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Transient protection for PV power

electronics

Transient protection for PV powerTransient protection for PV power

electronicselectronics

Michael RoppMichael Ropp

Electrical Engineering DepartmentElectrical Engineering DepartmentSouth Dakota State UniversitySouth Dakota State University

Brookings, SD 57007Brookings, SD 57007--22202220

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The need for transient protection

The need for transient protectionThe need for transient protection

South Dakota State University

Electrical Engineering Department

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The need for transient protection

The need for transient protectionThe need for transient protection

South Dakota State University

Electrical Engineering Department

Sources of transients

Lightning Switching

Types of transients

AC-sidefairly well quantified DC-side

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Transient suppression devicesTransient suppression devicesTransient suppression devices

South Dakota State University

Electrical Engineering Department

Most TSDs are shunt elements that operate by

presenting a high impedance to low voltages, but low

impedances to high voltages.

The most common ones in use today operate on the

same principle as zener diodes.

MOV

Control

DC

PV array

C1

C2

L

R

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Transient suppression devicesTransient suppression devicesTransient suppression devices

South Dakota State University

Electrical Engineering Department

Varistors

MOVs (most common TSD) SOVs

Silicon Avalanche Diodes (SADs)

(transorbs)

Ionizing gas tubes and spark gaps

(very high voltage DC) Filters

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Transient suppression devicesTransient suppression devicesTransient suppression devices

MOVs Generally the least expensive TSD

option, and readily available from many

manufacturers

Consist of a matrix of metallic oxide

(usually zinc oxide) or ceramicsandwiched between metal electrodes

Grain boundaries in zinc oxide act as

rectifying junctions, so the MOV isequivalent to an array of back-to-back

diodes

South Dakota State University

Electrical Engineering Department

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Transient suppression devicesTransient suppression devicesTransient suppression devices

SADs

Back-to-back zener diodes; one

forward, one avalanching

Excellent clamping properties

Also widely available in a range of

energy/voltage ratings

South Dakota State University

Electrical Engineering Department

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Critical parameters for selecting TSDsCritical parameters for selectingCritical parameters for selecting TSDsTSDs

South Dakota State University

Electrical Engineering Department

Clamping or pass-through voltage Surge current capability

Energy (MOV) or power (SAD) dissipation

Response to standard pulses

10/1000 sec pulse

8/20 sec pulse

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TSD comparisonTSD comparisonTSD comparison

Effect on

normal op

TS

effectivenessDevice Rel cost Degradation

MOV Low High High Good

Excellent

Good

Moderate

SAD Moderate Low/mod Low

Moderate Low LowIon tube

Moderate Moderate HighFilter

South Dakota State University

Electrical Engineering Department

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TSD comparisonTSD comparisonTSD comparison

MOV47 V 7400 V

SAD 500 V3 V

Gas tubes 700 V200 VDC

Clamp

Voltage

MOV 40 kA250 A

Peak

Current

SADGas tubes

1 A

80 kA2 kA

10 kA

South Dakota State University

Electrical Engineering Department

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Critical issues for transient suppressionCritical issues for transient suppressionCritical issues for transient suppression

South Dakota State University

Electrical Engineering Department

Proper device selectionneed to know what youre

protecting against. This is a problem for PV; DC-side

transients arent well specified.

Lead is criticalat transient frequencies, L dominates, not

R. Want very short, straight wires. (Story of our lives in

power electronics.)

TSDs do affect circuit designcapacitance of TSDs can be

significant. This is true for MOVs, less so for SADs.

Capacitors dont work well as TSDsespecially aluminumelectrolyticsbecause of their tendency to look more like

inductors at very high frequencies (ESL).

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UL and TSDsUL andUL and TSDsTSDs

South Dakota State University

Electrical Engineering Department

UL does list TSDs. Applicable standard: UL-1449.

UL suppression voltage ratings (SVRs) based on:

the average let-through voltage of two 6 kV

1.2/50m msec, 0.5 kA 8/20m msec impulses,

separated by a duty cycle test of ten 6 kV 1.2/50m

msec, 3.0 kA 8/20m msec impulses.

Also: IEEE-C62.41.1 and IEEE-C62.41.2standard

covering TVS in AC circuits of 1 kV or less

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Future trends in TSDsFuture trends inFuture trends in TSDsTSDs

South Dakota State University

Electrical Engineering Department

Doesnt appear to be a particularly active

area of research, but some developments

New materials

Selenium? (actually used in 1928!) Nanostructured materials?

New device structures based on

thyristors, cellular structures

Incremental performance improvements

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Future trends in TSDsFuture trends inFuture trends in TSDsTSDs

South Dakota State University

Electrical Engineering Department

Major issues remaining for TSD application:

Lead length: finding ways to minimize

inductance

End of life issues

SADs can fail open or shorted; monitoring?

Explosion protection Monitoring MOV degradation over time

Long-term energy absorption: GROUNDING

Preventing slow burn; thermal control

New structures with better long-term

properties?