CAPACITOR INSTALLATION SYSTEMS BENEFITS BASIC INFORMATION AND TUTORIALS

What Are The Benefits Of Installing Capacitors?

Power capacitors provide several benefits to power systems. Among these include power factor correction, system voltage support, increased system capacity, reduction of power system losses, reactive power support, and power oscillation damping.

Power Factor Correction.

In general, the efficiency of power generation, transmission, and distribution equipment is improved when it is operated near unity power factor. The least expensive way to achieve near unity power factor is with the application of capacitors.

Capacitors provide a static source of leading reactive current and can be installed close to the load. Thus, the maximum efficiency may be realized by reducing the magnetizing (lagging) current requirements throughout the system.

System Voltage Support.

Power systems are predominately inductive in nature and during peak load conditions or during system contingencies there can be a significant voltage drop between the voltage source and the load. Application of capacitors to a power system results in a voltage increase back to the voltage source, and also past the application point of the capacitors in a radial system.

The actual percentage increase of the system voltage is dependent upon the inductive reactance of the system at the point of application of the capacitors. The short-circuit impedance at that point is approximately the same as the inductive reactance; therefore, the 3-phase short-circuit current at that location can be used to determine the approximate voltage rise.

Increased System Capacity.

The application of shunt or series capacitors can affect the power system capacity. Application of shunt capacitors reduces the inductive reactive current on the power system, and thus reduces the system kVA loading. This can have the effect of increasing system to serve additional load.

Series capacitors are typically used to increase the power carrying capability of transmission lines. Series capacitors insert a voltage in series with the transmission line that is opposite in polarity to the voltage drop across the line, which decreases the apparent reactance and increases the power transfer capability of the line.

Power System Loss Reduction.

The installation of capacitors can reduce the current flow in a power system. Since losses are proportional to the square of the current, a reduction in current will lead to reduced system losses.

Reactive Power Support.

Capacitors can help support steady-state stability limits and reactive power requirements at generators.

Power Oscillation Damping.

Controlled series capacitors can provide an active damping for power oscillations that many large power systems experience. They can also provide support after significant disturbances to the power system and allow the system to remain in synchronous operation.

TEMPERATURE RISE CAUSES OF DIRECT CURRENT GENERATORS BASIC INFORMATION

What are the causes of heating of DC generators?

The losses in a dc machine cause the temperature of the parts to rise until the difference in temperature between their surfaces and the cooling air is great enough to dissipate the heat generated.

Permissible measured temperature rises of the parts are limited by the maximum “hot-spot” temperature that the insulation can withstand and still have reasonable life. The maximum surface temperatures are fixed by the temperature gradient through the insulation from the hot spot to the surface.

The IEEE Insulation Standards have established the limiting hot-spot temperatures for systems of insulation. The American National Standards Institute Standard C50.4 for dc machines gives typical gradients for those systems, listing acceptable surface and average copper temperature rises above specified ambient-air temperatures for various machine enclosures and duty cycles.

Typical values are 40°C for Class A systems, 60°C for Class B, and 80°C rise for Class F systems on armature coils.

Class H systems usually contain silicones and are seldom used on medium and large dc machines. Silicone vapors can cause greatly accelerated brush wear at the commutator and severe sparking, particularly on enclosed machines.

Heating of End Connections of Armature Windings. Small machines often have “solid” end windings banded down on insulated “shelf”-type coil supports. Larger machines are more heavily loaded per unit volume and usually have narrow coil supports, air spaces between the end turns, and ventilating air scouring both the top and bottom surfaces of the coil extensions.

With this construction, the approximate allowable product of ampere conductors per inch of outer circumference times the amperes per square inch for various rotor velocities is shown in Fig. 8-51 for a 40°C rise on the end turns.

STEAM TURBINE LOSSES BASIC INFORMATION

Clearance leakage. A 100% efficiency cannot be obtained because of friction in the blading and clearance between the stationary and rotating parts, and because the nozzle angle cannot be zero degrees.

Axial clearance increases in the stages further from the thrust bearing to satisfy the need to maintain a minimum clearance at extreme operating conditions when the differential expansion between the light rotor and heavy casing is at its worst.

To reduce this leakage, radial spillbands are used. These thin, metal-strip seals may be attached to the diaphragm or casing and extend close to the shroud bands covering the rotating blades.

This clearance can be kept quite close (0.020 to 0.060 in), and axial changes in the rotor position do not affect the clearance since the important on reaction stages with small blade heights because of the pressure drop across the moving blades.

Nozzle leakage. Leakage around the nozzles between the bore of the blade ring or nozzle diaphragm and the drum or rotor must be kept to a minimum.

This leakage is controlled through the use of a metallic labyrinth packing which consists of a single ring with multiple teeth arranged to change the direction of the steam as well as to minimize the leakage area.

Labyrinth packings are also used at the shaft ends to step the pressure down at the high-pressure end and to seal the shaft at the vacuum end.

Rotation loss. Rotation of the rotor consist of losses due to the rotation of the disks, the blades, and shrouds. Partial-arc impulse stages have a greater windage loss within the idle buckets. Rotation losses vary directly with the steam density, the fifth power of the pitch diameter, and the third power of the rpm.

In general, the windage loss amounts to less than 1% of stage output at normal rated output. At no-load conditions, windage loss for noncondensing turbines approximates 1.5% of the rating per 100 lb/in2 exhaust pressure, and on condensing units approximates from 0.4% to 1.0% of the rating at 1.5 inHg (abs) exhaust pressure.

Carryover loss. A carryover loss (about 3%) occurs on certain stages when the kinetic energy of the steam leaving the rotating blades cannot be recovered by the following stage because of a difference in stage diameters or a large axial space between adjacent stages.

Typically, this happens in control stages and in the last stages of noncondensing sections. The last stages of condensing turbines have the largest carryover losses (normally referred to as exhaust loss) because of the large variations in exhaust volumetric flow with exhaust pressure and the large variation of stage pressure ratio with load.

Stages preceding the last operate with essentially a constant pressure ratio down to very low loads and consequently can be designed for peak efficiency at a wide range of loads.

Leaving loss. Condensing turbines are frequently “frame sized” by last-stage blade height. It is sometimes economical to size the unit with exhaust loss equal to 5% deterioration in overall turbine performance at the design point (valves wide-open throttle flow and 1.5 inHg [abs] exhaust pressure) when the normal expected exhaust pressure will be higher or the unit will be operating at part load for a large part of the time.

Nozzle end loss, partial arc. Control stages and partial-arc impulse stages are subject to end losses at the interface of the active and inactive portions of the blading as the stagnant steam within the idle bucket passages enters the active arc of nozzles and must be accelerated.

There is also a greater turbulence in the steam jet at both ends of the active arc. In partial-arc impulse stages, the increase in efficiency due to larger blade heights (aspect ratio) is partially offset by increased rotation and end losses, and there is an optimum to this proportioning beyond which there is an overall loss.

Supersaturation and moisture loss. Moisture in the steam causes supersaturation and moisture losses in the stage. The acceleration of the moisture particles is less than that of the steam, causing a momentum loss as the steam strikes the particles.

The moisture particles enter the moving blades (buckets) at a negative velocity relative to the blades, resulting in a braking force on the back of the blades. Supersaturation is a temporary state of supercooling as the steam is rapidly expanded from a superheated state to the wet region before any condensation has begun.

The density is greater than when in equilibrium, resulting in a lower velocity as the steam leaves the nozzle. As soon as some condensation occurs at approximately 3.5% moisture, according to Yellot, a state of equilibrium is almost instantly achieved and supersaturation ceases.

CORE LOSSES OF DC GENERATORS BASIC INFORMATION AND TUTORIALS

The flux in any portion of the armature passes through p/2 c/r (cycles per revolution) or through (p/2 [(r/min)/60] Hz. The iron losses consist of the hysteresis loss, which equals Kb1.6fw watts, and the eddy current loss, which equals Ke(Bft)^2w watts.

K is the hysteresis constant of the iron used, Ke is a constant inversely proportional to the electrical resistance of the iron, B is the maximum flux density in lines per square inch, f is the frequency in hertz, w is the weight in pounds, and t is the thickness of the core laminations in inches.

The eddy loss is reduced by using iron with as high an electrical resistance as is feasible. Very high resistance iron has a tendency to have low flux permeability and to be mechanically brittle and expensive. It is seldom justified in dc machines. The loss is kept to an acceptable value by the use of thin core laminations, 0.017 to 0.025 in thickness.

Another significant loss is the pole-face loss. These losses are included in the core loss.

Unfortunately, there are other losses in the core that may differ widely even on duplicate machines and that do not lend themselves to calculation. These include:

1. Loss due to filing of slots. When the laminations have been assembled, it will be found in some cases that the slots are rough and must be filed to avoid cutting the coil insulation. This burrs the laminations and tends to short circuit the interlaminar resistance.

2. Losses in the solid spider, core end plates, and coil supports from leakage fluxes may be appreciable.

3. Losses due to nonuniform distribution of flux in the rotor core are difficult to anticipate. In calculating core density, it is customary to assume uniform distribution over the core section.

However, flux takes the path of least resistance and crowds behind the teeth until saturation forces it into the less used, longer paths below. As a result of the concentration, the core loss, which is about proportional to the square of the density, is greater than calculated.

Thus, it is not possible to predetermine the total core loss by the use of fundamental formulas. Consequently, core-loss calculations for new designs are usually based on the results from tests on similar machines built under the same conditions.

Such test results are plotted in Fig. 8-54 for machines using ordinary laminations 0.017 in thick and a limited amount of filing. They do not include the pole-face losses, which would increase the values about 30%.

DIELECTRIC LOSS AND CORONA BASIC INFORMATION AND TUTORIALS

Dielectric Hysteresis and Conductance

When an alternating voltage is applied to the terminals of a capacitor, the dielectric is subjected to periodic stresses and displacements. If the material were perfectly elastic, no energy would be lost during any cycle, because the energy stored during the periods of increased voltage would be given up to the circuit when the voltage is decreased.

However, since the electric elasticity of dielectrics is not perfect, the applied voltage has to overcome molecular friction or viscosity, in addition to the elastic forces. The work done against friction is converted into heat and is lost. This phenomenon resembles magnetic hysteresis in some respects but differs in others.

It has commonly been called dielectric hysteresis but is now often called dielectric loss. The energy lost per cycle is proportional to the square of the applied voltage.

An imperfect capacitor does not return on discharge the full amount of energy put into it. Sometime after the discharge, an additional discharge may be obtained. This phenomenon is known as dielectric absorption.

A capacitor that shows such a loss of power can be replaced for purposes of calculation by a perfect capacitor with an ohmic conductance shunted around it. This conductance (or “leakance”) is of such value that its PR loss is equal to the loss of power from all causes in the imperfect capacitor.

The actual current through the capacitor is then considered as consisting of two components—the leading reactive component through the ideal capacitor and the loss component, in phase with the voltage, through the shunted conductance.

Electrostatic Corona.

When the electrostatic flux density in the air exceeds a certain value, a discharge of pale violet color appears near the adjacent metal surfaces. This discharge is called electrostatic corona.

In the regions where the corona appears, the air is electrically ionized and is a conductor of electricity. When the voltage is raised further, a brush discharge takes place, until the whole thickness of the dielectric is broken down and a disruptive discharge, or spark, jumps from one electrode to the other.

Corona involves power loss, which may be serious in some cases, as on transmission lines. Corona can form at sharp corners of high-voltage switches, bus bars, etc., so the radii of such parts are made large enough to prevent this.

A voltage of 12 to 25 kV between conductors separated by a fraction of an inch, as between the winding and core of a generator or between sections of the winding of an air-blast transformer, can produce a voltage gradient sufficient to cause corona.

A voltage of 100 to 200 kV may be required to produce corona on transmission-line conductors that are separated by several feet. Corona can have an injurious effect on fibrous insulation.