Short Circuit Contribution from PV Power Plants.pdf

7/27/2019 Short Circuit Contribution from PV Power Plants.pdf

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Short Circuit Contribution from PV Power Plants

DOE/NREL/SNL Distribution System Modeling Workshop

La Jolla, California, July 27, 2012

Michael Behnke, Principal Engineer

BEW Engineering, a DNV Company


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DOE/NREL/SNL Distribution System Modeling WorkshopLa Jolla, California, 27 July 2012

Presentation Outline

Importance of Short Circuit Analysis in DG

Interconnection and Integration

Synchronous Generator Short Circuit

Characteristics

PV Inverter Short Circuit Characteristics

Status of Commercial Analysis Tools

Conclusions

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Purpose of Short Circuit Analysis

Power system faults (short circuit, ground faults) cannot be

eliminated

Utility protection systems must be designed to clear faultsthrough interruption of the source(s) and post-clearing

restoration of service to as many customers as possible

Short circuit analysis aids in achieving these objectives by:1. Quantifying the magnitude of fault current through interrupting devices

(circuit breaker, fuses, reclosers) to ensure that interrupting capacities

are adequate for fault clearance

2. Providing a basis for protection coordination so that the device(s) thatinterrupt the fault are the ones that minimize loss of load (selectivity)

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Synchronous Generator Short Circuit Characteristics

4

Source: Chapman, Electric Machinery

and Power System Fundamentals, 2002

Decay of

damper

winding

transient

Decay of

field

winding

transient


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Synchronous Generator Short Circuit Characteristics

Time-variant response represented by three separate

time-invariant models

Allows for Thevenin equivalent representation of

generator with constant voltage behind appropriate

impedance for time period of interest (Xd, Xd, Xd) for

balanced faults

Apply X2 and X0 for unbalanced faults

Short circuit calculation methods well documented inANSI and IEC standards

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DG Inverter Short Circuit Characteristics

6

IMPORTANT CAVEAT:

We will be talking about a certain class of inverters:

- 3-phase

- Voltage source topology

- High-frequency pulse width modulated (PWM)

-AC current regulated

This describes nearly all inverter models currently being

applied in utility and commercial scale DR

Some inverters (particularly some PV micro-inverters) donot fall into this class


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Three-Phase, Two-Level Voltage Source Converter Topology


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Sinusoidal Pulse Width Modulation

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Carrier and Modulating Signals


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9

Example of

AC Current

Regulation

Modulating signals

are generated from

AC Current Regulator id* and iq* are real

and reactive current

commands

High PI gains cause

inverter to behave as

stiff current source

ip

KK

s

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AC current regulator continues to regulate (and limit)

current during fault

Xd, Xd, Xd, X2 are only meaningful for a singleinverter operating point and one single fault location!

Danger! : Underestimation of fault current contribution

is possible with Thevenin representation whenimpedance is not changed to adapt to fault location

Line synchronization technique (e.g., PLL

implementation) has great impact on unbalanced fault

response

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Response to unbalanced faults: Manufacturer #1

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Response to unbalanced faults: Manufacturer #2

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Status of Commercial Analysis Tools

Disclaimer: these comments are general in nature are not directed at any

particular software vendors products

Approaches to modeling current-regulated inverters:

1. None (far most common)

- No option for user but Thevenin generator model

- What to do?

a. For balanced faults, use X = 1/Isc (Careful !!!

), orb. Manually iterate Thevenin impedance, voltage or both to achieve Isc

c. For unbalanced faults?

2. Iterative solution

- Automates manual process above, can work well for balanced faults- Provide user means to scale negative sequence current relative to positive

sequence current (but without user guidance) for unbalanced faults

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OK for one generator,

but for dozens?


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La Jolla, California, 27 July 2012

Conclusions

Inability to properly model inverter-based generation in utility industry tools will drive

penetration limits as utilities feel increasingly exposed on system protection issues

No relevant industry standards (IEEE, ANSI, IEC) currently exist

Commercial software packages are based on inapt Thevenin generator models

(though some vendors are making initial attempts at more suitable representation in

an uncoordinated way)

New computational methods are necessary

Generalizations about unbalanced fault response are difficult due to manufacturer

specific controls implementation

Consensus standard is badly needed, could be led by third party (DOE with National

Labs) and include key stakeholders:

- Utility Protection Engineers

- DG Inverter Manufacturers

- Software Vendors

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La Jolla, California, 27 July 201215

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