Probably 80% of all testing performed in electrical power systems is related to the verification of insulation quality. This technical article briefly describes the fundamental concepts of insulation testing including – insulation behavior, types of tests, and some test procedures.
AC or DC?
Most electrical equipment in utility, industrial, and commercial power systems uses either 50Hz or 60Hz alternating current. Because of this, the use of an alternating current source to test insulation would appear to be the logical choice.
However, insulation systems are extremely capacitive. For this and other reasons, DC has found a large niche in the technology. Before we can really evaluate the value of one system as opposed to the other (e.g. AC vs DC), let us examine how each type of voltage affects insulation.
Insulation Current Flow (AC)
Insulation may be simply modeled as a capacitor in parallel with a resistor as shown in Figure 1. The current flow that results will comprise two components: the capacitive current (Ic) and the resistive current (Ir).
Figure 2 shows the time domain graph of the two currents. For good insulation:
- Ic ≥ 100 x Ir
- Ic leads Ir by close to 90°
Insulation Current Flow (DC)
When DC current is involved, insulation may be modeled in a slightly different way. Consider Figure 3 below:
When switch S1 is closed, the DC supply is connected to the insulation system. In the DC model an extra capacitor has been added (dashed lines). The current that flows through this new capacitor is called the dielectric absorption current (Ida) and will be explained later.
Figure 4 show the time relationship for these three currents. The following paragraphs explain each of the three currents.
Capacitive Current (Ic)
The capacitive current charge the capacitance in the system. It normally stops flowing a few seconds (at most) after the DC voltage is applied. The short burst of capacitive current flow may put a rather substantial stress on any test equipment that is applied to very large insulation systems such as cables or large rotating machine.
Dielectric Absorption Current (Ida)
The applied insulation voltage puts a stress on the molecules of the insulation. The positive side of the molecules are attracted to the negative conductor and the negative side of the molecules are attracted to the positive conductor.
The result is an energy that is supplied to realign the molecules much like force will realign a network of rubber bands. Like Ic, Ida usually dies off fairly quickly as the molecules realign to their maximum extent.
Resistive (Leakage) Current (Ir)
This is the electron current flow that actually passes through the insulation. In good insulation the resistive current flow will be relatively small and constant.
Resource: TECHNICAL BULLETIN — 012a Principles of Insulation Testing by Cadick Corporation
If you think about Polarization Index, which is the ratio of DC insulation resistance between 1 minute and 10 minutes, the insulation system for most electrical equipment will exhibit changes in Ida well into minutes. This article gives the impression that this transient is over (It = Ir) in about 10 seconds, but I wish to point out that it continues on for much longer.
your topic needs more information to make it easy
Injecting the DC voltage has the problem, that only AC-faults can be detected. Bender eliminates that problem by adapting the injected DC voltage (very slow pulse) to the existing (extraneous) DC voltage in the system. This monitoring principle is patented under AMP (adapted monitoring pulse) by Bender Germany for the worldwide use.
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