internationreach

 

Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers

MV Testing

The Insulation Resistance Test (Megohmmeter or Megger Test)
One of the fundamental requirements of any high voltage system is that the system is well insulated between the three phases and most important, insulated from ground. For this obvious reason, a Megger test device has become an integral part of high voltage equipment testing.

Like most other types of equipment, high/mid voltage distribution gear has a limited ‘useful life’ span. As the equipment ages, the respectable quality of the equipment decreases, usually relating to a reduction in voltage insulation/protection. One of the most simple and convenient methods of measuring the quality of voltage insulation ( within a fixed portion of the distribution system ) is through the use of an insulation and resistance tester, otherwise known as a Megohmmeter or Megger.

A Megger applies a consistent voltage (usually between 500 and 5000 V) to an electrical component and then measures the current which the megger must supply to the component to keep the voltage at the required level. In effect, this supplied current is equivalent to the leakage current of the electrical component, allowing the meter to calculate the resistivity of the electrical components between the two points of measurement - a useful measurement of equipment quality.
megger.jpg (12880 bytes)


The Ductor Test

The ductor test is a simple test used in very low scale resistance measurement. The simple purpose of the fuse is to accurately limit current flow throughout the secondary distribution system.

Typically, primary fuses deal with a high current carrying load. This quantity of current must flow through a portion of the fuse which has a low resistive quality. The resistance of the fuse must be low or else the power dissipation of the fuse(s) would become quite significant. A ductor test measures the core resistance of the fuse and results are usually in the mW or pW ranges.

Although the test is done quickly and simply, the readings allow early detection of possible fuse breakdown. Once the resistivity of the core rises (compared to the core resistivity when new), the fuse will rupture prematurely, perhaps even under a low load.

 

The Power Factor Test

Power factor test are useful in detecting signs of insulation deterioration. Power factor in insulating testing is basically the same as the power factor in measurement. A piece of insulation applied in this test is practically the equivalent to a capacitor in parallel with a resistor. Mathematically, power factor is the cosine of the phase angle between the total current and the resistance component of the current. In insulation measurement the numerical value of the power factor is usually small, in the order of a few percent, zero being perfect.

Becouse normal power factor is so low measurements may be greatly distorted by dirt or moisture on the insulation surfaces. Incorrect readings can also result if tests are made near wet or grounded surfaces. Due to that, before making a power factor test on a piece of insulation, it has to be cleaned of all foreign matter.

For full and correct interpretation of measurement results in context of absolute withstand capability of insulation systems, it is necessary to establish a statistical correlation between power factor, voltage withstand capability, and service life for the kind of system being tested. In that case properly made and evaluated, power factor tests are useful in detecting signs of insulation deterioration.


High Potential Insulation Tests

The high potential test should be the primary filed insulation test used on switchgear installations. It should be separately applied on the magnetic breaker and stationary gear. The standard factory withstand test is specified to be 60 hertz for one minute. The alternating-current high potential test is the preferred test because it subjects the insulation to the same type of heat-conductive alternating stress the insulation must endure in service. Test of insulation resistance using a megohmmeter are not reliable indications of high voltage insulation quality. In complex apparatus this test can indicate law values of insulation resistance because of numerous parallel paths. The megohmmeter reading can also indicate high resistance value when the instructions has very little dielectric strength.

 

Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers