Review of activities

Measurements of switching overvoltages in transmission network and power plants

  • Measurements of transient overvoltages especially switching overvoltages in power systems with nominal voltage from 6 kV up to 420 kV
    1. by capacitive dividers in frequency spectrum cca 1 Hz - 1 MHz
    2. by electric field sensors in frequency spectrum 10 Hz - 10 MHz
  • Free potential measurements of transient overcurrents 0,2 A - 20 kA by Rogowski coils in frequency spectrum from 10 Hz to 10 kHz
  • Evaluation of measured data and their analysis
  • Modelling of measured transients using EMTP program
  • Analysis of extraordinary power network states - resonance, ferroresonance - measures for their elimination
  • Design, development and tests of HV dividers


  • Switching overvoltage measurements in 6 kV networks in Power Plants Tusimice, Pocerady, Dukovany, Melník - for Czech Power Company (CEZ)
  • Switching overvoltage measurements in 110, 220 and 420 kV transmission network substations - for Czech Power Transmission Company (CEPS)
  • No load line opening of long overhead line 400 kV - circuit breaker stresses (CEPS)
  • Measurement of overvoltage during disconnector switching in substation 420 kV Chrast (CEPS)
  • Unstability of voltage in circuit of tertiary winding of autotransformer 400/110/10,5 kV in substation Nosovice - elimination of VT ferroresonances (CEPS)

Analyses of extraordinary network events - failures and damages of HV equipment - investigations to causes of failures

(cases connected with some kind of overvoltage or electrical insulation degradation)

These studies generally involve:

  • Analysis of available event records - oscillograms of voltages and currents
  • Inspection of failed equipment
  • Field mesurements with aim to evoke circumstances of the investigated event - if possible - typically measurements of some type of switching
  • Simulation of measured phenomena in EMTP - approximation of computed data by measurement results
  • Extrapolation of simulation to variety of network states that could lead to investigated event
  • Laboratory HV tests of the equipment of the same type - if it is necessary
  • Assesment of failure causes
  • Recommendation of proper corrective measures for equipment or network


  • Failure of circuit breaker 420 kV in line bay V 413 in substation Reporyje - investigation of causes - Study for CEPS
  • Failure of SF6 current transformer 420 kV in bus coupler bay in substation Chrast and Sokolnice - investigation of causes - Study for CEPS
  • Failure of VT 110 kV in line bay V1527 in GIS Vyskov - investigation to causes - Study for North Czech Power Company
  • Field investigations of capacitive voltage transformers with failures in anti-resonance circuit (CEPS)
  • Failure of current transformer 420 kV in substation Slavetice due to lightning

Design of overvoltage protection by MOAs - coordination of insulation


  • Optimisation of overvoltage protection in substations
  • EMTP models of
    1. three phase full substation scheme including earthing system
    2. connected lines including models of towers
    3. Leader Development Model of flashovers (and backflashovers)
    4. Electro-Geometrical-Model of line conductor configuration for computation of shielding failures - direct strokes
  • Computation of lightning overvoltage penetration from lines into substation
  • Probabilistic evaluation of lightning overvoltage exceeding insulation levels and energy exceeding absorption capacity of MOAs
  • Optimization of MOA location
  • Selection of surge arrester types


  • Probabilistic studies of overvoltage protection effectiveness
  • Lightning performance of lines
  • Insulation coordination of lines and substation equipment


  • EMTP overvoltage protection analysis of five substations 400 kV based on modelling of complete substation schemes (CEPS)
  • Design of overvoltage protection of 420 kV substations Cebin, Slavetice (CEPS)
  • Overvoltage protection designs for circuits of tertiary windings of large power transformers (CEPS)
  • Design of overvoltage protection of Slovakian 420 and 220 kV substations Varin, Horna Zdana, Sucany (Slovak Power Company)
  • Design of electric motor drivers overvoltage protection in 6 kV cable network of power station Melnik (CEZ)
  • Type overvoltage protection designs for substations 110/22 kV and distribution transformers 22/0,4 kV (West Czech Power Company)
  • Cutting of grounding wires by stolers - influence on lightning performance of the overhead lines (CEPS)
  • Study of compact lines 400 kV for urban areas (CEPS)

Field measurements and investigations

  • Power transformers - partial discharge measurement
  • Metal oxide arresters - leakage current and on-line measurements of partial discharges
  • Instrument current and voltage transformers - on-line measurement of partial discharges
  • Manufacturing and purchasing of potential free partial discharge detector for inspection of HV equipment on-line
  • Interfering electromagnetic field measurements in the range of 0,15 - 300 MHz along UHV overhead lines and in substations (interference source searching)
  • Electric and magnetic field 50 Hz measurements of HV and UHV overhead lines and substations
  • Site measurements of pollution degree and measurements of pollution state of external insulation


  • Periodical off-line partial discharge measurements on the block transformers of nuclear power plant Dukovany and supervisory PD measurements on large power autotransformers in substations 420 and 220 kV of CEPS
  • Type oriented field on-line PD measurements of 220 kV and 110 kV current transformers with high percentage of units with degraded insulation
  • Diagnostic on-line PD measurements of large number of current transformers and surge arresters 110, 220 and 420 kV in substations of transmission network.
  • Measurements of radio interference levels and electric and magnetic fields 50 Hz of lines and substations for CEPS and distribution companies
  • Searching and localisation of RI sources on lines and in substations - measures to their elimination - for CEPS and distribution companies
  • Site pollution severity measurements for CEPS and for Czech Regional Distribution Companies (1978, 1980, 1989, 1995, 2002)
  • Cooperation in site pollution severity measurement for CIE Havana, Cuba (1984, 1986)

Power transformer monitoring system

  • Continuous monitoring of many diagnostic quantities (voltage vector diagram, energy balance, overvoltage, tg delta of bushings, PD activity) of large power transformer recorded and evaluated by measuring computer with continuous transfer of output data to distant terminal.


  • Two diagnostic monitoring systems more than 10 years in operation. The first one on the block transformer 400/15 kV in Nuclear Power Plant Dukovany, the second on the station autotransformer 400/110 kV in substation Reporyje.

Field tests

  • field lightning impulse voltage tests of reactors up to system voltage 420 kV
  • field lightning impulse voltage tests of GIS up to system voltage 420 kV


  • Acceptance tests of 420 kV GIS Dlouhe Strane and 420 kV GIS Chodov - for CEPS
  • Lightning impulse tests of 420 kV reactors after their reparation - for CEPS


  • preproject work and data preparation for HV and UHV overhead lines and substation designs
  • design of overhead lines parameters based on the application of insulation coordination
  • selection of insulation into polluted conditions
  • measures for increasing of outdoor insulation reliability in polluted conditions (recommendation for greasing, RTV silicon coating, cleaning and suitable device type selection and design for hot line washing)
  • residual service life of outdoor insulator strings
  • design of insulating assemblies (insulator strings for required operating and withstand voltages)
  • calculation of overhead line electric parameters
  • calculation of electric and magnetic fields of 50 Hz
  • calculation of RIV of overhead lines
  • design and calculation of insulating distances


  • All foregoing activities have been made for CEPS, Czech Regional Distribution Companies, Energovod (HV and UHV overhead line construction company), EGE - Engineering (overhead line design company)