Presentation on theme: "EE 369 POWER SYSTEM ANALYSIS"— Presentation transcript:
1EE 369 POWER SYSTEM ANALYSIS Lecture 9Transformers, Per Unit CalculationsTom Overbye and Ross Baldick
2Announcements For lectures 8 to 10 read Chapter 3 Homework 7 is 5.9, 5.14, 5.24, 5.26, 5.27, 5.28, 5.29, 5.32, 5.33, 5.37, 5.38, 5.43; due October 23.Homework 8 is 5.19, 5.23, 3.1, 3.3, 3.4, 3.7, 3.8, 3.10, 3.12, 3.13, 3.14, 3.16; due October 30.Homework 9 is 3.17, 3.18, 3.20, 3.23, 3.25, 3.26, 3.28, 3.29, 3.34, 3.37, 3.41, 3.45; due November 6. Turn in to grader at beginning of UT power plant tour.
3Per Unit Change of MVA Base Parameters for equipment are often given using power rating of equipment as the MVA baseTo analyze a system all per unit data must be on a common power base
4Per Unit Change of Base Example A 54 MVA transformer has a leakage reactance of 3.69% (on its own MVA base).What is the reactance on a 100 MVA base?
5Transformer Reactance Transformer reactance is often specified as a percentage, say 10%. This is a per unit value expressed as a percentage on the power base of the transformer.Example: A 350 MVA, 230/20 kV transformer has leakage reactance of 10%. What is p.u. value on 100 MVA base? What is value in ohms (230 kV)?
6Three Phase Transformers There are 4 different ways to connect 3f transformersD-DY-YUsually 3f transformers are constructed so all windingsshare a common core
10Y-Y Connection: Per Phase Model Per phase analysis of Y-Y connections is exactly the same as analysis of a single phase transformer.Y-Y connections are common in transmission systems.Key advantages are the ability to ground each sideand there is no phase shift is introduced.
16D-Y Connection: Per Phase Model Note: Connection introduces a 30 degree phase shift!Common for transmission/distribution step-down sincethere is a neutral on the low voltage side.Even if a = 1 there is a sqrt(3) step-up ratio
17Y-D Connection: Per Phase Model Exact opposite of the D-Y connection, now with aphase shift of -30 degrees.