Switchgear devices are important components in electricity supply systems. They are developed, constructed and manufactured to work correctly under a multitude of conditions. For this reason, it is necessary to check their performance during construction, manufacture, maintenance and repair.
In electrical energy transmission and distribution systems, switchgear devices are the connection to further parts of an installation. Throughout their operational lives, switchgear devices must constantly be able to connect, interrupt or disconnect operating parts. In the "open" status, they are a puncture-proof disconnection point, in the "closed" status, they carry and control short circuit currents.
Switchgear devices must survive mechanical and thermal stresses during operation without damage. Friction and abrasion influence the performance of the mechanical parts. The contact systems in the current-carrying circuits can deteriorate and thus increase the development of excessive heat.
In order to guarantee the operation of a switchgear device, the devices must be checked and maintained regularly.
When disconnecting the contacts in a high-voltage circuit breaker, a high-energy switch arc is created. It is necessary for circuit breakers to be able to extinguish short-circuit current arcs within a fraction of a second. For this purpose, circuit breakers feature extinction systems or extinction chambers.
The arc is created after disconnecting the switching contacts. Increasing pressure and flow in the extinction medium concerned cool the arc until it is finally interrupted. When the arc is extinguished, it is necessary for the contacts to move away from each other so that the insulating stroke is reached and no further striking of the arc occurs.
The arc contact establishes the first contact during the closing operation and has the last contact touch during the opening operation. The contacts wear during normal switching operations and also when short-circuit currents are interrupted. If the contacts are in poor condition, the circuit breaker becomes unreliable.
A high contact resistance within a switchgear device leads to high power loss coupled with thermal stress and possible serious damage to the switchgear device. Problems, such as high transfer resistance resulting from poor connections, can be identified by measuring contact resistance.
Regular measurements of the static and dynamic contact resistance allow an accurate assessment of the condition of the entire contact system. This ensures that maintenance requirements can be identified at an early stage and down times kept to a minimum.
For the static resistance measurement, the contact resistance is determined when the interrupter unit is closed. However, this measurement does not give an indication of the internal state, especially of the arc contacts.
An assessment can be made by an internal inspection of the contact, but it is very labour-intensive and time-consuming. To simplify analyses of circuit breakers, the dynamic resistance measurement was introduced.
The contact resistance is dynamically measured via a close-open operation. The contact characteristic and the arc contact can be reliably determined via the measurement results. During this switching operation a high test current is applied and the voltage drop is measured. The measurement of the complete switching operation shows the resistance characteristic of the entire contact travel.
The information given by the dynamic resistance measurement provides an overview of the entire contact status, particularly the arc contact and the eroded parts, which is not possible with the static measurement.
A - B:
At point A, the closing command is sent to the circuit breaker. The graph between A and B shows the time required for the ON coil to activate until the operating rod moves.
B - C:
At point B, the operating rod begins to move, which in turn moves the arc and main contacts of the breaker unit.
C - D:
At point C, the arc contact closes. The dynamic resistance, which is plotted together with the distance and the current, indicates the state of the arc contacts.
D - E:
At point D, the main contact closes and the contact resistance continues to decrease. The main contact continues to close until point E, where the circuit breaker is in the fully closed position.
E - F:
From point E to point F, the circuit breaker remains closed for the set time delay. During this time, the resistance value should remain constant. This time period is necessary to dampen oscillations.
F - G:
At point F, the OFF command is transmitted to the trip coil of the circuit breaker.
G - H:
At point G, the operating rod begins to move until the main contacts open at point H.
H - I:
At point H, the main contact opens and only the arc contacts remain closed. The signature between H and I indicates the status of the arc contacts.
I - J:
At point I, the arc contact opens and the resistance value increases. The operating rod continues to move until the breaker contacts are fully open.
The screenshot shows the characteristic of the dynamic resistance measurement. The characteristic shows the movement of the contacts. The transition to the arc contact is clearly visible. If the travel is measured, the length of the arc contact can also be determined. The display of the resistance characteristic and the length of the arc contact provides an insight into the internal status of the contact without having to open it.
Contact resistance measurements can be carried out with the PROMET ohm meters and can be incorporated within the test procedure. The test current can be set to a maximum of 600 A. Even very low resistance values in the single-digit micro-ohm range can be measured extremely accurately. The measured values are used for the evaluation of tests and are included in the test report.
PROMET resistance measuring devices in combination with ACTAS switchgear testing systems can be used to measure dynamic contact resistance simultaneously on three poles and on two or four breaker units per pole.
This means that the measurement can be carried out on all the contacts of a switchgear device in a single operation. This eliminates time-consuming connection and disconnection procedures and ensures that the measurement is carried out under identical conditions, allowing direct comparison of the contact resistances with one another.
Conclusion
Dynamic resistance measurement using PROMET resistance measuring devices in combination with ACTAS switchgear testing systems enables the condition of circuit breakers to be assessed. By recording the resistance curve throughout the entire switching process, information about the condition of the main and arc contacts can be obtained.
The method allows for reliable evaluation of the contact condition, detection of wear and tear and planning of maintenance measures. Thanks to the ability to test multiple poles simultaneously under identical conditions, test times are reduced and measurement results are more comparable.
Dynamic resistance measurement is therefore a meaningful test method that contributes to increasing operational safety and optimizing maintenance strategies for electrical switchgear.
