13-03-2014, 03:07 PM
Power Quality Seminar Report ‘03
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INTRODUCTION
Since last 25 years there has been an increase in the use of solid state electronic technology. This new, highly efficient, electronic technology provides product quality with increased productivity. Today, we are able to produce products at costs less than in the years passed, with the introduction of automation by using the solid state electronic technology .This new technology requires clear electric power.
The conventional speed control systems are being replaced by modern power electronic systems, bringing a verity of advantages to the users. Classic examples are DC $ AC drives, UPS, soft stators, etc. Since the thrusters converter technology is rapidly gaining in the modern industrial plants, the power supply systems are contaminated as the ideal sinusoidal current and voltage waveforms are getting distorted. This is in turn is affecting the performance of the equipment in the electrical network.
WHAT IS POWER QUALITY?
Adequate to superior power quality is essential for the smooth functioning of critical industrial processes. As industries expand, utilities become more interconnected and usage of electronically controlled equipment increases, power quality is jeopardized. Most large industrial and commercial sites are served by overhead lines with feeders that are subject to unpredictable and sporadic events, e.g. lightning and contact with tree limbs. Most distribution circuits have resoling devices that clear temporary faults through a timed series of trip and close operations. This minimizes the possibility of long-term outages but leads to a number of minor power disturbances. These typically occur several times a month. Many electric utilities have increased the voltage at which they distribute power. This allows a single circuit to serve more customers or deliver higher loads, and reduces energy losses in the system. But it often means the overhead distribution circuit is longer, with more exposure to disturbances.
HARMONICS-BASIC CONCEPTS
A pure sinusoidal voltage is conceptual quantity produced by an ideal AC generator build with finely distributed stator and field windings that operate in a uniform magnetic field. Since neither the winding distribution nor the magnetic field is uniform in a working AC machine, voltage waveform distortion is created, and the voltage time relation-ship deviates from the pure sine function. The distortion at the point of generation is very small (about 1%to 2%), but nonetheless it exists.
Because this is a deviation from a pure sine wave, the deviation is in the form of a periodic function and by definition, the voltage distortion contains harmonics. When a sinusoidal voltage is applied to a certain type of load, the current drawn by the load is proportional to the voltage and impedance and follows the envelope of the voltage wave form .These loads are referred to as linear loads (loads where the voltage and current follow one another without any distortion to their pure sine waves).examples of nonlinear loads are resistive heaters, incandescent lamps and constant speed induction and synchronous motors.
HARMONIC FILTERS
In some cases, reactors alone will not be capable of reducing the harmonic current distortion to the desired levels. In these cases, a more sophisticated filter will be required. The common choices include shunt connected, tuned harmonic filters (harmonic traps) and series connected low pass filters (broad band suppressors). They consist of a capacitor and an inductor which are tuned to a single harmonic frequency. Since they offer very low impedance to that frequency, the specific (tuned) harmonic current is supplied to the drive by the filter rather than from the power source. If tuned harmonic filters (traps) are selected as the mitigation technique, then multiple tuned filters are needed to meet the distortion limits which are imposed.
PULSE RECTIFIERS
12 Pulse drives are frequently specified by the engineers for heating, ventilating and air conditioning applications because their ability to reduce harmonic current distortion. In the mid 1960s when power semiconductors were only available in limited ratings, twelve-pulse drives provided a simpler and more cost effective approach to achieving higher current ratings than direct paralleling of power semiconductors.
A typical diagram of a large twelve-pulse drive appears in figure the drive's input circuit consists of two six-pulse rectifiers, displaced by 30 electrical degrees, operating in parallel. The 30-degree phase shift is obtained by using a phase shifting transformer. The circuit in figure simply uses an isolation transformer with a delta primary, a delta connected secondary, and a second wye connected secondary to obtain the necessary phase shift. Because the instantaneous outputs of each rectifier are not equal, an inter phase reactor is used to support the difference in instantaneous rectifier output voltages and permit each rectifier to operate independently. The primary current in the transformer is the sum of each six-pulse rectifier or a twelve-pulse wave form.
PULSE RECTIFIER
A typical diagram of a series connected eighteen pulse drive constructed from a standard six-pulse drive, two external rectifiers and a conventional 18 pulse isolation transformer appears in figure 1. The drive has terminals available to connect a DC link choke. These terminals are used to connect the two external rectifiers in series with the drives internal rectifier. The eighteen pulse transformer is designed to provide one third the normal input voltage to each of the three rectifiers at a 20 degree phase displacement from each other. The 20-degree phase shift is obtained by phase shifting the transformers secondary windings. The circuit in figure 1 simply uses an isolation transformer with a delta primary, and three delta connected secondary windings, one shifted + 20 degrees, one shifted -20 degrees and one in phase with the primary.
MOTOR TEMPERATURE REDUCTION
Motors operated on a VFD tend to run warmer than when they are operated on pure 60hz, such as in an across-the-line stator application. The reason is that the output waveform of the VFD is not pure 60hz,, but rather it contains harmonics which are currents flowing at higher frequencies. The higher frequencies cause additional watts loss and heat to be dissipated by the iron of the motor, while the higher currents cause additional watts loss and heat to be dissipated by the copper windings of the motor. Typically the larger horsepower motors (lower inductance motors) will experience the greatest heating when operated on a VFD. Reactors installed on the output of a VFD will reduce the motor operating temperature by actually reducing the harmonic content in the output waveform. A five percent impedance, harmonic compensated reactor will typically reduce the motor temperature by 20 degrees Celsius or more. If we consider that the typical motor insulation system has a "Ten Degree C Half Life" (Continual operation at 10 degrees C above rated temperature results in one half expected motor life), then we can see that motor life in VFD applications can easily be doubled. Harmonic compensated reactors are actually designed for the harmonic currents and frequencies whereas the motor is not.