ABB DP840 pulse counter or frequency measurement module 3BSE028926R1

Description

• DP840 This section contains the settings to configure the percent differential element, including

  all associated harmonic inhibit features. The 745 provides three independent harmonic

  inhibit features: HARMONIC INHIBIT, DP840 which implements an inhibit scheme based on 2nd or

  2nd + 5th harmonic which is ‘in-circuit’ at all times; ENERGIZATION INHIBIT, which allows

  changing the characteristics of the inhibit scheme during energization to improve

  reliability; and 5TH HARM INHIBIT, which implements an inhibit scheme based on 5th

  harmonic only, allowing inhibiting the percent DP840 differential during intentional overexcitation

  of the system.

• 
• This section contains the settings to configure the percent differential element. The main

  purpose of the percent-slope characteristic of the differential element is to prevent

  maloperation because of unbalances between CTs during external faults. These

  unbalances arise as a result of the following factors:

  • CT ratio mismatch (not a factor, DP840 since the 745 automatically corrects for this

  mismatch)

  • Onload tap changers which result in dynamically changing CT mismatch
  • CT accuracy errors
  • CT saturation

  The basic operating principle of the DP840 percent differential element can be described by the

  following diagram and its associated equations:

• The basic percent differential operating principle for three-winding transformers is

  illustrated by the following equations:

  (EQ 5.4)

  The basic percent differential operating principle for two-winding transformers is

  illustrated by the following equations:

  (EQ 5.5)

  where

  I restraint = per-phase maximum of the currents after phase, ratio, and zero

  sequence correction;

  I differential = per-phase vector sum of currents after phase, ratio, and zero

  sequence correction

  Note

  In the above equations, the 180° phase DP840 shift due to the wiring connections is taken into

  account, hence the + sign to obtain the differential current.

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