

· Direct strike in the last masts, possi-bility followed by rearward insulatorarcing.

· Conductor strike far away.

The necessary LIWV for the equipmentwas calculated in accordance with IEC60071 based on the maximum voltag-es and was compared with the prede-fined insulation levels. The resultsshow that the predefined insulationlevels are insufficient in the event of amast strike with rearward insulator arc-ing. The installation of surge arresterson the high-voltage side of the trans-formers was therefore recommended.Once these were dimensioned, thiswas accounted for in the simulationsand the simulations repeated. Thisproved that the equipment was nowsufficiently protected against lightningsurges with the specified insulationlevels (see Figure 2).

Figure 2: Maximum voltages without (upperplot) and with (lower plot) additional surgearresters

Implementation example II:TRV studyThe recurring voltage across the termi-nals of a circuit breaker after the deac-tivation of a short-circuit current iscalled the "transient recovery voltage"(TRV). To ensure safe deactivation, thegradient and maximum value of theTRV are not permitted to exceed de-fined test values specified in interna-tional standards.

The TRV is calculated as an example forthe SF6 circuit breaker in the GISswitchgear from implementation ex-ample I (Figure 1). A 3-pole fault in theoverhead line for bay No. 1 (far left) isconsidered. Figure 3 shows the simu-lated TRV across the "first-pole-to-clear"of the breaker if all of the bays in theswitchgear are in operation. Both, thegradient, as well as the maximum val-ue of the TRV are clearly within the en-velopes. If the fault is fed only fromone transformer bay supplied (1 over-head line bay and 1 transformer bayout of operation), this yields the re-sults in Figure 4. In the investigated

switching configuration, only the ca-pacitance of the short cable betweenthe GIS and the transformer is effectiveon the source side of the breaker forthe TRV. This significantly increases thegradient of the TRV compared withthat in Figure 3, and the maximumpermissible gradient is barely compliedwith. If a busbar connection is imple-mented instead of the short cable, thecalculated TRV values are outside ofthe permissible range. In this case, cor-rective measures would have to be in-vestigated and planned in the contextof the Engineering of the facility in or-der to comply with the normative re-quirements.

Figure 3: Simulated TRV – normal condition

Figure 4: Simulated TRV – transformer feedsfault

The permissible maximum TRV is de-fined as the envelope. If the voltageacross the breaker poles is within theenvelope over time, the normative TRVrequirements are complied with. Theenvelopes are dependent on the ampli-tude of the short-circuit current to beinterrupted. The lower the short-circuitcurrent, the higher and steeper are thepermissible envelopes.

Published bySiemens AG 2017

Energy Management DivisionFreyeslebenstrasse 191058 Erlangen, Germany

For more information, please contact:power-technologies.energy@siemens.com

AL=N, ECCN=EAR99Subject to changes and errors. The infor-mation given in this document only con-tains general descriptions and/or perfor-mance features which may not alwaysspecifically reflect those described, orwhich may undergo modification in thecourse of further development of theproducts. The requested performance fea-tures are binding only when they are ex-pressly agreed upon in the concluded con-tract.