6 Automation in the power supply system

• Generation highly concentrated

• System is quasi-static

• Generation is “totally” under

control

• Load is statistically predictable

• Load-driven system

• Power flow from transmission to

distribution is unidirectional

• Distribution is a totally passive

system

• More distributed generation

• Renewable sources are not totally predictable (uncertainty) and not under control

• Generation-driven system

• Power injection occurs also at distribution level (bottom-up power flow)

• The system is characterized by higher dynamics and low physical inertia

• Primary (≤ 30s)

  − represents the autonomous reaction for generators as well for loads

  − speed-droop characteristics of generators

  − reaction is individual for each turbine

  − steady-state frequency deviation

• Secondary (15s -15min)

  − change or shifting of speed-droop characteristic of generators

  − to return to nominal/initial frequency of the system under different load

  circumstances

  − centralized secondary control is used to prevent undesired power flows within

  different areas

  − to eliminate deviation through Area Control Error (ACE)

• Tertiary (>15min)

  − optimizes the overall system operation point (set-points)

  − sets the reference value for individual generation units so that sufficient secondary

  control is available

  − reserves are allocated among the available generators taking into account:

  o geographically wide distribution and

  o various economic factors of each generation unit

• Direct voltage control

− Tapped transformers with variable

transformation ratio

(On-Load Tap-Changer OLTC)

− Main disadvantage: voltage regulation in

discrete steps and slow

• Indirect voltage control

− Reactive power control and compensation

o Automatic Voltage Regulator (AVR) of

synchronous generators

o Synchronous condensers

o Static VAr Compensator (TCR/TSR,

TSC, TCR/FC, TSC/TCR)

o FACTS devices (e.g. Voltage Source

Converter based STATCOMs)

o Shunt/Series capacitors and inductors

• Primary Control (Primary Voltage Regulation PVR)

− Local, decentral, time response up to one minute

− Drives local voltage control equipment in case of disturbances to get voltage to its set-up

value

• Secondary Control (Secondary Voltage Regulation SVR)

− Regional control at slower speed (1-10 minutes)

− Coordinated control of PVR equipment by adjusting set-points and giving switching

commands

• Tertiary Control (Tertiary Voltage Regulation TVR)

− Central control every 30 min-1h

− Determination of optimal set-points to minimize system losses, maximize system

loadability, preserve or control reactive power flow between control regions

• transmission grid:

  • past: power-frequency-control, Maintaining the frequency and voltage, meshed grid

  • now: transport grid, Transmitting power from generators to consumers, Decentralized energy generation (offshore wind parks), Overload of the transmission lines, Grid bottlenecks, Maintaining the voltage at risk → Higher demand for reactive power, More complex control, Challenge: higher power transport over long distances

• medium voltage grids

  • past: ring grid with an open separation point, distribution, More unidirectional power flow with passive components

  • now: More decentralized generation, Bidirectional power flow at higher voltage level, Higher demand for control, Control of the feed-in, challenge: decentralised feed-in management

• low voltage grids:

  • past: radial grid, Distribution to end user, Voltage profile: steadily decreasing, unidirectional P flow, passive components

  • now: Active components at the low voltage grid, Dynamic feeding into MVG, Bidirectional P flow, control of the decentralised generation, Voltage profile: no longer continuously decreasing, challenge: decentralised feed-in management

• A cluster of micro-sources, storage systems and loads that represent a part of the grid as a single controllable unit providing both electricity and heat to local area

• independent control instances and the possibility of islanding with minimal interruption of supply

• Parallel mode

  − Coordinated power dispatch to local sources (economic optimization)

  ‒ Operation in parallel to the grid by implementing synchronization action

• Island mode

  ‒ Local sources operate typically according to a droop control logic for P and Q

  → Separate frequency control in the island grid

  ‒ Central controller may implement functionalities similar to secondary control or/and change droop coefficients

• Centralized hierarchal Control

  • Primary control (droop control)

  • Secondary control: to restore the nominal voltage and frequency

  • Tertiary control: controls the energy exchange between the microgrid and the rest of the grid

• Distributed agent-based Control