Others Waveform Distortion
G1: DC OFFSET
G2 Interharmonic
G3: Notching
Others PQ Disturbances
G4: Electrical Transient
G2: Voltage Unbalanced
G3: Long Duration Variation
Frequency Variation
Sources
Effects and Solution
Definition
Effects
Primarily caused by three-phase rectifiers or converters that generate continuous DC current
Causes
presence of a DC current and/or voltage component in an AC system
Effects in alternating networks
Operation of rectifiers and other electronic switching devices
Geomagnetic disturbances (GMD) causing GICs geomagnetically induced currents
generation of even harmonics in addition to odd harmonics
additional heating in appliances which may lead to a decrease of the lifetime of transformers and rotating machines and electromagnetic devices
electrolytic erosion of grounding electrodes and other connectors.
half-cycle saturation of transformer core
Recurring/periodic power quality disturbance due to the normal operation of power electronic devices (i.e. rectifier), when current is commutated from one phase to another.
G1: Voltage Fluctuation and Flicker
Parameters
Plt is a measure of long-term flicker severity obtained for a two-hour period
Pst is a measure of short-term flicker severity obtained for a 10 minutes interval
Source
Effect
G4 : Noise
Definiition
Mitigation
Causes
Solution
Unwanted electrical signal present in power system having broadband spectrum content lower than 200kHz superimposed with the power system voltage or current
Happen when the current
commutates from one phase to another
Isolation of the critical and sensitive equipment from the source of the power quality problem
- Filters
- Line conditioners
- Dedicated lines or transformers
- Arc furnace
- Faulty connection
- Power electronic devices
- Corona
- Improper grounding
Effects
Create problems in operation of electronic devices
Static frequency converters
Cyclo-converter
Rolling mill drives
Main winders
Electric arc furnaces
Large motors
Introduction of harmonic and non-harmonic frequencies
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What is Electrical Transient
May overload the electromagnetic interference filters and other similar high-frequency sensitive capacitive circuits
Sustained interruption
Suppression of DC currents
use a relatively large air gap between core and tank
Under voltages
Introduction
Over voltages
Electrical Transients are power quality disturbances that involves destructive high magnitudes of current and voltages or even both which only exist in very short duration for less than 50 nanoseconds to as long as 50 ms.
Definition: The root-mean-square (RMS) value deviations at power frequencies for longer than one (1) minute.
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Sources of Transient
Switching Activities
Tap Changing on Transformers
Capacitor bank switching
Opening and closing
Lighting Strides
Re-closing operations
Loose of connections in Distribution system that result in arcing, Accidents, human error, animal and bad weather
Impulse, Oscillatory
Type of Transient
causes and effect
causes
erratic operations of emergency generators or unstable power frequency sources
single generator or an inverter system not connected to the utility grid
effects
confuse logic systems and affect the operating speed of rotating machinery
it causes large power failure in the utility side
power frequency monitoring
the frequency of supply is to be maintained 1.5% of 50/60 except for momentary fluctuations
mitigation
Generation must be matched with loading and losses : :
mix of reserve generation and uninterruptible loads alleviate excursions
active power reduction depending on frequency
due to rectifiers and geomagnetically induced currents
Voltage Fluctuations : systematic variations of the voltage waveform envelope, or a series of random voltage changes
Flickers : the variation in the light output of various lighting sources
flicker is derived from the impact of voltage fluctuation on lighting intensity due to large loads that have rapidly changing active and reactive power demand
Equipment or devices that exhibit continuous, rapid load current variations
Loose connections
Small power loads
Capacitor banks
connected in parallel with inductive loads in order to compensate for the power factor of the installation
produces a voltage step proportional to the reactive power capacity of the capacitor bank and inversely proportional to the short-circuit capacity of the point of connection
Light Sources
Flicker affects our brain reaction and impairs vision, leading to discomfort and deterioration in work quality
Electrical Machines
Flicker can affect the production environment by causing personnel fatigue and lower work concentration levels
Synchronous Motors and Generators
Increase in losses
Premature wear of rotors
Changes in torque and power
Hunting
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Other devices could malfunction and have reduced efficiency
Computerized or automatic machines to produce defective parts
Unwanted triggering of UPS units to switch to battery
Security system malfunction
Testing equipment to give inaccurate data
Nuisance tripping due to misoperation of relays and contactors
Inconsistency of motor speed
Induction Motors
cause changes in torque and slip and affect the production process
result to excessive vibration, reducing mechanical strength and shortening the motor service life
Electro-heat Equipment
have lower operational efficiency
Electrolyzers
can reduce both the useful life and the operational efficiency of an electrolyzer equipment
elements of the high-current supply line can become significantly degraded, thereby increasing maintenance and/or repair costs
Static Rectifiers
a decrease in power factor and the generation of non-characteristic harmonics and interharmonics
commutation failure and damage to system components.
It characterizes an unusual case that falls between harmonics and transients.
The depth of the notch at any point in the system is influenced by:
Source inductance
The isolating inductance between the rectifier/converter
The point being examined
These frequencies are in the radio frequency range, that cause negative operational effects, such as signal interference introduced into logic and communication circuits
The width of the notch is the commutation angle
Definition by IEEE 1159: The decrease in the voltage supply level to zero for more than one (1) minute
They are measured and described by their duration since the voltage magnitude is always less than 10% of nominal
Outage does not refer to a specific phenomenon, but rather to the state of a system component that has failed to function.
Permanent in nature and require manual intervention for restoration.
They are specific power system phenomena and have no relation to the usage of the term "outage"
Interruption has no relation to reliability or other continuity of service statistics
Caused by permanent faults due to storms, trees striking lines or poles, utility or customer equipment failure in the power system or miscoordination of protection devices
Magnitude:
Short Interruption - Less than 0.10 per unit
Sustained interruption – 0.0 pu
Duration:
Short Interruption - ½ cycle to 1 minute
Sustained interruption - More than 1 minute
Ways to prevent interruptions:
Reduce incidents of system faults (i.e. arrester installation, feeder inspections, tree trimming and animal guards)
Limit the number of affected customers interrupted (i.e. single-phase reclosers and/or extra downstream reclosers)
Fast reclosing (i.e. Uninterruptible Power Supply (UPS), Back-up generator or Self-generation)
Definition
Due to heavily loaded circuits that lead to considerable voltage drop, switching on a large load or group of loads, or a capacitor bank switching off.
Expose electrical devices to problems such as:
overheating
malfunction
premature failure and shut down, especially for motors
Common symptoms of under-voltage
motors run hotter than normal and fail prematurely
dim incandescent lighting and batteries fail to recharge properly
Ways to solve the under-voltage problems:
Reducing the system impedance
Improving the voltage profile
Reducing the line current
Definition
According to IEEE as an increase in the AC voltage (RMS), typically to 110% - 120% of nominal, at the power frequency for duration longer than 1 minute
According to IEC 60071-1, overvoltage is any voltage between one phase conductor and earth or between phase conductors having a peak value exceeding the corresponding peak of the highest voltage for equipment
Classification
Internal origin – produced from phenomena related to system operation and in general due to abrupt variations of system conditions
External origin – due to atmospheric electricity
General causes:
switching off a large load
capacitor switching
dropping of load
mis-setting of voltage taps on transformers
The importance of temporary over-voltages
For surge arrester selection
For the design of both the internal insulation of equipment and the external insulation
Mitigation steps
Adjusting transformers to the correct tap setting
Manually or automatically switching off excess capacitor banks during light load or off peak hours
IEEE 1159: The decrease in the AC voltage (RMS), typically to 80% - 90% of nominal, at the power frequency for a period of time greater than 1 minute
DEFINITION
INTERHARMONIC: Waveform components that are at frequencies that are not an integer multiple of the supply frequency.
SUBHARMONIC: Interharmonics that are below the supply frequency
SOURCE OF HARMONIC
Electric arc furnaces
Arc welding machine
Power supply to traction system
Power electronic converter
When the end‐use of the electricity requires an AC voltage at a frequency other than at the network (fundamental) frequency (e.g. VSD and Cycloconverter)
When the PWM converters on the network side has no synchronism with the network frequency (the harmonics of the modulation frequency will be interharmonics)
Switched mode power supply for rectification
Induction motors
Ripple control
Automatic meter reading (AMR)
TYPICAL SPECTRUM OF INDUCTION FURNACE CURRENT
MAIN EFFECT
Light flicker.
Unwanted currents in the supply network generating additional energy losses.
Disturbed operation of electronic equipment
Acoustic noise and vibrations
Temperature increase and unwanted torques in induction motors
Interference causing mal-operation of equipment
Effect of Transient
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Unusual equipment damage due to insulation failures or flash-over
Damage to electronic components
Total failure, lockup or misoperation of computers or micro-processor based equipment.