Types of HVDC Circuit Breakers and Their Operation & Applications
The HVDC (High voltage Direct current) offer very efficient way of power transmission across very long distance & is used in various Green energy production nowadays. We use multiple protection devices to protect expensive equipment that are connected to such high voltage transmission lines. One of these equipment is a circuit breaker. We cannot use the same CB designed for AC because of the nature of the DC which renders them useless & may damage the system even more. That’s why we need HVDC circuit breakers in such type of applications.
A circuit breaker is a mechanical switch that automatically operates to protect a circuit from the damage caused by fault current. It automatically breaks the circuit upon sensing huge draw of current flow due to overloading or short circuit. It can also manually break open the circuit for maintenance or fault clearance. It can safely close & open a circuit to protect it from damage.
The main objective of a circuit breaker is to safely break open the circuit
- It should momentarily withstand the fault current
- It should safely break open the circuit
- It should quickly extinguish the arc.
- Its terminals should withstand the voltage after breaking.
- It should prevent the arc from re-striking.
The circuit breaker detects the fault current which is very high in range using various mechanisms such as
- Heating effect
- Electromagnetic effect
- Current sensors using CTs
The circuit breaker withstands the fault current momentarily & allows other circuit breakers to resolve the fault. The CB is designed to tolerate a specific range of fault current without damaging its terminals.
Once it detects the fault current, it trips & interrupt the current flow. It breaks open the circuit using some sort of stored mechanical energy such as spring or a blast of compressed air to separate the contacts. it can also use the fault current to break open the contacts using thermal expansion or an electromagnetic field using solenoid.
The next step that comes after the separation of contacts is the arc extinction. The arc is generated between the contacts due to the high voltage between them. It can damage the CB contacts or terminals due to excessive heat generated because of high current.
The electrical arc tries to make the circuit, so the current still flows in it. It must be extinguished & different kinds of circuit breakers use various insulating or dielectric arc extinction mediums such as.
- Insulating Oil
- Insulating gas such as SF6 (Sulphur hexafluoride)
Other than the medium being used arch quenching, various arc extinction techniques are used to quickly & safely eliminate an arc.
- Cooling of Arc: The arc heats up the air molecule which ionizes & reduces the resistance of the air. Cooling the arc will recombine the ionized particle into its natural state & increase the dielectric strength of the air molecule. As the resistance of the medium increases, the voltage required to maintain the arc also increases & the current starts to drop resulting in arc quenching.
- Air Blasting: Such method is used in air blast circuit breaker, where the arc is quenched using a blast of compressed air. The ionized air particles are replaced with non-ionized air molecules that have higher dielectric strength. It increases the resistance thus reducing the current which leads of extinction of the arc.
- Increasing the length of arc: The arc length is directly proportional to its voltage. Increasing the length of the arc by separating the contact terminals further apart will increase the voltage required to maintain it. Thus it will extinguish.
- Reducing cross section of arc: Another technique is to reduce the cross section of arc by reducing the contact sizes. Therefore, the voltage required for arc increases & extinguish it.
- Deflecting the arc: In this technique, a magnetic field is created to deflect the arc. it blow out the arc into a section of the circuit breaker called arc chute where it is cooled off & extinguishes.
- Dividing or splitting the arc: In this technique, the arc is split into multiple arcs by proving multiple contacts in between. The arc is split into numerous small arc in series which increases its length & the resistance. Therefore, reducing the arc current & eventually extinguishing it.
- Zero current quenching: this is the most common method used in AC circuit breaker. There are inherently multiple zero current in a AC waveform. The circuit is opened at exact point of zero current. So that the current does not rise to generate arc.
- Using charged capacitor in parallel: This technique is used in DC circuit breaker. The DC does not have natural zero current. Therefore, a charged capacitor with inductor is used in parallel to introduce artificial zero current in the line to extinguish the arc.
Comparison of AC Circuit Breaker & DC Circuit Breaker
As discussed above, the AC current fluctuate along zero line offering many natural zero crossing almost 100 times in a second at the 50 Hz. Therefore, the AC circuit breaker utilizes this feature of AC to extinguish the arc when the current is at zero point.
As we know, the alternating current changes its direction continuously which offers to allow multiple chances to extinguish the arc. On the other hand, the DC current stays in steady state & there is no chance of natural zero current. The arc generated in DC circuit breaking does not extinguish since the current never reached zero point.
Therefore, the DC circuit breaker utilizes extra circuitry to introduce artificial zero currents in the line to extinguish the arc. This is why DC circuit breaker are complex in design as compared to AC circuit breaker. Please read further about the “Comparison between AC & DC Circuit breaker”.
HVDC (DC) Circuit Breaker
HVDC (High Voltage Direct Current) circuit breaker is a special type of circuit breaker that is specifically designed to use for protection against fault current in a HVDC circuits.
As we have discussed earlier, the major part that differentiate the DC circuit breaker from other type of breakers is the arc quenching technique. In AC breaker, it is easier to break the circuit at zero crossing because the energy at that point is zero which does not allow generating arc at that voltage level. While in DC, the voltage & current never goes to zero. So there is always very high voltage & current between the contacts during its separation.
Requirement for HVDC Circuit Breaker
The HVDC circuit breaker operation & design is complex as compared to AC circuit breaker due to the absence of natural zero crossing. The arc generated in HVDC will never extinguish & it will heat up the contacts of the breaker & eventually destroy the contacts rendering the whole CB useless.
Not to mention, the circuit will still be complete & the equipment connected will get damaged due to the fault current. Therefore, the following requirement must be completed to ensure safe circuit breaking in HVDC circuit breaker,
- Creation of artificial zero crossing
- Dissipation of the stored energy inside the LC circuit
- Withstanding the voltage between its contact
- Prevention of arc restriking
In order to fulfill the above requirements, an LC circuit is introduced with the circuit breaker in parallel which will generate artificial zero current across the line to safely break open the circuit. The strength of the arc is directly proportional to the voltage level & the current. Therefore the fault current must be brought down to zero using an external circuit before breaking it.
- Related Post: Differences Between HVAC and HVDC – Power Transmission
Working Principle of HVDC Circuit Breaker
In order to generate artificial zero current in the system, an LC circuit is connected in parallel with the circuit breaker.
The following figure shows a typical HVDC circuit breaker & its working principle.
A pre-charged capacitor C is connected having reversed polarities as shown in the figure. An inductor L & a switch S is connected in series with the capacitor C. this extra circuit is connected in parallel with the main breaker M.
Under normal conditions, the switch S is open & the main breaker M is closed & the current flow through it. During current interruption or fault current conditions, the switch S is closed which completes the LC circuit & the main breaker M is opened.
During this time, the capacitor C start discharging & it pushes the current in reverse direction through the breaker M, forcing the arcing current to oscillate (reaching to zero point) which results in artificial commutation or zero crossing. This artificial zero commutation generated by the LC circuit allows the arc to extinguish at zero current point. The extra energy is dissipated with in the LC circuit.
This type of DC circuit breaker can work on a single power line & does not require the second opposite polarity line.
Another method of arc quenching in HVDC circuit breaker is explained using the following design that uses both lines of a DC transmission.
In this method, the Main breaker M is connected on the live or hot line. An LC circuit is connected in parallel with the main breaker M using two switches S1 & S2. The S2 switch connects the LC circuit to the ground through a high resistance R.
Under normal condition, the main breaker M & the switch S2 is closed, while the S1 is open. The current flows through the main breaker M to the load & also through the switch S2 to charge the capacitor. The capacitor is charged through the high resistance.
During current interruption or fault current, the switch S2 is opened & the S1 is closed. The charged capacitor starts discharging the current in reverse direction through the main breaker M. The LC circuit starts resonating & creates oscillating current that forces the current through the main breaker M to cross zero.
The main breaker M contacts separate at this zero current, resulting in the extinction of the arc. As soon as the M contacts open, the switch S2 is closed & S1 is opened. The S2 allows the extra energy to dissipate in the heavy resistance R. This prevents the arc from re-striking.
- Related Post: Types of HVDC Systems and MTDC Configurations
Uses of HVDC Circuit Breaker
The HVDC power has found its many applications apart from power transmission in the field of technology. Here are some of the application of HVDC that require HVDC circuit breakers,
Photovoltaic plant contains strings of photovoltaic panels that convert the solar energy into electrical energy in DC form. The solar radiation acting on the panels generates DC electrical energy. They are used for offering renewable energy to small houses as well as large industries.
Multiple strings of photovoltaic panels are combined together to increase their voltage & current ratings to power large compounds.
The equipment used for such high voltage (such as the converters & batteries) is very expensive & must be protected from fault conditions. Multiple DC circuit breakers are used for isolating different parts of the system in case of fault or maintenance to avoid any damage or casualties.
The DC power has found one of its best uses in the field of electric traction because of the speed-torque relation of the DC motor. The electric trains, trams & trolleybuses use DC motors. The advantage of DC supply is the use of single conductor that supplies current to the vehicle while the rail is used as the current return path. Thus it also reduces the number of conductors.
The DC power not only powers the motors but it powers the whole trains where various kinds of loads are connected. Thus proper protection system must be installed to protect all equipment. Therefore, multiple DC circuit breakers are used to protect each system.
- Related Post: Advantages of HVDC over HVAC Power Transmission
HVDC Power Transmission:
The HVDC is mainly used for power transmission over very long distances. HVDC terminals used for the conversion of the DC to AC or vice versa are quite expensive. They must be protected at all cost. Power transmission is mostly affected by fault currents that can easily damage any equipment connected with it.
Therefore, the HVDC circuit breakers are installed in the line to protect from fault current. Furthermore, they are also used for isolating the transmission line for maintenance work.
MTDC Grid Systems:
Multi Terminal Direct Current or MTDC is a complex type of HVDC transmission system which is used for its power transmission, flexibility & control over the power distribution between multiple grids.
The MTDC as its name suggest connects multiple terminals thus having multiple circuit branches. In case of faults or scheduled maintenance in a specific branch, it needs to be de energized using a circuit breaker. Therefore, multiple HVDC circuit breakers are used for separate branches.
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