An Overview of Power Quality

Akash Kewal Ram Engineer Sales AREVA T&D Pakistan Private Limited Karachi, 28 th January 2009 An Over-view of Power Quality

The term power quality means different things to different people “ the normal operation of electrical equipment which you can depend on to keep your household & business running properly and efficiently” “ If electrical equipment operates correctly and reliably without being damaged or stressed, we would say that the electrical power is of good quality” “ any deviation from normal voltage source can be classified as a power quality issue” “ When power quality is affected you may notice a brief interruption of power such as the blinking of your computer or dimming of your lights” What is Power Quality??

A utility may define power quality as reliability and show statistics demonstrating that its system is 99.9 percent reliable A manufacturer of load equipment may define power quality as those characteristics of the power supply that enable the equipment to work properly Power quality is ultimately a consumer-driven issue What is Power Quality?? “ Any power problem manifested in voltage, current, or frequency deviations that results in failure or misoperation of customer equipment”

A standard 100-watt light bulb requires 230 V to produce the designed light output. If the voltage drops to 207 V (-10%), the light bulb still works but puts out less lumens and is dimmer. If the voltage is removed as during a power outage, the light goes out. If however the voltage rises to 253 V (+10%), the light bulb will produce more lumens than it was intended to. This may result in: causing overheating and stress on filament – will reduce life increased need for air conditioning Variation of lumens output – not desirable Increased power consumption A simple example

Flickers Equipment damage Failure of data processing / electronic equipments Computer lockups Problem with switching of high loads Overheat neutrals Nuisance tripping Utility metering problems Electricity outage Power Quality Self Assessment IEC – 6100-4-30 (PQ measurement standard)

Electrical Disturbances cost $26 b to US companies (Per year) Cost of Poor Power Quality

IEC 61000-2-4 lays down what is considered as power quality criteria with regard to certain types of electrical power distribution systems. According to this approach power quality may be co-related with four topics: Voltage Frequency Harmonic distortion Power factor Power Quality as per IEC 61000-2-4

The impact of voltage variations is to cause higher energy consumption. For a given MW of power rating, the current drawn goes up inversely in proportion to the voltage - drop in voltage would result in increased current increased current causes increase in I²R losses of the network. For ex: a 20% drop in voltage would increase the losses in the network by 56% increased current contribute to increasing the voltage drop hence intensifying the problem Drop in efficiency of motors - characteristics of motors are such that a drop in voltage will mean a higher energy consumption to do the same job Power Quality & Voltage

Power Quality & Voltage variations in voltage are a frequent occurrence in power distribution networks. voltage drop could be as much as 40% of the rated value – increasing corresponding I²R losses by 277% - results in increased energy wastage and higher power demand Voltage variation: 1. factors arising from transmission & distribution of power 2. within a network due to the characteristics of the loads Other Voltage problems: 1. Under or over voltage 2. Dips and surges 3. Voltage impulses 4. Wave shape faults 5. Voltage swells 6. Blackouts, etc

Electricity Act 1937 – voltage variation ± 12.5% for MV & ± 5% for LV IEC - voltage variation ± 6% for MV & ± 5% for LV Most of motors are manufactured to operate in tolerance of ± 5% (IEC) to ± 10% ( NEMA/USA) Types of voltage variations: 1. Short term (may last up to 10 sec or less) 2. Exceeding 10 sec or up to hours Power Quality & Voltage

variations can occur, due to the load levels on the electricity supply system a highly overloaded power system will experience a drop in frequency. Mismatch in different section of a grid can cause power quality and power supply problems particularly, when it is important to have an integrated and interconnected grid Impact on energy conservation is less than other other parameters more stable than the voltage Power Quality & Frequency

due to feedback of harmonic currents from non linear loads is a form of electrical pollution on the network The presence of harmonic distortion has a significant impact in increasing energy consumption iron loss is also a function of the power of the frequency - presence of higher frequency components increases it, hence energy consumption will go up and this is of particular importance in the distribution transformer whose all day efficiency could be significantly reduced because of this aspect. Power Quality & Harmonics

high losses – increased temperature - faster ageing of the insulation increased I²R losses depends on skin effect - the phenomenon causes over heating - increasing the amount of energy consumption Allowable limit of voltage harmonic (5%) & current harmonic (10%) Power Quality & Harmonics

On Induction Motors Power consumption increases Thermal Losses Voltage harmonics causes extra losses in direct line connected motors The 5th harmonics creates a counter rotating field, where as the 7th harmonic creates a rotating field beyond the motors synchronous speed. The pulsating torque causes wear and tear on couplings and bearing 3% voltage distortion is permitted for Explosion Proof Motors as per EN60034-2o Effects of Harmonics

Miscellaneous Effects Capacitor failure Thermal Losses – reduced life Faulty operation of fuse and circuit breaker Unbalance, Distribution Transformers and Neutral Currents Increased stray losses in transformer resulting increase in iron, copper or eddy, de-rating Increased losses in generator/multiple zero crossing affect the timing of the voltage regulator, causing interference and operation instability Incorrect recording by utility meters Miss operation of drives Interferences in computer/telephone Increased maintenance cost Effects of Harmonics

Power factor measured as a ratio of active energy to apparent energy over a specified time period ideal power factor is unity - cannot be achieved due to the nature of loads used e.g. inductive & nonlinear loads lower power factor means more current drawn for the same load - causes increase in apparent power demand, i.e., kVA demand & increases I²R losses more system capacity is needed to supply the same load lower power factor results in higher energy consumption lowering of power factor also causes voltage drop Power Quality & Power Factor

Losses Due to Power Quality A rectangular supply voltage provokes 20% increase in losses A supply voltage with THDV* = 10% results in additional 6% losses Transformer losses increase 10 to 15% De-rating of generator will be 10% Increase in capital cost due to over sizes of equipment * total harmonic distortion value In general

The perfect power supply would be one that is always available within voltage & frequency tolerances and has pure noise free sinusoidal wave shape.” The Power Quality issues such as voltage variations, frequency variations, harmonic distortions and power factor combine together to reduce the overall operating efficiency of electrical networks and also results in increased power supply demand, unnecessary wastage of energy and malfunctioning / damage of equipment. Electric power is food of electric equipment, if food is healthy than equipment is healthy. Conclusion

The overhead and underground lines that deliver electricity to you every day are exposed to: Weather, trees, animals, vehicles and people. Events that commonly impact power quality Lightning strikes Wind and heavy rain that cause objects to be blown onto the exposed lines Trees or limbs falling onto the power lines Small animals / vermins that get onto the power lines or into ground mounted equipment; Cars that run off the road and hit power poles and ground mounted equipment; Construction companies that accidentally dig into the underground lines while building new homes or offices Causes of blackouts

BLACKOUT: Total loss of commercial power DROPOUT: A discrete voltage loss SAG: (complete or partial) for a very short period of time (milliseconds) constitutes a dropout. FLUCTUATION: A surge or sag in voltage amplitude, often caused by load switching or fault clearing. HARMONIC: A sinusoidal component of an AC voltage that is a multiple of the fundamental waveform frequency. IMPULSE: Transient voltage or current condition of positive or negative amplitude. NOISE: An undesirable signal which is irregular yet oscillatory that is super imposed on the desired signal. See common mode noise and normal mode noise. OUTAGE: An outage is a long-term power interruption. OVERVOLTAGE: A voltage greater than the rating of a device or component. SAG: A reduction in a voltage envelope. The duration is usually from one cycle to a few seconds. Usually, sags are caused by fault clearing or heavy load startup. SURGE: A short-term positive change in amplitude of a voltage. TRANSIENT: A high amplitude, short duration impulse superimposed on the normal voltage or current. NONLINEAR LOAD: Electrical loads in which the instantaneous current is not proportional to the instantaneous voltage, or, effectively, the load impedance varies with voltage.

An Overview of Power Quality

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