Electrical energy, one of the most important forms of energy that has become a primary need to be fulfilled as almost all walks of life need electricity to accomplish various works. Electrical energy generated at the power generation stations has to be transmitted and distributed for utilization and in the process has to meet up with many a challenges. Through proper right of way the energy is supplied to the consumers at the utility centers. The electrical power is made use for a wide range of loads at the consumer end. And the course of load demands varies widely posing problems in the abridgement of the quality of power delivered to the consumers.
Power Quality refers to the term used to describe electric power that drives an electrical load and the load's ability to function properly. Without proper power quality, the loads may not function properly and may even lead to the irrevocable damage to the load. Hence it is of vitality that power quality is maintained properly. To be precise, power quality actually refers to the quality of voltage and current delivered at the load end than power.
The quality of electrical power may be described as a set of values of parameters, such as:
• Continuity of service
• Variation in voltage magnitude
• Transient voltages and currents
• Harmonic content in the waveforms for AC power
Variation in voltage magnitude:
Ideally, AC voltage is supplied by a utility as sinusoidal having an amplitude and frequency given by national standards or system specifications. And under practical conditions, these may deviate in the following ways:
• Variations in the peak or RMS voltage.
• When the RMS voltage exceeds the nominal voltage by 10 to 80% for 0.5 cycle to 1 minute, the event is called a "swell".
• A "dip" also called as a "sag" is the situationwhere the RMS voltage is below the nominal voltage by 10 to 90% for 0.5 cycle to 1 minute.
• Random or repetitive variations in the RMS voltage between 90 and 110% of nominal value can produce a phenomenon known as "flicker" in lighting equipment. Flicker is rapid visible changes of light level.
• Abrupt, very brief increases in voltage, called "spikes", "impulses", or "surges".
• "Undervoltage" occurs when the nominal voltage drops below 90% for more than 1 minute. The term "brownout" is an apt description for voltage drops somewhere between full power (bright lights) and a blackout (no power – no light).
• "Overvoltage" occurs when the nominal voltage rises above 110% for more than 1 minute.
• Variations in the frequency.
• Variations in the wave shape – usually described as harmonics.
• Nonzero low-frequency impedance (when a load draws more power, the voltage drops).
• Nonzero high-frequency impedance (when a load demands a large amount of current, then stops demanding it suddenly, there will be a dip or spike in the voltage due to the inductances in the power supply line).
• Electrical line noise.
Causes of Voltage variations:
• Voltage Sags
Sags are most often caused by fuse or breaker operation, motor starting, or capacitor switching. Voltage sags typically are non-repetitive, or repeat only a few times due to recloses operation.
• Voltage Swell
A voltage swell takes place when the voltage is 110% or more above normal. The most common cause is heavy electrical equipment being turned off.
• Spikes
High-voltage spikes occur when there is a sudden voltage peak of up to 6,000 volts. These spikes are usually the result of nearby lightning strikes, but there can be other causes as well.
• Switching Transients
Switching transients take place when there is an extremely rapid voltage peak of up to 20,000 volts with duration of 10 microseconds to 100 microseconds. Switching transients take place in such a short duration that they often do not show up on normal electrical test equipment. They are commonly caused by machinery starting and stopping, arcing faults and static discharge.
• Frequency Variation
A frequency variation involves a change in frequency from the normally stable utility frequency of 50Hz.This may be caused by erratic operation of emergency generators or unstable frequency power sources
• Electrical Line Noise
Electrical line noise is defined as Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI) and causes unwanted effects in the circuits of computer systems.
• Brownouts
A brownout is a steady lower voltage state. The term "brownout" is an apt description for voltage drops somewhere between full power (bright lights) and a blackout (no power – no light). It comes from the noticeable to significant dimming of regular incandescent lights, during system faults or overloading etc., when insufficient power is available to achieve full brightness in (usually) domestic lighting. This term is in common usage has no formal definition but is commonly used to describe a reduction in system voltage by the utility or system operator to decrease demand or to increase system operating margins.
An example of a brownout is what happens during peak electrical demand in the summer, when utilities can't always meet the requirements and must lower the voltage to limit maximum power. When this happens, systems can experience glitches, data loss and equipment failure
• Blackouts
A power failure or blackout is a zero-voltage condition that lasts for more than two cycles. It may be caused by tripping a circuit breaker, power distribution failure or utility power failure. A blackout can cause data loss or corruption and equipment damage.
A considerable solutions for these power quality abridgement problems have been proposed which make use of various Flexible AC Transmission System (FACTS) devices. Enhancement of power quality with DSTATCOM is discussed below.
The enhancement of voltage sags, harmonic distortion and low power factor using Distribution Static Compensator (D-STATCOM) with LCL Passive Filter in distribution system can be achieved. power quality problems in transmission and distribution systems. The D-STATCOM is one of the most effective devices. A PWM-based control scheme can be implemented to control the electronic valves in the D-STATCOM. The D-STATCOM has additional capability to sustain reactive current at low voltage, and can be developed as a voltage and frequency support by replacing capacitors with batteries as energy storage.
DISTRIBUTION STATIC COMPENSATOR (D-STATCOM)
A D-STATCOM is a shunt compensating FACTS device. It consists of a two-level VSC, a dc energy storage device, controller and a coupling transformer connected in shunt to the distribution network. Figure below shows the schematic diagram of D-STATCOM.
Power Quality refers to the term used to describe electric power that drives an electrical load and the load's ability to function properly. Without proper power quality, the loads may not function properly and may even lead to the irrevocable damage to the load. Hence it is of vitality that power quality is maintained properly. To be precise, power quality actually refers to the quality of voltage and current delivered at the load end than power.
The quality of electrical power may be described as a set of values of parameters, such as:
• Continuity of service
• Variation in voltage magnitude
• Transient voltages and currents
• Harmonic content in the waveforms for AC power
Variation in voltage magnitude:
Ideally, AC voltage is supplied by a utility as sinusoidal having an amplitude and frequency given by national standards or system specifications. And under practical conditions, these may deviate in the following ways:
• Variations in the peak or RMS voltage.
• When the RMS voltage exceeds the nominal voltage by 10 to 80% for 0.5 cycle to 1 minute, the event is called a "swell".
• A "dip" also called as a "sag" is the situationwhere the RMS voltage is below the nominal voltage by 10 to 90% for 0.5 cycle to 1 minute.
• Random or repetitive variations in the RMS voltage between 90 and 110% of nominal value can produce a phenomenon known as "flicker" in lighting equipment. Flicker is rapid visible changes of light level.
• Abrupt, very brief increases in voltage, called "spikes", "impulses", or "surges".
• "Undervoltage" occurs when the nominal voltage drops below 90% for more than 1 minute. The term "brownout" is an apt description for voltage drops somewhere between full power (bright lights) and a blackout (no power – no light).
• "Overvoltage" occurs when the nominal voltage rises above 110% for more than 1 minute.
• Variations in the frequency.
• Variations in the wave shape – usually described as harmonics.
• Nonzero low-frequency impedance (when a load draws more power, the voltage drops).
• Nonzero high-frequency impedance (when a load demands a large amount of current, then stops demanding it suddenly, there will be a dip or spike in the voltage due to the inductances in the power supply line).
• Electrical line noise.
Causes of Voltage variations:
• Voltage Sags
Sags are most often caused by fuse or breaker operation, motor starting, or capacitor switching. Voltage sags typically are non-repetitive, or repeat only a few times due to recloses operation.
• Voltage Swell
A voltage swell takes place when the voltage is 110% or more above normal. The most common cause is heavy electrical equipment being turned off.
• Spikes
High-voltage spikes occur when there is a sudden voltage peak of up to 6,000 volts. These spikes are usually the result of nearby lightning strikes, but there can be other causes as well.
• Switching Transients
Switching transients take place when there is an extremely rapid voltage peak of up to 20,000 volts with duration of 10 microseconds to 100 microseconds. Switching transients take place in such a short duration that they often do not show up on normal electrical test equipment. They are commonly caused by machinery starting and stopping, arcing faults and static discharge.
• Frequency Variation
A frequency variation involves a change in frequency from the normally stable utility frequency of 50Hz.This may be caused by erratic operation of emergency generators or unstable frequency power sources
• Electrical Line Noise
Electrical line noise is defined as Radio Frequency Interference (RFI) and Electromagnetic Interference (EMI) and causes unwanted effects in the circuits of computer systems.
• Brownouts
A brownout is a steady lower voltage state. The term "brownout" is an apt description for voltage drops somewhere between full power (bright lights) and a blackout (no power – no light). It comes from the noticeable to significant dimming of regular incandescent lights, during system faults or overloading etc., when insufficient power is available to achieve full brightness in (usually) domestic lighting. This term is in common usage has no formal definition but is commonly used to describe a reduction in system voltage by the utility or system operator to decrease demand or to increase system operating margins.
An example of a brownout is what happens during peak electrical demand in the summer, when utilities can't always meet the requirements and must lower the voltage to limit maximum power. When this happens, systems can experience glitches, data loss and equipment failure
• Blackouts
A power failure or blackout is a zero-voltage condition that lasts for more than two cycles. It may be caused by tripping a circuit breaker, power distribution failure or utility power failure. A blackout can cause data loss or corruption and equipment damage.
A considerable solutions for these power quality abridgement problems have been proposed which make use of various Flexible AC Transmission System (FACTS) devices. Enhancement of power quality with DSTATCOM is discussed below.
The enhancement of voltage sags, harmonic distortion and low power factor using Distribution Static Compensator (D-STATCOM) with LCL Passive Filter in distribution system can be achieved. power quality problems in transmission and distribution systems. The D-STATCOM is one of the most effective devices. A PWM-based control scheme can be implemented to control the electronic valves in the D-STATCOM. The D-STATCOM has additional capability to sustain reactive current at low voltage, and can be developed as a voltage and frequency support by replacing capacitors with batteries as energy storage.
DISTRIBUTION STATIC COMPENSATOR (D-STATCOM)
A D-STATCOM is a shunt compensating FACTS device. It consists of a two-level VSC, a dc energy storage device, controller and a coupling transformer connected in shunt to the distribution network. Figure below shows the schematic diagram of D-STATCOM.
Voltage Source Converter (VSC)
A voltage-source converter is a power electronic device that connected in shunt or parallel to the system. It can generate a sinusoidal voltage with any required magnitude, frequency and phase angle. The VSC used to either completely replace the voltage or to inject the ‘missing voltage’. The ‘missing voltage’ is the difference between the nominal voltage and the actual. It also converts the DC voltage across storage devices into a set of three phase AC output voltages.
Controller
Proportional-integral controller (PI Controller) is a feedback controller which drives the system to be controlled with a weighted sum of the error signal (difference between the output and desired set point) and the integral of that value. In this case, PI controller will process the error signal to zero. The load r.m.s voltage is brought back to the reference voltage by comparing the reference voltage with the r.m.s voltages that had been measured at the load point. It also is used to control the flow of reactive power from the DC capacitor storage circuit. PWM generator is the device that generates the Sinusoidal PWM waveform or signal. To operate PWM generator, the angle is summed with the phase angle of the balance supply voltages equally at 120 degrees. Therefore, it can produce the desired synchronizing signal that required. PWM generator also received the error signal angle from PI controller. The modulated signal is compared against a triangle signal in order to generate the switching signals for VSC valves.
Energy Storage Circuit
DC source is connected in parallel with the DC capacitor. It carries the input ripple current of the converter and it is the main reactive energy storage element. This DC capacitor could be charged by a battery source or could be recharged by the converter itself.
LCL Passive Filter
LCL passive filter reduces the harmonics within the limits and thus the damage caused due to total harmonic distortion can be reduced.
The DSTATCOM is connected to the distribution system and thus the power quality can be improved.
There are also some other methods of improving power quality. Some of the solutions .There are five basic categories of solutions to some of the power quality problems, each having different capabilities, strengths and weaknesses.
1. Surge Suppressors
2. Voltage regulators.
3. Power conditioners
4. Uninterruptible power supplies.
5. Generators.
--------------------------------------------------------------------------------
Article By:
Minnie Florence V
Asst. Professor
EEE Department
Sphoorthy Engineering College
 |
| Sphoorthy Engineering College |
Your blog awesome, you shared valuable information about in #Power quality in electrical system. We provide more information in Harmonic distortions is one of the most common and irritating problems in industrial environment. We need to identify the source of harmonics and suppress them for a quality supply of power. Please visit Harmonic Analysis
ReplyDelete