A comparative between IEEE and EN in the transformer derating when supplying nonsinusoidal load current. A practical case.

Abstract

Nowadays, power quality is a challenge for the distribution companies since the new energy policies are directed to a distributed generation system with power electronic based technologies. The reduction of distribution transformers capability when supplying nonsinusoidal load currents has a major impact within capacity reduction in distribution networks produced by technical losses. IEEE Std C57.110-2018, EN-50464-3 and EN-50541-2 define procedures to derate transformers when supplying nonsinusoidal load currents. The aim of this paper is to compare these procedures through a real case distribution transformer that suffers problems due to high levels of current distortion.

Revisited Concept of Three-Phase Transformers’ Short-Circuit Resistances in Light of the Institute of Electric and Electronics Engineers (IEEE) Standard C57.110-2018

“…where I sz is the RMS value of the secondary currents of each phase (z = A, B, C) of the transformer, expressed by (11).…”

“…The load loss values (P cc ) of three-phase distribution transformers can be obtained directly, by applying the IEEE Standard C57.110 [6][7][8][9][10][11][12][13][14][15][16][17] and other well-known standards [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32], or, indirectly, based on their short-circuit resistances referred to as the secondary resistance (R cc ), according to the following expression:…”

“…The P DCN losses can be obtained through a direct current test [47] and the individual values of P ECN and P OSLN are defined by the IEEE Standard C57.90 TM [48], which sets P ECN = 2/3•P ACN , in oil-immersed transformers, and P ECN = 1/3•P ACN , in dry-type transformers. Equation ( 4) is not explicitly expressed in IEEE Standard C57.110-2018, although it is used in many references [8][9][10][11][12] to refer to primary currents. However, the standard does not clearly indicate the meaning of the currents in Equation (4).…”

Assessment of Load Losses Caused by Harmonic Currents in Distribution Transformers Using the Transformer Loss Calculator Software

“…The rated load losses (P LL−R ) of the transformers operating at the fundamental frequency (50-60 Hz) satisfy the following expression [24,25]:…”

“…The latter depend on the Root Mean Square (RMS) value of currents, while the losses caused by non-sinusoidal currents also depend on the frequency of the harmonics of these currents [16][17][18][19][20][21][22][23]. The IEEE Standard C57.110-2018 [24,25] establishes that, in the presence of non-sinusoidal currents, the load losses (P LL ) of the transformers are the sum of three others:…”