History of Direct Frequency Converters

Section 1.1 introduced converters containing either dc link capacitor(s) or inductor(s). They both are physically large and/or heavy in most cases. In addition electrolytic capacitors are subject to ageing, for example, and cannot be used in all applications as presented. The passive components also cause power losses. Moreover, the converters presented do not fulfil the idea of a purely semiconductor-based converter. Thus, direct frequency converters, converting ac power to ac power directly without dc link passive components, have been studied, too.

According to [Gyu70] and [Wyk94], the idea of direct frequency conversion was originally presented in the 1920s and even applied in the 1930s. The first semiconductor-based direct frequency converters were developed in the 1960s after the invention of the thyristor [Pel71], [Maz73], [Gyu76]. A possible main circuit of a phase-controlled thyristor-based three-to-three phase cycloconverter is presented in Figure 1.4. The circuit presented is a six-pulse cycloconverter, which assumes isolated loads so that a supply transformer is not necessary.

For simplicity, the cycloconverter in Figure 1.4 does not include circulating current reactors either, which are sometimes used to enhance load power quality with discontinuous load current [Pel71]. Although reactors and supply transformers are sometimes avoided, they are necessary in many cases to make the cycloconverter system possible in practice, e.g. the

Introduction 5 supply transformer is not avoided with non-isolated load because in this case all the supplies of all three bridges are required to be isolated from each other. In addition, the six-pulse three-to-three phase cycloconverter requires 36 and a twelve-pulse version requires 72 thyristors.

The load voltage and input current waveforms of the cycloconverter are heavily distorted and the fundamental power factor of the input is quite poor, too, irrespective of the fundamental power factor of the load [She04]. In practice, its load frequency is also usually limited to half of the supply frequency because normal loads cannot tolerate the voltage distortion produced with higher input-to-output frequency ratios [Pel71]. Thus, the advantages remaining are robustness of the thyristor technology and low losses. Considering the drawbacks and limitations mentioned, the cycloconverter cannot be seen as an optimal solution for medium power converters, but it is nowadays applied mostly with higher power levels where fully controllable semiconductor devices, like IGBTs or MOSFETs, cannot be used yet.

Load

A Load

B Load

C ab

c

Figure 1.4 Six-pulse cycloconverter with isolated load phases without circulating current reactors.

The idea of the silicon-based forced commutated cycloconverter was presented and analysed first in [Gyu70] and later in [Gyu76], the latter being known more widely due to its easier availability. Over the decades, matrix converter (MC) has become established as the name of this kind of direct frequency converter. The principle of a three-to-three phase MC is presented in Figure 1.5, where each ideal switch describes a bidirectional switch which can conduct current and block voltage in both directions depending only on the control signal of the switch.

a b c

A B C

Figure 1.5 Principle of matrix converter (MC).

Compared to Figures 1.1–1.4, the circuit in Figure 1.5 may appear simple. However, it presents only the principle. In practice, the ideal switches assumed are not available and a supply-side filter is also necessary. Thus, a more realistic circuit of a direct matrix converter (DMC) is presented in Figure 1.6a, where the ideal switches are replaced by IGBTs and diodes. The circuit has no dc link, but it still requires passive ac filter components, so that the MC cannot be based on semiconductors purely in practice.

A theoretically identical choice for the DMC is the indirect matrix converter (IMC), presented in Figure 1.6b, where p and n are the dc link bars. The idea of the IMC for MC analyses was introduced in the 1980s [Zio85] and an actual IMC circuit was proposed in the 1990s [Min93].

Below, the terms direct and indirect matrix converters, i.e. the DMC and the IMC, respectively, are restricted to mean a three-to-three phase version with the same power transfer capability as the circuits in Figure 1.6 have.

A rough comparison between the PWM converters discussed above is presented in Table 1.1.

A more extensive comparison between VSC and MC can be found e.g. in [Ber02].

(a)

B a

b c

A

C

(b) a b c

n p

A B C

Figure 1.6 (a) A direct matrix converter (DMC). (b) An indirect matrix converter (IMC).

Table 1.1 A rough comparison of the PWM frequency converters reviewed in Sections 1.1 and 1.2.

2-level converters 3-level converters Direct converters VSC BBVSC CSC 3LVSC 3LBBVSC Cycloconv. DMC/IMC Number of

diodes 12 12 12 24 36 None 18

Number of

active devices 6 12 12 12 24 36

thyristors 18 Dc link

components Capacitor(s) Capacitor(s) Coil Capacitors Capacitors None None Supply current Distorted Sinusoidal Sinusoidal Distorted Sinusoidal Distorted Sinusoidal Input-to-output

voltage ratio 0–1 0–2/ 3 (or more)

0–1

(or more) 0–1 0–2/ 3

(or more) 0–3 3/π 0– 3/2 Supply filter

inductor Usually Necessary Depends on

application Usually Necessary Usually transformer

Depends on application Supply filter

capacitor None Depends on

application Necessary None Depends on

application None Necessary

Introduction 7 The MC has no natural freewheeling paths for inductive current as has a VSC. On the other hand, the MC may short-circuit the supply, unlike the CSC. Thus, the commutation of semiconductor switches was a problem, too, until late the 1980s when the first safe commutation method was introduced [Bur89]. On the other hand, modern active semiconductors with fast switching capabilities were introduced just couple of years before as presented at the beginning of this chapter. First modulation methods for MCs were also presented in 1980s [Ven80a], [Ven80b], [Zio85] [Ale89], [Hub89], [Oya89], [Wie90], [Whe02]. Thus, the main problems with MCs were mainly overcome until 1990. In addition, an MC system recovers faster after a power grid failure than conventional BBVSC systems containing dc link capacitor requiring charging and causing inrush currents [Kan02]. The RBIGBTs may also provide some improvements for the MCs in the future as with the CSC.

Thus, it is possible that MCs will become common in practical applications.