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Automation of Complex Power Systems (Fach) / Begriffe (Lektion)

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• What is the goal of frequency control in the power system? To ensure a constant system frequency (keep its nominal value)
• The frequency control has a multi-level structure. Which is the time frame for each level? - Primary Control < 30s- Secondary control 15s – 15min- Tertiary control > 15min
• Describe the characteristics of primary control - Priority – to bring the frequency back to acceptable value- Local control action – automatic reaction of each turbine individually – autonomousreaction- Generators react according to their speed-droop characteristics- Steady-state frequency deviation – the control law is purely proportional
• Describe the characteristics of secondary control - Change or shifting of speed-droop-characteristic of generators to compensate thesteady-state frequency error after primary control and to return to original/nominalfrequency of the system under different load circumstances- In large systems secondary control cannot be realized individually – centralizedsecondary control is used to prevent undesired power flows within different areas –elimination of Area Control Error (ACE)
• Describe the characteristics of tertiary control - Supports frequency control at a larger timeframe- Optimizes the overall system operation point- Sets reference value for individual generation units so that sufficient reserves are available
• What is the goal of static state estimation? To determine the most likely state of the system based on the available measurements- To obtain the voltage phasor at all buses of the system
• What is the goal of voltage control? - Keep voltage profile stable- Keep voltage magnitude within predefined limits
• Why is voltage control important? - To provide a stable operation point- To keep the assets within their operation range: prolonged operation outside limits causes damage- To reduce reactive power losses: they affect efficient operation of transmission system
• Characteristics of primary voltage control? - Local, decentral, time response up to one minute- Local voltage control equipment brings voltage to its set-up value in case of disturbance
• Characteristics secondary voltage control? - Regional control at slower speed (1-10 minutes)- Operation of primary control equipment is coordinated, by adjusting set-points and giving switching commands
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• Characteristics tertiary voltage control? - Central control every 30 min-1h- Set points are determined optimally for minimising system losses, maximising system loadability and controlling reactive power flow between regions (optimal power flow)
• Direct and indirect voltage control – what is the main difference? - Direct voltage control includes equipment for controlling voltage directly: on-load tap-changer- Indirect voltage control includes equipment for controlling voltage through control of reactive power flow: synchronous generator, synchronous condensers, shunt / series capacitor and inductors, static var compensator (SVC), facts devices (static synchronous compensator (STATCOM) unified power flow controller (UPFC))
• Who is responsible for voltage control in today’s power system? TSOs
• What is the motivation to involve DSOs in voltage control? Presence of DGs brings challenges:- Local power production increases possibility for voltage profile variations- Reversed power flow affects operation of protection devices (bidirectional power flow)- Local power production can cause local power congestions or affect transmission system
• What are the difficulties for DSOs concerning control and operation of the distribution system? - Difficulties to maintain voltage profile at customer connection points = Monitoring of grid values is missing= Active voltage control is not in place (passive network)= DGs are not equipped to contribute to network control and operation - DSOs are not usually allowed to constrain DG generation in emergency situations= TSOs are responsible for generation curtailment (or under agreement of both. However, TSOs cannot monitor distribution network conditions (voltage, power flow)= DSOs can only react to DG actions and ask TSOs to control (V, Q or pf set-points) orconstrain DG generation
• Please describe the responsibilities of DMS-Tertiary Controller - Analyse data received by primary substation HV/MV – no direct control of DGs- Coordinate energy flow between DSO and TSO, to economically optimise network operation- Enforce control actions (DSO flexibility services), to guarantee system robustness- Restore network after faults – network reconfiguration- Enable market participation
• Please describe the responsibilities of Primary Sub-station Automation Unit (PSAU) - Monitor running operations at each feeder- Handle large amount of data- Report overall status to DMS and other PSAUs- Cooperate with SSAUs to achieve control functions for a common goal- Cooperate with DMS to control integration of DGs, in order to maintain grid stability
• Please describe the responsibilities of Secondary Sub-station Automation Unit (SSAU) of the hierarchical distribution automation system - Monitor all LV feeders (microgrids) connected to a common secondary substation- Evaluate data received from end-devices (smart meters)- DG smart meters: collect information of DG power level and connection status- Load smart meters: aggregate all resistive and inductive loads- Analyse current status of microgrids and take actions to guarantee grid connection- Report overall status to PSAU (upper level)- Take time-sensitive operation decisions and control LV feeders in real-time
• Wind interface type: Danish concept - Induction machine generator system* frequency flexilibility between wind turbine and the grid* Reactive power is required- Capacitor bank for reactive power compensation- No power electronics involved- Usually no pitch control (low power small size wind turbine)
• Wind interface type: Doubly-fed induction generator - Induction machine generator system- Current injected into the rotor winding is fully controlled by the power electronics devices* In order to control the slip frequency in the induction machine* Flexible speed control of pitch speed- Converter sized for a fraction of the power
• Wind interface type: Full converter - Converter sized for the full power* Fully decoupling of the wind turbine side and grid side- Permanent magnet synchronous generator system
• What are Grid Codes? - Connection guidelines for grid access in LV / MV / HV power systems- Specific requirements set by the utilities and system operators- Technical specifications and operational characteristics for operation, maintenance anddevelopment of the power systems
• On what terms the Grid Codes set requirements? - Power output and quality- Operational modes- Grid support, e.g. reactive power supply
• Please describe the microgrid concept A Microgrid is a section of a power grid connected through a main switch to the mainpower line and characterized by the following properties:- It is equipped with local generation and it is then able to operate independently from thegrid- It is equipped with a hierarchical control that can manage two main operating states* Parallel mode* Islanding mode- A central controller is in charge of detecting conditions that require to switch fromparallel to islanding mode
• Please describe the functions of the microgrid controllers - Central controller: Energy Management, protection coordination- Local controllers: Local implementation of the control commands from central controller
• Please describe the operation modes of a microgrid - Parallel mode* Coordinate power dispatch to local sources (economic optimization)* Operate in parallel to the grid implementing synchronization action - Islanding mode* Local sources operates typically according to a droop control logic for P and Q* Central controller may implement functionalities similar to secondary control or/andchange droop coefficients
• Please describe the control modes of a converter in a microgrid (grid-forming converters) Converters can be represented as an ideal ac voltage source with a low-output impedance, setting the voltage amplitude E* and frequency w* of the local grid.
• Please describe the control modes of a converter in a microgrid (grid-feeding converters) Converters are mainly designed to deliver power to an energized grid. They can be represented as an ideal current source connected to the grid in parallel with high impedance. This current source should be perfectly synchronized with the ac voltage at the connection point, in order to regulate accurately the active and reactive power exchanged with the grid.
• Please describe the control modes of a converter in a microgrid (grid-supporting converters) Can be represented either as an ideal ac-controlled current source in parallel with a shunt impedance, or as an ideal ac voltage source in series with a link impedance. These converters regulate their output active and reactive power to keep the value of the grid frequency and voltage amplitude close to the nominal.
• Demand Side Management (DSM) plays a vital role in integrating Distributed Energy Resources. Give a brief description of the DSM concept DSM is the planning, implementation, and monitoring of those utility activities designedto influence customer use of electricity in ways that will produce desired changes in theutility’s load shape, i.e., changes in the time pattern and magnitude of a utility’s load
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• Load Management is one way of DSM. Please name Load Management methods and give the relevant graphs. Peak shaving, load shifting, valley filling, strategic conservation, strategic load growth, flexible load shape
• What is meant by aggregation? The process of linking small groups of industrial, commercial, or residential customers intoa larger power unit to make them visible from the electric system point of view- Aggregation is a commercial function of pooling decentralised generation and/orconsumption to provide services to actors within the system
• What are Distributed Energy Resources (DER)? Small-scale power generation and storage technologies located close to the customer side
• Why is aggregation necessary? - DER units have to provide the flexibility and controllability to support the system operation- DER units are too small and too numerous to be addressed individually- Aggregation can involve distributed resources and/or distributed generation, since load orgeneration profiles of individual consumers and/or small generators appear as a singleunit to the electric system

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