An electrical grid is an interconnected network for delivering electricity from suppliers to consumers. It consists of generating stations that produce electrical power, high-voltage transmission lines that carry power from distant sources to demand centers, and distribution lines that connect individual customers.

A. Generation

Power plant stations are located in places where a fuel source is available, near to water source, or to take advantage of renewable energy sources, and are often located away from heavily populated areas to provide safety and noise and avoid risk and hazards for people.

The power generation is in the range of MVA, S=V*I, So what is the reasonable voltage limit to be generated?

Low voltage = High current that would lead to the following challenges

• Large generator conductors size to carry the currents = big generator stator size >> bigger rotor size >> bigger turbine >> increased fuel

• Large Cables cross-section area = Copper/Aluminium >> higher cost

• Increase Losses = (I)^2 * R

• Increase Voltage Drop = I * R

High voltage = Low current that would lead to the following challenges

• Large generator stator size due to bigger insulated material used >> bigger rotor size >> bigger turbine >> increased fuel

• The application for safety area due to the minimum safety distance of the high voltage will lead to

The Difficulty for operator access to the generator area for monitoring different sensors such as pressure, temperature, and vibration.

The control unit shall be replaced far away from the generator

However, the less cross-section area for cables will reduce the power losses as well as the voltage drop

Medium voltage = Optimal current that will get the benefits of

• Optimal Generator size >> optimal fuel used

• Easy access for operators in the generator area for supervision, monitoring, and controlling activities

However, the currents value to be transferred over long distances is not very low which means still there are power losses and voltage drop will exist. In this case, the advantage of generating high voltage could be applied in the form of stepping up the medium voltage generated through step-up transformer (Trafo/XFMR) to high or extra-high voltage levels.

B. Transmission

Transmission substations start after the Step-up transformer and carry the power through:

1. Overhead Transmission Lines (OHTL) >> applied in rural areas

Advantages

• Easy for Setup

• Easy to suspend over the motorway

• Better heat dissipation

• Easy for repairing and maintaining

Disadvantages / Challenges

1. • Humidity and dust

   • Accidents as lightning

   • Birds and low-flying aircraft

   • restricted by landscape

2. Under-Ground Cables/Feeder (UGC/UGF) >> applied in urban areas

Advantages

• No frequent faults

• Safe inside urban places

• Maintenance Free

Disadvantages / Challenges

2. • Geographic obstructions

   • In the case of Loose soil, Special trenches may be required.

   • Rerouting may be required due to other underground pipelines and sewage lines.

   • Long periods for repairing

   • Bad heat dissipation

   • High charging currents

C. Distribution

After the OHTL/UGF reaches the load location, the step-down stage will start. The question now is, what is the voltage value required?

The voltage value is based on the load and could be classified as:

• Residential loads >> Low voltage is required

• Commercial loads >> Medium voltage is required

• Industrial loads >> Medium and High voltage are required.

Also, in many cases, it is required to have a sub-transmission system network with a lower level voltage value than the transmitted value, hence stepping down the voltage from extra-high voltage to high voltage is mandatory, even the load doesn't require the high voltage level.

Switchgear are devices that switch, control, and protect circuits and devices. Their primary function is to "switch" power on or off as needed. These devices can service low, medium, or even high-voltage electrical circuits. They are necessary for switching power on and off for generators, motors, transformers, transmission lines, and power networks in domestic, commercial, and industrial distribution systems.

1. Switchgear Components include the following

• Busbar

• Disconnector Switches “Isolators”

• Earth Switches

• Circuit Breakers / Contactors with Fuses

• Protection Relays

• Instrument Transformers

• Lightning Arresters

2. Switchgear Insulation Medium could be:

• Air: Although the least expensive insulation option, air has a low dielectric strength, making it a relatively poor insulator. As a result, air-insulated devices are typically larger than devices insulated by other media.

• Gas: Pressurized gas also offers increased dielectric strength compared to standard air insulation. Although Sulfur Hexafluoride is a common insulating gas for switchgear systems, other gases may be used in its place.

3. Air vs. SF6 Insulated Switchgear

• • The construction of each type of switchgear impacts its longevity and maintenance. GIS uses the gas sulfur hexafluoride for insulation, whereas AIS uses air insulation in a metal-clad system. Sulfur hexafluoride is five times heavier than air and offers excellent extinction behavior.

  • Another significant construction-based difference is that a metal-clad AIS uses three-position draw-out circuit breakers (on, off, and test). GIS systems use fixed-mounted circuit breakers. The sealed-mounted breakers are a ‘sealed-for-life’ technology, whereas the AIS breaker can be removed for maintenance and troubleshooting.

There are many different electrical bus schemes but the selection of a particular scheme depends upon the system voltage, position of the substation in the electrical power system, flexibility needed in the system and cost to be expensed.

The Main Criteria for the selection of one Busbar arrangement Scheme among others are:

• System simplicity

• Easy for maintenance activities

• The minimum outage during maintenance activities

• Allowing future extensions with demand growth

1. Single Bus Single Breaker

Single Bus System is the simplest and cheapest one. In this scheme, all the feeders and transformer bays are connected to only one single bus.

Advantages

• Simple design.

• Cost-effective.

Challenges

• Busbar fault troubleshooting or maintenance cannot be possible without interrupting the power from the whole switchgear.

• Circuit Breaker fault troubleshooting or maintenance cannot be possible without interrupting the feeder and the busbar.

• Cable maintenance/troubleshooting is risky due to capacitive charges [Earth Switch must be used]

This scheme is often used in the low and medium voltage Switchgears due to its limitations, so using a withdrawable CBs that could be handled by the human would be a perfect solution for CB isolation to avoid disconnection of the busbar supply during the maintenance activities of the CB, however, still load will be interrupted during the circuit breaker maintenance.

2. Single Bus Single Breaker with Sectionalizer

This scheme is a modification of the previous scheme to increase its reliability. The idea here is to use Sectionalizer ((Bus Section CB / Bus Tie CB)), sometimes also called Bus Coupler CB but I don't prefer this definition in single bus systems.

Advantages

• During Busbar maintenance, only half of the switchgear will be interrupted in case of only one Bus-Section is used or a third of the Switchgear will be interrupted in case of two Bus-Sections are used, and so on. >> Higher Reliability

• Power sharing between the sections in case of loss of the Incomer supply for one of them.

Challenges

• Busbar fault troubleshooting or maintenance cannot be possible without interrupting the power from a part of the switchgear.

• Circuit Breaker fault troubleshooting or maintenance cannot be possible without interrupting the load.

• Cable maintenance/troubleshooting is risky due to capacitive charges [Earth Switch must be used]

Notes:

= Bus Riser is nothing but a Busbar with [L] shape to connect between the lower terminals of the Bus-Section CB and the Busbar (Two different levels).

= In the previous schemes, the Busbar shall be grounded during the maintenance activities. So, earth switches shall be installed

• In the Bus-Riser Panel/Bay (to utilize it as it is just a Busbar) or in other schemes it is installed in Bus-Section Panel/Bay = providing earthing facility to one section only.

• In a dedicated panel/bay called ((Bus Earthing/Measurement)), it is nothing but a panel/bay installed on the other section to provide earthing and VT measurements for the Busbar.

2. Double Bus Single Breaker

To increase the Switchgear reliability to be used with high voltage levels, Double Bus Single Breaker Scheme was one of the solutions. In Double Bus Single Breaker Scheme, two Busbars [(Upper and Lower) or (Front and Rear)], based on the installation way, are used in such a way that any outgoing or incoming feeder can be taken from any of the buses.

• Isolator/Disconnector Switches ((Q1, Q2, and Q9))

Actually, Feeders are connected to both buses in parallel through individual Isolators/Disconnector switches. The isolator/disconnector switch is playing the role of the withdrawable mechanism of the medium voltage switchgear illustrated above as the CB size, in this case, is huge and cannot be handled by humans. Also, another isolator/disconnector switch must be used on the line side to provide isolation from the line.

As isolator/disconnector switches are used for the busbar choices connection and isolation for CB during maintenance work, it is a motorized mechanism that is taking from 1.5 to 3 Seconds for closing or opening.

• Earth Switches ((Q51, Q52, Q53, and Q8))

Earth switches are used for safety purposes, to discharge the capacitive charges and induced voltage from either the Busbar side or the Line side.

As the scheme is applied for high voltage levels, long overhead lines or underground cables will have a huge amount of charges, and if a motorized mechanism earth switch is used, an explosion would occur, that's why a High-Speed Earth Switch is used.

• Bus-Coupler

The higher reliability requires continuity of supply and minimum interruption during operation. Bus-Couplers are used to allow transferring the feeder from one bus to another with load interruption. Bus-Coupler is closed >> then the Isolator/Disconnector Switch that was open >> then Open the Isolator/Disconnector Switch that was closed >> then Open the Bus Coupler again. Also, sometimes Bus-coupler is used to share the power between the two Busbars of the same section.

Advantages

• Two Busbars in the same section provide higher reliability and continuity of supply even one bus under maintenance.

• Power sharing among the sections in case of loss of the Incomer supply for one of them.

Challenges

• Circuit Breaker fault troubleshooting or maintenance cannot be possible without interrupting the load.

3. Double Bus Double Breaker

In Double Bus Double Breaker Scheme, two identical bus bars are used in such a way that any outgoing or incoming feeder can be taken from any of the bus similar to the Double Bus Single Breaker Scheme. The only difference is that here every feeder is connected to both of the buses in parallel through individual breakers instead only isolators. By closing any of the breakers and their associated isolators, one can put the feeder to the respective bus. Both of the buses are energized, and total feeders are divided into two groups, one group is fed from one bus and the other from another bus similar to the previous case. But any feeder at any time can be transferred from one bus to other.

There is no need for a Bus-Coupler because the operation is done by breakers instead of isolators.

4. One and Half Breaker

This is an improvement on the double breaker scheme to effect saving in the number of circuit breakers. For every two circuits, only one spare breaker is provided. The protection is however complicated since it must associate the central breaker with the feeder whose own breaker is taken out for maintenance.

During the failure of any feeder breaker, the power is fed through the breaker of the second feeder and Tie Breaker, therefore each feeder breaker has to be rated to feed both the feeders, coupled by Tie Breaker.

• During any fault on any one of the buses, that faulty bus will be cleared instantly without interrupting any feeders in the system since all feeders will continue to feed from the other healthy bus.

• This scheme is much more expensive due to investment for the third breaker.