Schweitzer Engineering Laboratories SEL-700G Series Instruction Manual Download

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4.9

Date Code 20170814

Instruction Manual

SEL-700G Relay

Protection and Logic Functions

Group Settings (SET Command)

For each restraint element (87R-1, 87R-2, 87R-3), the quantities are summed
as phasors and the magnitude becomes the operate quantity (IOPn). For a
through-current condition, IOPn should calculate to about 1 + (–1) = 0, at
rated load. Calculation of the restraint quantity (IRTn) occurs through a
summation of all current magnitudes. For a through-current condition, this
will calculate to about (| 1 | + | –1 |) = 2, at rated load.

Figure 4.5 shows how the differential element quantities are used to generate
the unrestrained 87Un (87U1, 87U2, 87U3) and restrained 87Rn/87HRn
(87R1, 87R2, 87R3, 87HR1, 87HR2, 87HR3) elements.

Unrestrained elements (87U1, 87U2, and 87U3) compare the IOP quantity to
a setting value (U87P), typically about 10 times TAP, and trip if this level is
exceeded. Elements 87U1, 87U2, and 87U3 are combined to form element
87U, as shown in Figure 4.7. Harmonic blocking or restraint is not performed
on the unrestrained elements. Use these elements to protect your transformer
bushings and end windings while maintaining security for inrush and through-
fault conditions.

Restrained elements (87R1, 87R2, and 87R3) determine whether the IOP
quantity is greater than the restraint quantity by using the differential
characteristic shown in Figure 4.2. This characteristic is modified by
increasing the restraint current threshold as a function of the second- and
fourth-harmonic content in the input currents for the harmonic restraint
elements (87HR1, 87HR2, and 87HR3). Set HRSTR = Y to activate the
harmonic restraint element 87HR.

In element 87Rn, for example, the IOPn and IRTn quantities determine
whether the relay trips. The logic enclosed within the dotted lines, sensitive
differential and high security differential logic, of Figure 4.5 implements the
respective sensitive differential and high security differential characteristics
shown in Figure 4.2. The differential element calculates a threshold as a
function of IRTn (f1(IRTn) or f2(IRTn)) as shown in Figure 4.5. IOPn must
exceed this threshold to assert 87Rn. The function, f1(IRTn), uses SLP1,
SLP2, and IRS1 settings, along with IRTn, to calculate the IOP value. The
function uses SLP2 in place of SLP1 when the Relay Word bit INRn is
asserted. This feature provides a high security mode of operation for 10 cycles
when the transformer is energized. The function, f2(IRTn), uses the IRS1
setting, along with the SLP12, SLP22, and IRTn, to calculate the IOP value.
The high security mode Relay Word bit, HSM, determines which
characteristic to use. If HSM = 0, the element uses the sensitive differential
logic and 87Rn is driven by 87SNn. If HSM = 1, the element uses the high
security differential logic and 87Rn is driven by 87SCn. More information on
HSM is covered later in this section (see High Security Mode Settings HSM,
O87P2, and HSMDOT). If E87 = GEN, 87Rn will produce a trip. If
E87 = TRANS, the relay still needs the results of the harmonic blocking and/
or harmonic restraint decision logic (see Harmonic Restraint and Harmonic
Blocking for more information). Note that the operating current elements
87OSNn (87OSN1, 87OSN2, 87OSN3) and 87OSCn (87OSC1, 87OSC2,
87OSC3) are not available as Relay Word bits.

Schweitzer Engineering Laboratories SEL-700G Series, Instruction Manual

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