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    One-Line Diagram: Simplified Single-Phase Balanced Three-Phase Single Line Apparatus Symbols

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    Safina Syg
    One-line diagram is a simplified single-phase circuit diagram of a balanced three-phase electric power system. It is indicated by a single line and standard apparatus symbols. The equivalent circuits of the components are replaced by their standard symbols.

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    One-Line Diagram: Simplified Single-Phase Balanced Three-Phase Single Line Apparatus Symbols

    Uploaded by

    Safina Syg
    52 views25 pages
    One-line diagram is a simplified single-phase circuit diagram of a balanced three-phase electric power system. It is indicated by a single line and standard apparatus symbols. The equivalent circuits of the components are replaced by their standard symbols.

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    Original Title

    CHAPTER 1 Continue

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    One-line diagram is a simplified single-phase circuit diagram of a balanced three-phase electric power system. It is indicated by a single line and standard apparatus symbols. The equivalent circuits of the components are replaced by their standard symbols.

    Copyright:

    Attribution Non-Commercial (BY-NC)
    Download as PPT, PDF, TXT or read online from Scribd
    Download as ppt, pdf, or txt
    52 views25 pages

    One-Line Diagram: Simplified Single-Phase Balanced Three-Phase Single Line Apparatus Symbols

    Uploaded by

    Safina Syg
    One-line diagram is a simplified single-phase circuit diagram of a balanced three-phase electric power system. It is indicated by a single line and standard apparatus symbols. The equivalent circuits of the components are replaced by their standard symbols.

    Copyright:

    Attribution Non-Commercial (BY-NC)
    Download as PPT, PDF, TXT or read online from Scribd
    Download as ppt, pdf, or txt
    of 25

    `

    One-line Diagram
    R Y B 3-phase system single-phase system

    One-line diagram is a simplified single-phase circuit diagram of a balanced three-phase electric power system. It is indicated by a single line and standard apparatus symbols.

    One-line Diagram
    The information on a one-line diagram is vary according to the problem at hand and the practice of the particular company preparing the diagram. Example :

    Load/ Power Flow Study

    Transient Stability Study

    Advantages of One-line Diagram  Simplicity.  One phase represents all three phases of the balanced system.  The equivalent circuits of the components are replaced by their standard symbols.  The completion of the circuit through the neutral is omitted.

    Impedance and Reactance Diagrams  Impedance (Z = R + jX) diagram is converted from oneline diagram showing the equivalent circuit of each component of the system. It is needed in order to calculate the performance of a system under load conditions (Load flow studies) or upon the occurrence of a short circuit (fault analysis studies).  Reactance (jX) diagram is further simplified from impedance diagram by omitting all static loads, all resistances, the magnetizing current of each transformer, and the capacitance of the transmission line. It is apply to fault calculations only, and not to load flow studies.  Impedance and reactance diagrams sometimes called the Positive-sequence diagram.

    Impedance and Reactance Diagrams


    Z diagram Derived from single line diagram Covers equivalent components of all circuit components X diagram Derived from Z diagram. All static loads, resistances, magnetizing current of each transformer, and the capacitance of the transmission line are omitted Consists only X Used to calculate fault analysis only More simpler Formulated as Z=jX

    Consists of R and X Used to calculate both load/power flow and fault analysis A bit complex Formulated as Z=R+jX

    Impedance and Reactance Diagrams


    Example : One-line diagram of an electric power system

    Impedance and Reactance Diagrams


    Impedance diagram corresponding to the one-line diagram of Example 1.2

    E1

    E2

    E3

    Generators Load A 1 and 2

    Transformer T1

    Transmission Line

    Transformer T2

    Load Gen. 3 B

    Impedance and Reactance Diagrams


    Reactance diagram corresponding to the one-line diagram of Example 1.2

    E1

    E2

    E1

    Generators 1 and 2

    Transformer Transmission Transformer Line T1 T2

    Gen. 3

    Equivalent circuit for each component?

    Per-unit Representation
    

    In power systems there are so many different elements such as Motors, Generators and Transformers with very different sizes and nominal values. To be able to compare the performances of a big and a small element, per unit system is used.

    Per-unit Representation
    

    Power system quantities such as voltage, current and impedance are often expressed in per unit or percent of specified values. Per unit quantities are calculated as:

    actual quantity Quantity in per - unit ! base value of quantity

    The advantages of per-unit quantities:


    The apparatus of the same general type of p.u. volt drops and losses are in the same order, regardless of size. The use of 3 in three-phase calculations is reduced. By the choice of appropriate voltage bases, the solution of networks containing several transformers is facilitated. Per-unit quantities more readily to digital computation.

    Per-unit Representation

    Per Unit Values


    S actual Spu ! Sbase I actual I pu ! I base
    Vactual Vpu ! Vbase

    Z actual Z pu ! Z base

    Conversion of Per Unit Values


    Z actual Sbase Z pu ! ! 2 Z actual Zbase Vbase
    Z actual ! Z base
    2 Vbase Z pu ! Z pu Sbase

    The formulas relate the various quantities for single-phase system:

    The formulas relate the various quantities for three-phase system:

    Per-unit Representation
    

    Usually, the nominal apparent power (S) and nominal voltage (V) are taken as the base values for power (Sbase) and voltage (Vbase). The base values for the current (Ibase) and impedance (Zbase) can be calculated based on the first two base values.

    Per-unit Representation

    Vbase ( given) I base Z base S base ! Vbase

    S base given

    Vbase V base ! ! I base S base

    Example in the module:

    Examples in the module:

    Exercise: The terminal voltage for a symmetrical Y-connected load with per phase impedance , ZL = 20 -300 is 4.5 kV line to line. The impedance for each line that link the load to the bus substation is ZTL = 1.5 at ; . Determine the line to line voltage 750the bus substation by using per unit analysis. Use 4.5kV and 130A as the base voltage and base current.

    Changing the Base of Per-unit Quantities


    

    The impedance of individual generators and transformers are generally in terms of % or pu quantities based on their own ratings (By manufacturer). For power system analysis, all impedances must be expressed in pu on a common system base. Thus, it is necessary to convert the pu impedances from one base to another (common base, for example: 100 MVA). Per-unit impedance of a circuit element

    (actual impedance, ;) v (base MVA) ! (base voltage, kV) 2

    The equation shows that pu impedance is directly proportional to base MVA and inversely proportional to the square of the base voltage. Therefore, to change from old base pu impedance to new base pu impedance, the following equation applies:
    2

    Per - unit Z new

    base kVold ! per - unit Zold base kV new

    base MVA new base MVA old

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