Components and parameters > Switches and protections

Differential protection is part of a circuit breaker and also refers to other circuit breakers and/or measurement fields, the other measuring points. In this way, an arbitrary object (rail, transformer, cable) or an arbitrary set of objects (direction) can be secured with the differential protection. The differential protection operates when the corresponding circuit breakers are no more than six branches apart. In that case the correct flow direction and transfer ratio of a possible intermediate transformer is taken into account.

Only this circuit breaker is tripped. Not the circuit breakers at the "other measurement points".

PARAMETERS

Parameter	Default	Unit	Description
Type			Type name of the differential protection
t,input	0	s	Pickup time
t,output	0	s	Operation output time
Sort			Sort
m			factor m (Time Multiplier Setting), at inverse characteristic
Other measurement points			List of circuit breakers and measurement units connected to this differential protection
dI >	0	A	First threshold value for activating the differential protection
t >	0	s	Time for trip signal
dI >>	0	A	Second threshold value for activating the differential protection
t >>	0	s	Time for trip signal
k1	0		Factor k1
k2	0	A	k2
k3	0		Factor k3
k4	0	A	k4
Release by the first current protection	no		The current has also to be bigger then I> of a fixed time characteristic
No own measurement			The circuit breaker itself does not provide a measurement

The operation is as follows. Suppose two currents I1 and I2 (as complex values) are measured, both directed towards the object to be protected. In the normal situation both flows are equal but opposite to each other: I1 = - I2. The absolute value of the sum of the measured flows Diff is then equal to zero.

Diff = | I1 + I2 |

In the event of a short-circuit in the object to be protected, Diff is no longer equal to zero, as there is current flowing from the object to the ground. If Diff is greater than the threshold value dI> for a time of at least t> a trip command will be given.

k1, k2, k3, k4

The values k1, k2, k3, k4 are used as an extra tripping criterions. Suppose that two currents I1 and I2 are measured as complex values. The sum of the absolute values will be calculated as follows:

Sum = | I1 | + | I2 |

The differential protection will only trip if the following three conditions are met:

Diff > dI>

Diff > k1 ? Sum + k2

Diff > k3 ? Sum + k4