GAS INSULATED TRANSMISSION LINE: A BACKBONE FUTURE TRANSMISSION SYSTEM?

The power transmission system of today will see basic changes in the near future and Gas Insulated Transmission Line (GIL) is one of them. When overhead (OH) lines cannot be built, then the GIL offers an alternative solution by going underground with the same quantum of power as the OH line.

The gas insulation technology was introduced to sub-stations in the late 1960s and is widely used today because of its significant advantages. Application of GIL was started way back in the 1975, when the Siemens co. installed a GIL inside a tunnel for one of the pumped storage power stations in Germany. 

These lines can be laid above the ground, under the ground directly in soil or in underground tunnels depending upon the requirements. The introduction of 2^nd generation GIL in 2001 using Nitrogen (N₂) and Sulphur Hexa-fluoride (SF₆) mixture and pipeline laying techniques to reduce the cost makes the GIL a long distance, bulk power transmission system with greater reliability and availability.

Construction of Gas Insulated Transmission Lines:

Gas insulated transmission line consists of a number of modular components which are assembled to make the complete GIL. They consists of an Aluminium conductor inside a tubular enclosure. The conductor is rested on cast resin insulators which keeps the conductor right in the centre of the outer enclosure. The outer enclosure is a strong aluminium alloy tube which provides the needed mechanical protection. The spacing between the conductor and the enclosure is filled with a mixture of Nitrogen and SF₆ gas to provide the required electrical insulation.

Usually the SF₆ percentage is small (about 20%). When these lines are buried directly into the ground, the outer ‘Al’ alloy enclosure is coated with a polyethylene layer throughout its entire length. Ultrasonic inspection technique is used to check the leakage of gas mixture. Fig 1 shows the construction of a GIL.

Fig.1: Construction of a GIL

Advantages of Gas Insulated Transmission Lines:

The advantages of Gas Insulated Transmission Lines (GIL), as compared to other transmission systems, are:

• higher power transfer capability (3000 MVA/system for a rated voltage of 550 kV), 
• superior Electro-Magnetic Compatibility (EMC), 
• low losses (less than 150 W/m for a loading of 1800 MVA) and 
• no fire or explosion hazard and hence higher safety. 

The insulating system is not subjected to aging and hence reduces the risk of internal failure. Since these lines are fully enclosed, hence are entirely protected from the environmental impacts. These lines have a very less maintenance requirement, only external inspection is needed. Because of these features the GIL system has an expected life of more than 50 years. The housing i.e. the outer enclosure is solidly grounded and therefore makes the GIL a safe system.  

The capacitance of GIL is very low (55nF/km) as compared to XLPE cables, and hence the compensation required in the form of reactors is usually not needed for lines up to 70 km. These lines are suitable for direct connection to sub-stations and no modification in the protection techniques is required.

Gas insulated transmission lines are gas tight and are sealed for lifetime and because of which they have superior operation throughout their comparatively higher lifetime. GIL have a very low magnetic flux density, 15 to 20 times less than conventional power transmission lines and hence are more suitable for power transfer through populated areas, EMC sensitive areas, and along with telecommunication systems. These lines are un-affected by high ambient temperature and severe atmospheric pollution.

Fig.2 shows the comparative analysis of magnetic flux density of GIL with other transmission systems.

Fig.2: Comparative analysis of magnetic flux density (in microTesla) of GIL with other transmission systems.

The GIL installation process consists of assembly of pre-fabricated modules at the installation site. The key elements are light in weight and can be easily transferred to the site location. During the installation gas tightness is to be ensured at all cost and which very much depends on the welding process.

Application of Gas Insulated Transmission Line: 

Gas insulated transmission line can be used in the voltage range of 245 to 550 kV with the current capacity up to 4.5 kA. Gas Insulated Transmission Lines in underground tunnels can be very viable option in future, as the land above the tunnel can be fully restored for agricultural use. Gas insulated transmission line can be installed vertically also, and hence can be used with underground power plants.  The tunnels used for GIL can also be used for ventilation purpose in the case of underground power plants which reduces the overall cost of the system. 

GILs have all the qualities to become the backbone of future transmission systems. They can also be deployed to transfer bulk power in the GW range from large off-shore wind farms through undersea tunnels with greater reliability. 

How useful is a Combiner Box in a solar PV system?

We are fast adopting renewable energy (RE) sources to fulfil our energy requirements and solar energy is going to be the dominant RE source in future, particularly in India. Two different systems of solar energy are employed now-a-days viz. solar thermal plants, and solar PV plants. Solar PV system is gaining a swift popularity among the mass, particularly the roof top solar PV system.

Two diverse types of roof top solar PV systems are in use. They are the standalone i.e. grid independent PV system and the second one is the grid-tied roof top solar PV system. The standalone solar PV system is relatively small in capacity; varying from a few hundred watts to a couple of kilowatts. On the other hand grid-connected or the grid-tied roof top solar PV system is comparatively larger in size; starting from a couple of kilowatts to few hundred kilowatts.

To get the required power output, the different PV panels are connected in series and/ or parallel. How a panel is connected depends on the rating of the charge controller, battery and the inverter. Factors such as voltage drop and power loss are also taken into consideration while going for a particular connection.

Let’s understand this aspect considering an example:

Suppose that we have to install a 400 W solar PV system. Four panels of 100 W each with a voltage (at maximum power) of 17.2 V and maximum current of 5.8 A are considered for installation. Now as per the load calculation and needed back up time an inverter of 850 VA and battery of 150 Ah are to be employed. Usually an inverter of 850 VA is available in the 12 V rating and so is the battery. Thus we have to use a charge controller of 12 V rating and matching the required current capacity. 

All the four panels should be connected in parallel so that the DC output after the charge controller is of 12 volts (input to the charge controller is a bit higher than 12 V). In this way, by parallel connection of panels, we can match the voltage rating of all the equipments i.e. inverter, the battery and the charge controller. Now if the inverter available in the market is of 24 V, then we have to use two batteries of 12 V each in series so that their voltage adds up. Now you have to connect your PV panels in series parallel combination. I hope the connection is clear to you.

What is a Combiner Box? 

       

Combiner boxes are an integral part of solar PV installation. They serve as the junction point where the several parallel connections from the PV panels come and join. Panels are connected in series and / or parallel as per the requirement as mentioned above.
               

The combiner box contains the necessary over-current fuses and circuit breaker, the bus-bars and the terminals for the required connections. Many solar PV installers fabricate their own combiner boxes to cut down the cost and to promote their products; otherwise there are so many combiner box manufacturers with many variants. Custom build or tailor made combiner boxes are also available on demand. Smart combiner box with data monitoring capability are also available which allows easy installation of data monitoring system.

 Solar PV systems above 5 kW usually have more wires and their connection becomes a difficult job without the use of a combiner box. Some grid-tied inverters come with the fuse protection and arrangements for parallel input connections and hence separate combiner box is not required.

Fig 1: A combiner box with Fuses & Circuit Breakers

Location of a Combiner Box

The combiner box should be as close as possible to the PV panels so that the length of wire required is reduced and the trouble shooting becomes easy as one can easily locate/ identify different wires. Since most of the combiner boxes are installed near the PV array i.e. they are installed outdoor and hence must be adequately weatherproof.

Voltage rating of a Combiner Box

Each combiner box is rated for a specific DC voltage. Most of the combiner boxes compatible with standalone PV system can handle 150 V DC at the maximum, whereas the grid-tied combiner box is rated up to 600 V DC.

Number of Terminals in a Combiner Box

Combiner boxes usually have a fixed number of input and output terminals. Grid interactive system requires fewer input and output terminals, as they usually work at higher DC voltage, while the battery backed PV system has more number of parallel wires. Large battery backed system may have multiple charge controllers and each controller may have its own combiner box. For an off-grid system it is a smart decision to have enough terminals in the combiner boxes for future extensions. To make the future additions an easy job, the combiner box and its output wires should be adequately sized.

Protection in a Combiner Box

As stated earlier both fuses and DC circuit breakers are used as protective devices in a combiner box, depending upon the DC system voltage. The fuse used in a combiner box cannot be opened up under load and hence cannot be employed as DC disconnectors. 

Power Conditioning Unit: A key element of Solar PV System

Power Conditioning Unit (PCU) is a very vital piece of equipment in any solar PV system and is also called solar power conditioning unit. The main components of any solar PV system are PV panels, charge controller, battery bank, inverters, cables, switches and the protection.

"Power Conditioning Unit (PCU) is a combined unit consisting of a solar charge controller, an inverter and a grid charger integrated in a single unit." 

Role of Power Conditioning Unit (PCU)

Power Conditioning Unit (PCU) is usually a DSP based PWM technology using IGBT and MOSFET. It facilitates the charging of battery bank through either solar PV panels or the grid/DG system. All the solar PCU has the ability to continuously monitor the state of battery, solar power output and the load. They not only monitors the state of affairs but even displays vital parameters such as PV voltage and current, load percentage, overload percentage, charging current etc.  

The solar energy is intermittent in nature and therefore a balance has to be made between the solar generation and the demand. When excess energy is generated the extra kWh is to be either fed into the grid, which is possible only in case of grid tied system, or to be stored into the battery bank. Due to over usage of power, if the battery voltage goes below a pre-defined level, the PCU will automatically transfer the load to the utility grid and simultaneously charges the batteries through the grid supply. 

Once the batteries attain a given voltage, the PCU cuts off the grid power to the system and returns back the load on to the solar system. The rest of the battery charging is now done by the solar PV system. In this way the PCU gives priority to the solar power over the grid, and uses the grid power only when the solar power and the battery power of required level is not available.

Fig.1: Back view of an off-grid Solar PCU

Significant advantages of PCU

The significant advantages of PCU are pure sine wave output with low Total Harmonic Distortion (a measure of power quality), higher efficiency, data logging monitoring etc. The commercially available PCU have efficiency more than 85% and nearly 5% Total Harmonic Distortion (THD) for linear loads. These units come with the deep discharge protection and thus ensure the health of batteries. PCUs have nearly 300% overload facility for a few milliseconds which helps during the starting of heavy loads. Thus these units have the inbuilt overload and short-circuit protection and therefore no need to worry about overloads and short circuits. Remote monitoring of the unit can be done with the help of RS232, Ethernet, GSM and GPRS.

Ratings of commercially available single phase PCU

The usual ratings of commercially available single phase off-grid PCU in India are 600 VA/24 V, 1kVA/24 V, 2kVA or 3kVA/48 V, 3/4/ 5/6 kVA/96 V, 7.5 kVA/120 V 

C-rating and Efficiency of a Battery

Batteries are the key elements used in Standalone Roof top solar PV systems and hence one should know a bit about these storage devices. Of the various types of batteries, based on the electrolyte material, the most commonly used battery type is the Lead-Acid type particularly in solar PV applications.

How batteries are rated?

Batteries are rated according to their:

1.   Voltage,

2.   Storage capacity, and

3.   Ability to deliver the stored energy over a given time period.

What is C-rating?

Energy storage capacity is given in Ampere-Hour (Ah) at some nominal voltage and at some specified discharge rate. The storage capacity is never fixed and depends mainly on how fast the energy is extracted from the battery. The manufacturers usually specify the ‘Ah’ capacity at a discharge rate that would drain the battery completely over a specified period of time at a specified temperature. The ability to deliver the stored energy over a given time period is called the ‘C’ rating. Batteries are available in market with different ‘C’ ratings such as C 5, C 10, C 20, and C 100. The batteries usually used for solar application in India are the C 10 type.

Fig.1: C-10 rating battery used in Solar PV application.

Let’s go through an example….

For example, a fully charged 12 V battery that is specified to have a 10 hour, 100 Ah capacity could deliver 10 A for 10 hours, after which the battery will get fully discharged. This ‘Ah’ specification is known as ‘C 10’ rate, where ‘C’ refers to the Ah capacity and the 10 is hours it would take to completely deplete.

Dependency of Storage Capacity.

As mentioned above the ‘Ah’ capacity or storage capacity of a battery is very much dependent on the discharge rate and the temperature. More rapid draining of a battery i.e. higher discharge rate results in lower ‘Ah’ capacity and vice-versa. In simple words, the above 100 Ah, C 10 battery would not last for 1 hour if you drain at a 100 A discharge rate and on the other hand it would last more than 100 hours if the rate is 1 A.

The battery capacity decreases significantly in cold conditions. There is an apparent improvement in battery capacity with the increase in mercury; but this does not mean that batteries are safe in hot climates. Rather the battery life is shortened by approximately 50% for every 10^oC rise above the optimum operating temperature of the battery normally 27^oC.   

Since the rated capacity of the battery is specified at a temperature and discharge rate (specified by the manufacturer), one has to adjust the battery capacity according to the prevailing temperature at the site and the discharge period. 

Does battery connection affect the ‘Ah’ capacity?

The ‘Ah’ capacity of the battery also depends on the connection of the battery. For example, two batteries connected in series will have the same current and hence the ‘Ah’ capacity remains the same. If the same batteries are connected in parallel, their current adds up and so is the ‘Ah’ capacity. But the energy stored in a battery bank, whether the batteries are connected in series or parallel, remains the same. Batteries when connected in series have higher voltage and lower current and hence the voltage drop and power loss are lesser. When batteries are connected in parallel, the weakest battery will bring down the voltage of the entire arrangement. Similarly, in series connected batteries, failure of one battery will completely shut down the system.

Why the manufacturer specifies both ‘Ah’ efficiency and ‘Wh’ efficiency?

Since the voltage varies throughout the discharge period, one cannot calculate the energy delivered by the battery during its discharge by simply multiplying 12 V x 10 A x 10 h = 1200 Wh. Therefore, battery storage capacity is mentioned in ‘Ah’ rather than ‘Wh’.

Manufacturer specifies the efficiency of their battery in terms of ‘Wh’ efficiency and ‘Ah’ efficiency. For example, the ‘Ah’ efficiency and ‘Wh’ efficiency of Luminous make Flooded Lead-Acid tall tubular batteries are greater than 90% and 80% respectively as claimed by the manufacturer.

So let’s have a look into these two efficiencies.

The amount of electrical energy stored in a battery is measured in ‘Wh’  

Energy efficiency in Wh 

= energy discharged in Wh / energy required in Wh to completely recharge

‘Ah’ efficiency 

= 'Ah' discharged / 'Ah' required for complete recharge

The energy or ‘Wh’ efficiency of a battery is always less than the ‘Ah’ efficiency because battery discharges at a lower voltage than they charge at.

A battery should never be discharged more than 80%, even under worst conditions. The more you extract from a battery every day, the more it will wear out. The wear out rate depends on the type of the battery and its cycle life. The life cycle, as mentioned in the Luminous battery catalogue, with 80% Depth of Discharge (DoD) is 1500 cycles, 50% DoD is 3000 cycles, and 20% DoD is 5000 cycles.   With this one can understand what the C rating, ‘Ah’ capacity and ‘Wh’ capacity is and how to interpret the nameplate rating of a battery.