Chapter 17. Load-Tap-Change Control and Transformer Paralleling

Chapter 17. Load-Tap-Change Control and Transformer Paralleling

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17 Load-Tap-Change Control and Transformer Paralleling James H. Harlow Harlow Engineering Associates 17.1 Introduction. .................................................................... 17 -1 17.2 System Perspective, Single Transformer . ....................... 17 -2 17.3 Control Inputs. ................................................................ 17 -3 Voltage Input . Current Input . Phasing of Voltage and Current Inputs 17.4 The Need for Voltage Regulation. .................................. 17 -4 Regulation of the Voltage at the Bus . Regulation of the Voltage at the Load 17.5 LTC Control with Power-Factor-Correction Capacitors . ....................................................................... 17 -8 17.6 Extended Control of LTC Transformers and Step-Voltage Regulators. .......................................... 17 -9 Voltage Limit Control . Voltage Reduction Control . Reverse Power Flow 17.7 Introduction to Control for Parallel Operation of LTC Transformers and Step-Voltage Regulators . ............................................... 17 -11 The Need for Special Control Considerations . Instrument Transformer Considerations 17.8 Defined Paralleling Procedures . ................................... 17 -12 Master = Follower . Circulating-Current Method . Negative Reactance Method 17.9 Characteristics Important for LTC Transformer Paralleling. ................................................ 17 -14 17.10 Paralleling Transformers with Mismatched Impedance. ..................................................................... 17 -14 17.1 Introduction Tap changing under load (TCUL), be it with load-tap-changing (LTC) power transformers or step- voltage regulators, is the primary means of dynamically regulating the voltage on utility power systems. Switched shunt capacitors can also be used for this purpose, but in the context of this discussion, shunt capacitors are presumed to be applied with the objective of improving the system power factor. ß 2006 by Taylor & Francis Group, LLC.
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The control of a tap changer is much more involved than simply responding to a voltage excursion at the transformer secondary. Modern digital versions of LTC control include so many ancillary functions and calculated parameters that it is often used with its communications capability to serve as the means for system-condition monitoring. The control of the tap changer in a transformer or step-voltage regulator is essentially the same. Unless stated otherwise, the use of the term ‘‘transformer’’ in this section applies equally to step-voltage regulators. It should be recognized that either type of product can be constructed as single-phase or three-phase apparatus, but that transformers are more often three phase, while step-voltage regulators are more commonly single phase. 17.2 System Perspective, Single Transformer This discussion is patterned to a typical utility distribution system, the substation and the feeder, although much of the material is also applicable to transmission applications. This first discussion of the control considers that the control operates only one LTC in isolation, i.e., there is not the opportunity for routine operation of transformers in parallel. The system can be configured in any of several ways according to the preference of the user. Figure 17.1 depicts two common implementations. In the illustrations of Figure 17.1, the dashed-line box depicts a substation enclosing a transformer or step-voltage regulators. The implementations illustrated accom- plish bus-voltage regulation on a three-phase (Figure 17.1a) or single-phase (Figure 17.1b) basis. Another LTC power
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This note was uploaded on 10/19/2010 for the course ENGINEERIN ELEC121 taught by Professor Tang during the Spring '10 term at University of Liverpool.

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Chapter 17. Load-Tap-Change Control and Transformer Paralleling

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