CT- Current transformer PT- Potential transformer

What are CT- Current transformer PT- Potential transformer ?

CT or Current Transformer & PT or Potential Transformer are measuring devices in AC system. These are also called as instrument transformers. As AC system deals with very high power hence we require Ammeter & voltmeter of humongous sizes to measure such high power which is impractical & expensive too.

A CT has following properties:-

1. It is a step up transformer.
2. It enhances the voltage & thus the current gets reduced for a fixed power, since P= V×I×cosΦ. Thus for a fixed amount of power, current can easily be measured.
3. Primary winding of a CT is always connected in series with the load & secondary winding is connected with the ammeter.

A PT has following properties:-

1. It is step down transformer.
2. It reduces the voltage for a fixed power & thus the voltage can easily be measured, since P= V×I×cosΦ
3. The primary winding of the PT is always connected in parallel to the load & secondary winding is connected to the voltmeter.

CT- Current transformer

PT- Potential transformer

CT and PT form the sub parts of instrument transformer .They are extensively used in power system for metering and protection purpose .These 2 form the most important part of any substation .Since in actual electrical power system we deal with high voltage and high current and it is not feasible and economical to manufacture devices to measure such high values .So to measure high values in power system use CT and PT.The primary of of these transformer is placed in main line and secondary is placed in any meter or relay depending on the application.The turns of primary and secondary are adjusted in such a manner that the current in secondary is small and easy to measure for CT and voltage in case of PT .Also they provide isolation between primary and secondary which is often required in electrical.

Shunt Reactors

 What is the use of a shunt reactor in a power system?

Shunt reactor is an equipment used for voltage control of the line in the power system. During the operation of a power line the load fluctuates from full load, light load or no-load causing voltage variations that need to be controlled either in steady-state or transient In these conditions is normal that the line voltage varies due to the capacitive current of the line. During very light load or no-load the line voltage increases. The phenomena is called Ferranti effect. The shunt reactor is an inductance connected from line to ground rated to absorb the capacitive current of the line thus reducing the voltage to avoid damages due to over-voltage to the customers.

Another type of reactor is the series reactor. This type of reactor is used to limit the short-circuit current of the system increasing the equivalent impedance of the line and thus reducing the short-circuit current.

AIS and GIS Switchgear

What's the different between AIS and GIS switchgear?

Let’s start with some definitions. AIS stands for air insulated switchgear and GIS stands for gas insulated switchgear. So far we can see that both are some sort of insulated switchgear like this.

So the remaining difference is air vs gas. These are the actual insulators in the switchgear. In air insulated switchgear the arc between the contacts is extinguished by the air. In the gas switchgear it is extinguished by the gas.

The differences are that GIS is more compact as the gas, usually sulfur hexafloruide - SF6 has a higher dielectric breakdown voltage than air. So less gas is needed to extinguish the arc. However the gas insulation raises costs. Therefore it tends to be used in areas where space is a premium such as in cities.

Air Insulated Switchgear

Gas Insulated Switchgear

However it should be noted that SF6 is a potent greenhouse gas. It is 23,500 times more potent than carbon dioxide, and can persist in the atmosphere for 1000 years. As the deployment of renewables increases so does the usage of SF6 gas. There are attempts being made to use alternatives to the gas whether its a different kind of gas or a combination of clean air and vacuum technology.

Series and Shunt Capacitors

Why are shunt capacitors preferred over series capacitors for improvement of power factor in distribution systems?

It is because series capacitors are not meant to be used for that purpose since it could increase the fault current level in the system, while shunt capacitors do.

Series capacitors are used to control the power flow within the grid by changing the transmission line’s reactance as well as improve the angular stability of the system after the fault clearance.

Power flow within grid: (a) without series capacitors, (b) with series capacitors

On the other hand, the shunt capacitors are used to provide reactive power needed by the load. By providing the reactive power for the load, the reactive power which is supplied by the grid will thus decrease, hence the power factor is improved.

Reactive Power Loss

Do we have reactive power loss like active power loss?

Yes.

The role and purpose of reactive energy are to maintain the em fields within the electrical network. Although reactive energy only flows within such networks it requires reactive current which together with the active component of current dissipates heat in the conductors and as such it too experiences or creates an active power loss within the transmission and distribution system. Note that the power losses are due to the MAGNITUDE of the current and not just the active component.

So S=|I|2Z=P+j∗Q=|I|2(R+jX)

Frequency Synchronization

Can any two buses in an AC power system have different frequencies? Is it possible to send real power from low-frequency bus to high-frequency bus region in an AC power system?

Yes, and it happens all the time. However the frequency deviations are very small and within the range of normal operations. One bus may be at 60.01 Hz and the other at 59.96 Hz and the system is still stable. Larger frequency deviations would be unstable however as the synchronous generators lose synchronization and their protection systems take them offline.

However is the frequency difference is much larger, then the two AC sections cannot be directly connected. In this scenario, an asynchronous connection using either a DC tie or variable frequency transformer is needed to connect the two grid sections.

DC Vs AC Transmission Line

When and why is DC used instead of AC for long-distance electric power lines? Is DC becoming more common now? What are its advantages and disadvantages?

The largest losses in long distance electrical power transmission come from energy lost in the resistance of the power line.
If P is the power transmitted, and R is the resistance of the line:
P=IV$P=IV$
Ploss=I2R=P2R/V2$P_{loss}=I^{2}R=P^{2}R/V^2$
If P is fixed by community demand, then you can reduce lost power dramatically by increasing the transmission voltage. As a result, all long-distance power transmission, AC or DC, is done at high voltage.

The advantage of AC has always been that it is easy to change the voltage up and down with a transformer; DC requires more equipment and some losses to convert.

That being said, transferring AC power between separate grids requires making sure the phase of the power transmitted matches from the two grids (so that the power from the two grids doesn't cancel or ring), which is difficult and expensive. This is not a problem for DC, so DC lines are used in cases such as where power is transferred from another grid to increase the capacity of an existing grid, or between countries that use different frequency power.

Capacitance between the AC phases (usually 3 phases are transmitted at once over a line) or between the line and the surrounding soil or water causes losses that are not a problem with DC. Therefore, undersea high voltage lines tend to be DC.

Overall line loss is also lower per 1,000 km, so very long distance transmission lines sometimes use DC.