Applications and forms of differential relays   

      Differential relays take a variety of forms, depending on the equipment they protect. The definition of such a relay is “one that operates when the vector difference of two or more similar electrical quantities exceeds a predetermined amount. It will be seen later that almost any type of relay, when connected in a certain way, can be made to operate as a differential relay. In other words, it is not so much the relay construction as the way the relay is connected in a circuit that makes it a differential relay.

ABB Residual Current Breaker with Overload Protection RCBo

   Most differential-relay applications are of the “current-differential” type. The simplest example of such an arrangement is shown in Fig. 14. The dashed portion of the circuit of Fig. 14 represents the system element that is protected by the differential relay. This system element might be a length of circuit, a winding of a generator, a portion of a bus, etc.

   A current transformer (CT) is shown in each connection to the system element. The secondaries of the CTÕs are interconnected, and the coil of an overcurrent relay is connected across the CT secondary circuit.

This relay could be any of the a-c types that we have considered.

Fig. 14. A simple differential-relay application

Now, suppose that current flows through the primary circuit either to a load or to a short circuit located at X. The conditions will be as in Fig. 15. If the two current transformers have the same ratio, and are properly connected, their secondary currents will merely circulate between the two CT’s as shown by the arrows, and no current will flow through the differential relay.

Fig. 15. Conditions for an external load or fault

But, should a short circuit develop anywhere between the two CT’s, the conditions of Fig. 16 will then exist. If current flows to the short circuit from both sides as shown, the sum of the CT secondary currents will flow through the differential relay. It is not necessary that short-circuit current flow to the fault from both sides to cause secondary current to flow through the differential relay. A flow on one side only, or even some current flowing out of one side while a larger current enters the other side, will cause a differential current.

In other words, the differential-relay current will be proportional to the vector difference between the currents entering and leaving the protected circuit; and, if the differential current exceeds the relay’s pickup value, the relay will operate.

Fig. 16. Conditions for an internal fault

It is a simple step to extend the principle to a system element having several connections. Consider Fig. 17, for example, in which three connections are involved

Fig. 17 A three-terminal current-differential application

The principle can still be applied where a power transformer is involved, but, in this case, the ratios and connections of the CTÕs on opposite sides of the power transformer must be such as to compensate for the magnitude and phase-angle change between the power transformer currents on either side. This subject will be treated in detail when we consider the subject of power-transformer protection.It is only necessary, as before, that all the CT’s have the same ratio, and that they be connected so that the relay receives no current when the total current leaving the circuit element is equal vectorially to the total current entering the circuit element. A most extensively used form of differential relay is the “percentage-differential” type. This is essentially the same as the overcurrent type of current-balance relay that was described earlier, but it is connected in a differential circuit, as shown in Fig. 18.

Fig. 18. A percentage-differential relay in a two-terminal circuit

The differential current required to operate this relay is a variable quantity, owing to the effect of the restraining coil. The differential current in the operating coil is proportional to I1 – I2, and the equivalent current in the restraining coil is proportional to (I1 + I2)/2, since the operating coil is connected to the midpoint of the restraining coil; in other words, if we let N be the number of turns on the restraining coil, the total ampere-turns are I1N/2 + I2N/2, which is the same as if (I1 + I2)/2 were to flow through the whole coil.

The operating characteristic of such a relay is shown in Fig. 19. Thus, except for the slight effect of the control spring at low currents, the ratio of the differential operating current to the average restraining current is a fixed percentage, which explains the name of this relay. The term “through” current is often used to designate I2, which is the portion of the total current that flows through the circuit from one end to the other, and the operating characteristics may be plotted using I2 instead of (I1 + I2)/2, to conform with the ASA definition for a percentage differential relay.

The advantage of this relay is that it is less likely to operate incorrectly than a differentially connected overcurrent relay when a short circuit occurs external to the protected zone.

Fig. 19. Operating characteristic of a percentage-differential relay

Current transformers of the types normally used do not transform their primary currents so accurately under transient conditions as for a short time after a short circuit occurs.

This is particularly true when the shortcircuit current is offset. Under such conditions, supposedly identical current transformers may not have identical secondary currents, owing to slight differences in magnetic properties or to their having different amounts of residual magnetism, and the difference current may be greater, the larger the magnitude of short-circuit current. Even if the short-circuit current to an external fault is not offset, the CT secondary currents may differ owing to differences in the CT types or loadings, particularly in power-transformer protection. Since the percentage-differential relay has a rising pickup characteristic as the magnitude of through current increases, the relay is restrained against operating improperly.

Figure 20 shows the comparison of a simple overcurrent relay with a percentage-differential relay under such conditions. An overcurrent relay having the same minimum pickup as a percentage-differential relay would operate undesirably when the differential current barely exceeded the value X, whereas there would be no tendency for the percentage-differential relay to operate.

Fig. 20. Illustrating the value of the percentage-differential characteristic

Percentage-differential relays can be applied to system elements having more than two terminals, as in the three-terminal application of Fig. 21. Each of the three restraining coils of Fig. 21 has the same number of turns, and each coil produces restraining torque independently of the others, and their torques are added arithmetically.

The percent-slope characteristic for such a relay will vary with the distribution of currents between the three restraining coils.

Fig. 21. Three-terminal application of a percentage-differential relay

   Percentage-differential relays are usually instantaneous or high speed. Time delay is not required for selectivity because the percentage-differential characteristic and other supplementary features to be described later make these relays virtually immune to the effects of transients when the relays are properly applied. The adjustments provided with some percentage-differential relay will be described in connection with their application.

   Several other types of differential-relay arrangements could be mentioned. One of these uses a directional relay. Another has additional restraint obtained from harmonics and the d-c component of the differential current. Another type uses an overvoltage relay instead of an overcurrent relay in the differential circuit. Special current transformers may be used having little or no iron in their magnetic circuit to avoid errors in transformation caused by the d-c component of offset current waves. All these types are extensions of the fundamental principles that have been described, and they will be treated later in connection with their specific applications.

    There has been great activity in the development of the differential relay because this form of relay is inherently the most selective of all the conventional types. However, each kind of system element presents special problems that have thus far made it impossible to devise a differential-relaying equipment having universal application.

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The Advantages of AC Power over DC Power

The Advantages of AC Power over DC Power

   Which is better, alternating current or direct current? Back in the 1880s, that question generated an all-out current war between two geniuses. On the one side, Thomas Edison pushed that direct current was safer and would cause less hazard of electrocution. However, it proved to be cheaper and easier to transmit the power of alternating current over long distances. This was the stance of Edison's competitor, Nikola Tesla.

More Power

   Probably the biggest advantage of AC over DC is that you can generate much more power from AC than DC. Alternating current is generated by large turbines. Direct current normally comes from batteries or sometimes from solar panels. Solar panels large enough to power entire cities would take huge amounts of land. Batteries use chemical reactions to produce electricity. Producing a huge battery would be expensive and impractical. Large turbines can be easily built and powered using steam, nuclear or hydraulic power.

Long Distance Transmission

   The ability to generate higher voltages with AC translates into the ability to transmit that power over longer distances. Alternating current is generated at power stations and transmitted through power lines to substations that can boost AC and keep it going farther distances.

Conversion

   In some operations, direct current may be preferred. However, it is easy to convert AC to DC by using simple transformers. It is much more difficult and costly to convert DC to AC.

Alternating Current in the Home

    One of the reasons that AC is better than DC in the home is that AC can be stepped up or down using transformers. On the other hand, when using DC, the power supply must match the load it powers. If you have a 120 volt lamp, then you need a 120 volt battery. If the battery is 240 volts, then the voltage can be reduced using a resistor, but, that would waste half the voltage.

Commercial Advantages

  Another advantage of the use of alternating current lies in manufacturing and other production facilities that drive the economy. Electric motors are used to drive conveyors and other equipment. The advantages of the AC motor are that they can yield a higher output of horsepower than DC motors and they are simpler in function than a DC motor. Also, transformers allow AC to be stepped up or down where it is needed to drive different sized motors.

What Is the Difference Between AC & DC Electricity?

     It includes all basic difference.

The Advantages & Disadvantages of AC and Dc

   AC may be better for one application, while DC may be better for another. Engineers need to Take various aspects of a particular application into account to decide between the two.

 VOLTAGE Transformation

     A big advantage that AC electricity has over DC electricity is that it can be easily transformed from a high voltage level to a low voltage level using a device known as a transformer. The cables used to transmit electricity over long distances resist that flow, so high voltages have to be used to push the electricity along them. These voltages would be dangerously high if they came into homes or business environments, so transformers are used to lower the voltage of the electricity before it is delivered to the end user. In the United States, electricity is delivered to end users at 120 volts.]

Reactance

       When electricity flows down a cable, it generates an electromagnetic field. When the current changes, as it does with AC current, a counter electro-magnetic field is produced that acts as a resistance to the power being transmitted. These means that AC transmission of electricity loses power due to both resistance and to reactance. Because DC power transmission never changes direction, it is not susceptible to power loss as a result of reactance.


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TYPES AND FUNCTIONS OF SUB-STATION

Types of Sub-station

Substations are of three types. They are:

>Transmission Substation

>Distribution Substation

>Collector Substation

Transmission Substation

A transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains high-voltage switches that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station may have transformers to convert the voltage from voltage level to other, voltage control devices such as capacitors, reactors or Static VAR Compensators and equipment such as phase shifting transformers to control power flow between two adjacent power systems. The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, many circuit breakers and a large amount of protection and control equipment (voltage and current Transformers, relays and SCADA systems). Modern substations may be implemented using International Standards such as IEC61850.

Distribution Substation

A distribution substation transfers power from the transmission system to the distribution system of an area. It is uneconomical to directly connect electricity

consumers to the high-voltage main transmission network, unless they use large amounts of power. So the distribution station reduces voltage to a value suitable for local distribution. The input for a distribution substation is typically at least two transmission or sub transmission lines. Input voltage may be, for example, 400KV or whatever is common in the area. Distribution voltages are typically medium voltage, between 33 and 66 kV depending on the size of the area served and the practices of the local utility. Besides changing the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line.

Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching and, switching and backup systems on the low-voltage side. Most of the typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side.

 Collector substation

In distributed generation projects such as a wind farm, a collector substation may be required. It somewhat resembles a distribution substation although power flow is in the opposite direction. Usually for economy of construction the collector system operates around 35 KV, and the collector substation steps up voltage to a transmission voltage for the grid. The collector substation also provides power factor correction, metering and control of the wind farm.

Functions of the substation

a. To Change voltage from one level to another.

b. To Regulate voltage to compensate for system voltage changes.

c. To Switch transmission and distribution circuits into and out of the grid system.

d. To Measure electric power quantity flowing in the circuits.

e. To Connect communication signals to the circuits.

f. To Eliminate lightning and other electrical surges from the           system.

g. To Connect electric generation plants to the system.

h. To Make interconnections between the electric systems of more than one utility. 

Substation Transformer Type

Further, transmission substations are mainly classified into two types depending on changes made to the voltage level. They are:

a. Step-Up Transmission Substations.

b. Step-Down Transmission Substations.

a. Step-Up Transmission Substation

A step-up transmission substation receives electric power from a nearby generating facility and uses a large power transformer to increase the voltage for transmission to distant locations.

There can also be a tap on the incoming power feed from the generation plant to provide electric power to operate equipment in the generation plant.

b. Step-Down Transmission Substation

Step-down transmission substations are located at switching points in an electrical grid. They connect different parts of a grid and are a source for sub transmission lines or distribution lines.

Layout

a. Principle of Substation Layouts

Substation layout consists essentially in arranging a number of switchgear components in an ordered pattern governed by their function and rules of spatial separation.

b. Special Separation

i. Earth Clearance: This is the clearance between live parts and earthed structures, walls, screens and ground.

ii. Phase Clearance: This is the clearance between live parts of different phases.

iii. Isolating Distance: This is the clearance between the terminals of an isolator and the connections.

iv. Section Clearance: This is the clearance between live parts and the terminals of a work section. The limits of this work section, or maintenance zone, may be the ground or a platform from which the man works. 

c. Separation of maintenance zones

Two methods are available for separating equipment in a maintenance zone that has been isolated and made dead.

i. The provision of a section clearance

ii. Use of an intervening earthed barrier The choice between the two methods depends on the voltage and whether horizontal or vertical clearances are involved.

i. A section clearance is composed of the reach of a man taken as 8 feet plus an Earth clearance.

ii. For the voltage at which the earth clearance is 8 feet the space required will be the same whether a section clearance or an earthed barrier is used.

Maintenances

Maintenance plays a major role in increasing the efficiency and decreasing the breakdown. The rules and basic principle are discussed.

Separation by earthed barrier = Earth Clearance + 50mm for barrier + Earth Clearance Separation by section clearance = 2.44m + Earth clearance

i. For vertical clearances it is necessary to take into account the space occupied by the equipment and the need for an access platform at higher voltages.

ii. The height of the platform is taken as 1.37m below the highest point of work.

Maintenance is done through two ways:

a. By Establishing Maintenance Zones.

b. By Electrical Separations.

a. Establishing Maintenance Zones

Some maintenance zones are easily defined and the need for them is self evident as in the case of a circuit breaker. There should be a means of isolation on each side of the circuit breaker, and to separate it from adjacent live parts when isolated either by section clearances or earth barriers.

b. Electrical Separations

Together with maintenance zoning, the separation, by isolating distance and phase clearances, of the substation components and of the conductors interconnecting them constitute the main basis of substation layouts. There are at least three such electrical separations per phase that are needed in a circuit:

i. Between the terminals of the bus bar isolator and their connections.

ii. Between the terminals of the circuit breaker and their connections.

iii. Between the terminals of the feeder isolator and their connections.

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power factor correction techniques

In this topic we are going to discuss about the various power factor correction technique used in the substation and they mentions as well as protection of this equipments.

       Under normal operating conditions certain electrical loads draw not only active power from the supply (kilowatts KW) but also reactive power (reactive KVA, KVAR). This reactive power has no useful function, but is necessary for the equipment to operate correctly. Loads such as induction motors, welding equipment, arc furnaces and fluorescent lighting would fall into this category.

Definition

The Power Factor of a load is defined as being the ratio of active power to total demand. The uncorrected power factor of a load is cos Ø (where Ø is the phase angle between the uncorrected load and unity), and the corrected power factor is cos Ø2 (where Ø2 is the phase angle between the corrected load and unity). As cos Ø approaches to unity, reactive power drawn from the supply is minimized

Compensating Capacitor

A capacitor inside an op-amp that prevents oscillations is called compensating Capacitor. Also any capacitor that stabilizes an amplifier with a negative-feedback path. Without this capacitor, the amplifier will oscillate. The compensating capacitor produces a low critical frequency and decreases the voltage gain at a rate of 20 dB per decade above the mid-band. At the unity gain frequency, the phase shift is in the vicinity of 270°.

When the phase shift reaches 360°, the voltage gain is less than 1 and oscillations are impossible. The series capacitor is connected to compensate for the line inductance and thus decrease the line reactance so that more power can be transferred through the line thus the system stability can be increased.

The question is about connecting Capacitors in SERIES. Series connection is done for improving STABILITY of the network and for transferring more power (by reducing the resultant reactance) i.e to improve the power transfer capability but not for improving power factor. Power factor will be improved by connecting capacitors in parallel to the load.

Power factor correction

In electric power distribution, capacitors are used for power factor correction. Such capacitors often come as three capacitors connected as a three phase load. Usually, the values of these capacitors are given not in farads but rather as a reactive power in volt-amperes reactive (VAR). The purpose is to counteract inductive loading from devices like electric motors and transmission lines to make the load appear to be mostly resistive.

Individual motor or lamp loads may have capacitors for power factor correction, or larger sets of capacitors (usually with automatic switching devices) may be installed at a load center within a building or in a large utility substation.

P.F Correction

When using power factor correction capacitors, the total KVAR on the load side of the motor controller should not exceed the value required to raise the no-load power factor to unity. Over corrective ness of this value may cause high transient voltages, currents, and torques that can increase safety hazards to personnel and possibly damage motor driven equipment.

Never connect power factor correction capacitors at motor terminals on elevator motors, plugging or jogging applications, multi-speed motors or open transition, wyedelta, auto-transformer starting and some part-winding start motors.

If possible, capacitors should be located at position 2. This does not change the current flowing through motor overload protectors. Connection of capacitors at position 3 requires a change of overload protectors. Capacitors should be located at position 1 for applications listed in paragraph 2 above. Be sure bus power factor is not increased above 95% under all loading conditions to avoid over excitation.

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