Dielectric spectroscopy

Dielectric spectroscopy is a powerful technique, which we use:

• as a diagnostic for electric ageing
• to examine how electrical charge moves through dielectric systems
• to characterise the dielectric properties of materials and insulating systems

We use a Solartron 1296A Dielectric Interface coupled to a frequency response analyser, and we have also developed high-voltage time domain techniques and ultra-sensitive bridge techniques for low-loss materials such as cross-linked polyethylene.

Measurements can be made:

• over the frequency range 10^-5 to 10⁷ Hz
• over the temperature range 0 to 250 deg. C (depending on the material)
• over a range of humidities and voltages
• at tan deltas as low as 10^-5.

Nano-Dielectrics

At Leicester we pioneered the first experimental work (reported in Nanotechnology) showing that insulation materials (e.g. polyethylene) filled with nanometric (e.g. 50nm diameter silica) particles have the potential to enhance dielectric properties properties significantly. 

The graph shows how space charge accumulated in epoxy filled with both "micro-particles" and nano-particles" led to increased fields under high voltage DC conditions.  The field, which may lead to enhanced ageing and breakdown, was much higher for conventional materials filled with micro-particles than those with nano-particles.

Advances in Understanding Electrical Breakdown in HV Insulation Systems

Electrical Tree

We have led the way in understanding electrical degradation and breakdown of polymeric and composite insulators. For example, work, supported by the National Grid Company, has led to the development of novel physical models and computer simulations of electrical tree growth, an important electrical breakdown mechanism in polymeric insulation.

The high voltage laboratory activities include:

• Experimental studies of electrical breakdown in a range of insulating materials and systems
• The use of ultra-sensitive partial discharge (down to 10^-15 C) and electroluminescence (single photon) techniques
• Development of condition monitoring techniques for HV insulators
• Forensic studies of insulator failure in cables, accessories, bushings, etc.

Theoretical understanding of electrical ageing leading to diagnosis and prognosis

Leicester has played a prominent role in the development of quantitative physical models for electrical ageing. This can serve as a basis for the identification of ageing markers and the prediction of service life.

Models have been developed for

• Electrical treeing.  The model quantitatively reproduces observations of treeing and accurately predicts both bush and branch-type treeing.
• Ageing due to the accumulation of space charge at defects. The models predicts characteristic life from generic features such as local ageing susceptibility and energy concentration. The model yields damage structures and relates failure statistics to local variations.

Space Charge Measurment

Under high field conditions, particularly under DC, charge may be accumulate inside an insulating material.  This can be detrimental as it increases and distorts the electric field, which may lead to premature ageing and failure.

The laboratory collaborated with Dr John Alison and Prof. Robert Hill at King's College London to develop a range of pusled-electro-acoustic measurement systems for measuring space charge distributions in films and slabs of solid insulation.

The apparatus has a resolution of around 1pC and 25 microns.  Fast measurements can be made with one of the systems, allowing, for example, the observation of the movement of charge with time. 

The graph shows charge moving in "packets" through a 0.15mm film cross-linked polyethylene under a field of 120kV/mm.