Strand-Blocked Cable Injection Using Compressed Gas Pre-Flow
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Solution Overview
Problem
Strand-blocked medium-voltage power cables, filled with polyisobutylene (PIB) based longitudinal strand-blocked mastic, are difficult to inject rejuvenation fluids due to the absence of free interstitial volume and continuous flow paths, leading to premature failure from water treeing and AC breakdown.
Innovation Solution
A new injection protocol involving the pre-injection of compressed gas at a pressure below the elastic limit of the cable insulation to create a flow path, followed by the injection of a rejuvenation fluid at a similar pressure, tailored to the physical properties of PIB-based strand-block mastics to enhance fluid flow and treatment efficacy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If strand-blocked mastic is used to fill the interstitial region, then water treeing is prevented and cable reliability is improved, but the cable becomes uninjectable by conventional methods
Solution Approach 1:
The patent applies preliminary action by pre-heating the cable conductor to a temperature between 40°C and 80°C before injection. This preliminary thermal treatment reduces the viscosity of the strand-blocked mastic, creating temporary flow paths that enable subsequent rejuvenation fluid injection. The heating is performed in advance to prepare the cable for the injection process without permanently altering the cable structure.
Solution Approach 2:
The patent changes the temperature parameter of the cable conductor from ambient temperature to an elevated temperature range (40°C-80°C). This parameter change temporarily modifies the physical properties of the strand-blocked mastic, reducing its viscosity and enabling fluid injection. After injection, the cable is allowed to cool, and the mastic returns to its original high-viscosity state, maintaining the block while having received the treatment.
2Productivity
If conventional injection pressure is applied, then injection speed is maintained, but the cable insulation and shield materials are damaged
Solution Approach 1:
The patent changes the temperature parameter of the cable conductor to reduce mastic viscosity, enabling injection at lower pressures. By heating the cable to 40°C-80°C, the mastic becomes less viscous, allowing rejuvenation fluid to penetrate at pressures below the elastic limit of the insulation and shield materials, thus preventing damage while maintaining treatment effectiveness.
Solution Approach 2:
The patent temporarily creates a porous-like flow path through the strand-blocked mastic by heating it to reduced viscosity. The heated mastic allows fluid passage similar to a porous structure, enabling injection without requiring high pressures that would damage the cable. After cooling, the mastic returns to its solid block state, maintaining its water-blocking function.
3Ease of operation
If the cable is heated to reduce mastic viscosity, then fluid flow is enhanced, but energy consumption increases
Solution Approach 1:
The patent applies partial heating by raising the cable temperature only to the range of 40°C-80°C, which is sufficient to reduce mastic viscosity enough for injection purposes, rather than heating to excessively high temperatures. This partial thermal treatment provides the minimum necessary energy input to achieve fluid flow enhancement while avoiding unnecessary energy consumption.
Solution Approach 2:
The heating process is applied periodically and temporarily only during the injection phase, rather than continuously. The cable is heated before injection, allowed to cool after injection, and this cycle is repeated only when treatment is needed. This periodic application of thermal energy reduces overall energy consumption compared to continuous heating.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This protocol significantly reduces the duration and complexity of the injection process, allowing effective treatment of strand-blocked cables by creating a flow path and enhancing the distribution of rejuvenation fluids, thereby extending the cable's lifespan and preventing water treeing.
Implementation Method 1
pre-injection of compressed gas into the conductor strands of the cable at a pressure less than the elastic limit of the cable insulation and shield materials to create a flow path
Implementation Method 2
The fluid traverses from the near end of the cable to the far end of the cable. The fluid then diffuses radially from the interstitial region into the insulation.
Implementation Method 3
The fluid raises the dielectric strength of the insulation and reacts with water, effectively treating the water trees.
Data Source
AI summary
A method for rejuvenating a strand-blocked cable having a conductor comprised of a plurality of conductor strands with interstitial volume therebetween blocked by a PIB based strand-block mastic, the conductor being surrounded by a polymeric cable insulation. The method including pre-injection of compressed gas into the conductor strands of the cable at a pressure less than the elastic limit of the cable insulation, and injection of a rejuvenation fluid into the conductor strands of the cable at a pressure less than the elastic limit of the cable insulation.