System design & power flow

Building Lines

  • Transfer power from one point to another
  • Limited prerequisites
    • Power
    • Voltage
    • Distance
  • One line at the time
  • Which technology
    • Power
    • Voltage
    • Distance

Optimizing grid

  • Do we ever ge the opportunities to build ultimate grid?
  • In areas where no distribution grid exist today
  • In very old grids
  • Collection grids
    • Wind power
    • Wave power
    • Tidal power
  • Make the best out of what we got
    • Optimal power flow

Power flow

Intoduction

  • Most important tool in power system operation and planning
  • See how the system work

On-line analyses

  • State estimation
  • Security analysis
  • Economic analysis
    • Optimal operation
    • Loss coefficients
    • Optimal pricing

Off-line analyses

  • Operation analyses
  • Planning analyses
    • Network expansion planning
    • Power exchange planning
    • Security and adequacy analyses
      • Faults
      • Stability

Problem description

  • A snapshot of the system
  • Knowing the demand and/or generation of power in each bus, find out:
    • bus voltages
    • load flow in lines and transformers
  • A single phase representation is usually adequate since power systems are usually balanced
  • The problem is described through a nonlinear system of equations
  • Need of iterative solution techniques
  • Solution technique: accuracy vs. computing time
  • Most used tool in steady state power system analysis

Problem

  • Associated with each bus are 4 quantities:
    • Real power
    • Reactive power
    • Voltage magnitude
    • Phase angle between voltages
  • Three types of buses are represented in the load flow calculations:
    • Slack bus provides the additional real and reactive power to meet the losses
      • Voltage magnitude and angle are specified
    • Voltage controlled buses
      • Voltage magnitude and real power are specified
    • Load buses
      • Real and reactive power are specified

Skriv slide 12-17 för hand

How to solve equations

  • We have a set of non-linear equations --> a network can only be solved iteratively (no direct solution)!
  • A load flow calculation is therefore done in a given pattern:
    • Check that sufficient variables are known (2n)
    • Give initial values to those voltages and angles that are unkown
    • Calculate the active and reactive power injections
    • Compare with known values of active and reactive power
    • Repeat the calculations until the accuracy between calculated and known powers is sufficient

OPF

Optimal power flow

  • The goal of an OPF is to determine the ''best'' way to instantaneously operate a power system
  • Usually ''best'' = minimizing operating cost
  • OPF considers the impact of transmission system
  • OPF is used as basis for real-time pricing in many electricity markets

From PF to OPF

Security constraints

  • OPF can include security constraints which represent operation of the system after contingency outages
    • allow the system operator to dispatch the system in a defensive manner --> security constrained OPF
    • if the contingency actually happened, the no limits are violated

Optimal power flow

  • Inequality contraints
    • transmission line/transformer/interface flow limits
    • generator MW limits
    • generator reactive MVAr limits or capability curves
    • bus voltage magnitudes
  • Available controls (i.e. control/optimization variables):
    • Generator MW outputs
    • OLTS transformer taps, phase-shift taps
    • Reactive power compensation devices (switched capacitor settings, SVCs, etc)
    • Load shedding
  • The power flow equations are introduced in OPF as demand-supply balances
  • The demand-supply balance is effected at each bus individually, not for the aggregated system
  • The optimum solution yields a set of generation variables that minimize cost while satisfying the physical laws of flow of electricity