Rectifier transformers: Technology update

January 29th, 2015, Published in Articles: Energize

 

Industry sectors such as the electrochemical and smelting rely on low voltage high current DC supplies for operation, which are supplied from sources known as rectifier transformers. This article is an overview of the technology available in this field.

The transformer and rectifier are provided as a combined unit. The low voltage and very high current creates a need to keep the connection between the transformer and the rectifier as short as possible to reduce losses and allow closer control of the transformer-rectifier combination. Furthermore, an integrated unit can be moved without affecting the connection or operation of the rectifier-transformer which allows units to be placed as close as possible to the process consuming the DC.

Process requirements

Electrolytic production of metals

Electrolysis is used in the production of many metals from processed ores, including aluminium, zinc, copper and others . Table 1 shows the requirements for different processes and typical sizes of plant.

Table 1: Power requirements for electrolytic production of metals.
Process Voltage
(per unit/cell)
Current (A)
Aluminium 4,5 180 000
Copper <1 160 – 400 /m2

Electrolysis of brine

Electrolysis is used to produce chlorine and caustic soda from brine. Power requirements are shown in Table 2 [2].

Table 2: Power requirements forelectrolysis of brine.
Cell voltage 3,10 V
Current density 7 kA/m2
Typical active cell area 2,72 m2
Total current/cell 19,04 kA

Electrochemical processes to produce other chemicals including organics are under investigation – most interesting is the reduction of carbon dioxide [1].

Arc furnaces

Arc furnaces use DC at high currents, ranging up to 120 kA, and at voltages of up to 1500 V.

Electroplating

Electroplating has a DC requirement but at a relatively low current and unless large plant is involved, conventional rectifiers can be used.

The rectification process

The aim of the rectification process is to produce a DC supply operating within the voltage limits required, and of the required quality. Rectification of AC produces a DC waveform containing a “ripple” component which is usually described in terms of harmonics, a term usually used in conjunction with AC waveforms. The DC output may be considered as a steady DC voltage with superimposed AC harmonics. The voltage harmonics result in AC current harmonics which are reflected back into the AC supply. The design goal of these high current rectifiers is to reduce the ripple as much as possible and hence reduce or eliminate the harmonics reflected back into the AC supply. Harmonics also affect the behaviour of the transformer, and the high harmonic currents involved result in high eddy current losses, and increased resistive losses.

The high currents involved mean that conventional ripple smoothing processes cannot be used and other methods have to be adopted to provide an output approaching a steady DC voltage. The most common methods used include combining a number of waveforms at the supply frequency but displaced by different phase angles from one another in the rectification process.

Notation

The rectifier configuration used is often referred to by counting the number of DC “pulses” output for every cycle of the input waveform. A single-phase, half-wave rectifier circuit would be called a “1-pulse” rectifier, because it produces a single pulse during the time of one cycle of the AC waveform. A single-phase, full-wave rectifier (regardless of design, center-tap or bridge) would be called a “2-pulse” rectifier, because it outputs two pulses of DC during the time of one AC cycle. A three-phase half-wave rectifier would be denoted a “3-pulse” unit and a three phase full-wave rectifier would be called a “6-pulse” unit.

Harmonics and pulse number

Table 3 is a listing of 3-phase harmonic currents for a 6-pulse rectifier and their respective frequencies. These harmonics are normally associated with nonlinear loads and have frequencies that are integer multiples of the 50 Hz fundamental [3].

Table 3: Harmonics associated witha 3-phase 6-pulse converter [3].
Harmonic number Frequency (Hz)
Fundamental 50
5th 250
7th 350
11th 550
13th 650

Note that the listed harmonic orders are one number below and one above “6” and “12,” respectively. In effect, the relationship of pulse number to harmonic order is expressed by the following equation:

h = kq ± 1                               (1)

Where

h = the harmonic order

k = any integer

q = the pulse number of the rectifier.

For a 6-pulse rectifier then, insert the number “6” for q and the integer “1” for k. Solving for h, we find that a 6-pulse rectifier will generate 5th and 7th order harmonics. By inserting the integer “2” for k, we find that the 11th and 13th harmonics also will be present. Thus, the table can be extended to include other normally present 3-phase harmonics with 6-pulse rectifiers, such as the 17th, 19th, 23rd, 25th, etc.

The pulse number chosen has a distinct influence on the harmonics present and certain pulse rates cancel harmonics. Harmonics decrease in amplitude as the harmonic number increases.

Three-phase half-wave rectification

3-pulse star connected system

Fig. 1 shows the diagram of a 3-phase half-wave rectifier using diodes and fed from a star configured secondary of a transformer. This produces the waveform shown in Fig. 2.
The rectified output of the three phases overlap to produce an output voltage varying between the peak voltage Vp and the voltage at the point of overlap of 0,5 Vp. Using equation 1 the harmonics present will be h = n * 3 ± 1 = 2nd and 4th harmonics as well as 5th and 7th , etc.

 

Fig. 1: Three phase star connected half wave rectifier.

Fig. 1: Three phase star connected half wave rectifier.

 

There is no voltage control available on this basic circuit. The disadvantage of this type of half wave rectification is that the DC component flowing through the core saturates the transformer.

Fig. 2: Output waveform of 3-phase half wave rectifier.

Fig. 2: Output waveform of 3-phase half wave rectifier.

6-pulse double star half wave rectifier

The configuration is shown in Fig. 3. In this configuration two star secondaries are connected in antiphase and the output of the rectifiers  connected in parallel. This produces a “6-pulse” configuration, as the “negative” cycles of the input wave are now rectified as well and added to the output, providing an additional set of pulses 180° out of phase with the input waveform.

The output waveform is shown in Fig 4. The output voltage varies between the peak value Vp and a lower point of  0,86 Vp. This configuration creates the equivalent to a full wave three-phase rectifier while distributing the load between two transformers.

Fig 3: 6-pulse double star half wave rectifier.

Fig 3: 6-pulse double star half wave rectifier.

Fig. 4: Output waveform : 6- pulse double star rectifier.

Fig. 4: Output waveform : 6- pulse double star rectifier.

This is a considerable improvement on the 3-pulse configuration but still has a ripple amplitude of 0,14 Vp and contains harmonics of the order:

h = n * 6 ± 1 = 5th and 7th, 11th and 13th, etc.

Interphase transformers

For the double star configuration to work correctly the voltages of the two transformers must be closely matched, and the current drawn from each transformer must be equal. A straight parallel connection will not guarantee this and additional measures are necessary to ensure balancing of the loads.

A common method of doing this is by inserting an auto transformer in the common connection. Fig. 5 shows this method. This acts as an interphase transformer and results in the load current being shared between the two rectifier arms. The output voltage of the rectifier is approximately equal to the average voltage of the two arms of the unit.

Fig. 5: Dual star rectifier with interphase transformer connection.

Fig. 5: Dual star rectifier with interphase transformer connection.

Saturable reactors

Further smoothing and regulation of the DC voltage and current in diode rectifiers is achieved by the inclusion of a saturable reactor (SR) in the line feeding the load (Fig. 6). The purpose of this unit is to achieve fine and continuous regulation of the DC voltage. The saturation of the core is controlled by a DC current fed into the control windings. This current is regulated by a sensor controller which monitors the current and voltage. The inductance of the core, and hence its reactance, will vary with the amount of saturation. At high degrees of saturation inductance decreases and impedance to AC harmonics decreases. Varying the degree of saturation controls the average voltage delivered to the load. A DC current flows through each driving circuit to control the magnetisation status of the core and with that, the voltage variation.

Fig. 6: Saturable reactor control.

Fig. 6: Saturable reactor control.

On line tap changers

For diode rectifiers the output voltage may also be varied by the use of on line tap changers (OLTCs) in the primary circuit of the transformer. These OLTCs are controlled by a monitoring and control circuits on the output of the rectifier. OLTCs are often used in conjunction with SRs to achieve fine control of voltage and current.

DC saturation of transformer cores-sizing

High DC currents flowing in half wave rectifier circuits could saturate the transformer core [1]. Generally, special transformers have to be built for these rectifiers. The power transformer has to be oversized by 21% at its primary side, and by 48% at its secondary side. In terms of average VA, the transformer needs to be 35% larger that the rating of the DC load. The larger rating of the secondary with respect to primary is needed because the secondary carries a DC component inside the windings. Besides, the transformer is oversized because of the circulation of harmonic currents, which do not generate active power. The core saturation, due to the DC components through the secondary windings, must also be taken into account for oversising the iron core.

3-phase bridge rectifiers

The 3-phase bridge or Graetz rectifier, is shown in Fig 7. The bridge may be fed from either a star or delta secondary. The rectifier produces a 6-pulse waveform of the type shown in Fig 4. The bridge configuration used results in the transformer carrying the full load continuously. Current flows in both positive and negative cycles of the input waveform, and there is thus no DC component flowing in the transformer core, reducing the possibility of core saturation. The rectifier produces harmonics of the order:

Fig. 7: 3-phase bridge rectifier.

Fig. 7: 3-phase bridge rectifier.

h = n * 6 ± 1 = 5th and 7th, 11th and 13th, etc.

To reduce ripple and harmonics, and to improve voltage control, additional bridges fed from supplies shifted in phase from the incoming supply may be combined to give the effect of additional phases and higher pulse numbers. Phase shift is indicated by the degrees of phase from the incoming source, e.g. +30º,-60º, etc.

Combined star-delta 12-pulse bridge rectifier

This is illustrated in Fig 8. Secondary windings in star and delta configurations are connected to 3-phase bridges and the output connected either in parallel or series. The phase angle between star and delta secondary voltages is 30°.

Fig. 8 Combined star-delta 12-pulse bridge rectifier .

Fig. 8 Combined star-delta 12-pulse bridge rectifier .

For instance in the case of a combined star/delta parallel system the difference between the two wave forms will be 30° ( Fig. 9). This has the effect of introducing another set of waveforms displaced by 30° giving a 12-pulse output. The voltage will vary between Vp and 0,96 Vp  and harmonics will be of the order of :

Fig. 9: Combined star delta bridge rectifier gives a 12–pulse output.

Fig. 9: Combined star delta bridge rectifier gives a 12–pulse output.

h = n * 12 ± 1 = 11th and 13th, as well as 23rd and 25th.

Higher pulse rates

Higher rates, up to 60-pulse systems are achieved by parallel connection of 12-pulse systems with output phases shifted relative to each other. Increasing the number of effective phases in the output increases the pulse number.

Phase shifts of angles other than the inherent phase difference in standard 3-phase configurations are obtained by the use of phase shifting transformers. For higher pulse rates the phase shift required would be submultiples. Typical phase angles are given in Table 4.

Table 4: Phase shift in multi-pulse systems.
Pulse count Phase difference
6 60°
12 30°
18 20°
24 15°
48 7,5°
60

Phase shifting transformers

A phase shifting transformer is a standard transformer with a single primary winding and several secondary windings. The secondary windings are interconnected in such a way as to give a resultant waveform that is shifted in phase from the primary waveform. The secondary windings may be of different sizes from the primary winding to give the desired phase shift. Fig. 10 shows an example of a phase shift transformer connection while Fig. 11 shows the vector diagram of the output. Various other configurations, such as zig-zag connections of the secondary, are used to achieve the desired effect. Phase shifting can also be applied to cancel out selected harmonics.

Fig. 10: Phase shift transformer.

Fig. 10: Phase shift transformer.

Fig. 11: Phase shift transformer vector diagram.

Fig. 11: Phase shift transformer vector diagram.

Thyristor control of voltage and current

The availability of high current and high power rated thyristors has led to the development of thyristor controlled rectifier transformers. The construction and configuration remain basically the same but thyristors are used instead of diodes, and control is exercised by varying the firing angle of the thyristor instead of using SR and OLTC devices. When used together with a saturable reactor both voltage and current can be controlled.

References

[1]    J Dixon: “Three phase controlled rectifiers”, Pontificia Universidad Católica de Chile http://web.ing.puc.cl/~power/paperspdf/dixon/21.pdf
[2]    G Botte: “Electrochemical manufacturing in the chemical industry” The Electrochemical Society Interface, Fall 2014, www.electrochem.org/dl/interface/fal/fal14/fal14_p049_055.pdf
[3]    J Dedad: “Six pulse conversion and harmonics”, Electrical Construction and Maintenance, Dec 2008, http://ecmweb.com/power-quality/six-pulse-conversion-and-harmonics

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