ELEC5205 Chap.3 Corona and Stepped Breakdown
Corona and Stepped Breakdown
Local activity before full breakdown
Corona occupies the extreme-field region near sharp hardware and can persist below whole-gap breakdown voltage. The active volume contains a streamer zone, an avalanche-only zone and an attachment-dominated exterior. Positive and negative space charge reshape the field differently, producing polarity-dependent pulses and glow.
Stepped leaders extend the same nonuniform-field reasoning to long sparks and lightning, where a propagating channel advances through successive high-field tips.
Locate the active region before naming the discharge
In a point-plane or wire arrangement, field magnitude falls quickly with distance from the stressed surface.
Ionisation can therefore be strong close to the electrode while attachment dominates farther out. Corona is sustained when charge creation, drift and field distortion maintain activity in that limited volume without forming a continuous conducting path.
The correct explanation marks the electrode polarity, identifies where effective ionisation is positive, and distinguishes a local luminous or pulsed zone from complete gap bridging.
Use space charge as feedback, not decoration
Electrons and ions move at very different speeds.
Their temporary separation changes the local field and can either extend or suppress subsequent avalanches depending on polarity and geometry. This feedback explains why positive and negative corona do not share one waveform or one stable operating range. A pulse train is consequently evidence of repeated growth and clearing, not merely electrical noise.
State the proposed charge distribution and its effect on the electrode field when interpreting a mode transition.
Extend the mechanism to long air gaps
A leader is a heated, conductive channel whose tip creates a new highly stressed region. The channel advances in steps because charge accumulation, local ionisation and connection events occur in sequence.
For lightning, cloud charge separation establishes the large-scale field; a descending leader and upward connecting activity then prepare the return-stroke path. The visible high-current stroke is not the same stage as leader formation.
Keeping the stages separate prevents a final channel current from being used to explain initiation.
Organise calculations around a path
A surface field substitution is only the first step in a nonuniform geometry. Evaluate the effective coefficient along radius or distance, find where it changes sign, and integrate over the active path.
An empirical polynomial must be applied with its stated pressure, field and unit conventions. Close the calculation with a mechanism statement: whether a critical avalanche or streamer condition is plausible, how space charge could alter the next event, and which geometric change would lower local stress.
What this chapter covers
- 01
Corona Inception
- 02
Streamer Zone
- 03
Space-Charge Feedback
- 04
Trichel Pulses
- 05
Stepped Leader
- 06
Lightning Protection
Worked example · free
Evaluate point-field inception
- 2Evaluate field magnitude at the point surface.
- 3Substitute into the empirical coefficient relation.
- 2Explain the difference between local inception and whole-gap breakdown.
Key terms
- Corona Inception
- Corona Inception is the voltage or field condition at which sustained local partial breakdown first appears.
- Space-Charge Feedback
- Space-Charge Feedback is the change in local field caused by slow ions left by discharge activity.
- Stepped Leader
- Stepped Leader is a propagating ionised path that advances through successive bursts in a strongly nonuniform field.
Corona and Stepped Breakdown FAQ
Why does corona not always become a spark?
Space charge can lower the field near the initiating electrode after a burst, suppressing further streamer growth. The active high-field region may also be too short for streamers to cross the weak-field remainder of the gap.
Why does polarity change corona behaviour?
Positive and negative ions occupy different regions and move at different rates, so they distort the electrode field differently. This changes inception, extinction, pulse frequency and whether a stable glow can form during each half-cycle.
How is lightning related to corona?
Both involve discharge in strongly nonuniform fields, but lightning extends the process over a much larger path. A leader tip concentrates the field ahead of it, advances in steps and can connect with an oppositely charged channel.
Exam move
Draw the three radial corona regions and explain what alpha prime does in each. Practise a point-field substitution followed by a statement about the missing integral. Then rehearse the lightning sequence from charge separation through field establishment, leader steps and channel connection.
Build a polarity comparison that names charge motion, space-charge field effect, pulse behaviour and stable region without claiming that one sign is universally safer. Redraw a leader with the highest field at its advancing tip, then connect one mitigation feature to the region of field it changes. Close by naming an observation that distinguishes local corona from a complete gap event.
Annotate which charge species moves quickly and which remains to reshape the next pulse, so polarity explanations retain a causal field picture.
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