Basler Electric BE1-11t - Manual (Page 103)

9424200995 Rev H 91
Phase Current Differential (87)
Protection
The phase-current differential (87) element monitors the differential current and provides primary
protection for power transformers.
Element logic connections are made on the BESTlogic
™Plus screen in BESTCOMSPlus®, and element
operational settings are configured on the Phase Current Differential (87) settings screen in
BESTCOMSPlus. A summary of the logic inputs and outputs and operational settings appears at the end
of this chapter.
BESTCOMSPlus Navigation Path: Settings Explorer, Protection, Current, Phase Differential (87)
HMI Navigation Path: Settings Explorer, Protection, Settings Group x (where x = 0 to 3), Current
Protection, Differential 87
Element Operation
The phase-current differential element compares the currents entering and leaving the protected
transformer. If a fault is detected, the BE1-11t initiates a trip signal to isolate the power transformer. This
action limits damage to the transformer and minimizes impact on the power system.
Functional Description
Figure 60 shows a detailed functional diagram of one phase of the phase differential protection function.
These functions and comparators are duplicated for each phase.
The measured currents are phase, zero-sequence, and tap compensated. The Configuration chapter
describes the setup of the BE1-11t for phase and zero-sequence compensation. Setup of the tap
adjustment compensation is described later in this chapter. The restraint current function uses the
compensated current to calculate the restraint current magnitude (in multiples of tap). Depending on the
setting, it calculates the maximum or average restraint current. The Operating Current function
determines the magnitude of the fundamental, second, and fifth harmonic differential current as the
phasor sum of those components of the compensated currents.
Figure 61 shows the characteristic of the phase current differential protection element. This comparator
has two slope settings and a minimum pickup setting. The slope settings are the ratios of delta operating
current to delta restraint current. The slope settings should be set above the maximum mismatch caused
by excitation losses, tap mismatch, and load tap changers. The minimum pickup setting determines the
minimum sensitivity of the restrained element.
Pickup and Trip
The Pickup output occurs first, followed by the Trip output.
Pickup
The Pickup output becomes true when the ratio of operating current to restraint current increases above
the slope setting. The operate current is above the Minimum Restrained Pickup setting for any of the
three phases. In BESTlogicPlus, the Pickup output can be connected to other logic elements to
annunciate the condition, control other elements in logic, and start the fault recorder (logic element
FAULTTRIG).
Assertion of the Pickup output initiates a timer that begins timing to a trip. The duration of the timer is
established by the Time Delay setting.
If the pickup condition subsides before the element delay expires, the timer and Restrained Pickup output
are reset, no corrective action is taken, and the element is rearmed for any other fault conditions.
BE1-11t 0BPhase Current Differential (87) Protection