Basler Electric BE1-87T - Manual (Page 35)

9171300990 Rev V BE1-87T Functional Description 3-1
SECTION 3 • FUNCTIONAL DESCRIPTION
General
BE1-87T relays are solid-state devices that protect transformers by providing output contact closure when
the scaled current into the protected transformer does not equal the scaled current out, within defined
limits. These relays are harmonically restrained to prevent tripping during initial energization and external
fault conditions. A through-current restraint also provides security against tripping for external faults. An
unrestrained tripping element is included to provide a high-speed trip in the event of a particularly severe
fault within the transformer.
Description
The functional block diagrams of Figures 3-1 and 3-2 illustrate the overall operation of the BE1-87T
Transformer Differential Relay. (Figure 3-1 shows Phase A or single-phase functions; Figure 3-2 shows
the additional functions for phases B and C.) Since the three phases are functionally similar, only phase A
is shown in detail in Figure 3-1. Note that in a three-phase unit, there may be one restrained output for
each phase (Output Type Option E in the third position of the Style Number), or one restrained output that
serves for all three phases (Option F in the third position). When Target Option C or D is specified for a
three-phase Unit (in the seventh position), an individual target is supplied for each phase.
Current Transformers
In the protected zone of the power system, CTs with a 1 ampere or 5 ampere secondary winding supply
the sensing current for each input. This is not
Scaling
shown in Figure 3-1 or Figure 3-2. Other relays may be
connected ahead of the BE1-87T. Sensing currents are, in turn, applied to relay internal input
transformers. These transformers provide system isolation.
Input currents are scaled by the front panel INPUT rotary switches that introduce resistances to the
internal CT secondaries. The switches are calibrated in 0.02 ampere increments from 0.4 to 1.78 ampere
for 1 ampere CT units (Options 2 or 4 in the second position of the Style Number), and in 0.1 ampere
increments from 2.0 to 8.9 amperes for 5 ampere CT models (Options 1 or 3 in the second). The many
graduations of adjustment are provided to allow each input to approach an ideal representation of the
actual operating per unit value.
Summing
Analog signals representing each input contribution are vector summed (shown as Summing in Figure 3-
1). This summing process produces the operating current (I
OP
) that is the phasor sum of the input
currents.
Ideally, and with perfectly matched CTs, a transformer without an internal fault should cause I
OP
to be
exactly zero on a continuous basis. When not zero, a fault would be indicated. However, saturation effects
caused by heavy through-current or magnetic inrush can cause a temporary imbalance even though no
internal fault has occurred. To prevent a false trip under such conditions, various types of restraint are
used. Each restraint is specific to a potential cause of misoperation. These are individually discussed in
the topic Restrained Trip Output.
30° Internal Phase Shift (Three-Phase Relays ONLY)
For three-phase units, the inputs to the Summing function are first routed through the 30° Phase Shift
circuit. There the signals may be advanced or retarded by 30° or passed through unchanged.
Compensating phase shift direction (shown as the Phase Shift Setting circuit in Figure 3-1) is determined
by the position of three jumpers on the internal Analog #2 Board. (The location of these jumpers is shown
in Figure 2-4.) The internal phase shift will accomplish the corresponding zero sequence blocking. The
current magnitude will be increased by
3
and must be taken into account in the tap setting (explained
later in detail in Section 4).