Powe Quality Issues 2007

Power Quality Issues And RemediesPresented By:

Nav GoelShashank YadavVikas Singh

Guided By:Mr.Sanjiv Kumar

INTRODUCTION

Since last 25 years there has been an increase in the use of solid state electronic technology. This new, highly efficient, electronic technology provides product quality with increased productivity.

With the advent of Power Electronics in the system, the electrical network is polluted due to non-linear characteristics of loads.

The conventional speed control systems are being replaced by modern power electronic systems, bringing a verity of advantages to the users. Classic examples are DC & AC drives, UPS, etc.

Since the thyristors converter technology is rapidly gaining in the modern industrial plants, the power supply systems are contaminated as the ideal sinusoidal current and voltage waveforms are getting distorted .

INTRODUCTION (contd.)

The Power Electronic devices are mainly Non-Linear loads.

The presence of Non-Linear loads is mainly responsible for introduction of Harmonics in the system.

The harmonics thus introduced affects performance of Electrical rotating machines, Transformers, Capacitors, Inductors, etc.

Also the quality of voltage that is made available to the consumer is not as per desired / permissible requirements .

And hence affects the performance of Electrical equipment of the consumer.

INTRODUCTION (contd.)

Now a day’s equipments have become more sensitive to voltage disturbances.

Electronics and Power Electronics equipments have especially become much more sensitive than its counterparts 10 or 20 years back.

Modern electronic equipment is not only sensitive to voltage disturbance; it also causes disturbances to other customers.

The customers are demanding reliable and good quality supply from utility and utility is also encouraging or panelizing the customers to maintain Power Quality.

What Actually Power Quality Is?

The term ‘‘power quality’’ means different things to different people.

As per IEEE Standard 1100 the definition of Power Quality is,’Power Quality is the concept of powering and grounding sensitive equipment in a manner that is suitable to the operation of the equipment’.

Power Quality is anything that affects the voltage, current and frequency of the power being supplied to the end users.

There is lot of disagreement about what power quality actually incorporates.

What Actually Power Quality Is? (contd.)

The terms ‘Power Quality’, ‘Quality of Power’, ’Voltage Quality’, ’Current Quality’, ’Quality of Consumption’, ‘Electromagnetic Compatibility’ are used interchangeably.

Power Quality was also an important issue in the past. But recently there is increased interest in the Power Quality.

Electrical power engineer have always been concerned about power quality. They see power quality as anything that affects the voltage, current and frequency of the power being supplied to end user.

As an engineer, there is a need to communicate to others the importance of understanding power quality and power quality problems.

Power Quality Issues

Power quality issue is defined as any problem manifested in voltage, current or frequency deviations that results in failure or malfunction of customer equipment.

Harmonics, voltage flicker, voltage regulation, unbalance, voltage sag, voltage swell, and interruption, usually characterize the quality of electric power.

Some of the power quality problems created by drives are harmonics, notching, etc, being heavily dependent on the supply system configuration, process equipment design, system switching, and protection practices.

Due to the wide use of adjustable AC & DC drives in all industrial applications, analysis of power quality problems is very important.

Power Quality Issues (contd.)

Contrary to common belief, the utility-supplied electricity is not the cause of all power-related process interruptions..

In fact, studies by the Electric Power Research Institute (EPRI),California indicate that as much as 80% of all power quality problems can be attributed to inadequate electrical grounding or wiring, or interactions between loads within the premises.

With a little knowledge of power quality issues , owners of industrial processes can learn to identify causes of electrical disturbances and take action to prevent their recurrence.

Power Quality Issues can be roughly broken into categories as follows:

Steady-state voltage magnitude and frequency,

Voltage sags,

Grounding,

Harmonics,

Voltage fluctuations and flicker,

Transients,

Blackouts and Brownouts,

Electrical Line Noise and

Surges and Spikes.

Electrical Noise, Harmonic Distortion and impulses:

Voltage sag, swell and outage:

Steady State Frequency And Voltage Magnitude:-

Frequency deviations can affect power electronic equipment that use controlled switching devices unless the control signals are derived from a signal that is phase-locked with the applied voltage.

In most cases, these effects have minimal economic impact and are not considered a real power quality problem.Steady state voltage regulation is a much more pronounced issue that can impact a wide range of end-use equipment.

Virtually all equipment, especially sensitive electronic equipment can be affected by voltages deviating outside the (±10%) range.

Voltage Sag:-

Under voltages of greater than (15–20%) can cause equipment to immediately trip. In most cases, such extreme under voltages are associated with system faults and the associated protection system.

These extreme under voltages are so important that they are classified in a power quality category of their own, called voltage sags.

Of course, areas closer to the faulted area will see a greater voltage sag due to the fault than other, more (electrically) remote areas.

Sags can originate anywhere in a system, but are more pronounced in utility distribution systems because of the greater exposure of low-voltage systems to the causes of short circuits.

Grounding:-

From a power quality perspective, improper grounding can be considered in three broad categories:

1. Ground loops,

2. Improper neutral-to-ground connections, and

3. Excessive neutral-to-ground voltage

Transients induced in one location can travel through the created ground loop, damaging equipment along the way.

Improper neutral-to-ground connections will create a ‘‘noisy’’ ground reference that may interfere with low-voltage communications and control devices.

Excessive neutral-to ground voltage may damage equipment that is not properly insulated or that has an inexpensive power supply

Grounding:- (Contd.)

For load equipment that produces significant voltage drop in the neutral, such as laser printers and copying machines when the thermal heating elements are on, the voltage from the neutral to the ground reference inside the equipment can exceed several volts.

In many cases, this voltage is sufficient to damage printed circuit boards, disrupt control logic, and fail components

Harmonics :- Harmonics are defined as electrical voltages and currents with

frequencies that are integer multiples of the electrical power frequency

As distorted current passes through an electrical distribution system, voltage distortion is then created according to Ohm’s Law (Volts = Current x Resistance).

Voltage Fluctuations And Flickers:-

Voltage fluctuations, and the corresponding light flicker due to them, are usually created by large power fluctuations at frequencies less than about 30 Hz.

In most applications, only

1. Large dc arc furnaces and welders,

2. Reactive power compensators, and

3. Cycloconverters,

are potentially problematic.

Because most utility supply companies limit voltage fluctuations, regardless of the frequency of repetition, to less than a few percent, equipment malfunction or damage due to flicker is very rare

Transients:-

The major sources of transients are:

1. Lightning,

2. Utility circuit switching and fault clearing,

3. Capacitor switching, and

4. Load switching

Transients, especially in the voltage supply, can create numerous power quality problems.

Blackouts and Brownouts:- A power failure or blackout is a zero-voltage condition that lasts for more than two

cycles. A blackout can cause data loss or corruption and equipment damage.

A brownout is a steady lower voltage state. 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.

Power Surges and Voltage Spikes:-

A power surge takes place when the voltage is 110% or more above normal. The most common cause is heavy electrical equipment being turned off.

Under these conditions, computer systems and other high tech equipment can experience flickering lights, equipment shutoff, errors or memory loss.

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.

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.

Sources of the problems include motors, relays, motor control devices, broadcast transmissions, microwave radiation, and distant electrical storms

What’s Harmonics?

Harmonic is defined as “a sinusoidal component of a periodic wave or quantity having a frequency that is an integral multiple of the fundamental frequency”.

Therefore, harmonic is the presence of voltage/current with the frequency of a multiple of fundamental voltage/current in the voltage/current of the system

Sources of harmonics: 1. Power generation equipment,

2. Induction motors,

3. Transformers,

4. Magnetic-ballast fluorescent lamps, and

5. Ac electric arc furnaces.

Causes of Harmonics:

In most cases, power electronic equipment is considered to be the cause of harmonics.

While switching converters of all types produce harmonics because of the nonlinear relationship between the voltage and current across the switching device.

Harmonic distortion of the voltage and current in an industrial facility is caused by the operation of nonlinear loads and devices on the power system.

A nonlinear load is one that does not draw sinusoidal current when a sinusoidal voltage is applied.

Examples of nonlinear loads are arcing devices such as arc furnaces, saturable devices such as transformers, and power electronic equipment such as adjustable-speed drives and rectifiers

Harmonics are caused by and are the by-product of modern electronic equipment such as

personal or notebook computers, laser printers, fax machines, telephone systems, stereos, radios, TVs, adjustable speed drives and variable frequency drives, battery chargers, UPS, and any other equipment powered by switched-mode power supply

(SMPS)equipment.

The above-mentioned electronic SMPS equipment is also referred to as non-linear loads.

In general, these non-linear loads require DC electric power.

In today’s environment, all computer systems use SMPS that convert utility AC voltage to regulated low voltage DC for internal electronics.

These switched mode power supply(SMPS) equipments draw current in high amplitude short pulses.

These current pulses create significant distortion in the electrical current and voltage wave shape,

Fundamental And Resultant Voltage Waveform With harmonics

Effects Of Harmonics

High levels of distortion can lower power factors, overheat equipment.

Harmonic currents increase the volt-amperes required for a load without increasing the watts. Because true power factor is equal to the watts divided by the volt-amperes, any increase in volt-amperes without a corresponding increase in watts will lower the power factor.

Overheating of transformers is another problem associated with harmonic currents.

Another effect of harmonic currents on the power system is the overheating of neutral wires in star-connected circuits. This effect occurs because the third harmonic and any multiples thereof do not cancel in the neutral as do the other harmonic currents

This is particularly the case for the third harmonic, which causes a sharp increase in the zero sequence current, and therefore increases the current in the neutral conductor.

Problems Created by harmonics

Transformers and reactors

Winding stray (eddy-current) losses due to non sinusoidal load currents rise in proportion to the square of the load current and the square of the frequency;

Hysteresis losses increase;

Possible resonance may occur between the transformer inductance and the line capacitance

Motor

Stator and rotor I2R losses will increase due to the flow of harmonic currents;

Core losses increases due to harmonic voltage;

Excessive losses in and heating of induction and synchronous machines

Also the quality of voltage that is made available to the consumer is not as per desired / permissible requirements and hence affects the performance of Electrical equipment of the consumer

Harmonic currents due to nonlinear loads demand the bigger neutral cables as the neutral current is vector sum of three phase currents drawn by the individual SMPS loads in each phase.

Harmonics appearing in the supply will tend to flow the neutral current even for three phase balanced fundamental currents in the source.

Proper attention for these neutral currents is to be given in the aspect of neutral conductor sizing

With the advent of newer SMPS equipment the harmonic problem will continue to get worse.

Problems created by Harmonics (contd.)

Methods Of Harmonic Reduction:-

Methods of Harmonics reduction

Passive Filter

Active Power Filter

Static VAR Compensat

or

Phase Shift Method

Passive Filters The simplest method of suppressing the harmonic distortion is to use

passive filters.

Passive filters contain a series of LC circuits tuned in on the harmonic, which should be mitigated.

By linking a few of such filters in a parallel way, the filter block can be constructed as so that it filters out all the harmonics.

The advantages of passive filters are:

Simplicity

Reliability

Low cost

Good efficiency

Basic Categories of Passive Filters:

Passive filters are designed (or can be purchased) in three basic categories as follows:

1. Single-tuned filters,

2. Multiple- (usually limited to double) tuned filters, and

3. Damped filters (of first, second, or third order).

The single-tuned and double-tuned filters are usually used to filter specific frequencies.

Active Power Filters:-

Active power filter(APF) is a new power electronic device to achieve the purpose to compensate harmonics by injecting a compensation current, the direction of which is contrary with the direction of harmonic current demanded to be eliminated and the magnitude of which is equal to the magnitude of harmonic current demanded to be eliminated in power system.

The active filter uses power electronic switching to generate harmonic currents that cancel the harmonic currents from a nonlinear load.

The concept of the harmonic current cancellation is such that the current being supplied from the source is sinusoidal.

Active Power Filter In OFF State:

Active Power Filter In ON State:

FACTS(Flexible A.C. Transmission System)

With ever increasing demand of electrical power , the existing transmission networks even in developed countries are found to be weak which results in poor power quality.

One way to improve this is the use of FACTS.

It employs high speed thyristors for switching in or out transmission line components such as capacitors , reactors ,etc.

The main objective of FACTS is to replace the existing slow acting mechanical controls required to react to the changing system conditions.

The power transfer between two systems interconnected through a tie line

is given by: p=V1V2sinδ/X

and can be controlled by three parameters, the voltages at the two systems,the reactance of tie-line and difference in voltage angles at the two ends.

FACTS (Flexible A.C. Transmission System) (contd.)

The FACTS devices are used to control one or more of these parameters

The various devices are as follows:

Static VAR Compensators(SVC)

Static Condensors (STATCON)

Static Var Compensator

Static VAR Compensator (or SVC) is an electrical device for providing fast-acting reactive power on high-voltage electricity transmission networks.

SVCs are part of the Flexible AC transmission system device family, regulating voltage and stabilizing the system.

The term "static" refers to the fact that the SVC has no moving parts. Prior to the invention of the SVC, power factor compensation was done by large rotating machines such as synchronous condensers.

The SVC is an automated impedance matching device, designed to bring the system closer to unity power factor.

Static Var Compensator (contd.)

If the power system's reactive load is capacitive (leading), the SVC will use reactors to consume VARs from the system, lowering the system voltage.

Under inductive (lagging) conditions, the capacitor banks are automatically switched in, thus providing a higher system voltage.

Static Condenser (STATCON)

These devices are known as STATCON or static condenser because they exhibit the properties of conventional synchronous condensers.

The basic elements of STATCON are

Inverter

Capacitor

Transformer

Voltage source inverter inverts d.c. voltage.

The 3-ø a.c. generated by inverter is synchronized to the a.c. line.

Static Condenser (STATCON) (contd.)

When the inverter fundamental voltage is greater than system line voltage , the STATCON works as a capacitor.

However when the inverter fundamental voltage is less than system line voltage , the STATCON works as a inductor.

The magnitude of d.c. voltage is controlled by adjusting the phase angle of inverter output voltage.

Transients

A transient as defined in IEEE 1100-1999 is a sub-cycle disturbance in the AC waveform that is evidenced by a sharp, brief discontinuity of the waveform. Transients can be categorized as:

Impulsive

Oscillatory

Lightning surges are the most common cause of impulsive transients on the utility system.

Capacitor switching is the most common cause of oscillatory transients. Both types of transients can affect industrial equipment.

Oscillatory Transients Switching on a capacitor can create an oscillatory transient on the system

voltage. When a capacitor is initially switched onto the distribution system, the voltage across the capacitor terminals rapidly falls to zero but the voltage across a capacitor cannot change instantaneously.

As the capacitor charges, the voltage oscillates so that a voltage overshoot rises to between 1.3 p.u. to 1.4 p.u. of the original system voltage.

An impulsive transient is typically characterized by large amounts of energy, and fast rise and decay times. These transients are typically caused by lightning or system faults.

An impulsive transient can be very damaging to a wide range of industrial equipment. The damage is caused by the high level of voltage and energy that is applied to equipment that is not rated for that level of energy.

Impulsive Transients

Voltage Sags

A voltage sag is defined as a decrease in RMS voltage magnitude lasting from 0.5 to 30 cycles.

These are usually caused by a fault in the utility transmission or distribution system.

If a fault occurs on a supplying distribution feeder, a customer would experience a brief interruption. All other customers on adjacent feeders would experience a voltage sag, the duration of which depends upon the clearing time of the protective device.

On other hand, a fault on the transmission system would cause a sag to all the customers downstream of that location .The diagram is shown in next slide:

Diagram Showing Distribution System Showing Fault Location and Affect on the Customer

Solutions to Voltage Sags

1.)Ferro resonant Transformer

Most voltage-sag solutions can be handled by ferroresonant transformers.

These power conditioners are also known as constant-voltage transformers (CVTs). CVTs are ideally suited for constant, low-power loads.

Unlike conventional transformers, the CVT allows the core to become saturated with magnetic flux, which maintains a relatively constant output voltage during input voltage variations such as under-voltages , over-voltages , and harmonic distortion.

The cost of a CVT is relatively low compared to other power conditioners. It also has the advantage of low maintenance because there are no moving parts or batteries to maintain. The approximate material cost of a CVT is $1000 per kVA .

Solutions to Voltage Sags (contd.)

2.) Uninterruptible Power Supply

A UPS system is designed to provide uninterrupted power to the protected load, regardless of the conditions of the supply system.

A rectifier/charger UPS always feeds the load through the UPS. The incoming AC line is rectified into DC power, which charges the batteries. This DC power is then inverted back into AC power to feed the load.

Solutions to Voltage Sags (contd.)

3.) Distribution capacitors

Are used to improve power quality in areas known to have poor power factor and low voltage.

Several customers may be small contributors to a power factor problem.

However, when taken in sum, the smaller poor power factors adds up to low and often fluctuating voltages .

This can over-heat conductors and lead to other utility hardware problems and reduced service quality.

Energy Consumption Pattern In India

Indutry 31%

Agriculture 31%

Domestic 21%

Commercial 5%

Others 12%

The sector-wise energy consumption at national level is as shown in pie chart:

Around 31% of total energy is used in industrial sector.

And around 31% is used in agricultural sector.

Conclusion

The industrial sector consumes nearly 31 per cent of the total commercial energy available in India.

This is basically due to the fact that Indian industries are often energy inefficient and have least concern for energy conservation.

Hence, experts believe substantial saving potential (nearly 30 per cent) through retrofitting in this sector.

Today the major part of electric losses consist of line losses, power thefts, and inefficient machines.

By reducing line losses and using energy efficient machines we can very much reduce the gap between demand and supply.

Conclusion

As we know that, the main cause of line losses is the harmonics present in the supply.

Thus if we can reduce the harmonics present in the transmission line we can very much ease off the burden on electricity production.

And finally we can move towards good “Power Quality”

thus making a Green Move…

Thank you

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