ABB Robot spare parts 3HAC029157-001 DSQC668 Servo Computer

Description

• DSQC668 Use of Standard CT Ratios
  • Standard CT ratios: CT2 / CT1=V1 / V2
  • Tapped relay windings / interposing CTs (inaccurate/expensive)

  Solution:

  • Wx Nom Voltage, Wx rated Load, DSQC668  Wx CT primary setpoints
  • Automatic correction for mismatch: (CT2 × V2) / (CT1 × V1) < 16

  Even ignoring the onload tap changer, the 1500/1 CT on winding 2 does not perfectly

  match the 500/1 CT on winding 1. A perfectly matched winding 2 CT ratio (at nominal

• 
•  

  winding 2 voltage) is calculated as follows:

  (EQ 5.1)

  CT2 (ideal) CT

  1

  V

  1

  V2

  × —–

  500

  1

  ———

  220 kV

  69 kV

  —————-

  –

  ×

  1594.2

  where CT1 = winding 1 CT ratio

  V1 = winding 1 nominal voltage

  CT2 = winding 2 CT ratio

  V2 = winding 2 nominal voltage

  Thus, for any load, the winding 2 CT secondary DSQC668  current is higher (per unit) than the

  winding 1 CT secondary current. The mismatch factor is 1594.2 / 1500 = 1.063.

  The transformer type is entered as S2 SYSTEM SETUP  TRANSFORMER  TRANSFORMER

  TYPE. The 745 calculates and automatically corrects for CT mismatch to a maximum

  mismatch factor of 16. The following information is entered as setpoints:

  S2 SYSTEM SETUP  WINDING 1  WINDING 1 NOM F-F VOLTAGE: “220 kV”

  S2 SYSTEM SETUP  WINDING 1  WINDING 1 RATED LOAD: “100 MVA”

  S2 SYSTEM SETUP  WINDING 1  WINDING 1 PHASE CT PRIMARY: “500:1 A”

  S2 SYSTEM SETUP  WINDING 2  WINDING 2 NOM F-F VOLTAGE: “69.0 kV”

  S2 SYSTEM SETUP  WINDING 2  WINDING 2 RATED LOAD: “100 MVA”

  S2 SYSTEM SETUP  WINDING 2  WINDING 2 PHASE CT PRIMARY: “1500:1 A”

  For a three-winding transformer, DSQC668 the setpoints under the S2 SYSTEM SETUP  WINDING 3

  menu must also be set.

  5.2.2.2 Onload Tap Changer

  • Onload tap changer
  • Variable voltage ratio
  • CT2 / CT1 = V1 / V2
  • Lower sensitivity on differential element

  Solution:

  • Tap position monitoring: V2 = Vmin + (n – 1)Vincr

  For example, the onload tap changer changes the winding 2 voltage, resulting in an even

  greater CT mismatch. A perfectly matched winding 2 CT ratio (based on the tap changer

  position) is calculated as follows:

  (EQ 5.2)

  where

  n = current tap changer position

  V2(min) = winding 2 minimum voltage (at n = 1)

  V2(tap) = winding 2 voltage increment per tap

  Thus, with the tap changer at position 33, the Winding 2 CT ratio must be 1428.6 / 1 to be

  DSQC668 perfectly matched. In this case, the mismatch factor is 1428.6 / 1500 = 0.952.

  The 745 allows monitoring of the tap changer position via the tap position input. With this

  input, the 745 dynamically adjusts the CT ratio mismatch factor based on the actual

  transformer voltage ratio set by the tap changer.

  Tap changers are operated by means of a motor drive unit mounted on the outside of the

  transformer tank. The motor drive is placed in a protective housing containing all devices

  necessary for operation, including a tap position indication circuit. This indication circuit

  has a terminal for each tap with a fixed resistive increment per tap. A cam from the drive

  shaft that provides local tap position indication also controls a wiper DSQC668 terminal in the

  indication circuit, as illustrated below.

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