Silicon Carbide (SIC) Lightning Arresters

Silicon Carbide Arresters (SIC):

The Non linear lightning arrester basically consists of set of spark gaps in series with the silicon carbide non linear resistor elements. Lightning arresters are connected between the phase conductors and ground. During normal system operating voltage conditions, the spark gaps are non conducting and isolate the high tension (HT) conductors from the ground. However whenever an overvoltge of magnitude dangerous to the insulation of the apparatus protected occurs ( these over voltages or over surges may be caused due to lightning strikes on the conductors or due to Extra High Voltage (EHV) switching) the spark gap breaks down and allows the high voltage surge current to flow through the ground.

Working Principle of Silicon Carbide (SIC) Lightning Arresters:

The volt-ampere characteristics of the non linear resistor in the lignting arrester can be approximately described by expression V = KI^β. Where K and β are dependent on the composition and manufacturing process of the Non linear Resistor (NLR). The value of β lies generally in the range of 0.3 and 0.45 for modern silicon carbide (SIC) lightning arresters. If the voltage across the Non Linear Resistor (NLR) doubles, the current would increase approximately by 10 times.

Therefore, with multiple spark gaps arresters can withstand high Rate of Recovery Voltage (RRRV). The non-uniform voltage distribution between the gaps (which are in series in lightning arresters) presents a problem. To overcome this, capacitors and non-linear resistors are connected in parallel across each gap. In case of lightning arresters employed for high voltage applications, capacitors and nonlinear resistors are connected across the stock of gaps and NLRs. With the steep voltage wave surge the voltage is mainly controlled by the capacitor and at the power frequency by the non-linear resistors. It is obvious that when the over voltages cause the break down of the series gaps, the current would be very high so as to make the voltage to subside very fast. The highest voltage that appear across the lightning arrester would be either the spark over voltage of the arrester or the voltage developed across the non-linear resistor during the flow of surge current. The lowest spark over voltage of the arrester is called the hundred percent impulse spark over voltage of the arrester. The voltage developed across the non-linear resistor during the flow of surge current is called residual voltage. The lower the value of the voltage developed the better the protection of the lightning arrester.

Disadvantages of Silicon Carbide (SIC) Arresters:

Some of the disadvantages of silicon carbide arresters compared to gapless arresters are given below:

• Silicon Carbide (SIC) arresters have inferior V-I Characteristics compared to Zno arresters (Metal oxide arresters). 
• Decrease in energy absorption (surge wave) capability compared to Zno arresters.
• Probability of sparking between the gaps.  

Advantages of Silicon Carbide (SIC) Arrester :

• Due to the presence of gaps the normal power frequency voltage during normal operation is negligibly less compared to gap less arresters. Hence no leakage current flow between the line and earth in SIC arresters  

Creepage Distance for Insulators in Substation

Creepage Distance for Insulators:

The creepage distance for insulators is the shortest distance along the insulator surface between the metal parts at each end of the insulator. Creepage distance can also be refer as leakage distance for insulators.
Insulators in substation are provided to avoid any leakage current from live electrical conductors to flow to the earth through supports. The atmospheric dust sticks to the insulator surface forming a conducting layer. The leakage current flows from the live conductor to the earth through such surface layers. The leakage properties (creepage properties) of an insulator s in substation are characterized by the length of the leakage path. While designing the insulator sheds, the leakage distance for insulators requirement should be satisfied. The requirement of the leakage distance or creepage distance for insulators depends on the

• Rated phase to ground voltage
• Degree of atmospheric pollution

If the surface of the insulator is clean, smooth and well gazed the dust particles tend to fall down. When an a.c voltage is applied to the apparatus, the dust particles have a tendency to align to the direction of electrical field lines on the surface of the insulator. If the surface of the insulator is rough and moist this alignment will be faster. The continuous application of voltage causes slow alignment of these particles. To prevent a continuous conducting track, the insulator should have sufficient ceepage distance or leakage distance.

During power frequency withstand test of an unclean insulator, the flash-over occurs along the dirty surface of the insulator. In case of internal gas filled or oil filled apparatus, the internal surface should also be free from moisture and dust. Otherwise internal flash over can occur along the surface by tracking. Some of the typical creepage distance for insulators in substation provided based on the level of pollution are tabulated below

S No.	Degree of Pollution	Recommended Creepage Distance for Insulators
1	Clean areas	16 mm/kV
2	Moderately polluted areas	20 mm/kV
3	Industrial areas	22 mm/kV
4	Heavily polluted areas	25 mm/kV

Underground Cable Fault Identification Methods

If a fault occurs in the underground cable, it is essential that the type of the fault and location of the fault should be determined as quickly and accurately as possible. Accuracy is important in order to avoid excessive trenching work. The type of fault which is most likely to occur is single conductor to ground fault. In multi-core cables, the fault current will likely give rise to excessive heating at the fault causing further damage to the insulation and extending the fault to remaining conductors. Open circuit faults may occur occasionally which be usually at cable joints. 

Cable fault type identification:

 Prior to the location of the fault on the power system it is important to determine the type of fault so as to make a better choice of the method to be used for fault location

• Isolate the faulty cable and test each core of the cable for earth fault. One terminal of the insulation tester is earthed and each conductor of the cable is in turn touched with other terminals. If the insulation resistance tester indicates zero resistance during any measurement, conductor to earth fault for the particular conductor is confirmed
• Then check the insulation resistance between the conductors. In the case it is a short circuit fault, the insulation resistance tester will indicate zero resistance
• After the above step, short and earth the three conductors of the cable at one end. Check the resistance between the conductors and earth and between individual conductors (at the other end). This procedure is carried out to check the open circuit faults
• In case in order to test any other faults. the insulation test of the individual cores with sheath or armour and between the cores is essential. The test should also be done by reversing the polarity of the insulation resistance tester. In the case of any difference in the readings, the presence of moisture in the cable insulation is confirmed. The moisture in the cable forms a voltage cell between the lead sheath and conductor because of the difference in the conductivity of these metals and the impregnating compound forms an organic acid when water enters 

Cable fault location Identification:

After the fault type identification, suitable fault location method should be employed to pinpoint the location of the fault. Some of the fault identification methods generally employed are:

• Murray loop test method
• Fall of potential test
• dc charge and discharge method
• Induction test
• Impulse wave echo test
• Time domain reflectometry test

Shunt Vs Series Capacitors Advantages

Capacitors aid in minimizing losses and helps in reducing operating expenses. Capacitors should be placed where the reactive power demand is more as capacitors generate reactive power and helps to maintain the voltage. Basically reactive power demand will be more at load side of the power system, hence capacitors should be placed as close as possible to load. In specific, capacitor banks should be placed where the low voltage problem occurs.Shunt capacitors are series capacitors are employed in power system for different purposes

• Shunt capacitors and series capacitors in the power system generate the reactive power to improve the power factor and voltage, thereby enhancing the power system capacity and reducing the losses. In series capacitors reactive power generation is proportional to the square of the load current (I²X_c), whereas in shunt capacitors reactive power generation is proportional to the square of the voltage (V²/X_c).
• The cost of installation of series capacitors is higher than that of the shunt capacitors. This is because the protective equipment for the series capacitors is often complicated. In addition to that, series capacitors are generally designed for higher power to cope up with the future increase in the load
• For the same voltage improvement, the rating of the shunt capacitors will be higher than that of series capacitors. Series capacitors compensation may create certain disturbances: ferro-resonance in transformers, sub-synchronous resonance during motor starting, shunting of motors during normal operation and difficulty in protection of capacitors from system faults.
• Series capacitors are more effective on distribution circuits with higher X/R ratio and for load variations involving a higher reactive content.
• Series capacitors are generally employed to improve the stability of the system and shunt capacitors are generally employed to improve the power factor of the system

Some of the factors which influence the choice between the shunt and series capacitors are tabulated below:

Objective	Series Capacitors	Shunt Capacitors
Improving Power Factor	Secondary	Primary
Improving Voltage level in overhead line system with a normal and low Power Factor	Primary	Secondary
Improving Voltage level in overhead line system with a high Power Factor	Not Used	Primary
Reduce Line Losses	Secondary	Primary
Reduce Voltage Fluctuations	Primary	Not used

Methods to Reduce Step Potential and Touch Potential in Substation

Step Potential and Touch Potentials are very important in substations because during ground faults all the ground current returns to the substation transformer (as the substation transformer is grounded). The current that returns through the earth can create a significant voltage gradient along the ground and between ground and conducting objects.A step potential in substation creates a path through the legs from one foot to the other. A touch potential in substation is normally considered a hand-to-foot or hand-to-hand contact.

Step potential and touch potentials are of concern during normal conditions and during ground fault.Under normal conditions, unbalanced currents can rise the neutral to earth voltage. This is not normally dangerous but it can cause shocks. 

Step potential and Touch potentials during faults are more dangerous. Therefore it is important to reduce the step potential and touch potential to be within limits during the substation design. 

 

Methods to Limit Step Potential and Touch Potential:

Step potentials and touch potentials can be reduced by employing one of the following methods:

1. By proving low resistance path to ground
2. By providing insulation layer between operating personnel and earth
3. By proper placing of ground conductors 

Some of the methods  employed to reduce step potential and touch potential in substation  are:

• Multi-grounded neutral helps to reduce the dangerous step potential and touch potential during line-to-ground faults. By creating low impedance path back to the source, faults are quickly cleared by fault interrupters. Multiple grounding electrodes tied together helps to reduce the touch potentials at the fault point. With multiple neutrals, step potentials are usually not dangerous since fault currents spreads between several grounding electrodes
• Using a reactor on the substation transformer neutral helps to limit the step and touch potentials. While utilities normally use the neutral reactors to limit the fault currents. the reduction of ground fault currents also reduce the step and touch potentials and reduces current in grounding and bonding connectors.
• By wearing electric hazard shoes. These shoes when dry can have offer millions of ohms of resistance which can save the operating personnel against these dangerous potentials. By using insulating materials such as rubber gloves can protect the personnel.
• By providing resistive surface layers in and around the substation. It is often provided with the surface of crushed rock or pebbles which acts as insulation medium between the operating personnel and earth. 

Factors affecting Soil Resistivity

Soil resistivity is the resistance of certain volume of the soil between opposite faces of the cube of soil with a volume of 1m³. Soil resistivity vary widely. Soil resistivity varies significantly with increase in the depth. Some of the important significant factors that affect the soil resistivity are listed below:

Soil Resistivity is  affected by:

Moisture content:

Moisture content in the soil is one of the major factors that determine the soil resistivity. The dryer the soil the more is the resistivity of the soil, wet soil will have low soil resistivity. Change in the moisture level of the soil through out the year during different seasons is the biggest reason for change in the ground electrode resistance. With increase in the moisture content in the soil, ground resistance or soil resistance decreases and about after 22% of moisture content there will be very little change in the soil resistivity

Temperature: 

Above the freezing point of the water, temperature does not impact the soil resistivity significantly but the temperature below the freezing point of the water soil resistivity rises and will have significant impact

Salts in Soil: 

The presence of soluble salts in the soil will significantly impact the resistivity of the soil. One of the option to reduce the electrode resistivity is by chemical treatment of soil near the electrode. Bentonite backfilles or salt treatment such as sodium chloride or calcium chloride can significantly reduce the soil resistivity. The only disadvantage is some chemicals enhance the metal corrosion rate which can damage the grounding rod to corrode.
If the grounding rod sustains significant current, the current flowing through the grounding rod may dry out the surrounding soil and that can increase the resistance of the electrode. When high currents flowing through the grounding rod or earthing electrode, this may be due to lightning or from faults the soil surrounding the electrode breaks resulting in the increase in the resistance

Ferroresonance in Power Transformers

Ferroresonance: 

Ferroresonance phenomenon can be triggered off through interaction of the system capacitance with a non-linear inductance, when transformer is at no-load. This oscillatory phenomenon or resonance phenomenon result in over-voltages which can damage the cable and the power transformer. Series ferroresonance is dangerous to power transformer 

Ferroresonance can occur when: 

• The power transformer is on no-load (even an active load which can be less than one tenth of the rated power can be sufficient to prevent the phenomenon of ferroresonance to occur by absorbing the energy).
• A large capacitance exists in line or the cable connected to at least one terminal of the transformer while the other terminals remain at normal potential
• An unbalanced situation is created by a single phase trip out, a break in the conductor or blowing of one or two fuses 

Methods to Prevent Ferroresoance: 

• Ferroresonance can be reduced by reducing the length between the transformer terminals and the section circuit-breaker or fuses 
• Avoid non simultaneous switching of power transformer 
• Earth the neutral point directly or through a resistor limiting the single phase fault current

Generating Station Grounding Principles

Grounding systems in the generating stations should able to meet the following grounding principles:

Grounding Principles:

• Metallic enclosure on electrical equipment and exposed non current carrying conductive materials in generating station which are capable of being energized due to insulation failure or accidental contact of the energized conductors shall be grounded
• The grounding should limit the step and touch voltage to acceptable limits during a ground fault on the enclosed equipment.
• The grounding in generating station should provide ground fault current return path so that ground faults can be detected by protective devices and can able to isolate the faults or can provide an alarm to the operator
• The grounding conductors and connections should withstand the ground fault current for the duration of the fault, without being damaged by thermal, thermo-mechanical or electromechanical stresses.
• The grounding conductors should be continuous; no switching device should be inserted in the grounding conductors (except where the operation of the switching device will also automatically disconnect all power sources to the equipment grounded by that conductor). Structural changes after installation should not interrupt the grounding conductor. In general, equipment enclosures should not be used as part of the grounding conductor.
• Ground conductors should able to withstand any applied mechanical stresses and should be reliable.Ground connections which are exposed should be accessible for inspection
• Grounding system in generating station should be designed in such a way to minimize the corrosion to adjacent structures and enclosures

Harmonics Disadvantages in Power System

Sources of Harmonics:

Several sources of harmonic currents that may be found on electrical power distribution networks are listed below:

• Variable speed motor drives
• Rectifiers
• Arc furnaces
• Wielding equipment
• Uninterrupted power supplies
• Switching mode power supplies
• Compact fluorescent lamps
• Electronic ballasts

Disadvantages of Harmonics:

Harmonics degrades the performance of power system. Some of the disadvantages of harmonics in the power distributed network are listed below:

• The harmonics flowing in the distribution network downgrade the quality of the electrical power supply. There can have several negative effects on the operation of the power system
• Increased losses on the distribution system due to increase in the effective rms current
• Over-load in neutral conductors due to cumulative increase in the third harmonics created by the single phase loads
• Overloads, vibration and premature ageing of the generators, transformers and motors as well as increase in the noise level
• Overloads and premature ageing of the power factor correction capacitors
• Distortion of the supply voltage that can disturb the operation of the sensitive loads
• Disturbances in the communication networks and telephone lines
• Resonance between the supply inductance and capacitance of the power factor correction capacitors

Factors affect Voltage Level Selection in Service System

Some of the factors that effect the selection of nominal voltage levels for the service systems are:

1. Type and size of the load to be served
2. Distance to which power is to be transmitted
3. Future Load Growth
4. Equipment availability for fault isolation
5. Permissible voltage regulations
6. Cost of utilization and service system equipment

Factors effect Selection of Voltage Levels:

• Nominal voltage supplied to the system depends on the type and rating of the loads to be supplied.In industres, generally below 200kW all motors are supplied with 415V supply and motor rated above 200kW are supplied with 6.6kV voltage level. Motors above 200kW can also supply with 415V supply however this results in increase in the cost of the motor and size of the motor which is not economical.
• Power transfer capability of a line depends on the nominal operating voltage at which power is transmitting in the system. For eg: 220kV transmission line can evacuate power about 200 to 250MW with temperature rise in permissible limits for a distance of about 150 to 200 KMs. 400kV transmission line can able to evacuate power about 500MW. In order to transmit high power for long distances higher operating voltage is chosen
• Short Circuit capability of the switching equipment increases with increase voltage rating. The short circuit capability generally determines the amount of the load that can be connected to the bus. Selection of the voltage levels depends on the total load and system fault levels. Therefore, auxiliary systems with large loads and high fault current levels tend to have high voltages systems with multiple medium voltage buses
• System voltage regulation and short circuit capability both contradict with each other. Short circuit current in the system is limited by the impedance present in the system. Cables and transformers are the impedances present in the power system. In order to reduce the short circuit transformer with high percentage impedance can be chosen. However, because of the high impedance high voltage drop exist during normal operation results in poor regulation. If short circuit constraint and voltage regulation doesnot satisfy for a particular voltage level, the short circuit capability of the switchgear shall be increased by selecting a higher nominal system voltage. The increase in the short circuit capability will permit the use of lower impedance transformer, thereby improving voltage regulation
• While chosing the voltage for a system, future load addition to the system should also be considered. Addition of new load will increase the voltage drop in the system and also increases the short circuit level of the system

Voltage Variations Effects on Power Plant Equipment

When the voltage at the terminal of the electrical equipment deviates from the value as specified from the name plate the equipment performance and life time of the equipment are effected. IEEE Standard 666 details regarding this. The effect on the equipment may be of minor or major serious depends on the voltage deviation magnitude and the sensitivity of the equipment for the voltage variations. 

Some of the equipment deviations which has certain effect due to supply voltage deviations are seen here:

Induction Motors:

Induction Motor voltage below name plate rating will result in decrease in the starting torque of the motor and increase in the full load temperature rise. 

On the other hand, increase in the supply voltage above the specified voltage will results in increase in the starting torque of the motor, increase in the inrush currents during starting and decrease in the power factor. 

Increase in the starting torque to above supply voltage results in acceleration of the coupling or driven equipment. Increase in the starting currents results in voltage drop in the bus connected to the motor. This effects the performance of the other loads connected to the bus.

Supply voltage above the rating will have less impact on the induction motor compared to under voltage condition but if voltage above 110% of the rated voltage will saturate the stator iron (flux is proportional to voltage / frequency) thereby damaging the magnetic properties and increase in losses.

Synchronous Motors:

Synchronous motors will have same impact as induction motor except the speed of synchronous motor remains same. Maximum or pull out torque varies directly with the voltage if the field remains constant. 

Transformers:

Under-voltage operation does not have significant impact on transformer, but due to under-voltage condition current drawn by the loads increases. If this increase in load current can create an over current condition. Transformer can be operated upto 110% at no load and 105% at full load without any harm. Further increase in voltage can results in the saturation of magnetic core thereby increasing the magnetizing current and losses. Continued operation in over excited region can result in failure of unit. 

 Lighting:

Fluorescent lamps operate satisfactorily over a range of + or - 10% of the ballast name plate rating compared to Incandescent lamps. Life of Fluorescent lamps are less affected compared to Incandescent lamps

Reactive Power Sources and Sinks in Power System

Dynamic Reactive Power Sources:

It includes reactive power compensators which have the ability to respond within duration of cycles and try to balance the reactive power requirement in the system. They are considered as transmission service equipment or devices. These devices respond to change in the reactive power requirement in the system quickly and provides or injects the sufficient reactive power into the system.Some of the dynamic sources which acts as sources to the reactive power are:

• Synchronous generators
• Synchronous condensers
• Solid state devices such as FACT devices (STATCOM, SVC)

Synchronous generators controls the reactive power to be delivered with the help of Automatic Voltage Regulator (AVR). The maximum capacity of synchronous machines (both synchronous generators and synchronous condensers) to deliver the reactive power (MVAR) is limited with in the operation region of P-Q diagram and system stability limits.

Static Reactive Power Sources: 

Static sources generally static in nature to the change in the reactive power requirement in the system. They deliver constant reactive power to the system irrespective of the system requirement. These devices are inexpensive but communication, control and associated switching and maintenance requires huge amount.Some of the static sources for the reactive power in the power system are:

• Shunt capacitors
• Filter banks
• Underground cables
• Transmission lines when lightly loaded
• Fuel cells
• PV systems 

Reactive Power Sinks: 

Reactive power generated by synchronous generators and other devices in the power system is absorbed or consumed by the loads. Therefore it is advisable to place the reactive power compensation devices at the load centers so that losses associated with transmission of reactive power to load centers is reduced. Some of the sinks for reactive power in the power system are listed below: 

• Induction motors(pumps, fans)
• induction loads (Arc furnace, heaters)
• Induction generators
• Synchronous machines (under excited)
• Transmission lines heavily loaded
• Transformers
• Shunt reactors

Voltage Control Interview Questions Answers

How the Voltage Control in power system is achieved?

Answer: Voltage control is achieved by suitably installing voltage control equipment at specified places such as at Generating stations, Transformer stations supplying power to feeders, and at load end

Name the different methods of controlling voltage in an ac supply system?

Answer: The different methods of controlling the voltages in an ac supply system are:

• Excitation control at generating stations
• By using tap changing transformers
• Induction regulators
• Booster transformers
• Series compensation
• Shunt Compensation
• Synchronous phase modifiers

What is a booster transformer?

Answer: Booster transformer is one which is often used towards the end of a power line in order to raise the voltage to a desired value

Why it is necessary to keep the receiving end voltage constant within specified limits?

Answer: For satisfactory operation of the equipment/machinery supplied by the electrical power network it is necessary to keep the receiving end voltage constant within the specified limits. Too wide variation of the voltage may cause the erratic operation or even malfunction of the consumer appliances

Why excitation control is not possible beyond certain limits?

Answer: Excitation below certain level may cause instability of the system and excitation above specified limits can lead to overheating of the machine's stator and rotor. So excitation control is not possible beyond certain limits

What are the some precautions must be taken when using a booster transformer?

Answer: There must be no fuse on the high voltage (hv) side or primary side since the booster transformer is similar to current transformer. If the fuse blows an extremely high voltage could built up on the secondary side

Power System Stability Interview Questions Answers

Define Infinite Bus in a Power System?

Answer: Infinite bus has constant voltage, constant electrical angle of the voltage, constant frequency, infinite fault level and therefore the quantities of the infinite bus do not get affected by connecting or disconnecting of individual generator or transmission line

Define Load angle of the generator?

Answer: Angle between rotor pole axis and stator rotating field axis (which is rotating at synchronous speed) is called load angle. It is expressed in electrical degrees or electrical radians

How steady state stability limit of a system can be improved?

Answer: Steady state stability of the system can be increased by either increasing the excitation of machine or group of machines, by reducing the transfer reactance

How transient stability limit can be increased?

Answer: Transient stability limit can be increased by:

• Increasing the system voltage
• By reduction in the transfer reactance
• By using high speed circuit breakers
• Automatic reclosing
• Single pole switching
• Turbine fast valving

Distinguish between steady state stability and transient stability?

Answer: Steady state stability refers to the conditions of the stability response to small and gradual changes of the load while transient stability corresponds to conditions of the stability in response to large sudden disturbances. A sudden large disturbances includes occurrence of faults in the system and clearing of faults, sudden load changes, in advent tripping of transmission lines or generators from the power system

On what basis can we conclude that synchronous generator goes out of stability?

Answer: If the power angle exceeds the critical angle of the load, synchronous machine goes out of stability

High Voltage (HV) Interview Questions Answers

Why Earth Wire is provided in the overhead transmisssion lines?

Answer: Earth wire is provided above the overhead transmission lines for protection against lightning strokes. The ground wire or earth wire protects the phase conductors or line conductors by attracting itself the lightning strokes which in its absence would strike the phase conductors. Also the ground wire reduces the voltage electrostatically or electromagnetically induced in the conductors by the discharge of a neighboring cloud. It also provides the additional protective effect by causing the attenuation of traveling waves set in the lines by acting a short circuit secondary of the line current

What is meant by protective angle? Give its value for reliable operation?

Answer: Protective angle is the vertical angle through the ground wire axis and the line passing from the ground wire axis to the outermost phase conductor. Its value for reliable protection is taken equal to 20-30 degrees

What is the difference between the lightning arrester and surge absorber?

Answer: Lightning arrester limits the duration and amplitude of the follow current while a surge absorber reduces the steepness of the wave front for a particular surge

What is meant by Lightning arrester or surge diverter?

Answer: A Surge Diverter is a device that is connected between line and earth, i.e, in parallel with the equipment under protection at the substation. It limits the duration and amplitude of the follow current 

Related Topics Transmission Line Protection Questions Fuse Advantages and Disadvantages High Voltage Induction Motor Protection Different Faults on Induction Motors Power System Protection Interview Questions 2 Power System Protection Interview Questions Different Transformer Faults High Voltage Interview Questions

What is meant by impulse ratio of any lightning arrester?

Answer: Impulse ratio of any lightning arrester is the ratio of breakdown voltage of the wave of special duration to breakdown voltage of a 50Hz wave

What is meant by voltage surge?

Answer: Voltage surge is sudden excessive rise in voltage that may be dangerous to the electrical equipment of an installation. Voltage surge cause damage to insulation of the system. Voltage surges are caused because of lightning, switching on and off the load at high voltages

How standard lightning impulse wave is represented?

Answer: Standard Lighting impulse wave is represented by 1.2/50 micro sec wave

Shunt Capacitor Advantages in Power System

Normal loads on ac supply system are inductive in nature (eg: motors, power transformers, voltage regulators, induction furnace, choke coils, magnetic systems, discharge tubes etc) Inductive power requires reactive power in addition to active power (active power is required to do the true work). The reactive power increases the load imposed on the system.

When a capacitor is installed across an inductive load (i.e, parallel to the load), the load starts receiving reactive power from the capacitor and thus capacitor power neutralizes the reactive power requirement of the inductive load. Consequently the supply system is relieved from producing reactive power to the load and it can able to deliver greater active power. When the power factor improves towards unity, the transmission losses decreases due to reduced current and the system voltage improves. A healthy supply enables machines and protection equipment to give optimum output in terms of performance and operational life span. Some of the advantages due to installing shunt capacitors in the power system are explained below

• For a particular active power (kW) the resultant demand (kVA) is subsequently reduced. So additional machines can be installed for given sanction of kVA load
• Since voltage drop is minimized, motor torque capability ( Torque α Voltage² ) improves, so starting time and the motor heating gets considerably reduced. Motor current requirement for the same output is lesser
• Because of  less heating, the ageing of the insulation becomes slow and thus the life of the machine and the cables increases
• Switchgear wear and tear is minimized because of lesser arcing energy dissipation (i.e, lesser acing time) at higher power factor
• Reduced losses in the feeders lead to lesser voltage drop, hence greater voltage regulation

Ungrounded or Isolated Neutral System Disadvantages

• There will be no flow in the zero sequence currents
• In ungrounded or isolated neutral system there will be little interference with the communication lines because of the absence of zero sequence currents
• In case of single line to ground fault or one phase becoming earthed, the voltages of the remaining two phases to the earth raise from normal phase to neutral voltage to full line value ((3)^1/2 times their normal value). This causes the stress on the insulation on all the machines and equipment connected to the system. The voltage rise of a phase above earth is sustained and thereby insulation failure is likely to occur on connected machines.
• The capacitance currents in the two healthy phases increases to ((3)^1/2) times their normal values



Related Topics Circuit Breaker Interview Questions Protection Relays Interview Questions Neutral Grounding in Power System Electromagnetic Relays Advantages Function of Instrument Transformers Ungrounded or Isolated Neutral System Fuse Advantages and Disadvantages High Voltage Induction Motor Protection

• The capacitance currents in the faulty phase becomes 3 times the normal value
• For operation of the protective device it is necessary that magnitude of current supplied should be adequate to operate them. In the case of earth fault of an isolated neutral or ungrounded systems, the fault current may be too small to actuate the protective devices. Thus in ungrounded system the earth fault relaying is more complicated
• The over voltages due to induced static charges are not discharged to the ground in an isolated neutral system. The voltage due to lightning surges do not find path to earth
• The danger to the equipment on the occurrence of line to line ground fault is appreciable and danger to the life in the proximity of the fault is often prolonged
• A capacitive fault current flows into the earth. Such a current if exceeds 4-5 amperes is sufficient to maintain an arc in the ionized path of the fault, even though the medium causing the fault has cleared itself. The persistence of  the arc due to the flow of capacitance currents gives rise to condition known as "Arcing Grounds" in which cyclic charging and discharging of the system capacity through the fault results in high frequency oscillations superimposed on the whole system and build up of very high voltage can occur. This results in phase voltage to rise to 5 to 6 times of normal voltage. The buildup of high voltage may result in insulation breakdown

Disadvantages of HVDC transmission compared to HVAC transmission

DC transmission has some advantages over AC transmission, but it cannot completely replace ac transmission system due to some limitations

High Cost: 

1. The converters required at both the ends of the line are much more expensive than conventional equipment. 
2. The converters produce lot of harmonics both on dc and ac sides which may cause interference with the audio frequency communication lines. Filtering and smoothing equipments are provided at the converter station in order to remove ripples from the dc output. It is also possible to provide filters at the ac side to absorb the harmonic current. These considerably increases the cost of converter substation
3. HVDC converters need complex cooling system
4. Cost of HVDC circuit breaker is higher compared to ac breakers
5. Converters have very little over load capacity and absorb considerable amount of reactive power. 

Maintenance of insulators in HVDC transmission line is more
Circuit breaking in multi-terminal dc systems is difficult and costlier

Therefore because of the cost factors HVDC transmission is economical if bulk amount of power (1500-6000MW) transmitting over large distance (beyond 500kms) compared to HVAC transmission

Circuit Breaker Making Capacity

During short circuit, circuit breaker contacts opens and automatically closes after few cycles to determine whether the fault is cleared. 90% of the faults occur in power system are transient in nature and lasts for few cycles and clears on its own eg: touching of transmission lines with each other beacause of wind, touching of tree branches to transmission line momentarily, etc. Such faults clears on its own and after the transient power system restores to normal operation. It is not advised to open the breaker contacts and isolate the line for such small transients. Therefore auto re-closers are employed which when relay senses the fault trips the circuit breaker contacts. After some time breaker contacts again try to close, if there is no fault in that instant power system restores to normal operation. However if fault further exists circuit breaker contacts open up permanently until it is reset. The cycles of closing and opening is designed  for the circuit breaker depends on the application.

Therefore circuit breaker should close under short circuit condition. The MAKING CAPACITY of the circuit breaker depends on the ability to withstand the effects of electromagnetic forces which which are proportional to the square of the peak value of the making current. Making current of circuit breaker when closed on short circuit is the peak value of maximum current wave (including dc component) in the first cycle of the current after the circuit is closed by the breaker

For determing the making current of the circuit breaker we must multiply symmetrical breaking current by (2)^1/2 to convert to peak value from rms value. and then by 1.8 to take into account the doubling effect of maximum asymmetry.

Rated Making Current = 1.8 x (2)^1/2 rated short circuit breaking current

                                   = 2.55 x rates short circuit breaking current

                                   = 2.55 x symmetrical breaking capacity

Surge Impedance Loading (SIL) of Transmission Line

Loading of any transmission line depends on,

• Thermal limitation (I²R limitation)
• Voltage regulation 
• Stability Limitation

This is defined as the load (of unity power factor) that can be delivered by the line of negligible resistance.

Where V_LL is the receiving end voltage in kV and Z_o is the surge impedance in ohms, and SIL is the surge impedance loading or natural loading of the line

The above  expression gives a limit of the maximum power that can be delivered by a line and is useful in designing the transmission line. This can be used for the comparison of loads that can be carried on the transmission lines at different voltages

From the above expression power transmitted through a long transmission lines can be either increased by increasing the value of the receiving end line voltage (V_LL) or by reducing the surge impedance (Z_o). Voltage transmission capability is increased day by day, this is the most commonly adopted method for increasing the power limit of the heavily loaded transmission line. But there is a limit beyond which is neither economical nor practical to increase the receiving end line voltage

By applying some methods such as introducing series capacitors (capacitors in series with the transmission line) or shunt capacitors (capacitors in parallel with transmission lines) can be used to reduce the value of surge impedance (Z_o).

Surge Impedance Loading (SIL) can be increased by reducing the Surge impedance of the line. From the above expression Z_o can be decreased by either increasing the capacitance (C) of the line or by reducing the inductance (L) of the line. Inductance (L) of the transmission line cannot be reduced easily
 By use of the series capacitors surge impedance (Z_o) and the phase shift get reduced due to decrease in the line inductance (L). This improves the system stability limit. These capacitors also helps in reducing the line drops and so voltage variations. But this method causes difficulty under short circuit conditions of system as capacitors will get damage.

By use of shunt capacitors though the surge impedance (Z_o) is reduced but the phase shift of the system increases this affects the poor stability in the system specially when synchronous machines are under the load. This method is not employed in long transmission lines specially when stability limits are present