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 substationcreates 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.

Neutral Grounding Practice in Power System

• Generally on neutral grounding is provided at each voltage. There will be several voltage levels between the generation of the power and distribution of the power in the power system. Only one ground is provided for each voltage level of the power system
• Grounding of the power system is provided at the source and not at the load end
• Each of the major bus section in the system are grounded
• For generator grounding, neutral of the generator is grounding through a resistance which limits the stator fault current. The value of the resistor employed for the grounding the generator decides the percentage of the generator windings left unprotected
• Synchronous motors and synchronous capacitors are provided with reactance type of grounding. This reactance grounding provides additional reactance which provides additional lagging currents which nullifies the capacitive grounding currents
• When several generators are connected to a common neutral bus, the bus is connected to the ground through a single grounding device. Disconnect switches are used to ground the desired generators to the neutral bus
• When several generators are operating in parallel, only one generator neutral is earthed. This is to avoid the interference between the zero sequence currents
• In generating stations there is a provision to ground neutral of at least two generators, though one at a time. The other generator neutral is grounded when the first generator is out of service
• When there are one of the two supply sources, no switching equipment is used in the grounding circuit.
• For the protection purpose, the neutral point of the star side of the power transformer is usually grounded
• The star connected secondary sides of the protective CTs and PTs are grounded at one point. This ensures stable neutral, proper measurement of the voltages and currents, kWh and kVA on the secondary side measuring instruments and controls
• For the circuits between 3 kV and 33 kV resistance or reactance grounding is used. But for low voltages less than 600V and high voltages above 33 kV solid or effective grounding is used. Effective grounding limits the voltages of healthy phases to line-to-neutral values in the events of ground faults and also eliminates the arcing grounds. The effective grounding causes the ground fault currents of very high magnitudes flow through the machine. But modern day protection systems are very sensitive and fast operating so that faults are cleared in very short time

Generator Protection Interview Questions & Answers

What are the common types of generator faults?

The common types of faults occurring in synchronous generators are:

Stator winding faults (phase to phase faults, phase to earth faults, inter turn faults), Rotor winding faults (conductor to earth faults, open circuit faults, inter turn faults), failure of prime mover, failure of the field, unbalanced loading, over loading, over-voltage at generator terminals, over-speed, ventilation failure, loss of excitation.

For what type of faults does differential protection is provided?

Differential protection responds to the phasor difference between two or more electrical quantities. It operates for the internal faults occur in generators or transformers. For external faults differential protection does not operate.

What is the disadvantage of ordinary differential protection? 

When differential relaying is used for protection, the CTs at both sides of the generator winding must be of equal accuracy. Otherwise if the CT errors are excessive it will cause the mal-operation of the relay. To safeguard against such disadvantages percentage differential protection is employed.

What is advantage of using percentage differential relay protection?

Advantages: 

It does not require CTs with air gaps or special balancing features

It permits a low fault setting to be used and this ensures maximum protection of the windings

It ensures complete stability under the most severe through fault conditions

Explain Differential Protection?

Differential protection is generally provided for the equipment or group of equipments which are to be protected against internal faults. They are the primary protection systems (operates faster) for any internal faults occurring within the protection zone.

Under healthy conditions the currents at both ends of the windings will be equal. EMFs induced in the secondaries of the CTs will be equal and so no current flows through the operating coil of the relay. When an earth fault or phase to phase fault occurs the condition no longer holds good and the differential current flows through the relay operating coils makes the relay to operate. Relay operates for the faults occurring within the zone of protection.

Why Over Current Protection is not necessary for modern generators?

Over Current protection is not considered necessary for modern alternators because these are capable of withstanding a complete short circuit at their terminals for sufficient time without much over heating and damage.

What type of protection is provided for the generators against over heating of the generator stator?

Resistance temperature detector

Which type of relays are used for the Merz-Price protection system for alternator?

Merz-Price protection is differential protection provided for the alternator. The relays used in the Merz-Price protection system of alternator are instantaneous electro-magnetic type protection.

Why large alternator is grounded with large resistance?

Large capacity of alternators are typically provided with resistance grounding. High value of resistor is connected to the neural path to the ground. If the generator is delta connected, then it is grounded with the help of zig-zag transformer or (Delta-Star) transformer such that a high resistor is connected between the neutral point and the ground. Resistance grounding is provided so as to limit the short circuit current flowing during earth fault to stator winding in order to provide protection against mechanical stresses and melting of winding during Line to Ground short circuit. In large generators fault current is limited as low as 10 to 15 amperes during short circuit.

Why it is not necessary to provide protection for turn to turn fault in alternator?

The coils of the modern alternators are single turn and therefore it is not necessary to provide protection for turn to turn faults.

Why it is necessary to suppress field immediately after disconnection of faulty alternator from the system?

In the event of fault on the generator windings even though the generator circuit breaker is tripped, the fault is continuous to fed as long as the excitation will exist because the emf is induced in the generator. Hence it is necessary to suppress the field immediately after disconnecting the faulty generator from the system.

Why it not necessary to provide over-voltage protection in turbo-alternator?

The over voltage occurs when the prime mover speed increases due to sudden loss of the load on the generator and the speed control governors in case of turbo-generators are very sensitive to the speed variations and therefore generator over voltage of significant duration or magnitude does not generally occur. This is the reason why generators are not provided with over voltage protection.

Advantages & Disadvantages Electromagnetic Relays

In Electromagnetic relays  operating current flows through the coil. When this operating current increases, coil energizes the electromagnet. When the operating current becomes large, the magnetic field produced by electromagnet is high such that this magnetic field pulls the armature or plunger making the trip circuit contacts to close. Some of the advantages, disadvantages and applications of electromagnetic relays are explained below

Advantages or merits:

• Electromagnetic relays have fast operation and fast reset
• They can be used for both ac and dc systems for protection of ac and dc equipments
• Electromagnetic relays operating speeds which has the ability to operate in milliseconds are also can be possible
• They have the properties such as simple, robust, compact and most reliable
• These relays are almost instantaneous. Though instantaneous the operating time of the relay varies with the current. With extra arrangements like dashpot, copper rings etc. slow operating times and reset can be possible.

Disadvantages or demerits:

• High burden level instrument transformers are required (CTs and PTs of high burden is required for operating the electromagnetic relays compared to static relays)
• The directional feature is absent in electromagnetic relays
• Requires periodic maintenance and testing unlike static relays
• Relay operation can be affected due to ageing of the components and dust, pollution resulting in spurious trips
• Operation speed for an electromagnetic relays is limited by the mechanical inertia of the component

Protective Relays

A special type of relay is one which monitors the current, voltage, frequency, or any other type of electric power measurement either from a generating source or to a load for the purpose of triggering a circuit breaker to open in the event of an abnormal condition. These relays are referred to in the electrical power industry as protective relays.

The circuit breakers which are used to switch large quantities of electric power on and off are actually electromechanical relays, themselves. Unlike the circuit breakers found in residential and commercial use which determine when to trip (open) by means of a bimetallic strip inside that bends when it gets too hot from overcurrent, large industrial circuit breakers must be “told” by an external device when to open. Such breakers have two electromagnetic coils inside: one to close the breaker contacts and one to open them. The “trip” coil can be energized by one or more protective relays, as well as by hand switches, connected to switch 125 Volt DC power. DC power is used because it allows for a battery bank to supply close/trip power to the breaker control circuits in the event of a complete (AC) power failure.

Protective relays can monitor large AC currents by means of current transformers (CT’s), which encircle the current-carrying conductors exiting a large circuit breaker, transformer, generator, or other devices. Current transformers step down the monitored current to a secondary (output) range of 0 to 5 amps AC to power the protective relay. The current relay uses this 0-5 amp signal to power its internal mechanism, closing a contact to switch 125 Volt DC power to the breaker’s trip coil if the monitored current becomes excessive.

Likewise, (protective) voltage relays can monitor high AC voltages by means of voltage, or potential, transformers (PT’s) which step down the monitored voltage to a secondary range of 0 to 120 Volts AC, typically. Like (protective) current relays, this voltage signal powers the internal mechanism of the relay, closing a contact to switch 125 Volt DC power to the breaker’s trip coil is the monitored voltage becomes excessive.

There are many types of protective relays, some with highly specialized functions. Not all monitor voltage or current, either. They all, however, share the common feature of outputting a contact closure signal which can be used to switch power to a breaker trip coil, close coil, or operator alarm panel. Most protective relay functions have been categorized into an ANSI standard number code.

PROTECTION INTERVIEW QUESTIONS WITH ANSWERS

1. Where Does Negative Phase Sequence Relay Is Employed?

Negative sequence relay is employed for the protection of generators and motors against unbalanced loading that may arise due to phase to phase faults.

2. What Is The Operation Principle Of Differential Relay?

Differential relay operates when the phasor difference of two or more similar electrical quantities exceeds a pre-determined amount.

3. Why Distance Protection Is Preferred As Primary Protection Compared To Over Current Protection For Transmission Lines?

Distance relay is superior to over current protection for the protection of transmission lines. Some the reasons are faster protection, simpler coordination, simpler application, permanent settings without need for readjustment, less effect of the amount of generation and fault levels, fault current magnitude, permits the high line loading.

4. Why Biased Differential Protection Is Preferred Over Simple Differential Protection?

Biased differential relay is preferred because its operation is not affected by the trouble arising out of the difference in the CTs ratios for high values of external short circuit currents.

5. Where Impedance Relay, Reactance Relay And Mho Relays Are Employed?

The Impedance relay is suitable for the phase faults relaying for the lines of moderate lengths Reactance type relays are employed for the ground faults while Mho type of relays are best suited for the long transmission lines and particularly where synchronizing power surge may occur.

6. What Is Percentage Differential Relay?

It is a differential relay where the operating current required to trip can be expressed as a percentage of load current.

7. What Are The Main Functions Of Differential Relays?

• High speed operation; High sensitivity
• Adequate short circuit thermal rating
• Ability to operate operate with low values of voltage
• Burden must not be excessive
• There should be no voltage and current creep

8. What Is Meant By “relay Settings”?

Relay settings means actual value of the energizing or characteristic quantity at which the relay is designed to operate under given conditions.

9. Define Plug Setting Multiplier?

Plug Setting Multiplier and is defined as the ratio of fault current in the relay coil to the pick up value.

10. Where Is Directional Relay Used?

Directional relay are used when graded time overload protection is applied to ring mains and interconnected networks.

11. For What Type Of Fault Does Buchholz Relay Is Employed?

Buchholz relay provides protection only against transformer internal fault.

12. How Definite Time Lag Is Achieved In Attraction Armature Relays?

The instantaneous type attraction armature can be made a definite time lag or inverse time lag by using a oil dash pot, an air escapement chamber a clock work mechanism or by placing a fuse in parallel with it.

13. What Is Protective Relay?

It is an electrical device designed to initiate the isolation of a part of the electrical installation, or to operate an alarm signal, in the event of abnormal condition or a fault. In simple words relay is an electrical device that gives signal to isolation device (eg: Circuit Breaker) after sensing the fault and helps to isolate the fault system from the healthy electrical system

14. What Are The Different Relays That Employed For Protection Of Apparatus And Transmission Lines?

• Over current relay; Directional relay; Distance relay; Under Voltage relay; Under-frequency relay
• Thermal relay; Differential relay; Phase sequence relays; Pilot relays

15. How The Electrical Power System Protection Is Divided?

• Generator protection; Transformer protection; Busbar protection
• Transmission line protection and Feeder protection

16. How Relays Are Connected In The Power System?

The relays are connected to the power system through the current transformer (CT) or potential transformer (PT).

17. What Are Different Types Of Principles Of Operation Of Electromechanical Relays?

Electromechanical relays operate by two principles: Electro-magnetic attraction and electro-magnetic induction. In electromagnetic attraction relay plunger is drawn to the solenoid or an armature is attracted to the poles of the electromagnet. In case of electro-magnetic induction, principle of operation is similar to induction motor. Torque is developed by electromagnetic induction principle.

18. Action Carried Out By The Relay And Circuit Breaker During Fault Condition?

After the relay sensing the fault condition, relay operates and close the trip coils. The effect of this will be circuit breaker operate to open the contacts.

Power capacitors for Power Factor Improvement

The more popular method of improving the power factor on low voltage distribution systems is to use power capacitors to supply the leading reactive power required.

The amount and location of the corrective capacitance must be determined from a survey of the distribution system and the source of the low-power factor loads.

In addition, the total initial cost and payback time of the capacitor installation must be considered.

To reduce the system losses, the power factor correction capacitors should be electrically located as close to the low-power factor loads as possible. In some cases, the capacitors can be located at a particular power feeder. In other cases, with large horsepower motors, the capacitors can be connected as close to the motor terminals as possible.

The power factor capacitors are connected across the power lines in parallel with the low-power factor load. The number of kilovars of capacitors required depends on the power factor without correction and the desired corrected value of the power factor. 

How to improve power factor?

To improve the power factor for a given load, the reactive load component (kvar) must be reduced.

This component of reactive power lags the power component (kW input) by 90 electrical degrees, so that one way to reduce the effect of this component is to introduce a reactive power component that leads the power component by 90 electrical degrees.

Several methods are used to improve the power factor in a system installation. One method that can be employed in large systems is to use synchronous motors to drive low-speed loads that require continuous operation.

A typical application for a synchronous motor is driving a low-speed air compressor, which provides process compressed air for the plant.

The synchronous motor is adjusted to operate at a leading power factor and thus provide leading kilovars to offset the lagging kilovar of inductive-type loads such as induction motors.

Synchronous motors are usually designed to operate at an 80% leading power factor and to draw current that leads the line voltage rather than lags it, as is the case with induction motors and transformers.

Electric motors & power factor influence

A low power factor causes poor system efficiency. The total apparent power must be supplied by the electric utility. With a low power factor, or a high-kilovar component, additional generating losses occur throughout the system.

To discourage low-power factor loads, most utilities impose some form of penalty or charge in their electric power rate structure for a low power factor.

When the power factor is improved by installing power capacitors or synchronous motors, several savings are made:

1. A high power factor eliminates the utility penalty charge. This charge may be a separate charge for a low power factor or an adjustment to the kilowatt demand charge.
2. A high power factor reduces the load on transformers and distribution equipment.
3. A high power factor decreases the I²R losses in transformers, distribution cable, and other equipment, resulting in a direct saving of kilowatt-hour power consumption.
4. A high power factor helps stabilize the system voltage.

What is a surge arrester?

A surge arrester is a device to protect electrical equipment from over-voltage transients caused by external or internal events.

Now let us understand these terms in the definition:

Transients: sudden change which lasts for a short time in other words a momentary variation in current, voltage or frequency.

External events: In electrical power system, the most common surge inducing event is lightning.

Internal events: switching events, for example sudden switching of loads, switching of reactive power sources( like inductor or capacitor banks), short-circuits and sparking etc.

What does the surge arrester do?

It basically diverts the high voltage current directly to the insulation or to the ground to avoid damaging the system. It does not absorb all of the high voltage that passes through it. It simply diverts it to the ground or clamps it to minimize the voltage that passes through it using MOV or Metal Oxide Varistor. Surge arrestors are NOT generally designed to offer protection against a direct lightning strike, but rather against electrical transients which might occur due to lightning in the vicinity of the line or conductor.

Wave Trap

• Wave trap is a parallel tuned inductor - capacitor tank circuit made to be resonant at desired communication frequency . It reduces corona losses in transmission lines in power system.
• Wave trap is installed in the substation for trapping the high frequency communication signal sent on the line from remote substation and diverting them to the telecom panel in substation control room. These high frequency signal should not be coming on the buses as these may damage the equipments .
• A wave trap is a device that allow only a particular frequency to pass through it that it filters the signals coming on to it . So a wave trap is connected between buses and the transmission line which allow only 50 Hz signal to pass through it.
• This is relevant in power carrier communication (PLCC) system for communication among various substation without dependence on telecom company network . The signals are primarily teleportation signal and in addition , voice and data communication signal.

SF6 Gas Properties

Introduction

SF6 is a combination of sulfur and fluorine its first synthesis was realized in 1900 by French researchers of the Pharmaceutical Faculty of Paris.

It was used for the first time as insulating material, In the United States about 1935. In 1953, the Americans discovered its properties for extinguishing the electric arc. This aptitude is quite remarkable.

 

Physical properties

It is about five times heavier than air, and has a density of 6.1 4kg / m3. It is colorless, odorless and non-toxic.

 

Tests have been carried out replacing the nitrogen content of air by SF6 (the gaseous mixture consisted of 79 % SF6 and 24 % oxygen): five mice were then immersed in this atmosphere for 24 hours, without feeling any ill effects.

 

It is a gas which the speed of sound propagation is about three times less than in air, at atmospheric pressure. The interruption of the arc will therefore be less loud in SF6 than in air.

The dielectric strength of SF6 in on average 2.5 times that of air, and, by increasing pressure, it can be seen that the dielectric strength also increases and than around 3.5 bar of relative pressure, SF6 has the same strength as fresh oil.

 The principal characteristics of the gas are as follows:

Molar mass                                   146.078

Critical temperature                     45.55°C

Critical pressure                           37.59 bars

In short, SF6 at atmospheric pressure is a heavier gas than air, it becomes liquid at - 63.2°C and in which noise propagates badly.

SF6 on the market

SF6 which is delivered in cylinders in liquid phase, contains impurities (within limits imposed by IEC standards No. 376)

 Carbon tetra fluoride             (CF4) 0.03 %

 Oxygen + nitrogen                   (air) 0.03 %

 Water                                            15 ppm

 C02                                                 traces

 HF                                                  0.3 ppm

 SF6 is therefore                        99.99 % pur.

 

SF6 Safety precautions:

Today there is no known dielectric and breaking agent combined better than SF6 gas.

 Initial state

In its initial state, before it has undergone thermal stress (usually the electric arc); SF6 is perfectly safe in normal conditions:

- It is non-toxic,

- It is uninflammable,

- It will not explode.

This does not mean that no precautions need to be taken: because of its lack of oxygen, this gas will not support life.

However, the concentration of SF6 would have to be high, since the International electro technical Commission (IEC) has shown that five mice left for 24 hours in an atmosphere of 79 % SF6 and 21 % oxygen will not only remain alive but will show no signs of abnormal behavior.

Man dies when the oxygen level of the gas he is breathing falls below 12 %.

 

 

CIRCUIT BREAKER TESTING

TEST	EQUIPMENTS USED
1.    Insulation Resistance Measurement	a.    5KV Insulation Tester (Make: Megger)        b.    3 Pole timer <Egil> (Make: Megger)        c.    Multimeter (Fluke)        d.    CPC 100 (Make: Omicron)
2.    Coil Resistance Measurement        (Trip and Close)
3.    Closing and Tripping coil pick up
4.    Breaker Timer Test        Close: <60ms        Open: <50ms
5.    Contact Resistance Measurement        Per joint: <20µ ohm
6.    Electrical & Mechanical Operational      check

*Electrical & Mechanical Operational Check Includes

·         Name Plate Details

·         Rack in/out check

·         Spring charge mechanism (Electrical & Mechanical)

·         Current drawn by spring charge motor & Spring charge time

·         Anti-pumping check, Open / Close indication check

·         Auxiliary contact checks & Counter Operation Checks

·         Tripping and Closing operation