POWER QUALITY AND IMPROVEMENT
#1

presented by:
Miss. Ankita B. Bahure
Miss. Arti S. Sathawane
Mr. Rohan R. Nibrad

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POWER QUALITY AND IMPROVEMENT
ABSTRACT:-

Electrical Energy is the life stream of progress for any nation and it is all the more vital in a developing country like INDIA. The energy in electrical form is most widely used since it can be converted into any other form efficiently. A power System Network consists of variety of apparatus for generation, Transmission and distribution spread over a large area serving to number of consumers. In this paper we are going to give information about power quality of system and the harmonics which are present and disturb the system due to non- linear loads.
Keyword: - Power factor, transient behavior, generation, transmission, distribution
INTRODUCTION:-
The Modern Power Systems are interconnected and operated in an integrated way due to technical reasons, economic benefits and for the sake of optimum utilization of available resources. The network when subjected to some kind of disturbance, be it routine and small or large, occurring somewhere at any instant of time, is usually associated with electromagnetic sub-transient phenomenon lasting for microseconds to milliseconds, electromechanical oscillations lasting for few milliseconds to many seconds and finally to a quasi-steady state. The so-called steady state hardly prevails in a practical system for any significant duration.
Now days the harmonics distortion is present in the electrical mains have been increasing due to non-linear loads. During past decades many power electronics equipment use in power system to control the flow of power. Such devices gained a great deal of popularity due to significant saving in energy cost that they provide. But unfortunately they are give rise to production of harmonics distortion .Due to harmonic distortion excessive heating, losses saturation in transformer, reduction of life, malfunctions of relays interference in communication line. So there is a need for better understanding of the harmonics, there adverse effect and measuring techniques and filtering.
CONCEPT OF POWER QUALITY:-
When the power system is polluted with harmonics, unbalance prevailing between phases and bad voltage profile due to poor power factors, the quality of power supply becomes very inferior which hampers the power apparatus of both the utilities and the customers .The power quality basically is associated with manifestation of many supply problems such as outage, dips/swells harmonics, flicker, unbalance, etc. concerning voltage, current and frequency both under steady state and transient conditions, which may affect power apparatus /customer equipment in some undesirable/adverse way and can lead to miss-operation or even failure .
The power quality addresses itself to maintain balanced supply of pure sinusoidal waves both for voltage and current at rated frequency in the three-phase system of desirable magnitudes at all nodes continuously and uninterruptedly. It may not be practically feasible to overcome or eliminate all kinds of disturbances, but their occurrences may be contained and countered with appropriate measures so as to maintain good power quality and operate the system with utmost reliability This takes into account automatically the sub-transient, transient and steady state phenomena of the power system.
LOAD BEHAVIOR:-
In particular, over the last two /three decades, the variety of loads employed by small, medium and bulk consumers are very diverse in characteristics behavior which are highly nonlinear, mostly inductive consuming large reactive –Q powers, fast responding and very sensitive due to the sophisticated electronic controllers, are posing serious problems for power Quality maintenances. It has become somewhat difficult to provide uninterruptible power supply and maintain voltages (in sinusoidal/undistorted wave forms) at all the feeder supply points as acceptable by the consumers, compensate the load reactive power and operate close to unity power factor always with well regulated voltage profile in the entire system so as to minimize the losses. At most feeding points in the distribution systems, a typical load is heterogeneous mixture of all kinds, having both single phase (unevenly distributed between phases) and three phase wit widely different characteristics, highly nonlinear, generate harmonics, distort both V & I wave forms, instrumental for flicker/voltage instability and unbalanced having large neutral currents/potential leading to very poor Quality of power supply.
CAUSE OF POOR POWER QUALITY:-
• Voltage sag (dip): A decrease between 10% and 90% in rms voltage or current at the power frequency for duration from 0.5 cycles to 1 min.
• Voltage swell (surge): An increase to between 110% and 180% in the rms voltage or current at the power frequency for durations from 0.5 cycles to 1 min.
• Over voltage: An increase in the rms ac voltage greater than 110% at the power frequency for duration longer than 1 min.
Under voltage: A decrease in the rms ac voltage to less than 90 percent at the power frequency for duration longer than 1 minute.
• Interruption: Momentary power interruptions of duration-1/2 second too greater tan 1 minute.
• Impulsive transient: A sudden and short duration disturbance by a very rapid change in the steady state condition of voltage, current, or both, that is unidirectional in polarity. Frequency Range :> 5 KHz (High Frequency);
Duration: 30-200(sec.)
• Oscillatory transient: A temporary rapid fluctuation in the steady –state condition of voltage, current, or both, that includes positive and negative polarity values. Frequency range: 500 Hz-2 kHz (duration from 0.5 cycles-30 cycles).
• Harmonics: Currents produced by nonlinear devices at harmonic (integer multiple of the fundamental frequency) frequencies injected into the system.
• Notching: Periodic voltage disturbance caused by the normal operation of power electronics devices when current is commutated from one phase to another.
• Noise: Any unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed upon the power system voltage or current in phase conductors, or found on neutral conductors or signal lines.
• Voltage fluctuation (flicker): Continuous, rapid variations in the load current.
• Frequency variation: The deviation of the power system fundamental frequency from its specified nominal value (e.g. 50 or 60 Hz).
CONVENTIONAL MEANS OF POWER QUALITY CONTROL:-
The conventional means of control are dealt in brief below –
Shunt Capacitors:-
The Shunt Capacitors are static devices, low cost, most efficient and bank can be formed with series-parallel combination of units of low voltage and low KVA rating to obtain the required capacity (Q & V). When a bank is installed at the point of common coupling /across a load directly, it compensates the reactive power, improves power factor, reduces KVA demand, line/feeder current for a given load decreases, voltages gets boosted and losses are reduced. They are widely used in both LT & HT systems due trouble free service, reliability and long life .The problems to be taken care of are: the transients during switching operations, scope for series /parallel resonance and the possibility for amplifying harmonics.
Series Capacitors:-
The Series Capacitors find application both in LT feeders and HT lines. They Compensate feeder/line reactance; improve the power handling capacity and stability, automatically compensate voltage drops for varying loads & damp out electromagnetic oscillations-thus reducing flicker problem to some extent. A series capacitor carries line current, subjected to varying V, I & Q values and need to be specially designed .In addition, special protective measures are necessary to protect against line faults. Now a days, thyristor switched capacitors are performed due to their attractive features.
Static Compensator:-
A Static Compensator essentially consists of Thyristor/Breaker Switched Capacitors in conjunction with Thyristor Controlled Reactors, employed where step-less control of reactive power is required both in LT & HT (to provide both lead & lag KV A). It is an ideal source for reactive power control but the cost is high, complexity in control and generates harmonics requiring filter circuits to suppress them.
Shunt Passive Filter:-
A Shunt Passive Filter with a capacitor and tuning reactor designed to offer low impendence path for a specific harmonic suppresses it, acts as capacitive circuit and inject reactive power at fundamental frequency, improves the p.f., boosts the voltage and reduces the losses. They are cheap, reliable but provide fixed KV Ar and the p.f may go leading at low load condition. Each dominant harmonic requires a separate filter and are not hundred % efficient. The harmonics can also be nullified by Active Filters, which produce voltages/currents of equal magnitude and opposite in phase through electronic circuits using IGBT’s. These are quite effective but costly for large ratings & sensitive.
There are some alternative techniques such as Isolation Transformer with zigzag connections neutralizing harmonics current induced fluxes and for reducing the neutral currents
WHAT IS HARMONICS? :-
‘Sinusoidal component having a frequency that is an integer multiple of the fundamental frequency’.
Ideally, voltage and current waveforms are perfect sinusoids. However, because of the increased popularity of electronic and other non-linear loads, these waveforms quite often become distorted. To qualify the distortion, the term total harmonic distortion (THD) is used. The term expresses the distortion as percentage of the fundamental (pure sine) of voltage and current waveforms.
SOURCES OF HARMONICS:-
Power electronic devices allow control of energy flow and variability of supply voltage and frequency. They have become increasingly numerous due to the energy saving and advantages when applied for motor control and therefore, power electronic devices have become the most significant sources of harmonic distortion. Some of the most common types of harmonics sources are as follows–
• Generators:
Improper pitch factor.
Non-uniform air gap.
Core saturation.
• Transformers:
Inrush current at the time of switching on.
Steady-state (no load) Magnetizing current.
Saturated condition due to over-excitation.
• Transmission/Distribution Circuits:
Normally passive components and does not result in harmonics generation.HVDC transmission generates a lot of harmonics and are corrected with necessary filters and continuously monitored.
Flexible AC transmission (Fact) using power electronics also produce a lot of harmonics and is corrected with necessary filters.
• Power Electronics:
Battery charges with thyristors.
Static frequency converters (variable speed drives).
Uninterrupted power supplies.
Solid state heater controllers.
CONSEQUENCE OF HARMONICS
Effects on the A. C. motors:
Harmonics current and voltage give rise to the additional losses in the stator winding, rotor circuit and stator and rotor lamination. Losses in the stator and rotor winding are greater if the stator and rotor has a skin effect winding due to this the machine gets overheated.
Effects on the Capacitor Banks:
As the reactance of the capacitor decreases as the frequency of the current passing through the capacitor increases. Hence this capacitor acts as a trap or sinks for the higher harmonic currents from the surrounding customer or the utility system. This causes the increase current to flow through the capacitor stresses that could lead to the capacitor bank failure.
Effect on the Neutral Wire :
Third and odd multiples of the third (9th, 15th, etc) are called as the triplen harmonic. If these triplen harmonic, Y-phase triplen harmonic and B-phase triplen harmonics are all in the phase with each other. They will add together rather than cancel on the neutral conductor of a 3 phase, 4 wire systems. This can overload the neutral if it is not properly sized to handle triplen harmonic current.
Effect on Transformer:
Triplen harmonics cause circulating currents to flow on the Delta winding of a delta-Star transformer configuration. The other adverse effect of the current and voltage harmonics are as follows in transformer.
Increased core loss.
Increased dielectric stresses.
Resonance between inductance of the transformer winding and capacitance of the feeder to which they are connected.
Effect on the Transmission System:
The flow of harmonic current in power system produces two main effects. Additional transmission losses occurred due to the increased RMS value of current waveform. Harmonic current flow results in harmonic voltage drop across various system impedances. In case of transmission by cable, harmonic voltage increases the dielectric stress in proportion to their crest voltages. This effect shortens the useful life of the cable. It also increases the number of the fault and therefore the cost of repairs.
Effect on Power Factor:
Harmonics in supply current causes power factor to decline. Because power factor is a measured of the supply utilization a low power factor means that the supply current is larger than needed for the transmission of the required energy to the load.
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