Corona Effect in Power System

Corona Effect in Power System

Electric power transmission practically deals in the bulk transfer of electrical energy, from generating stations situated many kilometers away from the main consumption centers or the cities. For this reason the long distance transmission cables are of utmost necessity for effective power transfer, which in-evidently results in huge losses across the system. Minimizing those has been a major challenge for power engineers of late and to do that one should have a clear understanding of the type and nature of losses. One of them being the corona effect in power system, which has a predominant role in reducing the efficiency of EHV(extra high voltage lines) which we are going to concentrate on, in this article.

What is Corona Effect in Power System and Why it Occurs?

For corona effect to occur effectively, two factors here are of prime importance as mentioned below:-

1) Alternating electrical potential difference must be supplied across the line.

2) The spacing of the conductors, must be large enough compared to the line diameter.

Corona Effect in Transmission Line

When an alternating electric current is made to flow across two conductors of the transmission line whose spacing is large compared to their diameters, then air surrounding the conductors (composed of ions) is subjected to di-electric stress. At low values of supply end voltage, nothing really occurs as the stress is too less to ionize the air outside. But when the potential difference is made to increase beyond some threshold value of around 30 kV known as the critical disruptive voltage, then the field strength increases and then the air surrounding it experiences stress high enough to be dissociated into ions making the atmosphere conducting. This results in electric discharge around the conductors due to the flow of these ions, giving rise to a faint luminescent glow, along with the hissing sound accompanied by the liberation of ozone, which is readily identified due to its characteristic odor. This phenomena of electrical discharge occurring in transmission line for high values of voltage is known as the corona effect in power system. If the voltage across the lines is still increased the glow becomes more and more intense along with hissing noise, inducing very high power loss into the system which must be accounted for.

Factors Affecting Corona Effect in Power System.

As mentioned earlier, the line voltage of the conductor is the main determining factor for corona in transmission lines, at low values of voltage (lesser than critical disruptive voltage) the stress on the air is too less to dissociate them, and hence no electrical discharge occurs. Since with increasing voltage corona effect in a transmission line occurs due to the ionization of atmospheric air surrounding the cables, it is mainly affected by the conditions of the cable as well as the physical state of the atmosphere. Let us look into these criterion now with greater details :

Atmospheric Conditions for Corona in Transmission Lines

It has been physically proven that the voltage gradient for Di-electric breakdown of air is directly proportional to the density of air. Hence in a stormy day, due to continuous air flow the number of ions present surrounding the conductor is far more than normal, and hence its more likely to have electrical discharge in transmission lines on such a day, compared to a day with fairly clear weather. The system has to designed taking those extreme situations into consideration.

Condition of Cables for Corona in Transmission Line.

This particular phenomena depends highly on the conductors and its physical condition. It has an inverse proportionality relationship with the diameter of the conductors. i.e. with the increase in diameter, the effect of corona in power system reduces considerably.

Also the presence of dirt or roughness of the conductor reduces the critical breakdown voltage, making the conductors more prone to corona losses. Hence in most cities and industrial areas having high pollution, this factor is of reasonable importance to counter the ill effects it has on the system.

Spacing between Conductors

As already mentioned, for corona to occur effectively the spacing between the lines should be much higher compared to its diameter, but if the length is increased beyond a certain limit, the Di-electric stress on the air reduces and consequently the effect of corona reduces as well.

Active Power and Reactive Power

Active Power and Reactive Power

Let us consider a single phase power circuit in which current lags behind the voltage by an angle φ.

Let the instantaneous electric potential difference v = V_m.sinωt

Then the instantaneous current can be expressed as i = I_m. sin(ωt - φ).

Where, V_m and I_m are the maximum values of sinusoidally varying electric potential difference and current respectively.

The instantaneous power of the circuit is given by

What is Electric Current?

What is Electric Current?

Electric current is nothing but the rate of flow of electric charge through a conductor with respect to time. It is caused by drift of free electrons through a conductor to a particular direction. As we all know, the measuring unit of electric change is Coulomb and the unit of time is second, the measuring unit of electric current is Coulombs per second and this logical unit of current has a specific name Ampere after the famous French scientist André-Marie Ampere.

If total Q Coulomb charge passes through a conductor by time t, then electric current I = Q / t coulomb par second or Ampere.

For better understanding, let give an example, suppose total 100 coulombs of charge is transferred through a conductor in 50 seconds. What is the electric current?

As the electric current is nothing but the rate at which charge is transferred per unit of time, it would be ratio of total charge transferred to the required time for that. Hence, here electric current I = 100 coulombs / 50 second = 2 Amperes.

'Ampere' is Sl unit of current.
Definition of Electric Current

While a potential difference is applied across a conductor, electrical charge flows through it and electrical electric current is the measure of the quantity of the electrical charge flowing through the conductor per unit time.

Three Phase Induction Motor Starting Methods

The starting methods of three-phase induction motor generally are direct-on-line starting, reduced-voltage starting and soft starter.

Direct-on-line starting
This kind of starting mode is the most basic and simplest in the motor starting. The method is characterized by less investment, simple equipment and small quantity. Although the starting time is short, the torque is smaller at starting and the current is large, which is suitable for starting small capacity motors.

Reduced-voltage starting
The reduced-voltage starting method can be introduced into medium and large size induction motors to restraint the starting current. When the motor finishes the starting, it will resume to full pressure working. However, the result of reduced-voltage starting will lower down the starting torque. Therefore, the reduced-voltage starting is only suitable for starting the motor under no-load or lightly loaded condition. Some common reduced-voltage starting methods are as follows.

• Stator circuit series resistance starting
  A three-phase electric reactor is inserted into the circuit of motor stator windings. The electric reactor can be simply considered as a coil, which can produce induced electromotive force to reduce the direct input power frequency voltage.
• Star-delta starting
  In the normal operation, 3 phase induction motor whose stator winding is stipulated to link in delta connection can be started in star while starting, to reduce the voltage of each phase of motor and then reduce the starting current. After finishing the starting, then it is connected in delta.
  Star-delta starting is widely used because of its advantages including simple starting equipment, low cost, more reliable operation and easy maintenance.
• Autotransformer starting
  Autotransformer reduced-voltage starting refers that the reduced voltage of grid power is attached to the motor stator windings until the speed approaches to a steady value and then the motor is connected to the power grid.
  At starting, the switch is pulled to the “start” position, and the autotransformer is linked to the grid followed by connection to the stator windings of the motor to achieve reduced-voltage starting. When the rotation speed approaches to the rated value, the switch will be pulled to “running” position, and the motor directly accesses to the grid under full pressure operation through cutting off autotransformer.
  
  Autotransformer reduced-voltage starting is introduced into the star connection for the large capacity motor or normal operation with certain load starting. According to the load, transformer tapping is chosen according to receive required starting voltage and starting torque. At this moment, the starting torque is still weakened, but not reduced by one-third (compared with the star-triangle reduced-voltage starting). However, the autotransformer is large-sized and lightweight with the high price and inconvenience maintenance, which is not allowed to move frequently.

Soft starter
A soft starter is a new type control device whose main advantages include soft starting, light load and energy saving, and quickness. One of the most important features is that the electronic circuit is conducted in the silicon controlled rectifier of a motor under the tandem connection of power supply. Using the soft starter to connect the power supply with the motor and different methods to control the conduction angle in silicon controlled rectifier can make the input voltage of motor increase gradually from zero and transfer all the voltage to motor from the beginning to the end, which is called soft starting. When starting in this way, the torque of the motor will gradually increase with enhancive speed. In fact, the soft starter is a voltage regulator that only changes the voltage without altering the frequency at starting.

PER UNIT CALCULATION

PER UNIT CALCULATION

a per-unit system is the expression of system quantities as fractions of a defined base unit quantity. Calculations are simplified because quantities expressed as per-unit do not change when they are referred from one side of a transformer to the other. This can be a pronounced advantage in power system analysis where large numbers of transformers may be encountered. Moreover, similar types of apparatus will have the impedance lying within a narrow numerical range when expressed as a per-unit fraction of the equipment rating, even if the unit size varies widely. Conversion of per-unit quantities to volts, ohms, or amperes requires a knowledge of the base that the per-unit quantities were referenced to. The per-unit system is used in power flow, short circuit evaluation, motor starting studies etc.

The main idea of a per unit system is to absorb large differences in absolute values into base relationships. Thus, representations of elements in the system with per unit values become more uniform.

There are several reasons for using a per-unit system:

• Similar apparatus (generators, transformers, lines) will have similar per-unit impedance and losses expressed on their own rating, regardless of their absolute size. Because of this, per-unit data can be checked rapidly for gross errors. A per unit value out of normal range is worth looking into for potential errors.
• Manufacturers usually specify the impedance of apparatus in per unit values.
• Use of the constant ${\displaystyle \textstyle {\sqrt {3}}}$ is reduced in three-phase calculations.
• Per-unit quantities are the same on either side of a transformer, independent of voltage level
• By normalizing quantities to a common base, both hand and automatic calculations are simplified.
• It improves numerical stability of automatic calculation methods.
• Per unit data representation yields important information about relative magnitudes.

The PU system deals primarily with values of power, voltage, current, and impedance. Any two variables are selected to be base values. The selection of two base values then fixes the other values. A per unit quantity then becomes the ratio of a selected parameter to a selected base value. Basically,

Per Unit = Present Value/ Base Value

Relationship between units