Operating Principles - GE Multilin DBT Instruction Manual

OPERATING PRINCIPLES

4. OPERATING PRINCIPLES
Figure 7 (Block diagram) shows the basic operation of the DBT (the most common case of a 3-phase version).
Seven units of measurement can be seen:
1. Voltage measurement for undervoltage function of breaker power supply.
2. Resistance measurement of coil 1 through contact 52/b.
3. Same through contact 52/a.
4. Resistance measurement of coil 2 through contact 52/b.
5. Same through contact 52/a.
6. Resistance measurement of coil 3 through contact 52/b.
7. Same through contact 52/a.
The first measurement unit serves an undervoltage comparator which will give a signal if the voltage is lower than the
setting, activating an adjustable timer between 0.2 and 20 seconds. If the undervoltage continues for at least the set
time, the timer output will activate the signal breaker voltage failure and also the signal general failure. The activation
of failure signals deactivate the corresponding output relays.
Note: 'deactivation of a relay' is understood to mean that it changes to its standby position as shown in the external
connections diagram, i.e. a NO closes and a NC opens.
The timer has a preset voltage drop time of 100 ms, thereby ensuring a minimum time in which the output contacts
are maintained for at least 100 ms.
Two resistance measurement devices are located after the undervoltage unit, both monitoring the first coil. The
resistance measurement is calculated through the voltage drop produced by the current injected by the current
sources.
Supposing that the breaker is closed, auxiliary contact 52/a would also be closed, whereas 52/b would be open. The
measuring unit connected to 52/b would show a higher voltage than that of the resistance setting, i.e. the DBT will
measure a higher resistance than the setting. On the other hand, the unit connected to 52/a will give a lower
resistance reading than the setting, taking into account that the coil circuit is correct.
Both resistance measuring unit outputs are the inputs of an equality comparator (this is a comparator that produces
output when both inputs are high or low, i.e. equal). In the example mentioned, one measuring unit gives a HIGHER
resistance than the setting whereas the other gives a LOWER resistance. Therefore, the comparator detects inputs
are not equal and does not produce an output signal, which means coil circuit is in a good condition. In case the coil
is broken, resistance measured by both units is infinite (i.e. equal) and the equality comparator produces a failure
signal. That signal is feed into a timer. After the set time elapses with a failure signal, two signals are generated, coil
failure and general failure.
As described for coil 1, the 3-phase DBT has circuits for a second and third coils.
In the case of a single-phase DBT, there are only resistance measuring unit for coil 1 (in addition to the undervoltage
unit).
GEK-105597C DBT Digital Trip Coil Supervision Relay 9