Electric power conversion systems – Current conversion – Having plural converters for single conversion
Patent
1992-12-09
1995-09-12
Stephan, Steven L.
Electric power conversion systems
Current conversion
Having plural converters for single conversion
307 82, H02M 777
Patent
active
054503094
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The construction of fast inverters comprising several single elements connected in parallel is only possible with rather large expenditure for control and electronic circuitry. This is only acceptable if the inverter must satisfy certain conditions which cannot be satisfied in another way, and/or if the parallel switching arrangement has certain decisive advantages in comparison with usual solutions comprising only a single element.
It is an aim of present-day builders of motors to produce motors which run ever faster. So-called "fast-running drives" require operational frequencies which exceed 500 Hz. In view of the power aimed at, in the range of several hundred kVA, usual thyristor- or GTO (gate turn off) inverters can at best provide these frequencies in the shape of elementary steps, as shown in FIG. 1.
The drawbacks of this crude, rectangle-shaped voltage and of the non-sinus shaped current curve which it entails, that is additional losses and peaks of the moment are well known. It has therefore been known for a long time to divide these voltage segments into smaller time units, in order to obtain a nearly sinusoidally-shaped curve of the motor current. With respect to the inverter this entails that the power switches must be switched on and off accordingly faster. The faster these switches work, the finer one can make the time slices of the voltage segments, and the more sinusoidally-shaped the motor current becomes, see FIG. 2.
If now the operational frequency of the usual 50 to 120 Hz is raised to over 500 Hz at fast-running drives, then the switching frequency of the switches must also be increased accordingly. This means that the thyristor- and GTO switches come even nearer to the limits of their switching speed.
Fast inverters with switching frequencies up to 100 kHz and able to deliver a sinusoidally-shaped current curve even with 500 Hz output frequency are normally available only up to a power of about 50 kVA. An increase of the power up to several hundred kVA with the same high clock frequency is scarcely imaginable with present-day techniques, hence the desire to use inverters which are connected in parallel.
The design of a parallel connected inverter differs in many ways from that of an inverter built in the usual way. Whilst hitherto the control, switching and the protection of the power rectifiers were the essential issues, general design considerations now come into the foreground. Protection problems cannot be considered as local issues any more, but must be viewed in relation to the overall system. Communication problems between the partial inverters must be solved, as must those which pertain to the reliability of a distributed system. One must never forget that the release time, that is the time available for collecting, processing, and distributing instructions to the entire system may not exceed a few microseconds. In what follows it will therefore be considered how a technically and economically acceptable arrangement of parallel connected inverters may look like.
In what follows different possibilities to connect inverters in parallel will be considered and a systematic description will be attempted. We will distinguish the following four main criteria:
The coupling of the energy, i.e. the collection of the energies delivered by the partial inverters, can be performed electrically or magnetically. If one uses an electrical coupling, the different phases of the partial inverters will be connected, and the load will be attached in the usual way. A magnetic coupling is said to exist for instance when using a motor with multiple winding.
In the case of an electrical coupling, the partial inverters can be mutually connected either through large connecting coils which have a function tied to the system as a whole, or through small coils which solely act as protection. This last case is called a direct coupling, see FIG. 4.
The timing relates to the switching times of the power switches. If these are switched in accordance with the same time grid for all
REFERENCES:
patent: 4328429 (1982-05-01), Kublick
patent: 4441032 (1984-04-01), Sakurai et al.
patent: 4673823 (1987-06-01), Tanaka
patent: 5200643 (1993-04-01), Brown
Berhane Adolf
Inventio AG
Stephan Steven L.
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