Short-Circuit Modeling and

System Strength

Mohamed Osman, Lead Engineer System Analysis

WECC SCMWG Meeting

May 08, 2018

RELIABILITY | ACCOUNTABILITY2

• Introduction

• Short-Circuit Sources and Modeling

• Short-Circuit Model Verification

• Short-Circuit Ratio Calculation Methods

• NERC’s ERS Measure 10 – System Strength

• Summary

• Questions and Answers

Short-Circuit Webinar Overview

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Introduction: Generation Resource Mix Evolution

0

20

40

60

80

100

120

140

160

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021

Actual Projected

GW

Figure 1. NERC-Wide Utility Scale Wind and Solar Actual and Projected Nameplate Capacity Doubled within the Past 5 Years

Source: 2016 LTRA Reference Case—Tier 1 Additions

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• Synchronous Generators and Condensers

▪ Model by using saturated subtransient reactance 𝑿𝐝𝐯"

▪ Major sources of fault current

• Motors▪ Model by using locked rotor transient reactance 𝑿′

• Transformers▪ Modeled by winding connections (e.g. delta, grounded wye)

• HVDC▪ Not a source of short circuit current

• Passive Elements ▪ Ignore system loads, positive sequence shunts, and line charging

Short-Circuit Sources and Modeling : Sources of Short-Circuit Current

MODEL

IGNORE

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Short-Circuit Sources and Modeling : Nonsynchronous Generators

Nonsynchronous Generator Types Sources of Fault Current

I. Squirrel-cage induction generator ▪ Dominated by the 𝑿′of the machine

II. Squirrel-cage wound rotor induction generator with external rotor resistance

▪ Similar to Type I machines but impedance 𝒁 will change due to external rotor

III. Doubly-fed asynchronous generator

▪ Crowbar resistance can dictate the AC fault contribution

IV. Full power converter generator ▪ Represented as a current source

V. Synchronous generator mechanically connected through a torque converter

▪ Typical synchronous generator behavior during faults

Table 1. Nonsynchronous Generation Types and Primary Sources of Fault Current

See White Paper for Detailed Modeling Information on Nonsynchronous Generators

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Perform Case Comparison Between 2 System Models 1. Compare the total number of generators (gen.) & total generation MVA.

Do the totals match? YES – Pass, NO – Fail

Short-Circuit Model Verification: Case Validation Process

Case NumberEntire System Gen (Offline & Online)

Online GenInternal Online Gen(Exclude Tie-Lines)

Case 1 : Count 2803 2020 1318Case 1 : Total MVA 358,045 361,437 201,017Case 2 : Count 6501 2478 2480Case 2 : Total MVA 920,863 214,637 211,978

2. Compare Case 1 & Case 2 tie-lines’ 3-phase symmetrical fault current. Is there a ± 10% difference? YES – Pass, NO – Fail

▪ If Pass (1 and 2), Proceed to Step 1 of the desired analysis

▪ If Fail (1 and/or 2), Evaluate case models more to determine data anomalies

Table 2 : Example Comparison of Generator Counts and Generator MVA Totals

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Short-Circuit Model Verification: Example Case Modeling of a Tie-Line Bus

• Example of transmission case modeling at the same 161 kV bus (Bus 1)

• 3-winding transformers are represented as 2-winding transformers

▪ Results in 12.30% difference at the same tie-line bus (Bus 2)

Reliability Emphasis on the Need to Coordinate Neighboring System Models

Figure 2 : Example Cases of Identical Tie-line Bus, Modeling Discrepancy & Impacts

Case 2 Model uses 3-Winding Transformers (Bus 1 to Bus 4)

Bus 1

Bus 2

Bus 3 Bus 2

Bus 3

Bus 4

Bus 5

Bus 6

Bus 4Bus 4

Case 1 Model uses 2-Winding Transformers (Bus 1 to Bus 4)

3-Winding Transformer

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Short-Circuit Model Verification: Network Reduction/Equivalent Network

• Purpose: To minimize the difference (± 10%) in fault current observed at study buses by performing network reduction using three buses away criteria▪ Replace a portion of the network with an equivalent circuit that contains

boundary buses with equivalent lines, generators, loads, and shunts from the external system which has been eliminated.

▪ Equivalent circuit is created such that the current-voltage relationship at the load of the original network is unchanged.

o See White Paper for recommendations on how to perform network equivalents at a tie bus and Sub-transmission bus (230 kV> bus kV > 100 kV)

• Different software platforms use different algorithms to create network equivalents

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Short-Circuit Ratio Calculation Methods: Calculation Fundamentals

• 𝑆𝑆𝐶𝑀𝑉𝐴: is the short-circuit MVA capacity at the bus in the existing network before the connection of the new generation source

• 𝑃𝑅𝑀𝑊 : is the rated megawatt value of the new connected source.

Short − Circuit Ratio 𝑆𝐶𝑅 =𝑆𝑆𝐶𝑀𝑉𝐴

𝑃𝑅𝑀𝑊

• SCR is a metric described as the voltage stiffness of the grid▪ Two-step process for calculating the SCR

o Step 1 : Perform Classical Three-Phase Fault Analysis

o Step 2 : Perform ratio of short-circuit capacity at the fault

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Short-Circuit Ratio Calculation Methods

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Short-Circuit Ratio Calculation Methods

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• ERSTF Framework Report recommendations included: ▪ Monitoring of events related to voltage (V) performance,

▪ Periodic review of short-circuit current at each transmission bus,

▪ Perform SCR calculations when the level of nonsynchronous generation is high or anticipated to increase.

• Application of the SCR Method:▪ SCR method is borrowed from screening for weak grids near HVDC

▪ SCR is now used to screen for weak grids near power electronic converters

▪ SCR helps identify areas with potential reliability risks:

o Fault-induced delayed voltage recovery (FIDVR) type events & misoperations

– Temporary loss of area’s V control, poses a risk of cascading to a larger area

– Weak grid can reduce V stability and will exacerbate FIDVR problems

▪ Other related voltage stability phenomena events

Background on ERS Measure 10

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NERC’s ERS Measure 10 : System Strength

• Measure 10: System Voltage and Reactive Strength Performance▪ Uses short circuit capabilities (three-phase fault analysis) in order to:

o A network screening which uses calculated short circuit ratios

o The short circuit ratio is used to identify areas with low system strength

o Low system strength indicates a need for further studies

– Identifies an areas’ potential risks to BPS reliability (i.e. BPS Operations and Planning)

– Additional studies may be needed such as, sub-synchronous oscillation studies, electromagnetic transient studies, and voltage stability studies

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Recommendations

• Benchmark planning cases against operations dynamic events or steady-state operating conditions

• Where possible, create criteria to compare common characteristics between various cases, such as:▪ total current at faulted tie-lines buses between neighboring cases,

▪ Count of network components and the network’s physical characteristics (e.g. line impedances, total generation MVA, etc.),

▪ generating resources availability (lengthy maintenance outages, mothballed, retirements and planned additions),

▪ transformer type, configuration (winding representation) and impedances

• When partitioning an area, the network equivalent should be done three buses away from the short-circuit bus under study

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