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Dr Tobias Bischof-NiemzChief Engineer
Small-Scale Embedded GenerationTraining and Knowledge Sharing Event
Grid impact studies
Pretoria. March 2019
Crescent Mushwana & Mpeli Rampokanyo - CSIR
Crescent Mushwanacmushwana@csir.co.za
2
Presentation Outline
• Issues to consider for grid Impact studies - CSIR
• Example in DigSilent Power Factory - CSIR
• MPE guideline on connecting small-scale RE in municipal LV and MV grids – Moeller & Poeller Engineering (MPE)
3
Grid impact of SSEG
Small-scale embedded generation (SSEG) refers to power generation under 1 MVA, located on residential, commercial or industrial sites where electricity is also consumed. (“Behind the meter installations”)
Integration of SSEG has an impact on the network – it is important to assess prior to connection so that if grid integrity / power quality are compromised, mitigating measures can be found and implemented.
Primary areas of concern are:
• Load flow impact (thermal loading and voltage profile)
• Fault level impact (safety, protection co-ordination)
• Power quality impact (harmonics ,flicker, voltage change)
Focus of this presentation is on assessment for a single application – however the Distributor should conduct regular assessments to determine cumulative impact of installed generation – especially on upstream network.
• Regularly updated register of installations is very important for such an analysis
4
Grid impacts to consider when interconnecting SSEG
• Voltage
• Thermal loading
• Fault currents
• Voltage variation
• Power quality
• Voltage unbalance
• Harmonics
• Flicker
- It is important to assess a number of plausible boundary conditions and ensure that for all situations – the network is OK.
- As a minimum, these boundary conditions are:
- High load, high gen
- Low load, high gen
- High load, no gen
- Low load, no gen
- If network conditions are acceptable, the SSEG is unlikely to have an adverse impact
after
before
5
Loading considerationsThe load or demand is a critical input to the assessment, your results depend on the assumed load
- Peak load
- Networks are usually planned to cater for a certain peak load – thus this information is well known by utilities and assessing the network under such conditions is not too much of challenge
- Embedded generation generally lower than peak load – no major issues expected
- Light load can present a challenge :-
- It is the condition of most concern due to the voltage rise that embedded generation brings about
- If SSEG is PV – minimum day time load is required, not absolute minimum which is often at night
- For residential customers this is usually during the day on weekdays
- For industrial / commercial customers this is usually during the day on weekends
- Coincidence / diversity of adjacent feeders will have an impact
- Accurate representation of load is key in obtaining credible results!
6
Technical Assessment Criteria
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Voltage
As per South African Distribution Code
• HV : Nominal voltage levels equal or greater than 44 kV up to and including 132 kV
• MV: Nominal voltage levels greater than 1 kV and less than 44kV
• LV : Nominal voltage levels up to and including 1 kV.
As per NRS 048 - 2
• Voltages > 500 V: Limits - 0.95 p.u < V < 1.05 p.u
• Voltages < 500 V: Limits - 0.9 p.u < V < 1.1 p.u
The presence of embedded generation generally results in voltage rise, and can either have a positive or negative impact on the overall network depending on loading conditions.
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Voltage
In radial feeders the impact of embedded generation on voltage can be estimated
- During peak loading, voltage along the feeder is low
- The addition of generation tends to improve the voltage profile of the feeder
- During light loading, voltage along the feeder is higher
- The addition of generation can result in over voltages if the load is very low
- In a meshed network, voltage impact is difficult to determine without conducting simulation studies as various voltage control elements will play a role in determining the resultant system voltages
- OLTC (on load tap changers HV/MV)
- Shunt devices (Cx and Rx)
- MV connected generators
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Illustration of voltage impacts of SSEG using a simple radial network
Grid
Au
to tra
nsfo
rme
r
Fe
ed
er
Lo
ad
11 kV
132 kV
OLTC
Fixed tap11kV/420V
HV/MV transformer, with MV
busbar voltage control
Distribution MV/LV
transformer
MV
voltage
drop
LV voltage
drop
HV source (generation,
transmission and sub-
transmission)Customer voltages
Must be between
90% and 110%
10
Peak load (5 MW)
Grid
Auto transform
er
Feeder
Load
11 kV
132 kV
OLTC
5%
voltage
drop
105%5MW
100kW
5% boost
5% voltage
drop100% 100%
10% voltage
drop
90%
105
100
9090
92.5
95
97.5
100
102.5
105
107.5
110
0 5
Vo
ltag
e (
%)
Distance (km)
11
Low load (0.5 MW)
Grid
Auto transform
er
Feeder
Load
11 kV
132 kV
OLTC
0.5%
voltage
drop
105%
500kW
10kW
5% boost
0.5% voltage
drop104.5% 109%
1% voltage
drop
108%
10% of peak load
105
104.5
109108
90
92.5
95
97.5
100
102.5
105
107.5
110
0 5
Vo
ltag
e (
%)
Distance (km)
12
Voltage profiles with no generation
Low load
1% rise
limit
Peak load
15% drop
limit
13
Peak load (5 MW) with generation
Grid
Auto transform
er
Feeder
Load
11 kV
132 kV
OLTC
2MW 30kW
3%
voltage
drop
105%3MW
70kW
5% boost
3.5% voltage
drop102% 103.5%
7% voltage
drop96.5%
105
102
103.5
96.595
97.5
100
102.5
105
107.5
110
0 5
Vo
ltag
e (
%)
Distance (km)
14
Low load (0.5 MW) with generation
Grid
Auto transform
er
Feeder
Load
11 kV
132 kV
OLTC
2MW 30kW
1.5%
voltage
rise
105%
20kW
5% boost
1% voltage
rise106.5%
112.5%
2% voltage
rise
114.5%
1.5MW
105 106.5
112.5
114.5
102.5
105
107.5
110
112.5
115
0 5
Vo
ltag
e (
%)
Distance (km)
15
Voltage profiles with generation
Network can absorb significantly less power than it can supply
16
Thermal loading
Thermal ratings of equipment should not be exceeded
• Ratings are obtained from datasheets
• For cables, method of installation must be considered; ducts and parallel cables reduce the current carrying capability
Verify the adequacy of the thermal ratings of all equipment whenever
• Network topology is changed, or
• New generator is connected,
• Tap setting of a transformer is changed.
If the generation exceeds the peak load in any part of the network, the thermal loading capability of the equipment must be checked.
17
Fault currents
Short-circuit capability of all equipment should not be exceeded.
For MV networks (up to the LV terminals of the MV/LV distribution transformers)
• The maximum three-phase short-circuit currents and the maximum single-phase to ground short-circuit currents should be calculated according to IEC60909:2016.
• The contribution of embedded generators should be considered. This may be obtained from data sheets.
In LV networks
• The maximum short-circuit currents can be assessed by considering the current-limiting properties of circuit breakers, if applicable.
The fault current contribution of generators should be checked for credibility by comparison with the following typical values:
• a) synchronous generators: eight times the rated current;
• b) asynchronous generators: six times the rated current;
• c) inverter based generators: rated current.
18
Voltage variation
Rapid voltage changes can occur due to:-
• switching of network components (e.g. transformers),
• sudden drop in active power output of a generator (e.g. passing cloud in the case of PV generation).
NRS 048 – 4 recommends that voltage change should not exceed 4%*
19
Power Quality
Unbalance
• Typically occurs in LV networks, where many customers have single-phase connections. It affects customers with multi-phase connections.
• As per NRS 048: Unbalance should not exceed 2%
Harmonics
• Generators must comply with NRS 048-2 and the South African Grid Code for Renewable Power Plants.
Long term flicker
• The South African Grid Code for Renewable Power Plants specifies the emission requirements for power plants up to 5MVA.
20
Summary of key technical assessment criteria
Category Criteria Basis Limit
Voltage
Over voltage
MV feeder voltage < 1.05 p.u
LV feeder voltage < 1.1 p.u
Voltage variationChange in voltage from 0% to 100% SSEG output
< 4%*
Loading Thermal Feeder/ transformer loading 100%
Protection Fault level Lowest switchgear rating CB rating
Power Quality
Unbalance
As per NRS-048 and Grid codeHarmonics
Flicker
21
There are various ways to assess grid impactsThe accuracy of the study and the speed of connection will depend on the study/analysis done
Acc
ura
cyo
f st
ud
ySp
ee
do
f con
nectio
n
Detailed Interconnection Studies
Hosting Capacity Analysis
Simplified connection criteria
22
Simplified connection criteriaIs handy for processing large volumes of application with speed, but be careful of being too conservative
Generic , conservative limits that allow for “fit and forget” of generation if it meets certain criteria.
• NRS-097-2-3
• GIZ voltage apportionment guideline
In other parts of the world some utilities use a percentage of feeder load or MV/LV transformer size
- California – USA – 15% of feeder load
- Western Power – Australia – Lesser of 30% of feeder load / transformer rating
In the UK
- LV installation where total aggregated Energy Sources are ≤ 16A/phase and use Type Tested Inverters can be connected without any grid impact studies
Simplified connection criteria:
‒ Based on typical network configuration, allows for quick and easy screening and connection
‒ Handy for processing volumes
‒ If set too conservative or too generic can result in
• many connection requests requiring detailed studies
• unused capacity
23
Simplified connection criteria – NRS 097-2-3If the simplified connection criteria is not sufficient, then grid impact studies are required
24
Simplified connection criteria – Voltage apportionmentModeling data (transformers, cables and load) details required for accurate assessment
*Extracted from MPE Guideline P13162 : Recommended practice for assessing the connection of small generators based on renewable energy sources to low-voltage and medium-voltage municipal grids
Max dU = 3% for single phase connections Max dU = 2% for three phase connections
25
Hosting capacity analysisHelps in ensuring that grid assessment for new connection can be done proactively – this expedites the process
Determines amount of generation that can be accommodated without adversely impacting power system under current configurations, without requiring mitigation or infrastructure upgrades
• Informs developers where generation can interconnect without system upgrades
• Streamline and potentially automate the interconnection process – studies only required when hosting capacity is exceeded.
• Inform distribution planning, such as where to proactively upgrade the grid to accommodate autonomous DER growth
Munics can proactively calculate hosting capacities for areas that have potential for high PV uptake in order to ease the burden when applications are received.
Detailed analysis
Power flow at each node until violation occurs. Stochastic analysis sometimes used to cater for uncertainty in PV size & locations as well as load
StreamlinedSimplified algorithms for each power system limitation
Shorthand equations Simple calculations
26
Detailed interconnection studies
Data required:-
• Accurate case file with relevant MV and LV networks modelled in detail
‒ All existing SSEG should also be included in the model
• Location of proposed SSEG on feeder
• Load profile of feeder (preferably annual)
• Size of generator
• Fault contribution of generator
‒ Can be determined from type of generation
‒ (synchronous vs non-synchronous)
Minimum studies to be conducted
• Load flow (check for voltage and thermal violations at various loading / generation conditions – as a minimum HLHG, LLHG)
• Fault level assessment
• Protection co-ordination
27
Reporting
It is important to compile a report summarising the findings of the grid impact assessment.
It must include the following as a minimum:-
• Background – details on connection request e.g. generator type, size, location, envisioned date of connection
• Study parameters – network assumptions, load assumptions, generator assumptions
• Technical analysis – results of studies conducted compared against technical assessment criteria – include before and after scenarios to see impact of SSEG
• Recommendation – can plant connect or not
• If upgrades are required in order to connect – include scope, cost and time
28
Studying cumulative impact of SSEG
As penetration levels of SSEG increase, their impact will no longer just be localised.
It is important to take stock every “x” kW or every “y” months to check the cumulative impact of embedded generation in the MV and LV networks.
A database of all approved and commissioned connections, as well as the related MV/LV transformer from which they are supplied is key to such a study.
29
References
• The South African Grid Code Version 9.0
• South African Distribution Code Version 6.0
• Grid Connection Code for Renewable Power Plants (RPPs) connected to the electricity transmission system (TS) or the
distribution system (DS) in South Africa
• NRS 048-2, Electricity supply – Quality of supply – Part 2: Voltage characteristics, compatibility levels, limits and assessment
methods.
30
References
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DigSILENT Power Factory example
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