Power Systems
What is Power Systems?
A system of calculation where all electrical quantities are expressed as fractions of chosen base values, simplifying power system analysis.
Key formula / rule: Base Current
Key points
- Understand the concept and purpose of the per unit system.
- Be able to select appropriate base values for a power system.
- Calculate base current and base impedance.
- Convert actual system quantities to per unit values and vice versa.
Common exam trap
Incorrectly calculating base impedance when changing base values.
Definitions
- Term
Per Unit System
- Meaning
A system of calculation where all electrical quantities are expressed as fractions of chosen base values, simplifying power system analysis.
- Term
Base Apparent Power (Sbase)
- Meaning
The arbitrarily chosen reference apparent power for a power system or a part of it, typically expressed in MVA or VA.
- Term
Base Voltage (Vbase)
- Meaning
The arbitrarily chosen reference voltage for a power system or a part of it, typically expressed in kV or V.
- Term
Per Unit Impedance
- Meaning
The ratio of the actual impedance of a component to the base impedance of the system it is part of.
Learning objectives
Understand the concept and purpose of the per unit system.
Be able to select appropriate base values for a power system.
Calculate base current and base impedance.
Convert actual system quantities to per unit values and vice versa.
Calculate per unit impedance of transformers and networks.
Formulae
- Name
Base Current
- Note
Used to derive base current from chosen base apparent power and base voltage.
- Expression
Ibase = \frac{Sbase}{Vbase}
- Name
Base Impedance
- Note
Used to derive base impedance from chosen base voltage and base apparent power.
- Expression
Zbase = \frac{Vbase}{Ibase} = \frac{Vbase^2}{Sbase}
- Name
Per Unit Voltage
- Note
Actual voltage divided by the chosen base voltage.
- Expression
Vpu = \frac{Vactual}{Vbase}
- Name
Per Unit Current
- Note
Actual current divided by the chosen base current.
- Expression
Ipu = \frac{Iactual}{Ibase}
- Name
Per Unit Power
- Note
Actual power divided by the chosen base apparent power.
- Expression
Ppu = \frac{Pactual}{Sbase}
- Name
Per Unit Impedance
- Note
Actual impedance divided by the chosen base impedance.
- Expression
Zpu = \frac{Zactual}{Zbase}
- Name
Per Unit Impedance Conversion (Changing Base)
- Note
Formula to convert per unit impedance from an old base to a new base.
- Expression
Zpu(new) = Zpu(old) \× \left(\frac{Vbase(old)}{Vbase(new)} ight)2 \× \left(\frac{Sbase(new)}{Sbase(old)} ight)
Prerequisites
Basic electrical engineering concepts (Ohm's Law, Power calculations).
Understanding of AC circuits.
Knowledge of power system components (transformers, generators, transmission lines).
Common mistakes
Incorrectly calculating base impedance when changing base values.
Using different base values for different parts of the system without proper conversion.
Confusing base voltage with actual voltage.
Forgetting to convert actual impedance values to per unit values before analysis.
Keywords
Per Unit System
PU System
Base Values
Base Voltage
Base Apparent Power
Per Unit Impedance
Power System Analysis
Normalization
Transformer Impedance
Load Flow
Fault Analysis
Practice preview
Which type of power plant typically has the lowest operating cost but high initial investment?…
easy
A medium transmission line is represented by a Nominal Pi model. If the series impedance is Z and shunt admittance is Y, what are the ABCD parameters of the line?…
medium
Which of the following is NOT a primary component of a typical electrical power system?…
easy