REPRESENTATION OF POWER SYSTEM COMPONENTS

 Understand about components of transmission lines which is corresponding to length and voltage of lines  Represent the model of generator, load  Calculate problems regarding base u

GENERATION AND TRANSPORT OF ELECTRICAL ENERGY

CHAPTER 3: REPRESENTATION OF POWER SYSTEM COMPONENTS

TON DUC THANG UNIVERSITY

FACULTY OF ELECTRICAL AND ELECTRONICS ENGINEERING

3.5 Base unit system

3.6 Per unit system

 Understand about components of transmission lines which is corresponding

to length and voltage of lines

 Represent the model of generator, load

 Calculate problems regarding base unit system

 Calculate problems regarding per unit system.

 Its length is less than 100 km

 There are only two parameters in the short length transmission line, resistance and reactance

 The complex component consisting of the two parameter is called impedance, Z=R + jX

3.1.1 Short length transmission lines

3.1 REPRESENTATION OF TRANSMISSION LINE

The voltage and current at the sending-end are calculated as the following expression

A Two port network

=

ABCD line constant in the equation is obtained by:

Consequently, the sending-end voltage and current are rewritten as below

3.1 REPRESENTATION OF TRANSMISSION LINE7

 Its length is in the range of 100-250 km

 When the length is from 200-250 km, the transmission line can be represented as either medium or long length models

 The model of medium length transmission line is either Pi or T both of them are equivalent

3.1.2 Medium length transmission lines

 Impedance of the line is located in the middle

 The shunt admittance is divided into two identical parts, one located in the sending-end and one located in the receiving-end

Nominal-Pi-representation

3.1 REPRESENTATION OF TRANSMISSION LINE

 ABCD line constant for the Pi model is obtained by:

The voltage and current at the sending-end are calculated as the following expression

 Shunt admittance of the line is located in the middle

 Impedance is divided into two identical parts, one located in receiving half and one located in the sending half

Nominal-T-representation

3.1 REPRESENTATION OF TRANSMISSION LINE

 ABCD line constant for the T model is obtained by:

The voltage and current at the sending-end are calculated as the following expression

 Its length is more than 250 km

 It is the combination of several Pi model or T model

 For the sake of simplicity, the long length transmission line is separated into several parts of

Pi model or T model

3.1.3 Long length transmission lines

 The entire equivalent circuit of a two-winding transformer

3.3.1 Two-winding transformer

3.3 REPRESENTATION OF TRANSFORMER

 When primary and secondary windings are represented as only one winding, the circuit is

as below

 In cases such as calculation of short circuit current, optimal operation of transformer station as well as voltage drop calculation, the simplest circuit suggested is a below.

3.3 REPRESENTATION OF TRANSFORMER

 The resistance and reactance of transformer are obtained by

với (kW) với (kVar)

3.3.2 Three-winding transformer

 The symbol (Fig.a) and circuit diagram (Fig.b) of a three-winding transformer

3.3 REPRESENTATION OF TRANSFORMER

 The equivalent circuit of a three-winding transformer

 Reactance of winding 1 is calculated by

 Reactance of windings 2 and 4 are obtained by the same equation

3.3 REPRESENTATION OF TRANSFORMER

 Resistance of each winding is dependent on the capacity of each winding

Case 1: high voltage winding/medium voltage winding/low voltage winding=100%/100%/100%

===

=

Case 2: high voltage winding/medium voltage winding/low voltage winding=100%/100%/67%

===

=

=

3.3.3 Autotransformer

 The symbol (Fig.a) and circuit diagram (Fig.b) of a three-winding transformer

3.3 REPRESENTATION OF TRANSFORMER

 The benefit parameter and resistance of each winding:

=1- 

 The reactance of each winding:

=0

3.4 REPRESENTATION OF LOAD

 Load bus is represented as the following symbol

 In solving problems regarding voltage, current, power, the load is represented as a impedance or two types of power:

jX + R

SLoad=P+jQ

 There are three types of Load:

 Resistive load: P=S, Q=0, power factor=1

 Inductive load: P>0, Q>0, power factor <1

 Capacitive load: P>0, Q<0, power factor <1

3.5 BASE UNIT SYSTEM

3.6 PER UNIT SYSTEM

 In this chapter, we have learnt:

 Representation of transmission lines, load, transformer and generator

 The method to apply base unit system and per unit system

EX1: An 22 kV, 500kVA electrical load with power factor of 0.8 (lagging) is supplied by a

22/0,4 kV transformer Chosen base voltage and power are respectively 22 kV and

1000 kVA Find

a. Load impedance in base unit system

b. Load power in base unit system

c. The current of load in per unit system

EX2: The data of a transformer are rate power of 1000 kVA, rate power loss of 10 kW,

UN of 6%, transformation ratio of 55 (22/0,4 kV) Find resistance and reactance of

it according to rate primary voltage and rate secondary voltage