Know the Star System of Distribution of Electric Energy

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Know the Star System of Distribution of Electric Energy

The Star System is composed of a three-phase transformer powered by the three phases of the primary circuit of electric energy distribution.

The primary bushings H1, H2 and H3 are fed by the 3 primary phases, protected by fuse switches (Matheus) and links specified according to the power of the transformer.

In the secondary output bushings X0, X1, X2 and X3, we will obtain the output voltages, as shown below:

Star System Connection Scheme

The system is powered at 13.8 kV, since the phases are D, E and F, subject discussed in the article on Delta System.

The bushing X0 corresponds to the NEUTRAL of the star system, X1 to phase A, X2 phase B and X3 phase C.

The nominal voltages between Neutral and Phase A, Neutral and Phase B, Neutral and Phase C are equal to 127 V and nominal line voltages equal to 220 V (127/220 V system).

Phases A, B and C are better known in the industry for R, S, and T.

Schematic of a star transformer

Schematic of a star transformer In the star system 220/380 V the nominal voltage between Neutral and Phase is 220 V, and the nominal line voltage is 380 V.

The expression used for voltage calculation in the three-phase system is as follows:

Where: VFN – neutral phase voltage

VFF – phase phase voltage or line voltage

V3 = 1.73 (approximate value, since it is periodic tithe)

According to the star formed by the 3 secondary coils (figure above), we notice that the phase angle between Phases A, B and C is 120º, which keeps the voltages out of phase as shown below:

Three Phase Voltage Diffusion Chart

Three Phase Voltage Diffusion Chart Author’s Note: RMS voltage, from the English Root Mean Square or Effective Value are the line or phase voltages.

Analogy between Star System and Delta System

In the Star System, because of the balanced voltages, we have been able to load loads much higher than the Delta System, which presents unbalanced voltages.

Due to the imbalance between the secondary voltages, the Delta System generates a very large load unbalance in the SEP – Electrical Power System, damaging it, whereas in the Star System, because of the balanced voltages, we can balance the loads with greater ease, keeping the SEP more stable and generating a smaller number of maintenance in the circuits of distribution, transmission and the generation of electric energy.


The Star System is infinitely better than the Delta System in all respects.

short  Adolpho Eletricista

Adolpho Eletricista – Your Electrician in São Paulo – SP!

Electrician Residential, Commercial, and Industrial

I attend in São Paulo, Greater São Paulo and South Zone of São Paulo – BR.

About Author

Adolpho Eletricista

Proprietário da Empresa Adolpho Eletricista. Técnico Eletrotécnico e Eletrônico habilitado. Eletricista residencial, predial, comercial e industrial autônomo com 42 anos de experiência. Atuação na Grande São Paulo e Zona Sul de São Paulo. Instrutor de NR-10. Atendimento Personalizado. Garantia de bons serviços. Redator de artigos técnicos profissionalizantes. Business Owner Adolpho Electrician. Electrotechnical and Electronic Technician enabled. Self-employed residential, building, commercial and industrial electrician with 42 years of experience. Expertise in the Greater ABC and Greater São Paulo region. NR-10 Instructor. Personalized service. Guaranteed good services. Writer of professional technical articles.

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