Page 49 - Energize June 2022

TECHNICAL

The best way to gather information values. In view of this, the test results should only be interpreted for comparative purposes.
about the condition of a circuit breaker This does not indicate the quality of the primary insulation system from the point of view of
would be to take it out of service and dielectric withstand-ability.
subject it to a comprehensive battery of
tests. Taking a breaker out of service for Contact resistance measurement
testing almost always presents challenges, Contact resistance (CR) measurement can give an indication of the state of the contacts,
given today’s pressing uptime demands both during static and operational states, obviating the need for visual inspection of the
and limited maintenance resources. contacts and opening of the switch enclosure. CR measurement is conducted at the normal
Offline testing requires the removal of current rating of the switch but using a DC source. Contact resistance is very low and is
the switch from service and an intrusive measured using a micro-ohm (μΩ) meter. It is recommended that for MV the resistance
test and examination of components. The test be made with 100 A or higher DC. The use of a higher current value gives more reliable
test is time-consuming and is usually only results than using lower current values.
undertaken if other non-intrusive tests
indicate problems. Static resistance measurement
Online testing comprises a set of Static resistance measurement is conducted with the switch contacts in the closed position.
non-intrusive tests undertaken with the The resistance is measured using a micro–ohm meter and a high current source. Contact
breaker in service but isolated from both resistance is measured by injecting a known DC current through the main contact system when
the supply and feeders, as well as being the circuit breaker is closed. By measuring the voltage drop, the resistance can be calculated.
earthed at both points. Online tests do The value of the main contact resistance reflects the condition of the conducting parts.
not provide the same level of information The static resistance value provides a reference value for all types of electrical contacts
as offline tests but can indicate whether and joints. The international standard IEC 56 states that this resistance is to be measured
offline testing or maintenance is required. using a current between 50 A and the breaker’s nominal current. Other international and
Monitoring requires the permanent national standards set forth similar guidelines in order to eliminate the risk of obtaining
installation of measuring equipment on erroneously high measurements if the test current is too low. In some cases, heat generated
the switch. Such equipment is used to by a high test current disperses any contact grease remnants or other impurities found on
measure and record the performance of contact surfaces (resulting from numerous high-current breaking operations). When the
the switch under real operating conditions. circuit breaker contacts are in poor condition, the values obtained can differ dramatically
Monitoring the switch performance from those measured at factory when the breaker was new. 3
under real operating conditions may
reveal different results from online test Dynamic resistance measurement (DRM)
conditions. This test is conducted by injecting DC current through the breaker main contacts and
measuring both the voltage drop and the current while the breaker is operated. These values
Non-intrusive test methods that are are then used to plot the resistance as a function of time. If contact movement is recorded
typically performed on-site include: simultaneously, it is possible to determine the resistance at each contact position. This
• Insulation resistance test method is used mainly for contact diagnosis, but can also be used for main contact timing.
• Static resistance measurement With DRM, the arcing contact length can be reliably estimated as shown in Figure 2.
• Timing analysis and coil current analysis The only alternative way of finding the length of the arcing contact is to dismantle the
• Motion analysis, circuit breaker. In SF6 breakers the arcing contact is commonly made of tungsten.
• Motor current
• Dynamic resistance measurements

Insulation resistance measurement test
The insulation resistance is measured
using a megohmmeter, which applies a
voltage of between 600 and10 000 V DC
to the switch to determine the value of
its resistance but does not indicate the
quality of the primary insulation.
Several factors should be remembered
when performing this test: The first is
that this test can indicate low values of
insulation resistance because of many
parallel paths, and the other is that an
insulation system having low dielectric
strength may indicate high resistance Figure 2: Dynamic resistance measurement (Megger)

energize | June 2022 | 47