Derating the three-phase power distribution transformers under nonsinusoidal operating conditions: A case study

The paper quantitatively investigates the problem of derating the in-service distribution power transformers that supply heavy nonlinear balanced loads. The adopted computation method basis on the international standards and only requires the basic power quality parameters measured at the secondary winding of the transformer and its certified test report. Hence, by computing the major transformer harmonic factors, an on-site derating report is generated, which provides the additional power losses generated by the harmonic currents and the corresponding transformer maximum permissible current (reduction in apparent power rating). The paper also critically analyzes other solutions that could avoid or minimize the harmonics detrimental effects on the transformers steady-state operation. The study is carried out on a 1600 kVA oil-filled type distribution transformer that serves both linear and nonlinear industrial loads.

I Introduction

The constant demand for increasing the energy efficiency in industrial low-voltage electrical installations determined the large scale usage of the power electronic equipments (inverters, rectifiers, soft starters etc.) [1] [3]. These intrinsically non linear loads generate important harmonic currents that adversely affect all the components of the local distribution power system [4] [6]. Thus, supplemental thermal, mechanical and even dielectric stresses are experimented by the power delivery electrical machines and apparatuses, causing their improper operation or failure and ultimately the reduction of their life expectancy. The harmonic currents could also generate the abnormal tripping of the protection devices (circuit breakers, fuses), overheating of the neutral conductor by exceeding its ampacity and the erroneous operation of the sensitive control equipment. Additionally, the voltages waveforms could also be distorted as a consequence of the harmonic current flows in power systems with large upstream impedances [7], [8]. Consequently, the three-phase power distribution transformers, as essential components involved in the power delivery process, are also negatively influenced by the harmonic currents flow [9] [11]. The transformers electrical and thermal rated parameters are indicated for perfect sinusoidal balanced load currents and supply voltages, at rated frequency. The nonlinearity of the modern industrial loads generates additional (windings eddy current and stray) losses within the distribution transformer, which may cause the exceeding of the device initially design thermal specifications. Thus, early windings isolation fatigue, malfunction, premature failure and a shorter lifetime of the transformer are to be expected [12]–[13].