POWER QUALITY MANAGEMENT
ABSTRACT
The present paper gives a brief introduction to power quality and ways for Energy Conservation. Electric power quality may be defined as a measure of how well electric power service can be utilized by customers. The term Power Quality means different things to different people. There is no agreed definition for power quality, it may be defined as the problems manifested in voltage, frequency and the effect of harmonics, poor power factor that results in mis operation/failure of customer equipment. The widespread use of high-power semiconductor switches at the utilization, distribution and transmission levels have made non-sinusoidal load currents more common. Certain type of power quality degradation result in losses and thus losses in transmission and distribution system have come under greater scrutiny in recent years.
This paper outlines the issues relating Reactive Power management to Power Quality and hence Capacitor Demand Meter has been discussed as a Power quality enhancer. The role of capacitor in power Quality Issues proves to be beneficial particularly when energy conservation is outlined.This paper also gives a view on various factors that affect power quality.
Introduction
Electric power quality may be defined as a measure of how well electric power service can be utilized by customers. When wave shapes are irregular, voltage is poorly regulated, harmonics and flicker are present, or there are momentary events that distort the usually sinusoidal wave, and the power utilization is degraded. This is referred as degradation of power quality. The widespread use of high-power semiconductor switches at the utilization, distribution and transmission levels have made non-sinusoidal load currents more common. Certain type of power quality degradation results in losses and thus losses in transmission and distribution system have come under greater scrutiny in recent years. This paper outlines the issues relating to Power Quality and their impact on Energy Conservation.
Power Quality
The term Power Quality means di¬fferent things to
different people. Power quality is the quality
of the electric power \supplied to electrical equipment
. Poor power quality can result in maloperation of the
equipment the electric utility may define power quality
as reliability and state that the system is 99.95% reliable.
Ideally the power would be supplied as a sine wave with
the amplitude and frequency given by national standards
(in the case of mains) or system specifications
(in the case of a power feed not directly attached to the mains
) with an impedance of zero ohms at all frequencies.
No real life power feed will ever meet this ideal.
It can deviate from it in the following ways (among others):
Variations in the peak or rms voltage (both these figures are important
to different types of equipment) When the rms voltage exceeds the nominal
voltage by a certain margin, a surge is produced. A dip is the opposite
situation: the rms voltage is below the nominal voltage by a certain margin.
A sag occurs when the low voltage persists over a longer time period
Variations in the frequency.
Variations in the wave shape - usually described as harmonics
Quick and repetitive variations in the rms voltage. This produces
flicker in lighting equipment.
Nonzero low frequency impedance (if the appliance draws more power
the voltage drops)
Nonzero high frequency impedance (if the appliance demands a large
amount of current or stops demanding it suddenly there will be a dip
or spike in the voltage due to the inductances in the power supply line)
rapid spikes and dips and longer term variations in voltage (usually
caused by the interaction of other equipment with line impedance)
POWER QUALITY IN POWER DISTRIBUTION SYSTEMS:
Most of the more important international standards
define power quality as the physical characteristics of the
electrical supply provided under normal operating conditions
that do not disrupt or disturb
the customer’s processes. Therefore, a power quality problem
exists if any voltage, current or frequency deviation results in
a failure or in a bad operation of customer’s equipment. However
, it is important to notice that the quality of power supply implies
basically voltage quality and supply reliability. A voltage
quality problem relates to any failure of equipment due to deviations
of the line voltage from its nominal characteristics, and the supply
reliability is characterized by its adequacy (ability to supply the load)
, security (ability to withstand sudden disturbances such as
system faults) and availability (focusing especially on
long interruptions).
Power quality problems are common in most of commercial,
industrial and utility networks. Natural phenomena, such a
s lightning are the most frequent cause of power quality
problems. Switching phenomena resulting in oscillatory
transients in the electrical supply, for example when capacitors
are switched, also contribute substantially to power quality
disturbances. Also, the connection of high power non-linear loads
contributes to the generation of current and voltage harmonic
components. Between the different voltage disturbances that
can be produced, the most significant and critical power quality
problems are voltage sags due to the high economical losses that
can be generated. Short-term voltage drops (sags) can trip
electrical drives or more sensitive equipment, leading to
costly interruptions of production. For all these reasons,
from the consumer point of view,
power quality issues will become an increasingly
important factor to consider in order to satisfy
good productivity. On the other hand, for the electrical
supply industry, the quality of power delivered will be one
of the distinguishing factors for ensuring customer loyalty
in this very competitive and deregulated market. To address
the needs of energy consumers trying to improve productivity
through the reduction of power quality related process stoppages
and energy suppliers trying to maximize operating profits while
keeping customers satisfied with supply quality, innovative
technology provides the key to cost-effective power quality
enhancements solutions. However, with the various power quality
solutions available, the obvious question for a consumer or utility
facing a particular power quality problem is which equipment
provides the better solution.
Power quality varies significantly from one area to
another. Some countries have very stable power
grids while others are extremely short on capacity.
Power disturbances are caused by the generation,
distribution and use of power, and lightning.
A power disturbance can be defined as unwanted
excess energy that is presented to the load
Causes of Power Disturbances
· Power disturbance originate both outside and inside customer facilities.
· Load switching causes surges because of collapsing fields (-e = l * di/dt)
· Over loaded power distribution systems can cause significant voltage variations between peak and off-peak hours.
· Significant momentary load changes, such as heavy inrush currents can cause severe voltage variations
· Black-outs can cause severe voltage surges both on loss and return of power.
· Circuit-breaker tripping and fuse blowing can cause severe surge voltages
· Large ups and variable-speed drives can cause various surge voltages inside buildings
Results of Power Disturbances
Ø Sags and under voltages can cause component overheating or destruction
Ø Surges and over voltages can cause component overheating, destruction or can trigger other electronic components such as SCR's.
Ø Component overheating reduces the life and deteriorates the real reliability as opposed to the estimated reliability based on steady-state conditions of the product.
Ø False triggering of other components can create nuisance alarm tripping or, worse, can cause overheating or destruction of other electronic components.
The manufacturer of the equipment defines power quality as characteristics of power supply that is required to make his equipment work properly but the customer is the one ultimately affected. While there is no agreed definition for power quality, it may be defined as the problems manifested in voltage, frequency and the effect of harmonics, poor power factor that results in mis operation/failure of customer equipment. According to this approach Power Quality may be co-related with four topics.
F Voltage
F Frequency
F Harmonic distortion
F Power Factor
Voltage
In the context this issue must be viewed from two different directions. The first direction is variation in supply voltage due to the factors arising from transmission and distribution of power. The second direction is variation in voltage within a network due to the characteristics of the loads connected therein. It is well known fact in many other developing Countries that the quality of voltage supplied by the utilities varies widely depending on the type of distribution network and the geographical locations of such networks. The problem of voltage variation in this regard becomes more acute in rural distribution network.
To complicate this problem further the voltage variation is also a function of the season of the year, for ex: rural feeders experience the lowest voltages when the drawl of power is the highest, which is invariably in a particular time the year depending on the agricultural output/crop of that area.
Similarly on particular feeders, which supply highly fluctuating loads of an industrial nature, it is common to find voltage variations beyond permissible limits. The impact of such voltage variations is to cause higher energy consumption due to a combination of factors. Some of the important factors are
ü For a given MW of power rating, the current drawn goes up inversely in proportion to the voltage. Consequently, a drop in voltage would result in increased current flowing on the network. This increased current then causes increase in I2R losses of the network. For ex: a 20% drop in voltage would increase the losses in the network by 56%. Further, this increased current will contribute to increasing the voltage drop and thereby intensifying the problem.
ü Drop in efficiency of induction motors: It is well known that a substantial part of electrical energy consumption occurs in induction motors. The characteristics of these motors are such that a drop in voltage will mean a higher energy consumption to do the same job. Hence, extra energy is consumed when there is a voltage drop on the network.
ü A variety of studies has shown that the variations in voltage are a frequent occurrence in power distribution networks. The voltage drop seen in such studies could be as much as 40%of the rated value, thereby increasing corresponding I2R losses by 277 %. This results in increased energy wastage and higher power demand from the system, i.e., the power generation equipment has to supply higher MW for the same load.
Voltage imbalance have the following adverse effects:
v Overheating of motors lead to insulation breakdown.
v Imbalanced currents.
v Negative voltage sequence
v Motor bearings failure.
v Speed variation in motors.
v Reduced production quality.
v Reduced motor efficiency.
v Wasted energy which leads to higher electric bills-KWD, KWH.
v Wasted investment and operation capital.
v Use of oversized machinery.
v More difficult to provide adequate overload protection.
v Increased noise and vibration.
v Increased maintenance of equipment and machinery
Three phase motors are even less tolerant of phase-to-phase voltage unbalance. A 5% unbalance will cause a 50% temperature rise in three phase motors and is defined by the National Electrical Manufacturers Association (NEMA) as the absolute maximum unbalance under which motors should be allowed to operate for short period of time as delineated in the NEMA specification MG1-14.34 subsequently, 5% voltage unbalance will result in 35% losses, where 4% voltage unbalance results in 25% losses, and 3% results in 15% losses, and wasted energy.
The variation of temperature rise with voltage unbalance is shown below
Brownouts
Brownout by definition is low voltage for an extended period of time (greater than half a cycle) in which the magnitude of the voltage is reduced.
Brownouts cause the following adverse effects:
Ø Temporary low line voltage.
Ø Shutdowns.
Ø Loss of microprocessor memory.
Ø Loss of control.
Ø Loss of control.
Ø Overheating of motors - insulation breakdown.
Ø Protective device tripping.
Ø Speed variation
Ø Reduced motor torque, which can lead to stalling
Frequency
While this is also an important factor, it is more stable than the voltage due to the fundamental nature of electricity generation, transmission and distribution. Frequency variations can occur, due to the load levels on the electricity supply system, for ex: a highly overloaded power system will experience a drop in frequency. Further, mismatch of frequency in different sections of a grid can cause power quality and power supply problems particularly, when it is important to have an integrated and interconnected grid. The issues relating to these topics are more relevant in the area of power system stability and load dispatch and are hence, not touched upon in this paper. Impact of this topic on Energy Conservation is less important in comparison with the other three topics stated above.
Harmonic Distortion
The problem of Harmonic distortion primarily occurs in modern electrical networks due to feedback of Harmonic currents from nonlinear loads. Harmonic Voltage distortion is created due to interaction of such Harmonic currents with source impedances. Consequently, this can be treated as a form of electrical pollution on the network.
This has resulted in a situation where it is not uncommon for a new consumer on the electricity grid to find that the incoming supply voltage consists of a basket of frequencies including the fundamental frequency. This is due to the pollution of the grid by other consumers. The presence of Harmonic distortion has a significant impact in increasing energy consumption. Some of the important reasons for this are listed below:
Ø All electromagnetic equipment such as transformers, motors etc, have two key constituents of losses namely, iron loss and copper loss. The iron loss is also a function of the power of the frequency. Consequently, presence of higher frequency components such as 5th harmonic, 7th Harmonic etc, will result in an increase in iron losses. Hence energy consumption will go up and this is a particular importance in the distribution transformers whose All Day Efficiency could be significantly reduced because of this aspect. It is also harmful for the transformers and motors since it causes faster ageing of the insulation due to higher temperature rise in the electromagnetic core.
Ø The phenomena of skin effect are well known. The flow of Harmonic currents therefore, increases the I2R losses depending on the occurrence of the skin effect. This phenomenon is well understood and causes over heating of equipment and current carrying parts thereby, increasing the amount of energy consumption for the same network load.
Harmonics cause the following adverse effects:
v Overheating of transformers (K- Factor), and rotating equipment.
v Increase Hysterisys losses
v Neutral overloading / unacceptable neutral-to-ground voltages.
v Distorted voltage and current waveforms.
v Failed capacitors banks.
v Breakers and fuses tripping.
v Unreliable operation of electronic equipment, and generators.
v Erroneous register of electric meters.
v Wasted energy / higher electric bills -KWD & KWH.
v Wasted capacity - Inefficient distribution of power.
Increased maintenance of equipment and machinery
Power Factor
Power factor is the phase shift between voltage & current. While the theoretical definition of Power factor is the ratio of active power to apparent power, it is well known that in electricity distribution systems this is measured as a ratio of active energy to apparent energy over a specified time period.
The ideal power factor is unity. However, this cannot be achieved in reality due to the nature of the loads used. For ex: inductive loads, nonlinear loads etc, A lower power factor means more current drawn for the same load. This causes increase in the apparent power demand i.e., kVA demand as well as increases I2Rlosses.
Consequently, more system capacity is needed to supply the same load. In other words a lower power factor results in higher energy consumption. Further, lowering of power factor also causes a drop in voltage which then brings us back to the ill effects of voltage variation as described earlier.
A lagging power factor is generally caused as a result of inductive loads, and particularly, motors that are not fully loaded.
Low Power Factor causes the following adverse effects
Ø Increased line losses I2R
Ø Wasted generation capacity (KVA)
Ø Wasted distribution /transformer/ capacity (KVA)
Ø Wasted system capacity (KVA)
Ø Reduced system efficiency (KW)
Ø Increased maximum demand (KVA), and related charge
Possible power factor charges
In line with the above problems, we can expect energy wasted and reduced power quality through:
•Increased maintenance of equipment and machinery.
•Wasted energy / higher electric bills – KWD and KWH.
•Wasted investment and operation capital.
The oldest solution for a low power factor in industry, in terms of counter balancing the lagging power factor, are capacitors. But, there are problems associated with capacitors which industry is staying away from because of the potential side-effects to today’s sensitive equipment (ex.electronics, computers, etc.).
Improving power factor is a great idea because it increases the efficiency of the distribution, reduces losses, and power factor charges are eliminated (if charged)
In addition to these problems intermittent supply failure and phase loss also adversely affects the power quality.
Intermittent Supply Failure:
Generally, intermittent supply failures are caused by the utility company switching loads, lines, and source supplies.
The fastest this switching can occur is three to five cycles.
During this period, there is a complete drop-out.
This may or may not be a concern for all industries but intermittent supply failure takes its toll on the operation and efficiency of equipment and machinery.
Supply failure causes the following adverse effects:
v Voltage control relay tripping.
v Phase imbalance relay tripping.
v Plant and equipment shutdown - downtime!
v Loss off critical microprocessor memory.
v Possible jogging, pinching and stalling of motors.
v Loss of control and resetting of equipment.
v Loss production
Supply failure causes the following adverse effects:
Voltage control relay tripping.
Phase imbalance relay tripping.
Plant and equipment shutdown - downtime!
Loss off critical microprocessor memory.
Possible jogging, pinching and stalling of motors.
Loss of control and resetting of equipment.
Loss production
Phase Loss
In case of phase loss, a lost phase from the remaining two phases causes interruption of power supply to industries and thus causing loss of valuable production time.
Phase loss causes the following adverse effects:
Ø Imbalanced operation of three phase motors, resulting in insulation breakdown and destruction.
Ø Increased downtime
Ø Loss of production.
Ø Major maintenance and replacement capital requirement
SOLUTIONS TO POWER QUALITY PROBLEMS:
There are two approaches to the mitigation of power quality problems.
The first approach is called load conditioning, which ensures that the equipment is
less sensitive to power disturbances, allowing the operation even under significant
voltage distortion. The other solution is to install line conditioning systems that suppress
or counteracts the power system disturbances. A flexible and versatile solution to voltage
e quality problems is offered by active power filters. Currently they are based on PWM
converters and connect to low and medium voltage distribution system in shunt or in
series. Series active power filters must operate in conjunction with shunt passive
filters in order to compensate load current harmonics. Series active power filters
operates as a controllable voltage source. In addition to it we can also use
capacitor demand meter for better power quality.
SERIES ACTIVE POWER FILTERS:
It is well known that series active power filters compensate current system distortion
caused by non-linear loads by imposing a high impedance path to the current harmonics
which forces the high frequency currents to flow through the LC passive filter connected
in parallel to the load. The high impedance imposed by the series active power filter
is created by generating a voltage of the same frequency that the current harmonic
component that needs to be eliminated. Voltage unbalance is corrected by
compensating the fundamental frequency negative and zero sequence voltage
components of the system
CONTROL SCHEME:
The block diagram of the proposed control scheme isshown in the fig below.
Current and voltage reference waveforms are
obtained by using the Instantaneous Reactive Power Theory. Voltage
unbalance is compensated by calculating the negative and zero
sequence fundamental components of the system voltages. These
voltage components are added to the source voltages through the
series transformers compensating the voltage unbalance at the load
terminals. In order to reduce the amplitude of the current flowing
through the neutral conductor, the zero sequence components of the line
currents are calculated. In this way, it is not necessary to sense the
current flowing through the neutral conductor
GATING SIGNAL GENERATOR:
This circuit provides the gating signals of the three-phase PWM
voltage-source inverter required to compensate voltage
unbalance and current harmonic components. The current and
voltage reference signals are added and then the amplitude of the resultant
reference waveform is adjusted in order to increase the voltage utilization
factor of the PWM inverter for steady state operating conditions. The gating
signals of the inverter are generated by comparing the resultan
t reference signal with a fixed frequency triangular waveform
(5 kHz). The triangular waveform forces the inverter switching
frequency to be constant.
The higher voltage utilization of the inverter is obtained if the amplitude
of the resultant reference signal is adjusted for the steady state
operating condition of the series active power filter. In this case,
the reference current and reference voltage waveforms are
smaller. If the amplitude is adjusted for transient operating conditions,
the required reference signals will have a larger value, which will
create a higher dc voltage in the inverter thus defining a lower
voltage utilization factor for steady state operating conditions.
Capacitor Demand Meter
A brief discussion regarding the Capacitor Demand Meter is
presented in this part of paper. Reactive Power Managemen
t is one of the aspects of Power Quality issues as this directly
relates to the customer receiving end voltage .When a
capacitor is added to the electrical network ,
the magnitudeof the resultant network
current shall change and this current
needs to be minimum at the load end terminals.
The basic objective of the development of the Capacitor
Demand Meter is to develop an instrument, which when
connected at the load end terminals shall directly indicate the
KVAR rating of capacitors to be connected at that point, to keep
the supply current drawn at minimum value. Capacitors, current
transformers and micro controller are the basic elements of
Capacitor Demand Meter.
The Role of Power Capacitors in Saving Energy :
As outlined above the problems of voltage drops are primarily
due to excessive flow of Reactive Power in the network
The use of Capacitor banks to carry out shunt compensation
as well as Series compensation would go a long way in
improving the voltage profile and making it more consistent.
Similarly, if all consumers of electricity can improve
their power factor closer to unity, the release of
system capacity and reduction in losses would be
very significant. Power Capacitors are the most well
proven and cost effective devices to achieve this objective.
As regards Harmonic distortion it is possible to reduce
the same by the use of passive and active harmonic filters.
Power capacitors of various types form an essential
constituent of such harmonic filtering devices.
Thus a capacitor plays a vital role in the Reactive
Power Management and thus in the Power Quality problems.
Conclusion
The Power Quality issues such as voltage variations,
Harmonic distortions and power factor combine
together to reduce the overall operating efficiency of
electrical networks and also result in increased power
supply demand and unnecessary wastage of energy.
Power quality can be improved by providing capacitor
demand meter, capacitor banks at the load side. In this
paper the use and advantages of applying Series active power
filters to compensation power distribution systems has been
presented. The principles of operation of series active power
filter have been presented
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