Dynamic Capacitance Measurement of HV Breakers
The latest trend in HV circuit breaker timing is DualGround testing using a method called dynamic capacitance measurement (DCM). DualGround is the trade name of Megger Programma, but other manufacturers making a similar instrument may call it Dual Earth testing. The DCM method can be explained as follows.
All HV circuit breakers exhibit the characteristics of a capacitive circuit when they are in the open state. As the breaker contacts start to move, the capacitance changes accordingly until the time at which they make contact and the circuit becomes a resistive circuit. The DCM test method measures the change in capacitance of the breaker during close cycle and quantifies it so it can be displayed with other breaker timing measurements. This DCM test may be performed with a full featured circuit breaker analyzer such as the Programma TM-1800 shown in Figure 7.27.
The DCM method of breaker timing is primarily used for HV breakers, such as 220 kV and above. The test results of a DCM test are presented in the same manner as a normal breaker tests. A typical timing graph of a HV breaker is shown in Figure 7.28.
This test method was developed to improve the safety of the test person- nel in the HV substations. When using this method, safety grounds are placed on both sides of the breaker while performing the test as is shown in Figure 7.29.
By leaving both grounds in place while testing, an effective safe zone is created around the breaker for the test personnel. This will protect both the test equipment and more importantly protect the test personnel from any unwanted induced currents and voltages in the test circuit from adjacent energized lines.