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Integration of High Penetration of Solar and Wind Power in Power Systems:
Experiences and Challenges Lecture 3-4 + Tutorial 2
Lennart Söder Professor in Electric Power Systems
KTH, Royal Institute of Technology, Stockholm, Sweden
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Set-up of Lectures L3-L4 + T2
Lecture L3: Solar power, examples and general challenges Lecture L4: Voltage control in low voltage grids with solar power Tutorial T2: Application of voltage control in radial and meshed grids with solar and wind power
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Aim of a power system 1. The consumers should get the
required power (e.g. a 60 W bulb), when the push the on-button. This should work no matter there is an outage in a plant, wind is changing etc. = keep a balance between total production and total consumption.
2. The consumers must have a realistic voltage, e.g. around 230 V, in the outlet.
3. Point 1-2 should be obtained at a realistic reliability. This is never 100,000... percent,
4. Point 1-3 should be obtained in an economic and sustainable way.
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Wind power integration challenge
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Solar PV at Ekerö - Stockholm 28 August 2017
Lennart Söder Professor Electric Power Systems, KTH
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”Renewable private house”
Wood burning for heating and solar power for electricity
5-6 tons/year min 4,9 kWh/kg
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Measurements at Lennart Söder: (ca 4300 kWh/year) (Nordpools price/hour for SE3 + certifikats + 3.5 öre/kWh + 390 SEK/year)
7
Energy invoice (competition): ≈140 actors
Grid invoice (monopoly)
• Sends data one time per day
• Sends hourly data • Uses GSM (GPRS)
+ El.tax + VAT
• Surplus caused by PV
• Sold for Nordpool price per hour
• Additional 60 öre/kWh in tax reduction
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Meter in January:
8
1 - 16 January 5 januari
+ tax + VAT
Measurements at Lennart Söder: (ca 4300 kWh/year) (Nordpools price/hour for SE3 + certifikats + 3.5 öre/kWh + 390 SEK/year)
New meter:
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Lennart Söders solar PV: Ekerö / Ellevio
• 4,5 kW (PV-max and converter) • Installed 17-19 July 2017 • Ca 4300 kWh/year ≈ yearly
consumption • 25 m2
Converter from PV (DC) to grid (AC)
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Production: 17-20 August, 2017 [Wh/5-minutes]: 300 3,6 kW
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Production: 17-20 August, 2017
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Production: 22-24 August, 2017
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23-aug
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Solar PV production Aug 22-24, 2017 [Wh/5-min]
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Three phase voltages: 22 August, 2017
226
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0
22 Aug - phase A [V]
22 Aug - phase B [V]
22 Aug - phase C [V]
Three phase voltages Aug 22, 2017 [V]
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400
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22-aug
22-aug
Production 22 August
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Three phase currents: 22 August, 2017
0
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22-aug
22-aug
Production 22 August
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22 Aug - phase A [A]
22 Aug - phase B [A]
22 Aug - phase C [V]
Three phase solar PV currents Aug 22, 2017 [A]
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Solar power (preliminary) economics
Costs: • Investment cost: Panels, construction, converter, grid connection.
Borrow funding from bank and/or do not use available money for other issues: Interest rate essential!
• Installation cost: Do it yourself or hire persons to do it • Possible subsidy: Can reduce investment cost • Possible grid connection fee: Who pays for grid? Consumers and/or
producers? Income: 1. Reduced purchase: Some of the solar power reduces purchase. Purchase
can include energy cost, grid tariff, taxes etc 2. Surplus: When production is higher than local demand this is fed to the
system. Net-metering Same value as for reduced purchase. At specific measurement Power price of some kind, sometimes feed-in tariff
3. Subsidies: Different types: Certificates, Guaranties of Origin, etc
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Ekerö solar power (preliminary) economics
Investment cost: 89500 SEK (PV-panels: 41720 (47%), other inv.: 17880 (20%), installation: 29900 (33%), ROT-decrease (-8055 SEK). Probably later INSTEAD a subsidy of 20% (-18350 SEK) Total cost of 0,8*90700 = 72560 SEK. (maybe -30% ?) Income: 1. Reduced purchase: 1,26 SEK/kWh 2. Surplus: Nordpool price (25 öre/kWh) + Certificates (5 öre/kWh) +
Guarantees of Origin (0 öre/kWh) + Tax reduction (60 öre/kWh) = 0,90 SEK/kWh
3. Assume 70% replaces consumption mean value = 0,70*1,26 + 0,30*0,90 = 1,15 SEK/kWh. 4300 kWh/year 4954 SEK/year.
Result: • 72560/4954 = 14,6 years • Or: ”Yearly interest rate of 5018/72560 = 6,8% which is much higher than
putting money in the bank,
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Solar power in distribution grids
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- 18 -
Steady state voltage limits
• Nearly all PV plants < 100 kWp in DE (total > 10 GWp) are connected to LV networks.
• Thermal limits of network assets (transformers, lines etc) are in many cases not the limiting factor!
• Violation of the voltage change (rise) is the main limiting factor! The steady state voltage change dU due to all generators connected to an distribution network may not changed by more than 3% (LV) resp. 2% (MV) compared to the situation without generation
PV
PP P
MS-Netz 20 kV
Trafo
0,4 kV Leitung HAS 1HAS 2
Last 1Last 2
PV
U
Länge
P
3~~
1,1 p.u. = 253 V
1,0 p.u. = 230 V
Hohe Einspeisung durch PV bei schwacher Last
Max. Last (mit Spannungstellung)0,9 p.u. = 207 V
Netzstation
Source: B. Engel, „Lastflussumkehr in Verteilnetzen durch dezentrale Einspeisung erneuerbaren Energien - Paradigmenwechsel und technische Lösungen, 22.4.2009
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An Overview from Control Structure of Double Stage PV Generator
PV Panels String
DC-DC Boost
DC/AC PWM
VSI
L\LC\LCL Lowpass
Filter
Tras. &
Grid
MPPT
Voltage control
Current Control
PLL, Synchronization
PWM1 PWM2
Anti-islanding
Grid V&f Support Active filter capability
Other Control possibilities
Temp.
19/10
MPPT =Maximum Power Point Tracking
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PV MPPT PWM DC-DC Converter
DC-Voltage Controller
abc
dq
abc
dq
PLL
dq
abc
PWM
d(t)
L\LCL Filter
PLL Operation
-
-
-
- DC/AC
20/10
No frequency & Voltage Power Control
AC-Voltage Controller
50,2 Hz problem in Germany
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Large Solar Power Integration in Local and Regional Grids (KTH PhD thesis: Afshin Samadi)
Research contribution: • Voltage control of distribution grids via reactive
power support of PVs
• A static equivalent of distribution grids with high PV penetration level
Results: • Developing coordinated active power dependent
(APD) voltage regulation, Q(P) • Developing coordinated droop-based voltage
(DBV) regulation, Q(V) • Developing a static equivalent model of
distribution grids with high penetration of PV systems embedded with a voltage support scheme
German Grid Code for PV
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© Fraunhofer IWES
22
Solar eclipse over Europe on 20 March 2015
http://eclipse.gsfc.nasa.gov/SEgoogle/SEgoogle2001/SE2015Mar20Tgoogle.html
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© Fraunhofer IWES
23
http://eclipse.gsfc.nasa.gov/SEgoogle/SEgoogle2001/SE2015Mar20Tgoogle.html
Solar eclipse over Germany on 20 March 2015
[source: HTW Berlin]
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© Fraunhofer IWES
24
Some Results
Estimated average maximum values of three studies 1. Worst Case:
• Decrease 11 GW, -11 GW/h, 250MW/min • Increase 19 GW, +15GW/h, 350MW/min
2. Best Case • Decrease 1.3 GW, 1.3 GW/h, 25MW/min • Increase 4 GW, 3.2GW/h, 65MW/min
[source: HTW Berlin und Fraunhofer ISE]
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© Fraunhofer IWES
25
PV Measurement & Forecast
-7.5 GW + 14.4 GW
+ 12.4 GW/h
- 7.5 GW/h
- 2.7 GW/15min (188MW/min)
+ 4.3 GW/15 min (~286MW/min)
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Some comments/challenges
1. Depending on load at maximal local PV production, there can be a “need” of more grid.
2. Voltage can be controlled in PV converters. Today “German Grid Code” is used, but flexible parameters can be settled.
3. There is a balance between how much the “grid capacity” can be handled by PV-converts and/or if grid has to be expanded or use of other voltage control equipment
4. A question is if it is acceptable to “spill” solar power. Probably economical at some level, but depends on regulation
5. In Germany it is said that there are 100000 batteries, since there are requirements of “local balancing” reduced need of grid.
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KTH ROYAL INSTITUTE OF TECHNOLOGY
Active / Reactive Power Control in EWR Grids High Penetration of Rooftop PV and Wind
Poria Hasanpor Divshali [email protected]
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28
Problem Definition
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Inverse active power High integration of RESs leads to reverse active power
29
Prob
lem
Def
initi
on
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Voltage profile
30
Reverse active power change the voltage profile Limit the Hosting capability
Overvoltage boundary Pr
oble
m D
efin
ition
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Standard EN 50160 Voltage Characteristics of Public Distribution Systems
• Static Characteristics
– @ LV and MV grids – mean 10-minutes voltage rms ±10% ; for 95% of week
• Dynamic Characteristics
– @ LV grid – Rapid Voltage change < 5%
31
Prob
lem
Def
initi
on
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Hosting Capacity (HC)
32
Prob
lem
Def
initi
on The amount of power could be produced by PVs
installed in LV and MV grids without causing any technical problem, is limited.
,
,max
min max
L, , BSS,i L, , BSS,
t,
. .
( ) I ,( ) ,
( ( ) ( ) P ( ), ( ) ( ) ( )) 0.
Max PV ii
k k
i
i i i PV i i i PV i i
HC P
s t
I tV V t VF P t k P t t Q t k Q t Q t
= ∀
<
< <− − − − =
∑ Maximizing the HC
Current limit Voltage limit
Power flow equations
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Solutions
Grid reinforcement On-load tap changers for MV/LV
Active power curtailment
Demand side management
Storage system & Electrical vehicles
Reactive power control of RES
33
Prob
lem
Def
initi
on
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Solutions
Grid reinforcement On-load tap changers for MV/LV
Active power curtailment
Demand side management
Storage system & Electrical vehicles
Reactive power control
34
More Economical Pr
oble
m D
efin
ition
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35
Grid Specification
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Network introduction
36
Inve
stig
ated
net
wor
k
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Feeder 1 Feeder 2 Total feeder length (km) 22.3 19.5 R/X ratio in MV grid 0.9 - 1.7 0.9 - 1.7 Customer # Small-scale 3700 2900 Large-scale 12 20 Max. load in 2015 (MW) 3.7 5.7 Total consumption Small-scale (GWh) 15.6 12.2 Large-scale (GWh) 1.2 19.2 RES penetration level Total (%) 76 88 LV connected (%) 32 13 MV connected (%) 44 75 LV connected PV Number 278 189 Installed capacity (MW) 4.4 3.2 MV connected RES Type / Capacity (MW) Solar / 7.3 Wind / 9.6 Distance to HV sub (km) 18.7 18.2 Max. revers power (MW) 7.2 7.9 Total production LV connected (GWh) 5.4 3.97 MV connected (GWh) 7.4 23.55
Network details In
vest
igat
ed n
etw
ork
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Maximum Voltage profile - Feeder #1
38
Inve
stig
ated
net
wor
k
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Maximum Voltage profile - Feeder #2
39
Inve
stig
ated
net
wor
k
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40
Load & Generation Profile Estimation
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Estimation method
1- Measurement of large units
2- Estimation of consumption profiles by Standard Load Profiles (SLP)
3- Estimation of Small-Scale production using the nearest large production profile
4- Detailed LV Model or aggregated load model in substations
5- Determine the PF by minimizing the reactive power error
6- Primarily Substation Model (controller of OLTC)
41
Est
imat
ion
met
hod
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42
Model Validation
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Active power validation Active power flow in a secondary substation located in feeder 1, one km away from the solar park.
43
Mod
el V
alid
atio
n
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Active power validation Active power flow in feeding point of feeder 1.
44
Mod
el V
alid
atio
n
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Reactive power validation
Reactive power flow in feeding point of feeder 1.
45
Mod
el V
alid
atio
n
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Voltage validation Voltage profile of a secondary substation located in feeder 1, one km away from the solar park.
46
Mod
el V
alid
atio
n
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Voltage validation Voltage profile of the solar park in feeder 1.
47
Mod
el V
alid
atio
n
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Central Battery Storage System
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Battery storage system
Battery bank • Price: 250-1350 (Euro/KWh) depend on technology
Convertor • Price: 200-600 (Euro/kVA)
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Reactive Power Control
Active Power Control
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Major Challenge High cost
• Sizing the BSS (Battery Bank & Convertor Unit) • Placement • Control of system based on generation and demand
profiles
Previous works: • Use sensitivity analysis for placement (Do not Work) • Neglect the reactive power injection • Not select convertor size separately from storage size
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ESS
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Major Challenge High cost of ESS
• Sizing the BSS (Battery Bank & Convertor Unit) • Placement • Control of system based on generation and demand
profiles
Previous works: • Use sensitivity analysis for placement (Not Work) • Neglect the reactive power injection • Not select convertor size separately from storage size
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Using Optimization to maximize the Hosting capacity
ESS
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Optimization Formulation
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( ) ( )( ) ( )( )
( )( )
( )( ) ( )
( ) ( )
, j ,
, j ,,
L, , BSS,i L, , BSS,
,max
min max
Max. , , , ,SOC(0),S
. .( ( ) ( ) P ( ), ( ) ( ) ( )) 0,
I ,
,
Charging 1
Discharging
BSS BSS j
BSS BSS j M MP t Q t
i i i PV i i i PV i i
k k
i
BSS
BSS
HC P t Q t j SOC
s tF P t k P t t Q t k Q t Q t
I t
V V t V
SOC t P tSOC t P t
SOC t
η
η
− − − − =
<
< <
++ =
−
( )( ) ( )
( )( )
( ) ( )2 2
,
,
0 ,
,
,
.
m M
m BSS M
m BSS M
BSS BSS M
SOC SOC t SOC
SOC T SOC
P P t P
Q Q t Q
P t Q t S
≤ ≤
=
≤ ≤
≤ ≤
+ <
Maximizing the HC
Grid Constraints
Battery Constraints
Convertor Constraints
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LV grid
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LV 2 Specification Number of LV bus node 283 Number of customer node 142 Number of Junction node 141 Number of PV plants 18 Capacity of PV plants (kW) 151
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The maximum HC LV grid voltage in the critical day at the maximum HC without BSS (24 profiles for t=1 to 24)
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Several Critical Busses
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Battery and Converter Size (1) The HC improvement (%) in different battery size (SOCM) and the convertor size (SM)
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SOCM (kWh) SM (kVA) 0 20 40 60 80
20 10,1 11,9 12,1 12,3 12,3 40 16,4 17,9 18,5 18,8 19,0 60 22,2 24,0 24,7 24,7 24,7 80 28,2 29,2 29,5 29,5 29,5
100 34,4 34,5 34,5 34,5 34,5 120 38,7 38,8 38,8 38,8 38,8 140 43,1 43,2 43,1 43,1 43,1 160 47,6 47,6 47,6 47,6 47,6
Case 1
Case 3
Case 4
Case 2
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Battery Li-Ion: 1000 €/kWh Convertor: 350 €/kW
• Case 1: 80 kWh battery bank, 20 kVA converter (87 k €)
• Case 2: 20 kWh battery bank, 80 kVA converter (48 k €)
• Case 3: 80 kWh battery bank, 80 kVA converter (108 k €)
• Case 4: 160 kVA converter without battery bank (56 k €)
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Battery and Converter Size (2)
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The Optimum active and reactive output power in different cases
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Battery and Converter Size (3)
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The maximum voltage of the LV grid (critical bus voltage) during time
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Battery and Converter Size (4)
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The minimum voltage of the LV grid during time
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Battery and Converter Size (5)
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The cumulative R/X ratio
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HV MV1 MVi MVi+1
MV Line 1
MV2
LV1 LVk LVk+1LV2
MV Line i
LV Line 1 LV Line k
Pinj, Qinj
,k , , , , .LV HV c k inj k c k inj kV V V R P X Q∆ = − +
, ,1 ,i / ,1 ,k 1
, ,1 ,i / ,1 ,k 1
... ... ,... ... .
c k MV MV MV LV LV LV
c k MV MV MV LV LV LV
R R R R R RX X X X X X
−
−
= + + + + + +
= + + + + + +
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The cumulative R/X ratio
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The maximum current change in MV/LV Transformer
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Battery and Converter Size (6)
SOCM (kWh) SM (kVA) 0 20 40 60 80
20 0,48 -0,49 -0,86 -1,15 -1,34 40 1,04 -0,29 -0,97 -1,50 -1,79 60 1,68 0,14 -0,61 -0,61 -0,61 80 2,39 1,64 1,35 1,34 1,34
100 3,20 2,38 2,04 1,86 1,86 120 4,08 3,15 2,69 2,48 2,46 140 5,04 3,93 3,47 3,12 3,12 160 6,09 4,70 4,25 3,92 3,83
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Energy loss of the LV grid (%), in regard to the total energy produced by PVs. (η=90%)
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Battery and Converter Size (7)
SOCM (kWh) SM (kVA) 0 20 40 60 80
20 0,135 0,139 0,143 0,148 0,153 40 0,141 0,145 0,148 0,154 0,158 60 0,151 0,154 0,157 0,162 0,167 80 0,165 0,158 0,160 0,166 0,167
100 0,182 0,180 0,179 0,181 0,180 120 0,203 0,206 0,198 0,195 0,187 140 0,229 0,231 0,232 0,206 0,210 160 0,258 0,262 0,264 0,242 0,229
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• Long distance to substation (large cumulative impedance)
• Closeness to large PVs
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Central BSS Place
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Central BSS Place
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• Almost same effect of active and reactive power in voltage regulation
• Battery bank has a higher price and less effect in compare to converter unit,
• Larger battery bank (active power control) can slightly reduce the maximum current and loss
• Larger power converter (reactive power control) increases slightly the maximum current and loss
• Reactive power control can improve the HC with much lower cost (D-STATCOM, TCR, … )
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Conclusions
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Thank you
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Tutorial T2 on voltage control at solar and wind power in radial and meshed grids
• Impact from solar and wind power: o Selection of power factor o Impact on local voltage o Hosting capacity o Impact on losses (where to produce reactive power?) o Possibility to supply feeding grid with reactive power (from where?) o Use of OLTC ( On Load Tap Changers) in transformer o Impact from grid strength. o Impact from R/X quota of grid. Can be different in different lines.
REACTIVE POWER MANAGEMENT WORKSHOP KTH 30TH OCTOBER 2017
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Tutorial T2 on voltage control and wind power
• Tool: Excel Load Flow program
REACTIVE POWER MANAGEMENT WORKSHOP KTH 30TH OCTOBER 2017
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Diff:
0,00000 Correct Blue numbers = input data: can be changed
Red numbers = ouput results: calculatedUn (kV) 11 Ub (kV) 11 R2: Ω/km 0,4Un (deg) 0 U1 (kV) 10,95 99,55% U2 (kV) 10,36 94,2% X2: Ω/km 0,4 U3 (kV) 10,06 91,5%Ssc (MVA) 300 U1 (grad) -0,814 U2 (grad) -2,633 B2: mS/km 0 U3 (grad) -3,623Ssc (deg) 90 km: 3Zsc (%) 100,00% 4,050 MW 2,050 MW PL: 0,050 P3: 2Rsc: Ω 0,000 Q3: 0,5Xsc: Ω 0,403 R1: Ω/km 0,4 0,000 MW R3: Ω/km 0,4
X1: Ω/km 0,4 0,000 MW X3: Ω/km 0,4 Off -1
P-in: 4,25 B1: mS/km 0 B3: mS/km 0 R4: Ω/km 0,4Q-in: 1,25 km: 3 P2: 2 km: 8 X4: Ω/km 0,4
Consumption Product. PL: 0,196 Q2: 0,5 PL: 0,000 B4: mS/km 0P2: 2 0 -1 Off km: 6P3: 2 0 Off -1 PL: 0,000P4: 0 0 R5: Ω/km 0,4 Windy-5 Off -1 0,000 MW
P5: 0 0 X5: Ω/km 0,4 97,6% R6: Ω/km 0,4 P4: 0P-tot 4 0 B5: mS/km 0 U5 (kV) 10,74 X6: Ω/km 0,4 0,000 MW Q4: 0PL-tot [MW] 0,246 km: 5 U5 (grad) 4,558 B6: mS/km 0 U4 (kV) 10,74 97,6%PL-tot [%]: 6,16% - PL: 0,000 P5: 0 km: 4 U4 (grad) 4,401PL-Net [%] 5,80% Q5: 0 PL: 0,000
Allahabad-3
Sunny-4
By Lennart Söder: November 2017
Voltages:
Sub-Station-1 Delhi-2
Feeding grid
Solve problem with current data
Assign Flat Start Base case 1 radial 11 kV
Base case 2 radial 66 kV
Base case 3 meshed 66 kV
Excel-instructions-171031.pdf Examples-171110.pdf
Power-system-2017.xlsm