Steven C. Rizzo

A novel power factor control scheme for high power GTO current source converter

A novel power factor control scheme for high-power gate-turn-off thyristor current-source power converters is proposed in this paper. Using both feedforward and feedback control techniques, the proposed scheme guarantees that the input power factor of the power converter can be kept at unity or a maximum achievable value. Another feature of this scheme is that it is parameter insensitive, that is, variations in the line and load impedance or changes in the filter capacitor size will not affect the process of tracking the maximum input power factor. No parameters in the control scheme should be adjusted to accommodate such variations or changes. Simulation and experimental results are provided to verify the operating principle of the scheme.

Symmetric GTO and snubber component characterization in PWM current source inverters

The pulsewidth-modulated (PWM) current-source inverter (CSI) used in AC motor drive applications can be implemented with symmetric gate turn-off thyristors (GTOs). One of the major difficulties in the optimization of the GTO switch and the snubber components of the inverter is the variation in different switching conditions encountered during normal operation. Past work has concentrated on the GTO and snubber components in voltage-source applications, where commutation of the GTO device is an independent process and does not affect the operation of the other inverter devices. This paper proposes the characterization of the GTO and the snubber components by formulation of the CSI equivalent circuit during the device commutation period. From the equivalent circuit, the state equations are derived, thereby obtaining accurate voltage and current waveforms of the GTO and associated snubbers. From the analysis, the component power loss can be calculated and optimization performed. Simulation results are verified by using both a laboratory prototype and medium-voltage drive system.

Medium voltage drives: are isolation transformers required?

Traditionally isolation transformers are used on medium voltage motor drives. Although this provides a means for reducing harmonics, reducing voltage stress on motors, it increases the size, cost, complexity and losses of the drive system. With the implementation of alternative topologies and new pulse width modulation switching patterns, these goals can be achieved without the use of an isolation transformer. The paper presents the topology and the techniques that can be utilized and proposes an overall AC motor drive system without the need for the transformer. Experimental results on a 600 hp, 4160 V motor drive are also included.

Application considerations for class-1 div-2 inverter-fed motors

Application considerations for fixed-speed motors utilized in hazardous locations are not new to the electrical community and have been discussed previously and are relatively well known in the petroleum industry. Documented guidelines for these applications are currently established, are primarily centered on motor-temperature-rise limitations and the use of nonsparking fans in class-1 division-2/zone-2 locations. However, there are additional considerations when motors and their associated loads are on inverter supply. Items such as the effect on motor temperature rise when operating at reduced speeds for units without independently powered blowers, the effect of nonsinusoidal supply at the motor terminals on motor temperature rise as well as rotor voltage related phenomenon are relevant items. This paper identifies and provides guidelines particular to inverter-fed applications for class-1 division-2 installations.

Application considerations for class 1 div 2 inverter-fed motors

Application considerations for fixed-speed motors utilized in hazardous locations are not new to the electrical community and have been discussed previously and are relatively well known in the petroleum industry. Documented guidelines for these applications are currently established, are primarily centered on motor-temperature-rise limitations and the use of nonsparking fans in class-1 division-2/zone-2 locations. However, there are additional considerations when motors and their associated loads are on inverter supply. Items such as the effect on motor temperature rise when operating at reduced speeds for units without independently powered blowers, the effect of nonsinusoidal supply at the motor terminals on motor temperature rise as well as rotor voltage related phenomenon are relevant items. This paper identifies and provides guidelines particular to inverter-fed applications for class-1 division-2 installations.

A CSI-based medium voltage multi-motor drive

A current source inverter (CSI) based multi-induction motor drive is presented. An active damping control is incorporated into the constant V/f control for drive stability improvement and LC resonance suppression. The dynamic performance of the DC link is improved by an inverter side DC voltage feedforward control. A new space vector switching pattern is developed such that the proposed control scheme can be implemented with a low switching frequency (500 Hz), which makes the proposed drive suitable for medium voltage and high power applications. The experimental results based on a 2O kVA GTO prototyping drive using a TMS320C31 DSP controller are included.

A new current source converter using a symmetric gate commutated thyristor (SGCT)

Extensive semiconductor development has gone into both bipolar and MOS structures for medium voltage (MV) applications. However the progression of MOS structures in MV applications has been difficult and the issues of module isolation, reliability, and dv/dt and motor/bearing life continue to limit its acceptance in these applications. A more suitable device structure and the natural choice for MV applications is the bipolar thyristor structure. The device that has been in use for many years, the GTO, is being replaced by the gate commutated thyristor (GCT). So far the GCT has been only thought of as fulfilling the needs for the voltage source topology. However, the symmetric GCT (SGCT) is viable and has significant advantages when implemented in a PWM current source inverter. This paper describes the design and characteristics of an 800 A, 6.5 kV SGCT and the effect of its implementation in a PWM-CSI. These effects include: operation at a higher switching frequency, elimination/reduction and modification of the snubber circuitry, reduction in size of the passive components, and a major impact on the cost of the converter. The paper includes experimental results on a 4160 V, 1250 hp, PWM-CSI AC drive.