AC System and DC System

DC system :

Advantages of DC Transmission :

There are two conductors used in DC transmission while three conductors required in AC transmission. There are no Inductance and Surges (High Voltage waves for very short time) in DC transmission. Due to absence of inductance, there are very low voltage drop in DC transmission lines comparing with AC (if both Load and sending end voltage is same). A DC System has a less potential stress over AC system for same Voltage level. Therefore, a DC line requires less insulation. In DC System, There is no interference with communication system. In DC Line, Corona losses are very low. In High Voltage DC Transmission lines, there are no Dielectric losses. In DC Transmission system, there are no difficulties in synchronizing and stability problems. DC system is more efficient than AC, therefore, the rate of price of Towers, Poles, Insulators, and conductor are low so the system is economical. In DC System, the speed control range is greater than AC System. There is low insulation required in DC system (about 70%). The price of DC cables is low (Due to Low insulation) In DC Supply System, the Sheath losses in underground cables are low. DC system is suitable for High Power Transmission based on High Current transmission.

The length DC Transmission lines are greater than AC lines.

Disadvantages of DC Transmission:

Due to commutation problem, Electric power can’t be produce at High (DC) Voltage. For High Voltage transmission, we cannot step the level of DC Voltage (As Transformer cannot work on DC). There is a limit of DC Switches and Circuit breakers (and costly too) Motor generator set is used for step down the level of DC voltage and the efficiency of Motor-generator set is low than transformer. So the system makes complex and costly. The level of DC Voltage cannot be change easily. So we cannot get desire voltage for Electrical and electronics appliances (such as 5 Volts, 9 Volts 15 Volts, 20 and 22 Volts etc.) directly from Transmission system. AC system : Advantages of AC Transmission System : AC Circuit breakers is cheap than DC Circuit breakers. The repairing and maintenance of AC substation is easy and inexpensive than DC Substation. The Level of AC voltage may be increased or decreased step up and Step down transformers. Disadvantages of AC Transmission  System: In AC line, the size of conductor is greater than DC Line. The Cost of AC Transmission lines are greater than DC Transmission lines. Due to Skin effect, the losses in AC system are more. In AC Lines, there is Capacitance, so continuously power loss when no load on lines or Line is open. Other line losses are due to inductance. More insulation required in AC System Also corona Losses occur In AC System,   There is telecommunication interference in AC System. There are stability and synchronizing problems in AC System. DC System is more efficient than AC System. There are also re-active power controlling problems in AC System follow us : https://www.youtube.com/channel/UC8uzr-PXQxLpAiVKA7S1rRQ?disable_polymer=true https://www.facebook.com/Engi.Prog/?ref=bookmarks

Classification and Types Of Power Diodes

Major Classification of Diodes:

·        Standard Diodes or General Purpose Diodes:

Standard or general purpose diodes have a comparatively high reverse recovery time, when compared to other diodes. Due to this reason they are used in applications which are not time sensitive and generally run on low speeds.

Usually the reverse recovery time for general purpose diodes varies between 20 micro seconds to 30 micro seconds which is quite a lot. Typical low speed applications for general purpose diodes include the power diode being used as a rectifier or in a converter, where the frequency input is quite low.

·        Fast Recovery Diodes

As their name suggests, these are the type of power diodes which have a relatively faster reverse recovery time, which usually varies from 2 micro seconds to 5 micro seconds. With such a fast recovery time, they can be easily used in high speed switching applications where the time is of great importance.

Due to their property of fast reverse recovery, they are also comparatively expensive as compared to the general purpose diodes.

·        Scotty Diodes:

Sometimes we face problem in charge storage in a pn-junction. This thing can be minimized to a great extent in a Scotty diode. A Scotty diode sets a barrier potential, i.e. a metal layer is deposited on n-type silicon.

As the rectification depends upon the majority charge carriers, so this layer prevents the recombination of these majority charge carriers, and a fast recovery can also be achieved in this way.

Types of Power Diodes:

·        Silicon Diodes

The diode detector verifies the radiation response and electrical characteristics of the PIN diode, which is a diode made out of silicon. Silicone is capable of handling high electrical fields without losing any of the electricity.

·        PIN Diodes

PIN photodiodes are used as radiation detectors that are used for X-ray measurements and for charged particles. The PIN diode spectrometer uses the radiation detector, signal processing electronics and a computer that analyzes and stores the information. Bias voltage is delivered to the detector. The charged particles or X-rays are sent through a preamplifier, an amplifier and a multichannel analyzer system (MCA). The charged particle creates a charged motion that influences the electric field, which produces a current pulse for the external circuit. An integrated current pulse is created that is proportional to the energy lost, whether that energy be the charged particles or the X-ray photons. The MCA is either a dedicated instrument or an analog-to-digital converter that is connected and interfaced to the computer.

·        Reverse-Biased Diodes

Diode detectors can be reverse-biased, which causes very little current to pass through the diode until the reverse breakdown voltage is surpassed, according to Newport.

·        Small Signal Diodes

Diode detectors operate at low voltages in the square law region. These diodes are known as small signal types, according to Newport. The higher voltage diodes operate more in the linear region and are known as the large signal types.

·        Log Detector Amplifiers

Log detector amplifiers use a series of amplifiers and diode detectors to detect output voltage. These detectors have a very good range for detection, but they are very large, which can make them inconvenient in some circumstances.

·        Gunn Diodes

Gunn diodes are diodes that are used as local oscillators that have negative resistance. This power diode produces very little noise and can operate at a very high frequency. These diodes are commonly used with microwaves because of the diode's efficiency and the frequency of the operation.

·        Epicap

The epicap a voltage-variable diode used for electronic tuning. When circuits are tuned electronically and electrically, they can function very quickly and reliably in a small form, according to Educypedia. These diodes are also more thermally stable and have less current leakage.

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Brushless Dc motor

In order to make the operation more reliable, more efficient, and less noisy the recent trend has been to use brushless D.C (BLDC) motors. They are also lighter compared to brushed motors with the 

same power output.

Why BLDC motors?

The brushes in conventional D.C motors wear out over the time and may cause sparking. This is illustrated in the Fig.1. As a result the conventional D.C motors require occasional maintenance. Controlling the brush sparking in them is also a difficult affair.

Figure.1))

Thus the brushed D.C motor should never be used for operations that demand long life and reliability. Fort this reason and the other reasons listed in the introduction, BLDC motors are used in most of the modern devices. Efficiency of a BLDC motor is typically around 85-90%, whereas the conventional brushed motors are only 75-80% efficient. BLDC motors are also suitable for high speed applications (10000 rpm or above). The BLDC motors are also well known for their better speed control.

The Basic working

The rotor and stator of a BLDC motor are shown in the Fig.2. It is clear that, the rotor of a BLDC motor is a permanent magnet.

(figure.2)

The stator has a coil arrangement, as illustrated; The internal winding of the rotor is illustrated in the Fig.3 (core of the rotor is hidden here). The rotor has 3 coils, named A, B and C.

(figure.3)        

Out of these 3 coils, only one coil is illustrated in the Fig.4 for simplicity. By applying DC power to the coil, the coil will energize and become an electromagnet.

(figure.4)

The operation of a BLDC is based on the simple force interaction between the permanent magnet and the electromagnet. In this condition, when the coil A is energized, the opposite poles of the rotor and stator are attracted to each other (The attractive force is shown in green arrow). As a result the rotor poles move near to the energized stator.

(figure.5)

As the rotor nears coil A, coil B is energized. As the rotor nears coil B, coil C is energized. After that, 

coil A is energized with the opposite polarity (compare the last part of Fig.6 with Fig.5).

(figure.6)

This process is repeated, and the rotor continues to rotate. The DC current required in the each coil is shown in the following graph

Types of Circuit Breaker

Many different classifications of circuit breakers can be made, based on their features such as voltage class, construction type, interrupting type, and structural features.

Low-voltage circuit breakers

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

             

An air circuit breaker for low-voltage
 (less than 1,000 volt) power distribution 
  switchgear .



Low-voltage (less than 1,000 V_AC) types are common in domestic, commercial and industrial 
application, and include:

• Miniature circuit breaker (MCB)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.

There are three main types of MCBs:

1. Type B - trips between 3 and 5 times full load current;
2. Type C - trips between 5 and 10 times full load current;
3. Type D - trips between 10 and 20 times full load current.

In the UK all MCBs must be selected in accordance with BS 7671.

• Molded Case Circuit Breaker (MCCB)—rated current up to 2,500 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.
• Low-voltage power circuit breakers can be mounted in multi-tiers in low-voltage switchboards or switchgear cabinets.

The characteristics of low-voltage circuit breakers are given by international standards such as IEC 947. These circuit breakers are often installed in draw-out enclosures that allow removal and interchange without dismantling the switchgear.

Large low-voltage molded case and power circuit breakers may have electric motor operators so they can open and close under remote control. These may form part of an automatic transfer switch system for standby power.

Low-voltage circuit breakers are also made for direct-current (DC) applications, such as DC for subway lines. Direct current requires special breakers because the arc is continuous—unlike an AC arc, which tends to go out on each half cycle. A direct current circuit breaker has blow-out coils that generate a magnetic field that rapidly stretches the arc. Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel.

TheDIN rail-mounted thermal-magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electricaldistribution boards throughout Europe. The design includes the following components:

1. Actuator lever  - used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off/tripped). Most breakers are designed so they can still trip even if the lever is held or locked in the "on" position. This is sometimes referred to as "free trip" or "positive trip" operation.
2. Actuator mechanism - forces the contacts together or apart.
3. Contacts - allow current when touching and break the current when moved apart.
4. Terminals
5. Bimetallic strip - separates contacts in response to smaller, longer-term overcurrents
6. Calibration screw - allows the manufacturer  to precisely adjust the trip current of the device after assembly.
7. Solenoid - separates contacts rapidly in response to high overcurrents
8. Arc divider/extinguisher 

Magnetic circuit breakers 

Magnetic circuit breakers use a solenoid ( electomagnet ) whose pulling force increases with the current . Certain designs utilize electromagnetic forces in addition to those of the solenoid. The circuit breaker contacts are held closed by a latch. As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch, which lets the contacts open by spring action. Some magnetic breakers incorporate a hydraulic time delay feature using a viscous fluid. A spring restrains the core until the current exceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by the fluid. The delay permits brief current surges beyond normal running current for motor starting, energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breaker

Thermal magnetic circuit breakers

Thermal magnetic circuit breakers, which are the type found in most distribution boards, incorporate both techniques with the electromagnet responding instantaneously to large surges in current (short circuits) and the bimetallic strip responding to less extreme but longer-term over-current conditions. The thermal portion of the circuit breaker provides an "inverse time" response feature, which trips the circuit breaker sooner for larger overcurrents but allows smaller overloads to persist for a longer time. On very large over-currents during a short-circuit, the magnetic element trips the circuit breaker with no intentional additional delay.

Common trip breakers

When supplying a branch circuit with more than one live conductor, each live conductor must be protected by a breaker pole. To ensure that all live conductors are interrupted when any pole trips, a "common trip" breaker must be used. These may either contain two or three tripping mechanisms within one case, or for small breakers, may externally tie the poles together via their operating handles. Two-pole common trip breakers are common on 120/240-volt systems where 240 volt loads (including major appliance or further distribution boards) span the two live wires. Three-pole common trip breakers are typically used to supply three-phase electric power to large motors or further distribution boards.

Two- and four-pole breakers are used when there is a need to disconnect multiple phase AC, or to disconnect the neutral wire to ensure that no current flows through the neutral wire from other loads connected to the same network when workers may touch the wires during maintenance. Separate circuit breakers must never be used for live and neutral, because if the neutral is disconnected while the live conductor stays connected, a dangerous condition arises: the circuit appears de-energized (appliances don't work), but wires remain live and some RCDs may not trip if someone touches the live wire (because some RCDs need power to trip). This is why only common trip breakers must be used when neutral wire switching is needed.

Medium-voltage circuit breakers

Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers (up to about 40.5 kV). Like the high voltage circuit breakers described below, these are also operated by current sensing protective relays operated through current transformer . The characteristics of MV breakers are given by international standards such as IEC 62271. Medium-voltage circuit breakers nearly always use separate current sensors and protective relays, instead of relying on built-in thermal or magnetic overcurrent sensors.

Medium-voltage circuit breakers can be classified by the medium used to extinguish the arc:

• Vacuum circuit breakers—With rated current up to 6,300 A, and higher for generator circuit breakers. These breakers interrupt the current by creating and extinguishing the arc in a vacuum container - aka "bottle". Long life bellows are designed to travel the 6–10 mm the contacts must part. These are generally applied for voltages up to about 40,500 V.  which corresponds roughly to the medium-voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.
• Air circuit breakers—Rated current up to 6,300 A and higher for generator circuit breakers. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.
• SF6 circuit breakers extinguish the arc in a chamber filled with sulfure hexafluoride gas.

Medium-voltage circuit breakers may be connected into the circuit by bolted connections to bus bars or wires, especially in outdoor switchyards. Medium-voltage circuit breakers in switchgear line-ups are often built with draw-out construction, allowing breaker removal without disturbing power circuit connections, using a motor-operated or hand-cranked mechanism to separate the breaker from its enclosure. Some important manufacturer of VCB from 3.3 kV to 38 kV are ABB, Eaton, Siemens, HHI(Hyundai Heavy Industry), S&C Electric Company, Jyoti and BHEL.

High-voltage circuit breakers

Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electrotechnical Commition (IEC). High-voltage breakers are nearly always solenoid-operated, with current sensing protective relay operated through current transformers. In substations the protective relay scheme can be complex, protecting equipment and buses from various types of overload or ground/earth fault.

High-voltage breakers are broadly classified by the medium used to extinguish the arc.

• Bulk oil
• Minimum oil
• Air blast
• Vacuum
• SF6
• Co2

Due to environmental and cost concerns over insulating oil spills, most new breakers use SF₆ gas to quench the arc.

Circuit breakers can be classified as live tank, where the enclosure that contains the breaking mechanism is at line potential, or dead tank with the enclosure at earth potential. High-voltage AC circuit breakers are routinely available with ratings up to 765 kV. 1,200 kV breakers were launched by Siemens in November 2011, followed by ABB in April the following year.

High-voltage circuit breakers used on transmission systems may be arranged to allow a single pole of a three-phase line to trip, instead of tripping all three poles; for some classes of faults this improves the system stability and availability.

Hgh voltage direct current  circuit breakers are still a field of research as of 2015. Such breakers would be useful to interconnect HVDC transmission systems.

Sulfur hexafluoride (SF₆) high-voltage circuit breakers

A sulfur hexafluoride circuit breaker uses contacts surrounded by sulfur hexafluoride gas to quench the arc. They are most often used for transmission-level voltages and may be incorporated into compact gas-insulated switchgear. In cold climates, supplemental heating or de-rating of the circuit breakers may be required due to liquefaction of the SF6 gas.

Disconnecting circuit breaker (DCB)

The disconnecting circuit breaker (DCB) was introduced in 2000 and is a high-voltage circuit breaker modeled after the SF₆-breaker. It presents a technical solution where the disconnecting function is integrated in the breaking chamber, eliminating the need for separate disconnectors. This increases the availability , since open-air disconnecting switch main contacts need maintenance every 2–6 years, while modern circuit breakers have maintenance intervals of 15 years. Implementing a DCB solution also reduces the space requirements within the substation, and increases the reliability, due to the lack of separate disconnectors.

In order to further reduce the required space of substation, as well as simplifying the design and engineering of the substation, a fiber optic current sensor (FOCS) can be integrated with the DCB. A 420 kV DCB with integrated FOCS can reduce a substation’s footprint with over 50% compared to a conventional solution of live tank breakers with disconnectors and current transformers, due to reduced material and no additional insulation medium.

Carbon dioxide (CO₂) high-voltage circuit breakers

In 2012 ABB presented a 75 kV high-voltage breaker that uses carbon dioxide as the medium to extinguish the arc. The carbon dioxide breaker works on the same principles as an SF₆ breaker and can also be produced as a disconnecting circuit breaker. By switching from SF₆ to CO₂ it is possible to reduce the CO₂ emissions by 10 tons during the product’s life cycle.

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