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: V 10kA, 20kA / V 5mA 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: Al 3+ ; Sb 3+ ; 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.
MOV block crack failures under electrical impulses are due to brittle fracture. 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.
ZnO MOV manufacturing process flow: Process step Critical requirements • Raw materials - Metal oxides - Water quality (purity) - Organic additives - Trace contaminants • - “Most critical step” Powder (Spray dried) • Compaction - Pressure profile • Firing - Temperature profile and atmosphere • Surfacing, glazing and metallizing - Proper handling • Block mechanical control - Micro cracks detection (Ultrasonics) • - Weed out “weak” blocks Electrical testing
Critical spec. parameters MOV Raw materials • Metal oxides: ZnO,Bi 2 O 3 , Co 3 O 4 , - Part. Size Distr.; Spec. Surf. Area Sb 2 O 3 , 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 - Optimize compaction pressure profile. - Reduce spring-back effect - Minimized pressure gradients in blocks. • Fixed die and opposing rams compaction (Preferred method) - Tooling material and design. Better control of body neutral plane - Neutral plane at half way of height. Improved mechanical consistency of the - Avoid generation of edge micro green body (critical for HE MOV blocks) flaws, cracks, laminations, end cap, etc.
Powder character.: Parameters affecting Particle shape, size and distrib.; Spec. Surf. Area; block compaction: flow time; hardness. Body geometry: Compact properties: Organic system: Height; density, (Aspect Specified density; minimized Binder type and qty.; ratio). density gradient; green plasticizer; defloculant; strength; (min. defects). lubricant. 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): Peak temperature crystal growth Temperature to be tailored for binder system and disc type Shrinkage Uniform densification: • Cooling off : (Porosity elimination, LPS Natural or forced convection BBO Bi 2 O 3 compounds) High Temp. dwell: • Time Homogeneous crystal growth (narrow and uniform)
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: Close – up Micro-fissures / voids Failure at 100 kA / 4 / 10 µs: fracture surface Fracture origin
ZnO MOV firing bulk defects: XRD spot analysis Close – up Micro-fissures / voids Failure at 100 kA / 4 / 10 µs: fracture surface Fracture origin
Flat surface grinding: Flat surface specs: Lapping • No scratches • Grinding (Preferred) No edge chips • Flat parallel surface Surface suitable for molten Intense US washing, rinsing • metal adhesion. and degreasing.
Testing of “black” MOV block before metallizing and passivation: • Visual inspection Ultrasonic testing (Internal cracks, other defects) •
Metallisation process: Passivation coating: Air spray of a glass powder Flame spray • • suspension. • Arc spray (Preferred) Lead glass (Environm. Issue) • Masking procedure • Leadless glass (Cost issue) (Critical for edge • definition) Glass coating thickness can be an issue Accurately defined margin • • for HASD test (Distr., class 1 and 2). geometry for every block
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 of 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
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