Friday, July 20, 2012

Dynamic Voltage Restorer (DVR)

Voltage variations are a nightmare for the industries like Semiconductor fabrication plants, Paper mills, Food processing plants and, automotive assembly plants etc. Fast variation in source voltage can affect the overall performance of the industry and also cause damage to critical loads in these industries resulting in great financial loss. The most common disturbances that occur in the source voltage are voltage sags or swells that can be due to disturbances in the transmission line or system, adjacent feeder faults and Fuse or breaker operation. Voltage sags of even 10% lasting for 5-10 cycles can result in costly damage in critical loads and heavy machineries in an industry. The voltage sags can be symmetrical faults in which negative and zero sequence components are also present.
Presence of uncompensated nonlinear loads in the distribution system can cause harmonic components in the supply voltages which are extremely dangerous. To mitigate the problems caused by poor quality of power supply, series connected compensators are used. These compensators are called Dynamic Voltage Restorer (DVR), as their primary application is to compensate for voltage sags and swells. Their configuration is similar to that of SSSC (Static Synchronous Series Compensator). However, the control techniques are different. A DVR is expected to respond fast (less than ¼ cycles) and thus employs PWM converts (Pulse-width modulation converts) using IGBT (Insulated gate bipolar transistor) or IGCT (Integrated Gate-Commutated Thyristor) device.
A DVR has to supply energy to the load during the voltage sags. If a DVR has to supply active power over longer periods, it is convenient to provide a shunt converter that is connected to the DVR on the DC side. As a matter of fact one could envisage a combination of DSTATCOM (Distribution Static Compensator) and DVR connected on the DC side to compensate for both load and supply voltage variations.  The combined compensator is termed as Unified Power Quality Conditioner (UPQC) which has similar to UPFC (Unified Power Flow Controller). However the control objectives are different.
The DVR was first installed on Duke Power System in USA in August 1996, with a rating of 2MVA with 660 kJ of energy storage capacity, it is capable of compensating 50% voltage sag for a period of 0.5 second (30 cycles). It was installed to protect an automated yarn manufacturing and rug weaving facility. Since then, several DVRs have been installed to protect microprocessor fabrication plants, paper mills etc. typically, DVRs are made of modular design with a module rating of 2 MVA or 5 MVA. They have been installed in substations of voltage rating from 11 kV to 66 kV.

Friday, March 18, 2011


Circuit breakers are mechanical devices designed to close or open contact members, thus closing or opening of an electrical circuit under normal or abnormal conditions. Air blast circuit breakers are using compressed air or gas as the circuit breaking or interrupting medium. Gases such as nitrogen, carbon dioxide, hydrogen or Freon can be used as arc interrupting medium. But compressed air is the most accepted arc interrupting medium. The reasons are, nitrogen has circuit breaking properties similar to compressed air and there is no advantage of using it. Carbon dioxide has a draw back as it is difficult to control owing to freezing at valves and other restricted passages. Hydrogen has increased breaking capacity but it’s costlier.

Freon has high dielectric strength and good arc extinguishing properties, but it is expensive and it is decomposed by the arc into acid forming elements.

Desirable features of air blast circuit breakers are High speed operation, which is very necessary on large inter connected networks in order that system stability can be maintained and in the air blast circuit breaker this is achieved because the time interval between the receipt of a tripping impulse and contact separation is very short. Suitability for frequent operation, repeated switching by an air blast circuit breaker is possible simply because of absence of oil, which rapidly carbonizes with frequent operation, and because there is an insignificant amount of wear and tear at the current carrying contact surfaces. High speed reclosure by automatic means is an advantage on hv inter connected networks to assist and maintain system stability during the clearance of transient faults, a type of fault which is perhaps in majority on over head line. Negligible maintenance, the ability of the air blast circuit breaker to cope with repeated switching also means that negligible maintenance is required. Elimination of fire hazard, because of the absence of oil the risk of fire is eliminated.

Reduced size, the growth of dielectric strength is so rapid in air blast circuit breakers that final gap required for arc extinction is very small. This reduces the size of the device.

The air blast circuit breaker requires an auxiliary compressed air system which supplies air to the breaker air receiver. When opening is required compressed air is admitted to the arc extinction chamber. It pushes away the moving contacts. In doing so the contacts are separated and the air blast takes away the ionized gases along with it and assist arc extinction.

Thursday, November 12, 2009



Circuit breakers are devices which can automatically break a circuit under fault conditions and it can make the circuit manually after the fault. Circuit breakers are classified into different types. One of the main circuit breaker among them is Vacuum circuit breaker. Here vacuum is used to extinct the arc. Arc extinction is the main process of a circuit breaker

Before that, it is needy to explain how an arc is produced. Circuit breaker has two contacts- Fixed and movable, otherwise called as electrodes. Normally the contacts remain closed but if any fault occurs these contacts open automatically and of course can be opened manually.

We know that circuit breakers work under the instructions from relay. When the trip coils from relay energize, circuit breaker identifies fault occurred and hence the contacts are pulled apart. A high intensity current flows through the contacts of the circuit breaker before they are opened by the system. At the same instant contacts begin to separate and causes heavy heat and temperature in between the contacts. This is termed as an arc. This heat is highly dangerous and can cause heavy damage to the whole system. So it has to be extinguished in no time.

Vacuum is having superior qualities over other arc quenching medium. It has high insulating strength. When the contacts are opened in vacuum, the arc is produced in between them. It got extinguishes quickly since the metallic vapours and ions produced during the arc rapidly condense on the surface of the circuit breaker contacts, resulting in quick recovery of dielectric strength.

The contacts of Vacuum circuit breaker and arc shied is mounted inside a vacuum chamber. The chamber is connected to the control mechanism by means of stainless steel bellows. A glass vessel is used as the outer insulating body. This is all about its construction.

Vacuum circuit breaker is really the most reliable one with low cost, no fire hazards, no gas generation, low arc energy, little maintenance and low inertia.

Sunday, November 8, 2009

What is an Actuator?

Actuator is a typical mechanical device used to move or control a mechanism or a system. This device gains energy for its working from air, electricity or liquid and then converts it into some kind of motion.

In engineering field actuators have many more applications in different sectors. In this field actuators are used as mechanisms to provide motion and are again used to stop the motion of that particular device. So it is clear that actuator just actuates the motion. May be start or stop, depends on the machine.

In electronics engineering, actuators are subdivisions of transducers. They are used to transform input signal into motion. Normally input signal would be electrical. E.g. electrical motors, pistons, relays, pneumatic actuators, piezo electric actuators etc. Actuators can be sometimes used as hardware components. Different types of actuators are available like plasma actuators, pneumatic actuators, electric actuators, linear actuators.

It’s a type of tool used to put something into automatic action. Actually actuators are used with number of sources. Depending on the type of actuator we are using, different tools will be assisting us to put the device to motion. Mostly they are used in office work area since most of them are used to move valve door in systems.

Sometimes they are used to maneuver certain mechanical devices on work. Depending on the shape and style actuators are divided into different classes. Linear actuators, valve actuators, hydraulic actuators are the best known and used among them. Each of them has assigned functions e.g. hydraulic actuators are used to allow more pressure, size and movement with the object.

All of these different types of actuators are best when knowing what type of motion control you need. They also provide speed, Belt drive, acceleration and accuracy with linear motion No matter what type of actuator is needed, there is a tool that makes it easier to maneuver a certain object or space in your work area.

Wednesday, November 4, 2009


What is Digital Signal Processing?

It’s a major branch of science that deals with the representation of signals as symbols or sequences and after that these signals are getting processed. There are two subfields of processing available- Analog signal processing and Digital signal processing.

This digital signal processing also includes subfields like audio and speech processing, sonar and radar processing, sensor array processing, spectral estimation, statistical signal processing, digital image processing, signal processing for communication systems, biomedical signal processing, seismic signal processing etc. So from the above description it’s clear that DSP is a vast branch in engineering field that requires genuine attention and development.

Generally these processing measures the continuous analog signals and is then converted into digital form by means of an Analog to Digital converter (A-D converter). For certain, the required output would be in Analog form, so the signal is then converted to Analog using Digital to Analog converter (D-A converter). We know that this process is very complex when compared with analog processing and hence error detection and correction methods are used to clarify the process.

The main notable advantage of DSP is that it is less affected to noise and hence non vulnerable. DSP algorithms are carried out in special processors like Digital Signal Processors which is inbuilt in the systems. Since the process is very lengthy and time consuming, we usually undergo the process in this type of processors. DSP’s include microprocessors, digital signal controllers, field programmable gate arrays etc. DSP’s can be implemented using specialized microprocessors like DSP56000, TMS320 and the SHARC.

Usually the digital signals are denoted in time domain, frequency domain, spatial domain etc. With the available characteristics, it is decided that on which domain the signal can be denoted. Sampling is another important point to be specified. Sampling can be done in signal processing during two stages discretization and quantization. In the dicretization stage the space of signals are divided into equivalence classes and quantization is carried out by replacing the signals with representative signals.

DSP has applications in the fields of audio signal processing, audio compression and in the fields as mentioned earlier.

Friday, October 30, 2009

What are Lightning Arresters?


Lightning arresters or surge diverters are used to protect the earth screen and ground wires in an electrical system against direct lightning strokes. They conduct the high voltage surges to the ground without getting affected to the system. The lightning arrestor provides a cone of protection which has a ground radius approximately equal to its height above the ground.

The surge diverters consist of a spark gap in series with a nonlinear resistor. The function of non linear resistor is very important. As the gap sparks over, due to the over voltage, the arc would get short circuited and may cause power flow current in the arrester. Since the characteristic of the resistor is to offer high resistance to high voltage, it prevents the effect of a short circuit. After the surge is over, the resistor offers high resistance to make the gap non-conducting.

One end of the arrestor would be connected to the terminal of the equipment to be protected and other end would be grounded. The length of the spark gap is set in such a way that normal line voltage is enough to cause an arc across the gap but high voltage would break down the air insulation and form an arc.

Different types of surge diverters are available depending on the purpose. Some of them are Rod gap arrester, Horn gap arresters, Multigap arresters, Expulsion type lightning arrester and valve gap arrester. Each of them has variety of range of operating conditions.
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