Improvement of power quality for 6-kV industrial power system with motor load

Industrial power systems most often are characterised by a high amount of motor load and reactive power compensation. This configuration results in parallel resonance circuits, with resonance frequency in a typical range of 200 Hz up to 1 kHz. Typical harmonics in the system will be amplified and operational disturbances are the results. A 6-kV-system in a petrochemical installation is supplying a pumping station with a load of approximately 5 MW. All motors and the feeding 30/6-kV-transformer are equipped with reactive power compensation with blocking device against ripple-control signals. This arrangement is vulnerable to harmonics, as the impedance is increased due to resonances which are between 400 Hz and 900 Hz depending on the operating conditions. These resonances are close to the major harmonics, e.g. 5th or 7th. An amplification of the harmonic voltages is resulting from this, mal-operation of circuit-breakers, use-melting and fluctuating motor torque had been recognised. After carrying out intensive measurement and assessment of the harmonics under different load conditions, blocking reactors for the capacitors were supposed as countermeasure. This scheme solved both applications, i.e. avoid reduction of the ripple-control signals and protecting the capacitor from impermissible currents caused by resonances in the supply system.