simulation of power systems with large share of renewable generationdigsilent.pdf · simulation of...

Simulation of Power Systems with Large Share of Renewable Generation

International Seminar on „Impact of Generation from Renewable Sources

on Conventional Power Generation and Grid“

31st January 2014, New Delhi - India

Flavio Fernández DIgSILENT GmbH, Germany

1

Overview

• Background

– What makes renewable generation different?

– Integration challenges from the power system analysis perspective

• Power system analysis functions

– Overview typical analysis needs

– Simulation tools for the steady-state and time domain

• Modelling aspects

– Considerations about dynamic models of renewable generation

• Conclusions

Impact of Generation from Renewable Sources on Conventional Power Generation and Grid, New Delhi -31.01.2014 2

Integration challenges

• Wind and solar energy is non-dispatchable and less predictable

– Limited prediction accuracy using weather forecasting

– Influenced by geographical distribution

– Impact on capacity building

– Impact on grid expansion engineering and strategic planning

• Power output inherently variable

– Increased variability (of the net load) in the power system

– Stepper ramp rates with increasing penetration levels

– Needs for higher load-following flexibility (interconnection between

balancing areas, demand response, smart grids)

– Impact on the system operation procedures



• Renewable energy is mainly based on inverted fed generation

systems that gradually replace traditional synchronous generation

– Technology allows a higher control flexibility

– New dynamic characteristics, influencing short-circuit currents and

network stability

• Wide range of manufacture-specific technical solutions in order to

comply with grid code requirements worldwide

– Testing and validation of simulation models has become necessary to

guarantee high model accuracy

– Benchmarking


Smarter transmission network

• System operation

- Optimized operation close to

real time

- Efficiently and accurately

create the required system

configuration

- Increased data volume



Variable generation

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MW

Large generation

Inflexible generation

Active distribution networks

Smart grids &

meters,

energy

storage

Active demand

Time of use tariffs

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Time of Day

Ele

ctr

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2020 Demand ~ 15GWh (daily) - 1.5million vehicles

Typical winter dailydemand

Pe

ak

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uti

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12,000 miles p.a.

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ak

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Optimal Charging

Period

Distributed generation • System planning

- Efficient use of existing

transmission capacity (getting

closer to technical limits)

- Investment in new transmission

assets

Increased need for detailed network analysis


Grid Safety Analysis; DACF, D2CF Congestion Management

ATFA

Scheduled outages Annual, month, week LDF, N-1

Outage planning Operational planning Control Center

D-365 D-1 D+1 D-7 D

Long Term Planning LDF, N-1, SHC, Protection, etc

Strategic planning Operation

D-years

Intraday operation

EMS

PowerFactory

Dynamics, transients, harmonics, etc

System integration: architecture solution


Power System

Analysis Functions


Load flow analysis

• Stochastic load flow calculations

– Inherent variability of renewable

generation

– Time dependent characteristics,

based for instance on wind speed

measurements or solar radiation

patterns

• Quasi-dynamic simulation

– Medium to long term simulations

based on steady-state analysis

– Synthetic signals via flexible

scripting functions

– Statistical analysis (e.g. network

losses)


Contingency analysis

• Work out post-fault actions to secure

network operation under (n-x)

contingencies

– Optimization of phase shifters,

transformer taps, reactive power

compensation devices, etc.

• Simulation model shall account for:

– Dynamic circuit ratings (short term and

continous ratings)

– Wide area balancing mechanisms

(power/frequency controllers)

– Demand side response / smart grids

• … hence maximize usage of existing

transmission assets


Short-circuit calculation

• Dynamic voltage support (reactive power injections) not currently

supported by standard short-circuit calculation standards (IEC/ANSI)


Short-circuit calculation

• Static generator model in PowerFactory

– Dynamic voltage support acc. to k-factor settings of the converter, for

instance acc. to ENTSO-E

– Active for transient current calculation: Iks

– Based on a current iteration method


Du

imax

10%

k

1

i

Capacity value and reserve margins

• The Capacity value is the measure of the “firm” capacity of a

renewable source that can be counted on as a reliable contribution to

the sum of all grid capacity

– „Firm“ capacity is the generation capacity (MW) available at all times

– Capacity Credit is usually expressed in percentage of the rated power.

• The capacity credit of a variable power source is system dependent

parameter determined by:

– Penetration level of variable power sources

– Availability of conventional generation

– Load

– Transmission constraints



• Purpose of the calculation:

– General assessment of the value of variable generation.

– Capacity Planning.

– Generation Reliability assessment.

– Remuneration strategies definition

• Determination of adequate reserve margins (or reserve capacity)

– Available capacity above the normal capacity needed to meet peak

demand

– Regulatory bodies usually require producers to maintain a constant

reserve margin of 10-20% of normal capacity



• Calculation based on reliability-based model of the power system

– Probabilities for each generator to drop out

– Variable power source model based (non-dispatchable generation)

– System Load curves

• Loss of load probability (LOLP, typically <10%, on seasonal peak):

– Probability that yearly/seasonal peak load cannot be covered by available

generation capacity.

• Loss of load expectancy (LOLE, typically < 6 hs/year))

– Expected number of hours per year (time probability) during which the

available capacity cannot cover the load.


Harmonics analysis and flicker emission

• Harmonic calculation, as traditionally used for

distorting loads, may lead to excessively

conservative results

• Therefore, alternative methods have been

proposed (as eg. IEC 61000-3-6):

– Summation laws with weighting factors to

better assess the contribution of large wind

parks or distributed generation

• Voltage Flicker Assessment

– Short- and long term flicker emission factors

– Relative voltage changes


,1

N

h m hm

U U

,1

N

h m hm

I I

Resonances


• Risk of parallel and/or series resonances

– In particular offshore wind farms have vast underground cable systems

and reactive power compensation equipments

– Resonances may therefore arise due to the interaction with the power

system (inductive)

– Frequency scans can be used to identify resonance frequencies

• Special attention shall be driven to:

– The impedance of the network at the PCC can vary significantly (large

number of system operation scenarios, outages, etc.)

– Impedance locus of the network at PCCs shall be used instead of

classical max/min short-circuit power equivalent

– Automatic frequency scan for all possible configurations

Stability analysis

• Fault ride-through (FRT) capability

– With increasing penetration levels, the system cannot afford a large loss of

renewable generation

– Simulations shall verify that the generator stay connected during a grid

fault causing a voltage dip at the PCC (requirement acc. to grid codes)

• Frequency response

– Response of the renewable generation to a change in the grid frequency

– Contribution to frequency control (primary control, synthetic inertia)

– Minimum frequency range of operation (50.2 Hz problem)

• Small Signal Stability

– Assist in damping of power system oscillations (enough synchronizing

torque in the system?)


Stability analysis: fault ride-through


• Assessment of grid code requirements, typically:

– Do not disconnect above the red line

– Max active power recovery rate

– Temporary disconnection and re-synchronisation

• Analysis is done by means

of time domain simulations

– Accurate dynamic models

of the renewable

generation required

• RMS/EMT time domain simulations

– Multi-phase AC/DC networks

– Fast, adaptive step-size algorithms

– A-stable numerical integration algorithms

– Simulation events of any kind of event

• Small signal analysis

– Selective eigenvalue calculation


Stability analysis: simulation

Dynamic models of renewable generation

• Development of dynamic model not an easy task

– Wide range of control strategies to comply with grid code requirements

worldwide

– Technology is continuously evolving, so that models have to be

continuously updated

– The model shall be adequate to the simulation purpose (trade-off between

accuracy and complexity)

• Several efforts worldwide started

– IEC TC88 WG27 working group on wind generation models

– Western Electricity Coordinating Council (WECC), Renewable Energy

Modeling Task Force on wind and PV generation models


Dynamic models of renewable generation

• Vendor-specific models

– Normally available from the manufactures

– Highly accurate models

– Not always publicly available but may require non-disclosure agreements

– In PowerFactory, models of all major manufactures are available

• Generic models

– Not vendor specific

– It can be parameterized to reasonably simulate the dynamic behavior of

the generator

– Standard generic models are needed for portability across different

software simulation packages and data exchange between TSOs.


Dynamic modelling environment in PowerFactory

• Models of renewable generation are implemented in PowerFactory

using the dynamic modeling environment DSL (DIgSILENT Simulation

Language)

– General-purpose dynamic modeling language allowing to describe any

system of linear or non-linear differential equations

– Modeling environment with a graphical editor, with which any model can

be defined by drawing a block diagram

• Especially in wind generation applications, a flexible modeling

environment is essential because of the large variety of generator

types and control concepts used by different turbine manufacturers

that have to be reflected in the corresponding models.


Wind Power Integration – Modelling in PowerFactory

Dynamic Modelling Environment in PowerFactory

PLL ElmPhi*

Fmeas

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Vac StaVmea*

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PQ StaPqmea*

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PQ Control ElmDsl*

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Slow PLL ElmPhi*

Active

Po

we

rRed

uctio

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Elm

Dsl

Generator ElmGen*,ElmVsc*

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Iac StaImea*

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Current Controller ElmDsl*

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WTG with fully rated converter:

u

Qin

Pin

Fm

ea

s

sinref

iq_ref

pred

cosref

id_ref

ii

ir

u1i_in

u1r_in

24

S

G/

IG

Step Up TransformerGrid Side Converter

Conclusions

• High integration level of renewable generation challenges classical

power system planning and operation

– In general, increased need for detailed system analysis and optimization

– Analysis automation (to allow system optimization closer to real time

operation)

– Higher level of integration of power system simulation tools with external

systems (EMS, DMS, weather forecast systems, etc.)

• Large variety of power system analysis functions available such as

steady state, quasi-dynamic, dynamic and transient network analysis

• Ongoing efforts towards the standardization of dynamic models of

renewable generation (wind/PV generators)



Thanks for your attention

Flavio Fernández

DIgSILENT GmbH

[email protected]

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