Liquid Insulation

In current power industry nowadays, High Voltage Apparatus with gas (SF6) insulation is the main player. Nearly all new main intake substations are using Gas Insulated Switchgears (GIS). It seems that a lot of people in the industry tends to think that GIS is better than liquid insulated switchgears. There are some who feels that liquid insulated switchgears are already obsolete and GIS is the way of the future. I have asked some of my colleague on which is the better insulation between SF6 and oil. Not suprisingly all of them says SF6 has better insulation capability than oil.

My Staff filling up the Transformer Insulation Oil into On Load Tap Changer Chamber

However this is not true. In the previous post in Gas Insulation, Paschen's Law states that the breakdown voltage is the function of pressure multiply by distance of gap (Vs=f(p x d). ). We know that liquid is denser than gas which means that liquid has more pressure. When we relate this with Paschen's Law, it is proven that liquid acts as a far better insulation than gas. Liquid also quench the arc faster than gas and has the capability to dissipate the heat better than gas.

Electron movement in Gas Insulation

Electron Movement in Liquid Insulation

The picture above shows the movement of electron from cathode to anode in two different types of insulation. In gas insulation medium, the distance between molecules of the gas is wide apart so electron can move easily. So avalanche can easily occur which in turn will create insulation breakdown. However in liquid insulation medium, the configuration of molecules is very near and the number of molecules are much higher than in gas insulation medium. This means that liquid is much denser than gas and leave a little space for electron to move.

The concept is as same as when you are speeding on a highway like a Mat Rempit. If there are a lot of cars on the highway, you cannot ride your motorcycle very fast. But if there are only two or three cars on the highway, then you can speed like the highway is owned by your father. A word of advice, please ride and drive carefully.

Brief Info on Sulphur Hexaflouride (SF6)

Sulfur hexafluoride (SF₆) is an inorganic, colorless, odorless, non-toxic and non-flammable greenhouse gas. SF₆ has an octahedral geometry, consisting of six fluorine atoms attached to a central sulfur atom. It is a hypervalent molecule. Typical for a nonpolar gas, it is poorly soluble in water but soluble in nonpolar organic solvents. It is generally transported as a liquefied compressed gas. It has a density of 6.12 g/L at sea level conditions, which is considerably higher than the density of air.

SF₆ is used in the electrical industry as a gaseous dielectric medium for high-voltage circuit breakers, switchgear, and other electrical equipment, often replacing oil filled circuit breakers (OCBs) that can contain harmful PCBs. SF₆ gas under pressure is used as an insulator in gas insulated switchgear (GIS) because it has a much higher dielectric strength than air or dry nitrogen. This property makes it possible to significantly reduce the size of electrical gear. This makes GIS more suitable for certain purposes such as indoor placement, as opposed to air-insulated electrical gear, which takes up considerably more room. Gas-insulated electrical gear is also more resistant to the effects of pollution and climate, as well as being more reliable in long-term operation because of its controlled operating environment.

(Taken from http://en.wikipedia.org/wiki/Sulfur_hexafluoride)

Gas Insulation

Air around us is a good insulator. It is abundance and self restored after electrical breakdown process. Air is commonly used as insulator in switchgears and overhead transmission line. However as demand for electricity supply increases especially in dense urban area, the need for a smaller and compact high voltage apparatus arise. Researchers realize that air is not the viable to fulfill this needs. Thus a new gas insulation technology is born - The Sulphur Hexaflouride (SF6). Air has Critical Breakdown Voltage of 24.5kV/cm which means that for one centimeter gap between a different phase, or between phases and ground, air can withstand 24.5kV. If the voltage exceeds this value, breakdown will occur.

Breakdown Voltage of Air

On the other hand, SF6 gas has Critical Breakdown Voltage of 88.4kV/cm which means that for one centimeter gap between a different phase, or between phases and ground, air can withstand 88.4kV. Breakdown will occur if voltage exceeds this value.

Breakdown Voltage of SF6 gas

Explanation on why SF6 has higher breakdown voltage than air

In the previous section, I have stated that the breakdown voltage of SF6 is higher than air. In this world everything happens for a reason and same goes to this. SF6 has added characteristics which makes it more superior than air in terms of breakdown voltage. The reason is SF6 has higher dielectric strength and has arc quenching capability.

Molecular Structure of SF6 Gas

The Townsend Breakdown criterion for air is given by:

 where gamma is the Townsend Second Ionization Coefficient and alpha is Townsend Second Ionization. However for SF6 gas, the Townsend Breakdown Criterion is given by:

 which we can see there is an additional coefficient added and it is called the attachment coefficient. The attachment coefficient is introduced to Townsend Breakdown Criteria in order to describe the behaviour of SF6 gas molecules which will attach itself to excess electrons during the first ionization and the second ionization. This behavior is the one which contribute to more superior dielectric strength and higher arc quenching capability compare to air.

Effect of Pressure Towards Gas Breakdown Capability (Paschen's Law)

In 1889, Friedrich Paschen made a discovery when he was experimenting the breakdown voltage of a parallel plate in a gas with respect to gap distance and pressure. He realize that as pressure of gas decrease, the density of gas will also decrease thus providing more space for electron to move. At some point, increment in gap distance will also increase breakdown voltage. Below is the Paschen's Law

where p is the gas pressure and d is the gap distance. The graph below shows different Pachen's Curve for CO2, Air and H2.

 

Noted that each gasses shows increment in breakdown voltage one the (pd) value increase. In my experience, 33kV GIS switchgear has a pressure of 1.2 bar compared to 132kV GIS switchgear which could reach to 5 bar.

Types of High Voltage Insulation

In order to deliver high voltage electricity to consumers, good insulation must be used to ensure that electrical energy is not lost and safety is always present. Generally there are four types of electrical high voltage insulations available. There is gas insulation, liquid insulation, solid insulation and vacuum insulation. All four have their own unique characteristics, have their advantages and disadvantages.

World Electricity Consumption

Example of Gas Insulated High Voltage Apparatus

1) The picture below shows ABB ZX2 33kV switchgear used in some TNB Substation. 

ABB ZX2 Double Bus Gas Insulated Switchgear

2) Next one is an SF6 Ring Main Unit Switchgear. The picture was taken when I was doing preventive maintenance in Pencawang Elektrik Padat D/O Bungalow in Balik Pulau, Pulau Pinang.

F&G SF6 Ring Main Unit Switchgear in Pencawang Elektrik DO Bungalow in Balik Pulau

3) Below is the picture of 11kV Pelka Imalat Vacuum Circuit Breaker (VCB) in Bayan Bay Main Distribution Substation. The picture was taken when I did a switching for a breakdown nearby.

Pelka Imalat 11kV Vacuum Circuit Breaker

Example of Liquid Insulated High Voltage Apparatus

1) Below is the picture of 11kV 750kV Transformer taken from Kuarters TNB 'B' Substation in Bayan Baru. The winding of the transformer is immersed in Hyrax transformer oil which acts as insulation

750kVA 11kV/0.433kV Hermetically sealed Transformer

 2) Next is the picture of winding inside a 750kVA 11kV/0.433kV Hermetically sealed Transformer. The winding and accessories shown are all immersed in oil insulation

Inside 11kV/0.433kV 750kVA Transformer

3) Below is the conservator tank for a 90MVA 132kV/33kV. The picture was taken during our visit to Malaysian Transformer Manufacturing (MTM) factory in Hulu Klang. See how huge it is compared to a person standing next to it.

Conservator Tank for 90MVA 132kV/33kV Transformer

Example of Solid Insulation in High Voltage Apparatus

1) This is the picture of 33kV Aerial Bundle Cable (ABC) Straight Through Joint. The joint is put on a tray fitted on a 10 meter spun concrete pole. The picture was taken when I was on attending a fault between PMU Sungai Kecil and SSU NTPM in Nibong Tebal, Pulau Pinang. The insulation is based on polyethylene products.

33kV ABC Straight Through Joint

2) Next is the picture of 11kV Premoulded Underground Straight Through Joint. This picture was taken when I attended a breakdown somewhere in 2008. The insulation used is resin and plastic. The resin is in liquid form at first and it hardened to solid once the resin colds down.

11kV Single Core Premoulded Straight Through Joint

3) The picture below shows an 11kV Termination Joint on an Oil Link Unit switchgear. The orange colour covers is the cable sleeves which limit the high voltage stress of the cable termination. It is made from polyethylene. The picture was taken in 2006 when I was having my final year practical in TNB Distribution Banting.

11kV Termination Joint

The Wonders of High Voltage World

Electricity has become a necessity to mankind. Since the discovery of electricity by Sir Michael Faraday, electricity has evolved from a privileged to a selected few to a need for everyone regardless of their level in society. Electricity has brightened each home in urban area and even to remote islands. Without electricity, many daily activities of mankind will be interrupted and even cause lost of lives.

TNB Temenggor Hydro Electric Station

With this in mind, it is important to secure a stable and uninterruptable electricity supply. Scientist, researchers and engineers strive ahead to ensure electricity is delivered with no problem. After many experiments, theories and field trial, it is known that electricity delivered in high voltage reduce losses and increase supply performance. Thus a new knowledge field is born which is called High Voltage Technology.

TNB Transmission Line in Perak