Considerations in Developing High Energy ZnO MOV Blocks RAMN PUYAN - - PowerPoint PPT Presentation

Considerations in Developing High Energy ZnO MOV Blocks

RAMÓN PUYANÉ

• R. Puyané

Advanced Varistor Consulting Limited

Caraban House Caraban – Ravensdale Dundalk – IRELAND

MOV blocks are the working component in a surge

• arrester. The two major demands on high energy ZnO

MOV blocks are:

• Protect equipment and networks
• Survive overvoltage transients

• System protection:
• Good voltage clamp:

V10kA, 20kA / V5mA

as close to 1 as possible To achieve it, it is required to have:

• Good intrinsic conductivity at high current (10 kA; 20 kA)

Proper doping of ZnO grains. Main donors: Al3+ ; Sb3+ ; F+

Caution: incorrect doping can increase leakage at COV.

 Electrical impulses:

• Duty cycle impulses (Square waves: long duration – low current)
• Lightning impulses (Short duration – high currents)

 Long life under load

• Reliable: improves with time

 Moisture attack

• Critical issue for arrester manufacturer.

This presentation will focus on survival under electrical impulses.

 Electrical impulse failures:

• Crack mode failure due to fast generation of

thermally induced mechanical stresses.

• Puncture mode failure due to current concentration

at a non-uniform spot with lower threshold voltage.

 A brittle fracture in a MOV block originates at a very precise weak spot acting as the crack

initiator, such as:

• Micro flaws
• Voids
• Inhomogeneities

How can these weak spots be eliminated or minimized? By implementing the proper ceramic manufacturing procedures.

MOV block crack failures under electrical impulses are due to brittle fracture.

ZnO MOV manufacturing process flow:

Process step Critical requirements

• Raw materials
• Metal oxides
• Water quality (purity)
• Organic additives
• Trace contaminants
• Powder (Spray dried)
• “Most critical step”
• Compaction
• Pressure profile
• Firing
• Temperature profile and atmosphere
• Surfacing, glazing and metallizing
• Proper handling
• Block mechanical control
• Micro cracks detection (Ultrasonics)
• Electrical testing
• Weed out “weak” blocks

MOV Raw materials

Critical spec. parameters

• Metal oxides:

ZnO,Bi2O3, Co3O4,

• Part. Size Distr.; Spec. Surf. Area

Sb2O3, MnO, NiO, …

• Trace contaminants (Clamp)
• Organic additives

Deflocculant, Binder,

• Low ash content

Antifoaming agent,

• “Easy” burn off

Plasticizer, …

• No gelling

Varistor powder oxide formulation

• Electrical requirements:
• Better peak pulse stability
• Low current power losses
• Improved energy handling capability
• Reduced residual voltage by enhanced doping
• Better control of ceramic crystalline microstructure:
• Enhanced grain uniformity to reduce residual voltage and leakage at low voltages
• Physical and mechanical aspects of the ceramic body:
• Higher mechanical strength for better energy handling capability

Varistor powder organic formulation:

The organic additive system requires:

• Binder plasticity
• Organic system formulation:

It is critical for “good” compaction

• Powder compactability
• Specified density
• Solid body lubricity
• No compaction defects
• Mechanical strength of green discs

ZnO MOV ceramic powder:

Spray dried agglomerates: Powder requirements:

• Fully homogenized powder.
• No segregation.
• Well controlled Part. Size Distr.
• Single Spray Dry
• Good flowability (small flow time)

(Class 1 and 2)

• High flow and tap density
• Spherical solid particles
• Double Spray Dry with calcination

(No “doughnuts”, no “egg-shells)

(Class 3, 4 and 5)

• Solid lubrication present
• No foreign contamination (Particles, fibres,

aggregates, etc.)

ZnO MOV ceramic powder: SEM pictures

Spray dried agglomerates: Close-up:

ZnO MOV ceramic powder: SEM pictures

Contaminant fibres:

ZnO MOV ceramic powder: SEM pictures

Organic contaminant particle: Close-up:

ZnO MOV ceramic powder: SEM pictures

Organic contaminant particle IR spectrum:

ZnO MOV ceramic powder: SEM pictures

Organic contaminant particle IR spectrum:

Block compaction methods: Requirements

• Floating die
• Fixed die and opposing rams compaction

(Preferred method)

• Better control of body neutral plane
• Improved mechanical consistency of the

green body (critical for HE MOV blocks)

• Optimize compaction pressure profile.
• Reduce spring-back effect
• Minimized pressure gradients in blocks.
• Tooling material and design.
• Neutral plane at half way of height.
• Avoid generation of edge micro

flaws, cracks, laminations, end cap, etc.

Parameters affecting block compaction:

Compact properties:

Specified density; minimized density gradient; green strength; (min. defects).

Body geometry:

Height; density, (Aspect ratio).

Organic system:

Binder type and qty.; plasticizer; defloculant; lubricant.

Powder character.:

Particle shape, size and distrib.; Spec. Surf. Area; flow time; hardness.

Compaction technique:

Die filling, pressing profile (ramps and dwells) ; maximum force; ejection procedure.

MOV block compaction defect:

Radial crack visualized on a sintered block.

ZnO MOV firing profile:

The temperature profile and the firing atmosphere must be controlled. With proper control, watt loss at low voltage and residual voltage at high current will improve:

• Binder Burn Off (BBO):

to be tailored for binder system and disc type

• Uniform densification:

(Porosity elimination, LPS Bi2O3 compounds)

• High Temp. dwell:

Homogeneous crystal growth (narrow and uniform)

BBO

Shrinkage Peak temperature crystal growth

Temperature Time

Cooling off : Natural or forced convection

ZnO MOV firing:

• BBO is a critical step: Most mechanical defects are generated at

this stage: Voids, cracks, microfisures

• Uniform densification: Avoid thermal gradients; atmosphere control
• HT sintering:

Optimize t and T to promote homogeneous crystal growth.

ZnO MOV firing bulk defects: Micro-fissures / voids Close - up

ZnO MOV firing bulk defects: Void / Larger grain size

• Mechanically weak spot
• Larger grain size reduces

threshold voltage

ZnO MOV firing bulk defects: Micro-fissures / voids Close – up

Failure at 100 kA / 4/10 µs: fracture surface Fracture origin

ZnO MOV firing bulk defects: XRD spot analysis Micro-fissures / voids Close – up

Failure at 100 kA / 4/10 µs: fracture surface Fracture origin

Flat surface grinding:

• Lapping
• Grinding (Preferred)
• Flat parallel surface
• Intense US washing, rinsing

and degreasing.

Flat surface specs:

• No scratches
• No edge chips
• Surface suitable for molten

metal adhesion.

Testing of “black” MOV block before metallizing and passivation:

• Visual inspection
• Ultrasonic testing (Internal cracks, other defects)

Metallisation process:

• Flame spray
• Arc spray (Preferred)
• Masking procedure

(Critical for edge definition)

• Accurately defined margin

geometry for every block

Passivation coating:

• Air spray of a glass powder

suspension.

• Lead glass (Environm. Issue)
• Leadless glass (Cost issue)
• Glass coating thickness can be an issue

for HASD test (Distr., class 1 and 2).

Final electrical testing: The only possible outcome is to weed out weak blocks that are non performing under energy impulses.

Conclusions and recommendations:



More homogeneous blocks will show better survival performance under temporary over-voltages.



MOV blocks with higher mechanical strength will have a higher probability

• f survival under energy impulses.



Stronger MOV material will make it possible some reduction of block size. Smaller blocks with similar energy characteristics can make it possible to reduce the size of the arresters. However, block over heating must be considered when reducing MOV block size.



A block size reduction can have an impact on costs for the block and arrester manufacturers as well as for the arrester user since a smaller and lighter arrester will be cheaper to install and to replace.

Thank you for your attention!

Ramón Puyané ADVANCED VARISTOR CONSULTING LIMITED Dundalk, Co. Louth Ireland & Montpellier, L’Hérault France