SAQ

-Primary (≤ 30s)

-Secondary (15s

15min)

-Tertiary (>15min)

Primary (≤ 30s)

• Represents the autonomous reaction for generators as well for loads (‘inertial response’)

• Speed-droop characteristics of generators

• Reaction is individual for each turbine

• Steady-state frequency deviation

Secondary (15s-15min)

• Change or shifting of speeddroop 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 (‘reservepower’) is available

• Reserves are allocated among the available generators taking

  • geographically wide distribution,

  • and various economic factors of each generation unit into account.

  
  

• Decrease in Load?

  • -> Frequency increases

• Loss of Generation?

  • -> Frequency decreases

1. Danish concept

2. Doubly-fed induction generator

3. Full converter

• Elimination of complex turbine inverter

• No transformers -> less probability of failure -> higher reliability

• Decentralized power electronics have higher maintenance costs

  • Less power electronics -> less maintenance costs

    
    

Nested contrl loop solutions consists of two cascaded loops

1. Voltage control (outer and slower loop)

2. Current control (inner and faster loop)

• The inner current loop can be done without considering the outer voltage loop.

• For the design of the voltage control loop,the current control loop is represented by its dominant pole

• Bandwith of internal loop > Bandwith of external loop

  
  

1. Grid-forming converter

2. Grid-feeding converter

3. Grid-supporting converter

Grid-forming:

• ideal voltage source with low-output impedance

Grid-feeding:

• ideal current source connected to the grid inparallel with high impedance

Grid supporting (two ways):

1. ideal current source in parallel with shunt impedance

2. ideal voltage source in series with link impedance

A section of a power grid connected through point of common coupling (PCC) with properties:

• local generation (DERs) and loads

• being able to operate independently from the main grid

1. Parallel mode (grid connected)

   • Coordinated power dispatch to local sources (economic optimization)

   • Connected to larger power system through a PCC by implementing synchronization action

2. Islanding mode

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

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

   • Grid-forming converter is necessary

1. Primary

2. Secondary

3. Tertiary

• Controls local variables such as frequency and voltage, establishes power sharing

• Droop controllers at each DER converter

• Cause steady-state grid frequency to deviate from nominal

• Compensation of steady

  state errors in microgrid voltage and frequency

• Active power sharing amongst different DGs

• Reactive power sharing and voltage regulation

• Optimises microgrid operation: economic dispatch

• Controls active and reactive power references for each DER

• Also manages eventual congestions

• Power flow between microgrid and utility grid in parallel mode

1. Primary

   • milliseconds to seconds, responsible for immediate adjustments to match supply and demand.

2. Secondary

   • seconds to minutes, focuses on optimizing the operation of the microgrid over short to medium time horizons.

3. Tertiary

   • minutes to hours or even days, deals with strategic decision making such as scheduling and economic dispatch.

1. Minimal impact as primary control is mostly based on local measurements.

2. Can cause delays in restoring system variables to their nominal values, potentially leading to instability and oscillations.

3. Delays can affect the optimization processes, leading to suboptimal power exchanges and economic inefficiencies.

• Droop gain

• Loading condition

• Line impedances

The idea of synchronverter is to control the inverters to mimic the behavior of Synchronous Generators

• During the the short circuit fault in theMVDC grid based on DAB converters

  • T_0: short circuit occurs

  • T_1 DC link voltage reaches zero

  • T_2 DC link voltage starts being recovered

• Stages:

  • Natural response: Capacitive discharge state (T0 - T1)

  • Free whelling diode stage (T1 - T2)

  • Grid feed-in stage (T2 - fault clearance): Forced response of DC distribution systems

Capacitive discharge stage (stage 1)

• The curve of the discharge current depends on the type of the storage and the passive system parameters (R,L,C)

• The capacitive discharge is the natural response of the dc system and uncontrollable for the converters

Free wheeling diode stage (stage 2)

• This stage exists ONLY when the system parameters in the circuit are in underdamped condition, which means that there is zero crossing in the DC link voltage during decreasing

• When the DC link voltage reaches zero, the residual current in the DC line flushes in the free wheeling diodes, which starts the free wheeling diode stage

• Surge current flowing through the free wheeling diode in this stage can lead to thermal damage on the power electronics devices

Absence of free wheeling diode (stage 2)

• The free wheeling diode stage could also be absent if the system parameters in the circuit are in overdamped condition, which means that there is NO zero crossing in DC link voltage during the fault

• In this case the DC link voltage does not reach zero during the fault and the fault enters grid feeding stage (stage 3) directly from capacitive discharge stage (stage 1)

Grid feeding stage (stage 3)

• The grid feed-in stage starts when the DC link voltage starts being recovered by the increased feed-in of grid side current

• The recovered DC link voltage level in the steady state is depending on the capacity of the grid and also the controlling target of the converters

• The process of linking small groups of industrial, commercial, or residential customers into a larger power unit to make them visible from the electric system point of view.

• Aggregation is a commercial function of pooling

  decentralised generation and/or consumption to provide services to actors within the system.

Small scale power generation and storage technologies located close to the customer side.

• 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 or generation profiles of individual consumers and/or small generators appear as a single unit to the electric system

• DER aggregation enables aggregators to operate DER and to provide services to the power system, e.g. system balancing

• DER aggregation helps implementing smart grids concepts by reaping some of its benefits to integrate DER units more efficiently

Demand Side Management (DSM) is the planning, implementation, and monitoring of those utility activities designed to influence customer use of electricity in ways that will produce desired changes in the utility’s load shape, i.e., changes in the time pattern and magnitude of a utility’s load

• Changes in electric usage by end use customers from their normal consumption patterns in response to changes in the price of electricity over time

• Incentive payments designed to induce lower electricity use at times of high wholesale market prices or when the system is jeopardized

• Includes all intentional modifications to consumption patterns of electricity of end use customers, intended to alter the timing or level of instantaneous demand or the total electricity consumption

• Bottom up approach: begin at the consumer level and work their way up through the system

1. Peak clipping/shaving

2. Load shifting

3. Valley filling

4. Strategic conservation

5. Strategic Load growth

6. Flexible Load shape

• Common Information Model (CIM): Common model for describing the components in power systems for use in a common Energy Management System (EMS) Application Programming Interface (API)

• Purpose: Standardization and Interoperability

• A transformer is represented by a single PowerTransformer container class

• Windings are represented by the objects TransformerWinding within the PowerTransformer container

• Analogue for the TapChanger and TransformerWinding

• The winding is physically connected to the network and conducts electricity → inherits from ConductingEquipment

• Transformers don’t conduct electricity

  → PowerTransformer inherits from Equipment

• TapChanger inherits from PowerSystemResource, as it is not a separate piece of equipment, but a part of TransformerWinding

• Aggregation relationship - a PowerTransformer is made up of 1 or more TransformerWindings

• Aggregation relationship - a TransformerWinding is made up of 0 or more TapChangers

• Connection directly through the windings

• Transformer can be represented with 4 CIM objects:

  • Two TransformerWindings

  • One TapChanger

  • One PowerTransformer

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