PV-systems.pdf

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Fraunhofer IWES Fraunhofer IWES

Slide 1, Power System Service Seminar, 31.03.2011

Power System Services of PV-systems:Requirements, testing and application in Germany

D. Geibel, Dr. G. Arnold, Dr. T. Degner

Fraunhofer Institute Wind Energy and Energy Systems Technology Fraunhofer IWES

DERlab Workshop on Power System Services31.03.2011, Risoe


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Power System Services of PV-systems:Requirements, testing and application in Germany Introduction

Power System Services of PV-systems

Grid code requirements

Test procedures for PV-systems

Application of power system services Voltage control by reactive power

Voltage control by active network components

Conclusions


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1. IntroductionGenerating capacities in Germany

Generation capacity inGermany

137.5 GW (in 2009) intotal

46 GW of RES

Wind

Increase since 1994

25 GW in 2009

PV

Increase since 2004

17.3 GW in 2010

52 GW in 20201 Diagram: Fraunhofer IWES, Data: Erneuerbare Energien in Zahlen, BMU, 2010

1: National development plan for renewable energies


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1. IntroductionInterconnection of PV-systems

Interconnection of PV-systemsmainly to LV due to power

ratings

Interconnected of PV-systems toHV or MV

18%1 (end of 2008)

Jan. 2009 to Sept. 2010

9335 MWp new installed PV-power

Share of PV-systems withrated power > 100kWp: 3278MWp

Grid integration of PV ondistribution system level

Diagram: Fraunhofer IWES; Data: Information of new installed PV-systems of BNetzA

1EEG Statistikbericht 2008; BNetzA


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1. IntroductionGrid integration constraints

Power System Stability

Rotor angle stability

Frequency stability

Voltage stability

Network Constraints

Capacity of lines, cables,transformers

Power quality

Steady-state voltage limits

Harmonics

Flicker

Source: Definition and Classification of Power System Stability, IEEE/CIGRE JointTask Force on Stability Terms and Definitions, IEEE Transactions on Power Systems,

Prabha Kundur et. al.


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1. IntroductionGrid integration challenges and approaches

Transmission network (HV/EHV)

Challenges Power generation not located to

load centers

Integration of offshore windparks

Approaches

Grid reinforcement

Power System Services of DERunits

Central storages

FACTS

Distribution network (MV/LV)

Challenges Bidirectional power flows

Voltage control

Approaches

Power System Services of DERunits

Active network components

Manageability of loads

Local storages

Grid reinforcement


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2. Power System Services of PV-systemsGrid code requirements Overview

Pre-condition for connection

Fulfilling of technical minimum standards concerningelectrical behaviour (6 EEG)

Link to grid codes

Approval of Conformity by certificates (64 EEG)

Grid codes in Germany

Describe behaviour of DER units in order to meet systemneeds

High voltage / Extra high voltage

Transmission Code 2007

Medium voltage

BDEW MV-guideline

Low voltage

E VDE-AR-N 4105 (Draft version)


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Distribution code DER plants connected MVdistribution grid

Technical requirements for DER plants (steadystate / transient)

Basic procedure for approval of conformity

Link to Technical Guidelines of FGW for verification

of requested electrical behaviour

FGW TR3: Testing procedures

Link to IEC 61400-21

Adaptation of test procedures for PV

FGW TR4: Modelling

FGW TR8: Certification


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Fraunhofer IWES Fraunhofer IWESSlide 9, Power System Service Seminar, 31.03.2011


Static requirements

Limitation of power-qualitycharacteristic parameters

Harmonics, interharmonics andhigher frequency components

Flicker

Switching operation

Active power control

Active power reduction bynetwork operator

Active power reduction at over-frequency

Reactive power control

Dynamic requirements

Fault-ride-through (FRT) capabilitywith no disconnection of DER plantsduring the voltage dip

No change of active powergeneration after faults

Feed-in of reactive power during thefault for voltage stabilization

Limitation of short-circuit current


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2. Power System Services of PV-systemsGrid code requirements Active power control

Automatic active powerreduction depending on grid

frequency

Grid support in case ofpower surplus

Avoiding grid instabilities

due to immediatedisconnection of largegeneration capacities

Under normal gridconditions no impact on

energy yield For inverter no additional

hardware required


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2. Power System Services of PV-systemsGrid code requirements Active power control

Measurement results of aPower-One PVI-55.0 Central

inverter at Fraunhofer IWES

Start of reduction at fGrid >50.20 Hz

Instantaneously available

power at 50.20Hz is used foractive power reductioncalculations

Amount of active powerreduction is determined by

droop with of 40% per Hz Internal hysteresis: Rerise

of active power if fgrid 50.05 Hz

Power-One PVI-55.0 CentralMeasurement results Fraunhofer IWES


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2. Power System Services of PV-systemsGrid code requirements Reactive power capability

Maintenance of grid voltage

Generating unit must beable to provide reactivepower during normaloperation

Required power factor cos

at network connection pointof plant:

0.95underexcited to0.95overexcited

Impact on inverter design

Apparent power has to beincreased



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2. Power System Services of PV-systemsGrid code requirements Fault-Ride-Through FRT)

Source: Erzeugungsanlagen am Mittelspannungsnetz. BDEW, Release June 2008

Generating units must stay connectedduring grid faults

Different FRT-curves

Type 1: Direct coupled synchronousgenerators

Type 2: all other generating units

Requested behaviour of generating unitdepends on mainly two factors:

Depth of voltage dip

Duration of voltage dip

Behaviour of unit:

Stay connected during fault

Short time disconnection withresynchronisation within 2s at most

Reactive current injection + shorttime disconnection

No requirements


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Reactive current injection of unitaccording to Transmission Code 2007

Amount of reactive power isdetermined by k-factor

k = Ib/In/ U/Un

Depth of grid fault

Reactive current before fault

Voltage before fault Response time 20 ms

Max. reactive current: Ib 1.0 In

Max. duration of reactive currentinjection: time of fault clearing +500ms

In terms of unsymmetrical faults arelease of overvoltage protection hasto be avoided


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Influence of reactivecurrent on grid

voltage during fault

For both cases thesame voltage dip isused

k-factor 0: noreactive current

k-factor 2: due toreactive currentvoltage is raised



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2. Power System Services of PV-systemsGrid code requirements Implementation

Source: SMA Solar Technology AG


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2. Power System Services of PV-systemsTest procedures for PV-systems Testing environments

Recuperation unit

AC-network simulator

Impedance network

PV-simulator AC-network simulator

with additional

impedance network

90 kVA

Linear amplifiers

4-quadrant

operation PV-simulator 30kW

Special testingequipment for PV-systems with higher

power ratings


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6 MVA FRT-container (MV)

1 MVA AC-network simulator (LV)

1 MW DC-source

Signal generator

Loads (resistive, inductive, capacitive)

LV and MV test networks


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2. Power System Services of PV-systemsTest procedures for PV-systems Testing procedures

Test of behaviour of DER unitsduring grid faults normally donewith FRT-containers

FRT containers designed for highpower ratings (100 kW < P < 6 MVA)

Not applicable for string inverters (P< 20kW)

NS-FRT tests with network simulator

Calculation of voltage phasors at

LV network for faults (3phaseand 2 phase) in HV and MVnetworks

Real reproduction of voltagecurves with low ohmicprogrammable AC-networksources

Real emulation of networkimpedance by adjustable R-L-network

1_HS

1_MS

2_MS 2_NS

110

kV

Netz

20 kV

Kabel

0,4 kV

Kabel

110 kV

FL

EUT


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2. Power System Services of PV-systemsTest procedures for PV-systems Testing procedures

Unbalanced 2 phase fault

MV/LV transformer: Dd

Unbalanced 2 phase fault

MV/LV transformer: Dy


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3. Application of power system servicesVoltage Control

Source: Fraunhofer IWES, Project Aktives, intelligentes Niederspannungsnetz


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3. Application of power system servicesVoltage Control Reactive power

Selective voltage changeby power factor control of

DER unit

k: network impedanceangle

SSC: Short circuit power

SA,max: nominal power ofDER unit

( )

SC

kVA

aVS

S

u

+

=

cosmax,

Source: B. Valov, Volle Nutzung der Netzkapazitt und Spannungsstabilisierung durch neues Auslegungskonzept fr PV-Kraftwerke, 24.Symposium Photovoltaische Solarenergie, Bad Staffelstein, 2009


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3. Application of power system servicesVoltage Control Case studies

Generic network case studies

Case A: Single feeder network,

single PV-system

Case B: Single feeder network, fivePV-systems

Case C: Four feeders, each with 5 PV-

systems Different feeders length are

considered (100m, 300, 600, 900m)


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3. Application of power system servicesVoltage Control Potential of reactive power

Case C: 4 feeder, 5 PV-systems perfeeder, cos =0.9, max. voltage rise3%

Relative increase of connectable PVsystems due to reactive powervoltage control varies between 1.5and more than 2.

At larger network impedance angles

(resp. short feeder) the thermallimits of the network transformer orthe cables may be reached.

Example (Sk = 1,2 MVA, k = 25 ,4x5 PV-systems)

cos = 1.0: 216 kW in total

cos = 0.9: 378 kW in total(factor of 1.75)

0

0.5

1

1.5

2

2.5

3

3.5

4

15202530354045network impedance angle []

max.PVpow

er@

cos

=0.9/

max.PVpower@cos

=1

[k

W/kW] cos0,9u

cos= 1

P_PV > thermal limit ofcables

P_PV > SrT (630kVA)


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3. Application of power system servicesVoltage Control Active network components



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3. Application of power system servicesVoltage Control Active network components

Extended Use of the available voltageband

Case C: 4 feeder, 5 PV-systems per

feeder Switchable MV/LV transformers provide an

alternative way to control the voltage inLV networks.

transformer with steps 2x 2.5%

allowed voltage rise increases from 3%to 8%

max. connection power increase by afactor of 2.7

In particular useful for longer feeders

Example (Sk = 1,2 MVA, k = 25 , 4x5 PV-systems)



Smart transformer: 538 kW in total

10

20

30

40

50

60

70

80

15202530354045

network impedance angle []

P_

PV

/Sk[%]

cos = 0,9u

switchable MV/LV

transformer

cos = 1

P_PV > SrT transformer

thermal lim it of cable NAYY


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Backup


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3. Application of power system servicesVoltage Control Effect of network parameters Effect of different cable types

Sk and k decrease with increasing distance to the substation

The cable type determines how fast Sk and k decrease

At long distances the effect of the cable type dominates

Example: 600m distance, NAYY 4x150mm Sk = 1,2 MVA; k = 25

0

1

2

3

4

5

6

0 200 400 600 800 1000

distance [m]

netw

orkshort-crcuitpower[MV

A]

NA2XRY 4x70mm

NAYY 2x4x150mm

NAYY 4x95mm

NAYY 4x150mm

NAYY 4x240mm

10

15

20

25

30

35

40

45

50

0 200 400 600 800 1000

distance [m]

networkimpedanceangle[

NA2XRY 4x70mm

NAYY 2x4x150mm

NAYY 4x95mm

NAYY 4x150mm

NAYY 4x240mm


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3. Application of power system servicesVoltage Control Maximum permissible connection power Power factor cos =1, max.

voltage rise 3%)

Sk

and K

at the end of thefeeder are sufficient todescribe maximumpermissible PV power

The serial placement of PVsystems leads to a higheramount of connectable

power because of the lowervoltage rise of the PVsystems connected tostronger network points

Case C (parallel feeders)results in similar curves, but

per feeder PV power islower, because of thevoltage rise in thesubstations LV-busbar

1

2

3

4

5

6

7

8

15202530354045

network impedance angle []

(feeder length 100m to 900m)

P_PV

max/Sk[%]

5 PV Systems, Transf.

630kVA uk 6% and cables 5 PV Systems, Transf. 630kVA,

uk 4%, and cables 4x150mm

5 PV Systems, Transf.

630kVA, uk 4% and cables

4x95mm1 PV-System, transformer

630kVA, uk 4% and cables

4x150mm1 PV System, transf. 630kVA, uk

4% and cables 4x95mm

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