���������� # � To Find out the 50% Critical Impulse Flash-Over Voltages on the 11KV Pin-type Insulator with Positive & Negative Impulse
Control Panel
Charging Unit
Insulator
Impulse Generator
0
CRO
Apparatus:
Test Specimen (11 kV Insulator)
Impulse Generator
Charging Unit
Control Board
50% Critical Impulse Flash-Over Voltage: The 50% critical flashover voltage is the crest value of the impulse wave that under specified conditions causes flashover on 50 percent of the applications.
Procedure: The Impulse Generator shall first be adjusted to deliver a negative 1.2/50 µs waveform. It is applied on to the insulator until the flashover occurs. When the flash over occurs then it is tested for the voltage below it. If it again flashes, voltage is decreased to check. If it does not flashes then voltage is increased until it flashes. This process is repeated for one subsequent voltage value, where there is a 50% chance of Flashover. All this process is repeated for the positive waveform as well.
���������� # � Study of relationship between String Efficiency & the no of Insulators (units) used in a String Insulator.
A V1
B V2
C Water Resistance
E V2 230V
IVR
V 0-460V
V3
D V4
0-150KV
V 0 0
Apparatus:
Insulator string 1. 3 units 2. 4 units 3. 5 units
Electrostatic Voltmeter
150 kV Testing Transformer with Control & Protection Gear
Procedure: The circuit arrangement of the experiment is shown in fig. A specific voltage is applied across the string. Voltage of each unit is measured with respect to ground using an electrostatic type Voltmeter. Thus voltage distribution across each unit can be found out. Protective resistance is inserted to protect the secondary of high Voltage Testing Transformer
���� ������� ����������� ��� in the case of short circuit during breakdown. Three different readings are taken by changing the no. of insulators from 3 to 5. String Efficiency i s calculated for each string.
Observations & Calculations: (i)
3 Insulators
Voltage applied on the string = V = _____ KV
Voltage
Value
V2 V1
Voltage Across Insulator
Equation
A
V1
B
V2 - V1
C
V - V2
Value
Efficiency = Voltage across string n x Voltage across unit near power conductor = ___________ %
���������� # � Study of Relationship between String Efficiency & the No. of Insulators (units) used in a String Insulator with Guard Ring
A V1
B V2
C Water Resistance
E V2 230V
IVR
V 0-460V
V3
D V4
0-150KV
V 0 0
Apparatus:
Insulator string 1. 3 units 2. 4 units 3. 5 units Electrostatic Voltmeter 150KV Testing Transformer with Control & Protection G ear
Procedure: The circuit arrangement of the experiment is shown in fig. A specific voltage is applied across the string. Voltage of each unit is measured with respect to ground using an electrostatic type Voltmeter. Thus voltage distribution across each unit can be found out. Protective resistance is inserted to protect the secondary of high Voltage Testing Transformer in the case of short circuit during breakdown. Three different readings are taken by changing the no. of insulators from 3 to 5. String Efficiency is calculated for each string.
Theory: The flash over voltage of an insulator is considerably lower if its surfaces are wet. These wet surfaces form a conducting path and the effective insulated path for flashover is then shortened. Insulators are designed as that even under rain conditions, portions of the sheds remains dry and provide effective insulation. In the laboratory rain tests are often made o
by spraying water at an angle 45 to the insulator subjected to voltage.
Procedure: Make connection as shown above where E-P-C should be approximately in line. This three-point method involves passing a current into the electrode to be measured and measuring the voltage between the ground electrode (E) under test and a test potential electrode (P). A test current electrode (C) is driven into the earth to permit passage of current into the electrode to be tested. Potentials are measured with respect to the ground electrode under test which is assumed to be at zero potential. It is shown in the following figure
For battery check, needle should be within blue belt.
Theory: The purpose of electrical ground testing is to determine the effectiveness of the grounding medium with respect to true earth. The earth may provide the return path for fault currents, and for safety, all electrical equipment frames are connected to ground. The resistivity of the earth is usually negligible because there so much of it available to carry current.
The limiting factor in electrical grounding systems is how well the grounding
electrodes make contact with the earth, which is known as the ground rod interface. This interface resistance component, along with the resistance of the grounding conductors and the connections, must be measured by the ground test. In general, the lower the ground resistance, the safer the system is considered to be. There are different regulations which set forth the maximum allowable ground resistance, for example: the National Electrical Code specifies 25 ohms or less; MSHA (Mine Safety and Health Administration) is more stringent, requiring the ground to be 4 ohms or better (less resistance).
