High Resistance Interruption

[caption id="attachment_1285" align="alignnone" width="300"]

High Resistance Interruption[/caption]

High Resistance Interruption is a topic of switchgear protection. In this method are resistance is made to increase with time so that the current becomes insufficient to maintain the arc.

The rate at which the resistance is increased or the current is decreased is not abnormal so as to cause harmful induced voltages in the system.

Because of the resistive nature of the arc discharge, most of the energy in the system will be received by the circuit breaker.

The main drawback of this interruption is that the energy dissipation is high.

Hence it can be used only in low and medium circuit breakers and in d.c. circuit breakers.

Arc resistance can be increased by:

(a) Lengthening the arc:

Arc resistance is directly proportional to the length of are so as to increase resistance, the separation between the contacts is increased.

(b) Cooling the arc:

Cooling helps in deionization of medium thus increasing arc resistance.

(c) Splitting the arc:

The resistance can be increased by splitting the is into a number of smaller arcs in series. Each arc experiences the effect of lengthening and cooling. Arc may be split by introducing some conducting plates between the contacts.

(d) Reducing cross-section of the arc:

When the area of the arc is reduced, the voltage required to maintain the arc to be increased i.e. resistance is increased. Allowing the arc to pass through a narrow opening can reduce the cross-section area.

Berry Type Transformer

Limitations of Terzaghi Theory

Limitations of Terzaghi Theory The value of the coefficient of consolidation has been assumed to be constant. The distance d of the drainage path cannot be measured accurately in the field. The thickness of the deposit is generally variable, and an average value has to be estimated. There is sometimes difficulty 1n locating the drainage face, sometimes thin previous seams that can act as good drainage face are missed in the boring operations. The equation is based on the assumption that the consolidation is one-dimensional. In the field, the consolidation is generally 3-dimensional. The lateral drainage may have a significant effect on the time rate of consolidation. The initial consolidation and secondary consolidation have been neglected. Sometimes these form an important part of the total consolidation. In actual practice, the pressure distribution may be far from linear or uniform. Read More Muller-Breslau principle

Price Guard Wire Method

Price Guard Wire Method Some form of Price Guard Wire Method is generally used to eliminate the errors caused by leakage currents over insulation. Fig. 3.14 illustrates the operation of This Method. In fig 3.14(a), a high resistance mounted on a piece of insulating material is measured by the ammeter voltmeter method. The micro-ammeter measures the sum of the current through the resistor (IR) and the current through the leakage path around the resistor. The measured value of resistance computed from the readings indicated on the voltmeter and the microammeter, will not be a true value but will be in error. Figure 3.14 Application of guard circuit for measurement of high resistance In fig, 3.14 (b), the guard terminal has been added to the resistance terminal block. The guard terminal surrounds the resistance terminal entirely and is connected to the battery side of the micro-ammeter. The leakage current IL now bypasses t

Negative Booster

Negative booster A negative booster is employed to conform to the regulation that the potential difference between any two points of the rail return shall not exceed 7 V. Two boosters, positive and negative, are used which are mechanically coupled together and driven by a DC motor. The positive booster is connected to the trolley wire (near the generating station) and the negative booster (separately excited) is connected to the track rail. The 'positive booster' adds voltage to the line while the 'negative booster lowers the potential of the point it is connected to. As we go along the trolley wire away from the generating station/sub-station, the potential drop increases, and the voltage of the trolley wire falls. Since the current returns via the track rail points away from the generating station acquire high potentials. This potential is brought down by the negative boost provided by the negative booster. When the load is sufficiently far away from the generating stati

Self Study World

Search This Blog