What is Role of DC Supply in Power System Protection?

IN high voltage substations, an independent source of supply is necessary for actuating the protective gear, operation of trip relays, release of trip order to the circuit breakers, interlocking, annunciation, audible alarm, remote position indication and other similar purposes.

The supply is particularly important for trip circuits. A reliable source of supply is necessary which can perform the above functions at the time of system faults. The choice is to select one of the following two systems:

1. AC supply fro station auxiliary transformer.

2. DC supply from independent source that is from storage batteries.

If we use AC supply from the station transformer, it can go to very low voltage levels in case of faults very close to the station. Therefore this source is not reliable and can not be used for station protective relays.

An independent supply from DC storage batteries is ideal for protective relays and switchgear trip circuit. This supply is not affected by the system faults. Lead acid accumulators or Alkaline accumulators can be used for this purpose. Lead acid is most commonly used. Complete DC equipment for a substation may be divided into three parts that is, storage batteries, charging equipment and distribution board.

It is utmost important to monitor that dc supply circuits and trip circuits remain always healthy and dc supply is available at points designed in the scheme, for this purpose trip circuit supervision relays are used.

Relays in Power System Protection

A relay is a device which makes a measurement or receives a controlling signal in consequence of which it produces a sudden pre-determined changes in one or more electrical output circuits.

A protection relay is a relay which responds to abnormal conditions in an electrical power system and controls a circuit breaker so as to isolate the faulty section of the system with the minimum interruption to the service.

Electromechanical relays

Electromechanical relay is a conventional relay in which the measurement is performed by moveable parts. The operation of such relays is based upon the following effects of electrical current:

1. Electro-magnetic attraction

2. Electro-magnetic induction

3. Thermal effect (Heat generation, some electromechanical relays respond to gas pressure generated due to heat of arc like Buchholz relay)

Static Relays

The expansion and growing complexity of modern power system required high performance and sophisticated characteristics from protection relays. This was possible with the use of semiconductors and other components in static relays.

Numerical Relays

The Numerical relays purely work on mathematical solution of different equations. They are micro processor based relays. Tripping decisions are not made by any measuring elements but are done by micro computers who continuously calculate and monitor the system data. Main advantages of Numerical Relays are:

1. Highly economical

2. Continuous self monitoring

3. More availability

4. Less work at panel fabrication

5. High flexibility in use

6. Memory

7. Possibility of remote control

8. Possibility of downloading/uploading information to computers

Important requirements of relays are:

1. Reliability

The protection relay should be reliable. It should not  fail to operate when faults and abnormal conditions, to which it is meant to respond appear. At some times it should not perform false operation.

2. Selectivity

Protection is arranged in zones, which should cover the power system completely, leaving no part unprotected.

When a fault occurs the protection is required to select and trip only the nearest circuit breakers. This is also known as “Discriminate tripping”.

A Proper coordination of the operating and tripping characteristic is essential to achieve selective tripping of protective device connected in series in the network and thus assure high level of reliability of supply required by the consumers.

Today main challenge for protection engineer is to design a very intelligent protection scheme which in case of fault would isolate the minimum part of the network. Sometime a fault on one point causes tripping of unrelated breakers resulting in unwanted tripping.

Protection engineer needs to analyze this and requires sufficient data from affected circuit to recommend the remedial steps to have better security of the system.

3. Sensitivity

The relay should have sufficient sensitivity so that it picks up under minimum fault conditions within its own protective zone. It should operate for faults at farthest end of zone under conditions of minimum generation.

4. Fast

The relay should clear the faults in shortest possible time. However, speed must not be obtained at the cost of selectivity.

5. Stability

Ability of relay to withstand the changes outside its zone.

What is the purpose of Power System Protection

The power system protection can not prevent system faults, but it can limit the damage caused by short circuits while protecting people and equipment from damage. Goals are to selectively clearing faults in milliseconds and protecting equipments from overload conditions. The damages are due to short circuits in a system, abnormal conditions in the system and majorly human mistakes.

The basic purpose of installation of protective relays are:

1. To protect very expensive equipment like transformers, generators, motors, cables and other installed components.

2. To ensure the interruption of supply to minimum number of consumers in case of system fault.

3. To minimize the production losses of the factories.

4. To avoid major shutdowns.

Since the protection system limits the damages/losses and keeps the system running therefore it is considered a vital importance for the system.

What is Soft Starter and How it Works?

Technically a soft starter is any device that will reduce the torque delivered to the power train. Mechanically, this can be a clutch, fluid drive, magnetic coupling, short coupling or any of the variety of devices that allow the motor to start-up across the line while slowly applying the shaft torque to the load to avoid “torque shock”. Electrically it can be any system that reduces the torque by virtue of reduced voltage or a change in motor connection.

Changing the motor terminal voltage reduces the torque because the motor output torque varies by the square of applied voltage. So if 50 percent voltage is applied to a motor, it will produce  25 percent of its available torque at that point.

Reduced Voltage starting can be accomplished in several ways as well, A common method is to use an auto transformer that drops the motor voltage during starting, then is switched out so that the motor gets full voltage when running. This method is called reduced voltage auto transformer starting. Similar to this are reactor and primary resistor starters which drop the voltage through those devices as well. All of the above technologies can be and often are referred to as “Soft Start” devices, but more recently this terminology has come to usually mean one specific type, the solid state reduced voltage starter.

The solid state reduced voltage starter(SSRV) uses high speed switching devices call SCRs(silicon controlled rectifiers) to switch on for only a portion of each half of the sine-wave line power. By doing so, the RMS voltage getting to the motor is reduced proportionally by the amount of time the switch is delayed. So if the SCR is not allowed to be conducting until the sine wave is already 1/2 over with, the output RMS will be 1/2 of the line voltage. By moving the gate point further back in the sine wave, the RMS voltage is increased until the SCR is being gated at zero crossing point and the motor is getting full line voltage. The speed at which the SCR gating is backed up is called the Ramp Time, and can typically be anywhere from a fraction of a second to 60 seconds. Although longer times are technically possible, most AC motors applications will not allow this because the increased current caused by the reduced voltage will begin to exceed the thermal safety limits of the motor itself, particularly the rotor. In addition the ramp time can be over ridden by a current limit setting, which determines the motor current through feedback sensors and stops the gate advancements in order to maintain the particular current setting. This feature is useful when the power system has limited delivery capabilities, such as weak utility lines or portable generators.

Finally once the motor is at full voltage the SCR firing becomes unnecessary and it is often beneficial to use a bypass contactor to shut power around SCRs. SCRs are not perfect conductors, and will reject approx 1.5 Watt of heat per running load ampere per phase. So on a  phase 100A motor, the SCRs will be rejecting 450W of heat into the enclosure continuously. A bypass contactor is a good way of avoiding that heat buildup without introducing dust, moisture or other contaminants into the enclosure.