An Overview of Power Quality

Akash Kewal RamEngineer Sales

AREVA T&D Pakistan Private Limited

Karachi, 28th January 2009

An Over-view of Power Quality

2 2AREVA T&D Presentation (long version)

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)


Ideal Power Quality


Poor Power Quality


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 power2. within a network due to the characteristics of the loads

Other Voltage problems:1. Under or over voltage2. Dips and surges3. Voltage impulses4. Wave shape faults5. Voltage swells6. 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





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 linesTrees 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


Occurrence of PQ Problems


BLACKOUT: Total loss of commercial powerDROPOUT: A discrete voltage lossSAG: (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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