HVDC 5th UNIT NOTES

DC REACTORS:

The purpose of a DC-link reactor is to reduce the harmonic currents on the DC side of the converter and to reduce the risk of commutation failures by limiting the rate of rise of the DC line current at transient disturbances in the AC or DC systems. Small losses and small size and weight in relation to capacity are typical features for smoothing reactors.The DC reactor is more effective than the input reactor in improving the power factor. We recommend joint use of the input reactor, which is effective in suppressing external surges, when facilities where the inverter is to be applied require high reliability.

Smoothing reactors are vital components in HVDC systems. The purpose of the reactor is to reduce the so called current ripple on the DC side of the system.The direct current that comes from the rectifier in the DC system has superimposed harmonic components, also called ripple. The smoothing reactor is connected in series with the rectifier and the whole load current flows through it. The purpose of the reactor is to provide high impedance to the flow of the harmonic currents, reduce their magnitude and thus making the DC current more smooth.

ADVANTAGES:

Ø  Delta or vertical arrangement depending on space availability

Ø  -40 ºC / + 55 ºC ambient temperature range, customer specific design

Ø  Fiberglass resin spacers are used

Ø  AN (air-natural) cooling method

Ø  Reduction of electrical losses

Ø  Extended equipment lifecycle

Ø  High mechanical strength to withstand elevated short-circuit forces

Ø  Conservative temperature rise to ensure extended service life

Ø  Surface treatment for protection against UV radiation and pollution

Ø  Customized space-saving solutions for installations in compact areas

Ø  Environmentally friendly solution

Ø  Maintenance-free design

DC REACTORS CIRCUIT:

PROTECTION AGAINST OVER VOLTAGES:

Overvoltages:

Overvoltages are extremely high voltages that damage or evencompletely destroy insulation and hence impair or completelydisrupt the function of electrical and electronic components of allkinds. The basic principle of over voltage protections is the same in DC systems as in AC systems. These are given below:

Ø  The overvoltage stress in equipment with non-self-restoring insulations must be limited t all times by providing surge arresters. The protection level of the arresters must be lower than the breakdown voltage of the insulation.

Ø  Self-restoring insulations such as air may be allowed to breakdown where there is no danger to the safety of the personnel.

Ø  The operation of the surge arresters or flashover of air insulation must not be frequent. Frequent discharges of arresters may damage. This implies that the protective level of arresters must be higher than the maximum operating voltage in the system .

Ø  There must be proper coordination of the insulation and over voltage protection in different parts of the system, taking into account the characteristics of insulation, the nature of over voltages.

Three Categories OverVoltages:

ü  Direct lightning strikes

ü  Indirect effects of lightning strikes

ü  Operating or switching overvoltages

Overvoltage Protection in a converter station:

 For a system with two 12-pulse converters per pole, there are about 40 arresters per pole. The arresters are selected with adequate energy dissipation capabilities which vary with the locations of the arresters. For example, the valve arrester protecting the commutation group at the highest potential can be subjected to higher energies than other arresters when a ground fault occurs between the valve and the converter transformer in the upper bridge. This is due to the discharge of the line and the DC filter.The closing of a bypass switch across a converter results in increasing the DC voltage across the remaining converter. The converter unit arrester is stressed in such cases.The protection firing of a valve is the backup protection that is available for overvoltages in the forward direction.

PROTECTION AGAINST OVER CURENTS:

Overcurrent:

Overcurrent is the condition where the current in amperes is greater than the rated current of the equipment or conductors, resulting from an overload, short circuit, or ground fault.The most fundamental requirement in any electrical system is proper overcurrent protection of conductors and equipment.An overcurrent protection device protects the circuit by opening the device when the current reaches a value that will cause an excessive or dangerous temperature rise in conductors. Most overcurrent protection devices respond to both, short-circuit or ground-fault current values as well as overload conditions.

The faults producing over current are classified into 3 types:

ü  Internal faults which cause high over currents but are very in frequent. The thyristor surge currents ratings must be chosen to withstand these over currents.

ü  Line faults which cause over currents in the range of 2 to 3 p.u. These are limited by current control the protect against line faults which are frequent will be discussed in this chapter.

ü  Commutation failures at inverters may be quite frequent; however because of continuous conduction during commutation failure, the current reference has to be reduced.  

The over current protection in converters is based on principles similar to those used in AC systems. The factors that must be considered in designing a protection system are

1.      selectivity   

2.      sensitivity          

3.      reliability     

4.      back up

The basic protection against converter faults is provided by value group differential protection, which compares the rectified current on the value side of converter transformer to the DC current measured on the line side of the smoothing reactor . The differential protection is employed because of its selectivity and fast detection. The over current protection is used as back up. The level of over current required to trip must be set higher than that of the differential protection to avoid tripping with faults outside the station  (that can be cleared  by the control action).The pole differential protection is used to detect ground faults which may not be otherwise detected, such as faults at the neutral bus.

CIRCUIT BREAKER:

A circuit breaker (popularly known as CB) is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

Introduction to Circuit Breaker:

The modern power system deals with huge power network and huge numbers of associated electrical equipment. During short circuit fault or any other types of electrical fault these equipment as well as the power network suffer a high stress of fault current in them which may damage the equipment and networks permanently. For saving these equipment’s and the power networks the fault current should be cleared from the system as quickly as possible. Again after the fault is cleared, the system must come to its normal working condition as soon as possible for supplying reliable quality power to the receiving ends. In addition to that for proper controlling of power system, different switching operations are required to be performed. So for timely disconnecting and reconnecting different parts of power system network for protection and control, there must be some special type of switching devices which can be operated safely under huge current carrying condition. During interruption of huge current, there would be large arcing in between switching contacts, so care should be taken to quench these arcs in safe manner. The circuit breaker is the special device which does all the required switching operations during current carrying condition.

Functionsof Circuit Breaker:

 A circuit breaker’smain functions are:

ü  Sense the current flowing in the circuit

ü  Measure the current flowing in the circuit

ü  Compare the measured current level to its pre-set trip point

ü  Act within a predetermined time period by opening the circuit as quickly aspossible to limit the amount of energy that is allowed to flow after the trippoint has been reached

Components of Circuit Breaker:

ü  The five basic components of a circuit breaker are:

ü  Frame, or case made of metal, or some type of electrical insulation

ü  Electrical contacts

ü  Arc extinguishing assembly

ü  Operating mechanism

ü  Trip unit, containing either a thermal element, or a magnetic element or both

OPERATION:

The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source.

Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting (opening) the circuit. Contacts are made of copper or copper alloys, silver alloys, and other highly conductive materials. Service life of the contacts is limited by the erosion of contact material due to arcing while interrupting the current. Miniature and molded case circuit breakers are usually discarded when the contacts have worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.

When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium the arc forms in. Different techniques are used to extinguish the arc including:

• Lengthening / deflection of the arc
• Intensive cooling (in jet chambers)
• Division into partial arcs
• Zero point quenching (Contacts open at the zero current time crossing of the AC waveform, effectively breaking no load current at the time of opening. The zero crossing occurs at twice the line frequency i.e. 100 times per second for 50 Hz and 120 times per second for 60 Hz AC)
• Connecting capacitors in parallel with contacts in DC circuits

Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit.

Types of Circuit Breaker:

According different criteria there are different types of circuit breaker .According to their arc quenching media the circuit breaker can be divided as

1) Oil Circuit Breaker
2) Air Circuit Breaker
3) SF6 Circuit Breaker
4) Vacuum Circuit Breaker

According to their services the circuit breaker can be divided as

1) Outdoor Circuit Breaker
2) Indoor Breaker

According to the operating mechanism of circuit breaker they can be divided as

1) Spring operated Circuit Breaker
2) Pneumatic Circuit Breaker
3) Hydraulic Circuit Breaker

According to the voltage level of installation types of circuit breaker are referred as

1) High Voltage Circuit Breaker
2) Medium Voltage Circuit Breaker
3) Low Voltage Circuit Breaker

SURGE ARRESTERS:

Surge arresters are an essential aid to insulation coordination in electrical power supply systems. Valuable equipment can be protected against lightning- and switching overvoltages.If basic rules are kept, modern Metal oxide surge arresters offer complete protection against overvoltages. In addition they offer a failure rate of nearly 0% and a lifetime of more than 20 years.

Costs of surge arresters in electrical power supply systems are less than 1% of the worth of the equipment they protect.High availability and low costs of surge arresters facilitate new applications like surge arresters for transmission lines. As a result surge arresters convince with a quality improvement of electrical power supply systems.

A surge arrester is installed between communication equipment and coaxial cable connector or between two communicationsequipment’s to protect communication equipment from damage caused by transient state voltage formed by lightning induction. It adopts quarter-wave technology, is designed according to VSW theory and frequency spectrum of lightning wave. It has features of quick reaction, big current passing capacity, wide frequency band, low VSWR, low insertion loss, easy installation and free maintenance. It can be used to meet protection requirements of various communication equipment’s and lightning sensitivity.

Arresters of special design are also built to meet specific application requirements. The Mogard Surge Arrester consists of a stack of Zinc Oxide discs mounted in a sealed porcelain housing. Each disc is wedged by means of a silicon rubber wedge, which offers better heat transfer capability and protection against physical damage during transport. The pressure relief device is an integral part of the Mogard Arrester and prevents violent shattering of the Arrester if wrongly operated

Features

􀂃 Standard size

􀂃Extremely fast response time

􀂃 Very high current rating

􀂃 Standard size

􀂃 Extremely fast response time

􀂃 Very high current rating

􀂃 Stable performance over life

􀂃 Very low capacitance

􀂃 High insulation resistance

􀂃 RoHS-compatible

§  Low-voltage surge arrester 

Apply in Low-voltage distribution system, exchange of electrical appliances protector, low-voltage distribution transformer windings

§  Distribution arrester

 Apply in 3KV, 6KV, 10KV AC power distribution system to protect distribution transformers, cables and power station equipment

§  The station type of common valve arrester

 Used to protect the 3 ~ 220KV transformer station equipment and communication system

§  Magnetic blow valve station arrester

 Use to 35 ~ 500KV protect communication systems, transformers and other equipment

§  Protection of rotating machine using magnetic blow valve arrester

 Used to protect the AC generator and motor insulation

§  Line Magnetic blow valve arrester

 Used to protect 330KV and above communication system circuit equipment insulation

§  DC or blowing valve-type arrester

 Use to protect the DC system’s insulation of electrical equipment

Applications of SURGEARRESTERS

􀂃 Line protection

􀂃 Station protection

􀂃 Base stations

SURGEARRESTER

LIGHTING ARRESTOR:

            A lightning arrester (in Europe: surge arrester) is a device used on electrical power systems and telecommunications systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth.

Smaller versions of lightning arresters, also called surge protectors, are devices that are connected between each electrical conductor in power and communications systems and the Earth. These prevent the flow of the normal power or signal currents to ground, but provide a path over which high-voltage lightning current flows, bypassing the connected equipment. Their purpose is to limit the rise in voltage when a communications or power line is struck by lightning or is near to a lightning strike.

If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices. Lightning-produced extreme voltage spikes in incoming power lines can damage electrical home appliances.

A lightning arrester may be a [spark gap] or may have a block of a semi-conducting material such as siliconcarbide or zinc oxide. Some spark gaps are open to the air, but most modern varieties are filled with a precision gas mixture, and have a small amount of radioactive material to encourage the gas to ionize when the voltage across the gap reaches a specified level. Other designs of lightning arresters use a glow-discharge tube (essentially like a neon glow lamp) connected between the protected conductor and ground, or voltage-activated solid-state switches called varistors or MOVs.

Lightning arresters built for powersubstation use are impressive devices, consisting of a porcelain tube several feet long and several inches in diameter, typically filled with disks of zinc oxide. A safety port on the side of the device vents the occasional internal explosion without shattering the porcelain cylinder.Lightning arresters are rated by the peak current they can withstand the amount of energy they can absorb, and the break over voltage that they require to begin conduction. They are applied as part of a lightning protection system, in combination with air terminals and bonding.

Types/Classifications

 Originally, there were three types of surge arresters. They are:

o   Expulsion type

o   Nonlinear resistor type with gaps (currently silicone-carbide gap type)

o   Gapless metal-oxide type.

·         There are four (4) classifications of surge arresters.  They are:

o   Station class

o   Intermediate class

o   Distribution class (heavy, normal, and light duty)

o   Secondary class (for voltages 999V or less)

There are some basic considerations when selecting the appropriate surge arrester for a particular application:

·          Continuous system voltage

·          Temporary overvoltages

·          Switching surges (more often considered for transmission voltages of

345KV and higher, capacitor banks, and cable applications)

·          Lightning surges

·          System configuration (grounded or ungrounded/effectively

Ungrounded

Advantages:

ü  Provides an enhanced area of protection

ü  Unique design with shielded down-conductor for sensitive structures

ü  Special software based placement program to determine number and location of terminals

ü  Complies with most stringent lightening protection standards

ü  Easy to install and low on maintenance

ü  An economic protection against lightening hazards

Applications:

ü  Telecommunications & Broadcasting

ü  Petrochemicals, Oil and Gas

ü  High rise buildings and hotels

ü  Sports Centre such as Golf Course, Race Tracks and Stadiums

ü  Aviation-Civil and Military

ü  Mining-Coal, Gold, Nickel, Iron, Copper, Bauxite and many others

ü  All kinds of industrial facilities

ü  Defence-communications, surveillance and storage of armor

ü  Power Generation and Distribution

ü  Rail and Transportation Systems