High Voltage Circuit Breakers (HV/EHV/UHV) act as the most effective protection mechanism against electrical mishaps in the high voltage environment of Substations. Since Circuit Breakers (CBs) are mechanical devices, they are vulnerable to breakdowns if timely performance tests are not conducted to ensure their functional healthiness.
In our earlier post, we broadly covered the 9 fundamental must-do tests to ensure the optimal performance of CBs. In this post, we will learn more about Dynamic Contact Resistance Measurement (DCRM) which is widely considered as the most comprehensive test for High Voltage CBs.
What Is DCRM And Why Is It So Important?
While there has been a rapid evolution in the testing and maintenance methodologies of CBs over the years, condition-based maintenance and testing remains widely popular.
Before DCRM tests were introduced in 1992, High Voltage CBs were tested using the Static Contact Resistance Measurement methodology wherein the static contact resistance was measured by injecting a DC current through the CB’s contacts in a closed condition and measuring the milli-voltage drop across the contacts.
However, a major lacuna in the Static Contact Resistance Measurement methodology was that it did not give comprehensive information about the condition of the CB’s contacts and mechanism like their state of erosion, contact misalignment, damages to the driving mechanism, etc.
This warranted the need for an advanced test in the form of DCRM for comprehensively assessing the condition of HV CB contacts.
How Are DCRM Tests Conducted?
A DCRM test is conducted by injecting a 100 Amps DC current through the CB’s contacts during its Close-Open (C-O) operation and measuring the corresponding voltage drop at a high speed (generally at 10kHz or more). A Travel Transducer is also used to capture the travel characteristics of the CB.
The CB Analyzer then calculates and plots the resistance across the CB contacts as a function of time. A high sampling frequency will ensure proper data capturing during the Closing and Opening of the Contacts.
To get a good DCRM signature, the time interval between the close trip operations is kept at 300 milliseconds. The obtained Resistance Vs Time graph along with the current variations and travel characteristics is considered as a ‘fingerprint’ for the CB contact and is used as a benchmark for comparing the future results of the same CB.
DCRM testing is especially useful for SF6 and Air-blast CBs whose arcing contacts are usually made of tungsten-copper. Since these contacts burn off and erode with each live operation of the CB, a DCRM test facilitates a reliable estimation of the length of the CB’s arcing contacts.
When conducting any DCRM test, a pertinent point to be kept in mind is that the test should be conducted only during the CB’s Close-Open (C-O) action and never during it’s Open-Close (O-C) action or during its Opening (O) action.
Attempting to measure the CB’s condition during its O-C action is impractical because a sudden change in the resistance variation from infinite to the arcing contact resistance is difficult to measure. Further, the transient DC current at the moment of arcing contact touch generates an undesired noise that results in incorrect measurements.
DCRM testing is widely considered as the most comprehensive testing methodology to assess the healthiness of CB contacts and is equally beneficial for CB manufacturers as well as Utility companies.
For CB manufacturers, DCRM plays a pivotal role as manufacturing errors can be detected and rectified at the factory itself before dispatch.
For Utility companies, DCRM helps in identifying CBs with problems in their contacts or in their operating mechanism. Thus, the scheduled shutdown and necessary maintenance activities can be undertaken and CBs can be saved from unexpected failures.
In the next post, we will see how SCOPE’s state-of-the-art DCRM kit and analytical expertise helped one of India’s largest Electric Power Transmission Utility to cohesively assess the condition of their HV CBs before deciding for an overhaul of their HV CB infrastructure.
In the meantime, please feel free to get in touch with us on firstname.lastname@example.org if you have any queries on DCRM testing.