1 Transient Protections Introduction Part I

8/8/2019 1 Transient Protections Introduction Part I

1/31

Transient Protections

Part I


2/31

Effective transient suppression requires

that the impulse energy is dissipated in the

added suppressor at a low enough voltageso the capabilities of the circuit or device

are not exceeded.

onsemi AN843


3/31

When the fuse blows out,

voltage at load1 goes high due

to inductive kick due to di/dt.


4/31

Due to capacitance across primary and secondary,

huge surge can appear at the secondary.


5/31

If there is a break at the primary when thetransformer has reached max flux (at Zero

Cross condition), there can be a huge

transient.


6/31

WAVESHAPES OF SURGE

VOLTAGES

Indoor Wave shapes:

Measurements in the field, laboratory, and theoretical

calculations indicate that the majority of surge

voltages in indoor low-voltage power systems havean oscillatory wave shape.

This is because the voltage surge excites the natural

resonant frequency of the indoor wiring system.

These surges are oscillatory, can have different

amplitudes and wave shapes in the various places of

the wiring system.

The resonant frequency can range from about 5 kHz to over

500 kHz. 100 kHz is most typical.


7/31

0.5 s 100 kHz Ring Wave


8/31

Outdoor Locations

Both oscillating and unidirectional

transients have been recorded in outdoor

environments (service entrances and other

places nearby).

Unidirectional Wave Shapes


9/31

Peak Surge Voltage versus Surges per Year

EIA paper, P587.1/F, May, 1979, Page 10


10/31

The low exposureportion of the set of

curves is data

collected from

systems with littleload-switching activity

that are located in

areas of

light lightningactivity.


11/31

Medium exposuresystems are in

areas offrequent

lightning activity

with a severeswitching

transient problem.


12/31

High exposure

systems are rare

systems supplied by

long overhead lines

which supply

installations that

have high spark

over clearances and

may be subject toreflections at power

line ends.


13/31

When using graph in the figure in previous

slides, it is helpful to remember that peak

transient voltages will be limited to

approximately 6 kV in indoor locations due

to the spacing between conductors using

standard wiring practices.


14/31

Where the transient energy will go?

The energy contained in a transient will be dividedbetween the transient suppressor and the sourceimpedance of the transient in a way that is determinedby the two impedances.

With a spark-gap type suppressor, the low impedance ofthe Arc after breakdown forces most of the transientsenergy to be dissipated elsewhere, e.g. in a currentlimiting resistor in series with the spark-gap and/or in thetransients source impedance.

Voltage clamping suppressors (e.g. zeners, MOVs,rectifiers operating in the breakdown region) on the otherhand absorb a large portion of the transients surgeenergy.


15/31

Three categories of service locations

Category I: Outlets and circuits a long distance from

electrical service entrance. Outlets more

than 10 meters from Category II or more

than 20 meters from Category III (wire

gauge #14 #10)


16/31


Category II: Major bus lines and circuits a short

distance from electrical service entrance.

Bus system in industrial plants.

Outlets for heavy duty appliances that are

close to the service entrance.

Distribution panel devices.

Commercial building lighting systems.


17/31


Category III Electrical service entrance and outdoor

locations.

Power line between pole and electrical

service entrance. Power line between distribution panel and

meter.

Power line connection to additional near-by

buildings. Underground power lines leading to pumps,

filters, etc.


18/31

Surge Voltages and Currents Deemed to

Represent the Indoor Environment

Depending Upon Location

Notes:

1. Open Circuit voltage

2. Discharge current of the surge (not the short circuit current of the

power system)

3. The energy a suppressor will dissipate varies in proportion with the

suppressors clamping voltage, which can be different with different system

voltages (assuming the same discharge current).


19/31

LIGHTNING TRANSIENTS

several mechanisms There are several mechanisms in which

lightning can produce surge.

1. Direct lightning strike to a primary (before

the substation) circuit. When this high current,that is injected into the power line, flowsthrough ground resistance and the surgeimpedance of the conductors, very large

transient voltages will be produced. 2. The lightning misses the primary power line

but hits a nearby object.


20/31

LIGHTNING TRANSIENTS

several mechanisms 3.When a primary circuit surge arrester operates and

limits the primary voltage, the rapid dv/dt producedwill effectively couple transients to the secondarycircuit through the capacitance of the substation

transformer windings in addition to those coupled intothe secondary circuit by normal transformer action.

4. If lightning struck the secondary circuit directly, veryhigh currents may be involved which would exceedthe capability of conventional surge suppressors.

5. Lightning ground current flow resulting from nearbydirect to ground discharges can couple onto thecommon ground impedance paths of the groundingnetworks also causing transients.


21/31

TRANSIENT SUPPRESSOR

TYPES

Carbon Block Spark Gap Oldest and most commonly used transient suppressor

in power distribution and telecommunication systems.

The device consists o

ftwo carbon block electrodesseparated by an air gap, usually 3 to 4 mils apart.

One electrode is connected to the system ground andthe other to the signal cable conductor. When atransient over-voltage appears, its energy isdissipated in the arc that forms between the twoelectrodes, a resistor in series with the gap, and alsoin the transients source impedance, which dependson conductor length, material and other parameters.


22/31


TYPES

Carbon Block Spark Gap

The carbon block gap is a fairly inexpensive

suppressor but it has some serious problems.

It has a relatively short service life

There are large variations in its arcing voltage. The

nominal 3 mil gap will arc anywhere from 300 to 1000

volts. This arcing voltage variation limits its use

mainly to primary transient suppression with moreaccurate suppressors to keep transient voltages

below an acceptable level.


23/31


TYPES

Gas Tubes

It is made of two metallic conductors usuallyseparated by 10 to 15 mils encapsulated in a

glass envelope which is filled with severalgases at low pressure.

Gas tubes have a higher current carryingcapability and longer life than carbon block

gaps. The possibility of seal leakage and the

resultant of loss protection has limited the useof these devices.


24/31


TYPES

Selenium Rectifiers

These do not have the voltage clamping

capability of zener diodes.

This is causing their usage to become more

and more limited.


25/31


TYPES METAL OXIDE VARISTORS (MOVs)

An MOV is a non-linear resistor which is voltagedependent and has electrical characteristics similar toback-to-back zener diodes.

It is made up of metal oxides, mostly zinc oxide withother oxides added to control electricalcharacteristics.

When constructing MOVs, the metal oxides aresintered at high temperatures to produce a

polycrystalline structure of

conductive zinc oxideseparated by highly resistive inter-granularboundaries. These boundaries are the source of theMOVs non-linear electrical behavior.


26/31

METAL OXIDE VARISTORS (MOVs)

The MOV polycrystalline body is usually formed into

the shape of a disc.

The energy rating is determined by the devicesvolume, voltage rating by its thickness, and current

handling capability by its area.

The energy dissipated in the device is spread

throughout its entire metal oxide volume, Hence they are well suited for single shot high power

transient suppression applications where sharp

clamping capability is not required.


27/31

METAL OXIDE VARISTORS (MOVs) The major disadvantages are that they can only

dissipate relatively small amounts of average powerand are not suitable for many repetitive applications.

Another drawback with MOVs is that their voltageclamping capability is not as good as zeners.

Perhaps the major difficulty with MOVs is that theyhave a limited life time even when used below their

maximum ratings. For example, a particularMO

V witha peak current handling capability of 1000 A has alifetime of about 1 surge at 1000 A pk, 100 surges at100 A pk and approximately 1000 surges at 65 A pk.


28/31

TRANSIENT SUPPRESSION

USING ZENERS

Zener diodes exhibit a very high impedance

below the zener voltage (VZ), and a very low

impedance above VZ.

Because of these excellent clippingcharacteristics, the zener diode is often used to

suppress transients.

Zeners are intolerant of excessive stress so it is

important to know the power handling capability

for short pulse durations.


29/31

TRANSIENT SUPPRESSION

USING ZENERS

Zener transient suppressors are designed totake large, short duration power pulses.

This is accomplished by enlarging the chip and

the effective junction area to withstand the highenergy surges.

The package size is usually kept as small aspossible to provide space efficiency in the circuit

layout, and since the package does not differgreatly from other standard zener packages, the

steady state power dissipation does not differgreatly.


30/31

ZENER versus MOV TRADEOFFS

The clamping characteristics of Zeners

and MOVs are given for the following

transient waveform.


31/31

Better

clamping is

seen forthe Zener

1 Transient Protections Introduction Part I

Documents

Transcript of 1 Transient Protections Introduction Part I