Thyristor converters

Converts a.c to d.c. and can be ower diode fixed voltage for a.c. drives (phase controlled thyristor converter for d.c. drives)... 

Variable a.c. voltage and frequency IGBT or thyristor converter (7) Inductor... 

DC methods mostly use shunt or compound wound motors. Occasionally series wound motors are used when high torque at low speeds is required. These machines are fed with DC voltage derived from a three-phase AC source using a thyristor converter. The thyristor converter rectifies the AC into DC but with control over the magnitude of the average DC voltage. Thyristors are also called silicon controlled rectifiers . 

It seems to be a mis-conception, however, amongst some mine operators that MG sets are old fashioned, maintenance intensive and inefficient and if you are going to upgrade or refurbish a DC winder, you have to replace the MG set with a thyristor converter They fail to understand that, particularly when the power supply to a mine is less robust, subject to voltage fluctuations and sensitive to harmonic and other disturbances, a thyristor converter is possibly the worst form of drive that should be used. Operation of the winder at slow speed produces a very low power factor draw on the AC supply (typically as low as 0.2). Furthermore, if the winder s drive motor is of an old design with a sohd rather than a laminated field frame, its power output would need to be de-rated by is much is 15% in some cases because of additional heating created by the AC components of the DC supply developed by a thyristor converter. 

The associated equipment consists of a solid-state automatic voltage regulator that controls a thyristor converter which in turn supplies the generator field via a field circuit breaker, generator slip rings, and brush gear. The main power output from the generator to the step-up transformer is by means of a forced air-cooled, isolated phase bus duct, with tap offs to the unit service transformer, excitation transformer, and potential transformer cubicle. 

Since 1960 the direct current for electrolysis has been provided exclusively by silicon rectifiers. A set of rectifiers can supply up to 450 000 A. Voltages up to 4.0 kV per diode are feasible, but usually for safety, a peak a.c. voltage of 1500 V, corresponding to a d.c. output of 1200 V, is not exceeded. Liquid cooling of the diodes permits a compact design, and self-contained equipment reduces leakage losses. Modem electrolysis cell plants also use continuously variable thyristor converters in place of silicon diodes [64]. 

A modification of this basic drive system uses solid-state rectifiers and thyristors to convert the wound-rotor, variable-frequency slip power first to direct current and then to hne-frequency power (60 Hz in the United States). This in turn is fed back to the power system as useful energy. 

Diode bridge rectifier (converter) Inverter unit IGBT or thyristor, depending upon the size of machine,... 

These are unidirectional and uncontrollablet static electronic devices and used as static switches and shown in Figure 6.14. A diode turns ON at the instant it becomes forward biased and OFF when it becomes reverse biased. By connecting them in series parallel combinations, they can be made suitable for any desired voltage and current ratings. Whether it is a transistor scheme or a thyristor scheme, they are used extensively where a forward conduction alone is necessary and the scheme calls for only a simple switching, without any control over the switching operation. They are used extensively in a rectifier circuit to convert a fixed a.c. supply to a fixed d.c. supply. 

Figure 6.24(b) Thyristor cubicles for high-power static converter units (Courtesy Siemens)... 

Variable AC voltage and frequency converter (IGBT or thyristor)... 

The exciter is an AC generator with a stator-mounted field. Direct cur rent for the exciter field is provided from an external source, typically u small variable voltage rectifier mounted at the motor starter. Exciter oui put is converted to DC through a three-phase, full-wave, silicon-diode bridge rectifier. Thyristors (silicon-controlled rectifiers) switch the cur rent to the motor field and the motor-starting, field-discharge resistors These semiconductor elements are mounted on heat sinks and assembled on a drum bolted to the rotor or shaft. 

A relatively new innovation for use in electric motor compressor drives is the variable frequency power source. Fundamentally, the power source converts an existing three-phase source into DC then uses an inverter to convert back to a variable frequency supply. Thyristors or transistors are used to switch the output at the required frequency. 

A power converter generates a pulsating DC voltage from the three-phase mains. DC motor speed is determined by the amount of DC voltage generated that can be altered via the thyristor control angle of the power converter. 

The power conditioner was an 18-pulse, stepped-wave, gate-turn-off thyristor (GTO) converter manufactured by GE. It was capable of four-quadrant operation, i.e., it was bi-directional and VAR capable. It was configured with three, six-pulse converters which were connected in series on the a.c. side and in parallel on the d.c. side. It was rated at 10 MW, and the input window voltage was rated 1750 Vdc to 2800 Vdc. A Westinghouse monitoring and control system provided the BESS with a highly automated supervisory control and data-acquisition capability [18,19]. 

The power-conditioning system was built by GE and consists of paired, six-pulse converters that form a 12-pulse converter module, and three of these are paralleled to achieve the required power rating. The switches are GTO thyristors. The power conditioner incorporates harmonic filtering and provides for four-quadrant operation, i.e., the equipment is bi-directional and provides VAR control [21,22]. 

Before thyristors and power transistors were introduced for AC to DC and AC to DC to AC converter systems, there were a number of special designs of AC motors that gave better performance than standard squirrel-cage motors. These motors required connections to the rotor windings. They had better speed control, superior torque versus speed characteristics and some methods were energy efficient. However, they were more complicated and hence more expensive. 

In this group there are several systems that use power electronics to provide a variable magnitude voltage at a variable frequency. Most of the systems use rectifiers and thyristors in the form of converters and inverters. 

Cyclo convertor drives were developed from the DC thyristor drive and consist of three DC converters arranged to produce a variable low-frequency three-phase sine wave to an AC motor. The development of high-power Cyclo convertor drives and advances in AC motor technology led to this combination being widely employed worldwide in large hoist apphca-tions with both synchronous and induction motors. Indeed there are still apphcations today being installed with Cyclo convertors. 

In cathodic protection stations 220 V ac rectifiers are used with silicon diodes or steered thyristor rectifiers, the output parameters of which are automatically regulated. As a result of the development of electronics, field power transistors have been introduced and used for the construction of cathodic protection stations in the form of superacoustic transistor converters. Such stations have over approx. 30% energetic efficiency, much smaller dimensions and mass, they work quietly, and are easily regulated automatically over the range of out-... 

A control unit topped by trigger equipment for static power converters triggers the thyristors, so the output voltage of the chargers is independent of the mains and load fluctuations. An automatic switchover unit and a corresponding set point to the controller allow these devices to yield constant voltage or constant DC current. 

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