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

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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)

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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

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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

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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!

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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%

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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)

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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)

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Voltage profiles with no generation

Low load

1% rise

limit

Peak load

15% drop

limit

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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)

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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)

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Voltage profiles with generation

Network can absorb significantly less power than it can supply

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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.

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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.

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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%*

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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.

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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

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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

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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

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Simplified connection criteria – NRS 097-2-3If the simplified connection criteria is not sufficient, then grid impact studies are required

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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

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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

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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

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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

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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.

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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.

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References

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DigSILENT Power Factory example