1.12.3 Merits and demerits, conditions of application

The considered circuit has much in common with three-phase bridge rectifier: there are same a pulsation ratio U_d and the waveform of i₁, but the transformer is a little bit worse used. S_TAV depends on depth of regulation and, hence, summarized power S_T+S_Tbr is depended on a range of regulation of angle α. This circuit is applied to low voltage and high current rectifiers because current I_d flows through a two parallel connected thyristors instead a two series connected as in three-phase bridge rectifier. Application of double three-phase star with the current-balancing reactor rectifier may give essential reduction of number of thyristors and higher efficiency. The given circuit has found wide application in electrometallurgy.

1.13 Equivalent polyphase circuits

Equivalent polyphase circuits could be realized by parallel or series connection of rectifier circuits:

- For reduction of pulsations of a waveform of U_d;

- For decrease of the higher harmonics of a waveform of i₁;

- Those units are applied for high currents I_d and high voltage U_d.

In a parallel way and in series way it is possible to connect different kinds of circuits: three-phase with center cap, with the current-balancing reactor, bridges. It is necessary to establish the phase displacement between secondary voltage and primary voltage in the angle depending of a number connected circuits for purpose setting up equivalent polyphase conditions.

1.13.1. Series connection of two three-phase bridge rectifiers

Conditions: α=0, L_a=0, L_d=∞

A

B

C

i^||_1A

5^|

3^|

1^|

i^|_2A

i^||_1C

i^|_1A

i^||_1AC ↑

i_1A

I

II

i^||_2A

6^|

4^|

2^|

5^||

2^||

6^||

3^||

4^||

1^||

L_d

Figure 1.42

U

e_dI

U_dI

5’

6’

1’

2’

3’

4’

5’’

6’’

1’’

2’’

3’’

4’’

U_dII

e_dII

U_d

U_d=U_dI+U_dII

U

1’

4’

4’’

1’’

2’’

5’’

i’’_1A

i_1A

Figure 1.43

Bridges I and II operate independently each from other. Transformers of ones are fulfilled with different group of transformer connections. In this case, in order to establish the phase displacement between outputs voltages of bridges, transformer connection for bridge I-Υ/Υ, for the bridge II – Δ/Υ. So waveforms of U_d1 and U_d11 are displaced on 30 degrees.

According to the aforesaid

U_dI = U_dII; I_d = I_dI = I_dII; P_dI = P_dII.

For purpose of realization of these conditions, line voltage ratios of the transformers for both ones should be equal:

Secondary currents also are same:

Waveforms of primary phase currents also coincide with waveforms of secondary currents and, hence, coincide for bridges I and II, but its value differ one from other on times because as line K_Tr of both transformers are equal as phase K_Tr are differ

Thus, a peak primary phase current for winding connection Υ/Υ is and for Δ/Υ -

The primary current i₁ is neighborhood of sine curve.

Primary windings could be connected in star, and secondary - one in a star, another in a delta then it is possible to use one transformer with two secondary windings.

Creation of similar circuits probably and with the big number of bridges but then it is necessary to use phase-shifting transformers. Sequential circuits are expedient for applying on the high voltage.