Topic 2: Short Duration Voltage Variation
Group 3: Islam, Hisyam, Amad Aslam
Group 1: Faiq, Fatin & Aiman
Mitigation Technique
Voltage Swell:Increase in voltage magnitude (110% to 180% of nominal voltage) for a duration of 0.5 cycle to 1 min
Group 2: Ibrahim, Nazim, Dayana
Effect of V.sags
Standard of V.sags
Other characteristic of v.sag
Estimating v.sag performance
Source of V.Sags
1: Power system faults
a) Lightning, TNB grid system will operate to
clear any fault due to lightning.TNB's protection system operates instantly isolating and causing voltage sag.
b) Wind/Ice
c) Insulator contamination
d) Animal contacts
e) Accidents (Crane encroachment /Fires objects etc)
2: Energization of transformer
3: Starting of Induction motors
4: Sudden load changes
Voltage Sag indices 1
Indices are used to indicate the quality and reliability of the service provided by Utility companies and compare power quality in different network,They should be kept at a minimum, easy to access and be representative of the disturbance they characterize.
Voltage Sag indices 2
Utility system level
Individual customer level
Serve as a specific metric of compatibility between customer processes and the electrical environment
Serve as a general quality metric to be used by the power provider for proactive planning and maintenance
Voltage sag indices 3
Magnitude-Duration Characterization of RMS Voltage Variations
Voltage sag indices 4
Voltage sag indices 4
1. Single-event index: A parameter indicating the severity of a voltage or current event, or otherwise describing the event.
- Single-site index: A parameter indicating the voltage or current quality or a certain aspect of voltage or current quality at a specific site.
- System index: A parameter indicating the voltage or current quality for a whole or part of a power system
Voltage sag indices 5
SARFI related index and curve
SARFI index
Voltage sag indices 6
✓ SARFI-90: below 90% of voltage reference
✓ SARFI-80: below 80% of voltage reference
✓SARFI-110: above 110% of voltage reference
✓ Short-Duration Index: it only 1⁄2 cycle and 60 seconds
Voltage Sag indices 7
SARFI-CBEMA : Count or rate of voltage sags and interruptions with retained voltage and duration
SARFI-ITIC: : Count or rate of voltage sags and interruptions with retained voltage and duration
SARFI-SEMI: : Count or rate of voltage sags and interruptions with retained voltage and duration
Voltage Sag indices 8
Group 4: Zamira, Muhammad Khalid, Basir Safi
Voltage Sag: Parameter- Voltage sag duration and Ma
gnitude
Interruption
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An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min. The result of ; Power system faults, equipment failures and control malfunctions
Measured by their duration since the voltage magnitude is always less than 10% of nominal
The duration of an interruption due to a fault on the utility system is determined by the operating time of utility protective devices.
Instantaneous reclosing generally will limit the interruption caused by a non-permanent fault to less than 30 cycles
Delayed reclosing of the protective device may cause a momentary or temporary interruption.
The duration of an interruption due to equipment malfunctions or loose connections can be irregular
Some interruptions may be preceded by a voltage sag when these interruptions are due to faults on the source system
The voltage sag occurs between the time a fault initiates and the protective device operates.
Three phase rms voltages for a instantaneous interruption due to a fault and subsequent re closer operation
Sudden connection of heavy load will cause the voltage at recieving to drop.
Induction motors draw excessive current 4 to 5 times the rate current causing voltage dip during starting or sometimes can cause interruption by operation of protective devices if excessive curretn drawn lasts longer.
Temporary interruption plots. ( Courtesy of Dranetx BMI/Electrotek.)
Inrsuh transformer current during energization is also very high causing voltage drop
Voltage waveform
Short Supply Intteruption
The common methods of reducing the impact of costly interruptions include on-site and off site alternative sources of electrical supply
End user may install on-site sources, such as battery-operated uninterruptible power supplies (UPS) or motor-generator sets
Utility may provide an off-site source that includes two feeders with a high-speed switch that switches to the alternate feeder when one feeder fails
Voltage Dip Duration
magnitude of voltage sag and its calculation methodology(monitoring and theoretical calculations)
PARAMETERS
The typical power system including generators,loads and coupling impedance is a single,integrated and dynamic system
any change of voltage,current ,impedance,else,at one point immediately brings about a change in every other point in the system
Voltage dip duration is mainly determined
by operating time of protection devices such as
Fuses, circuit-breakers and relays
which designed to have an inverse time characteristic
in a manner lower short circuit current would have longer fault clearance time
Solution Classes
Method
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magnitude-duration plot common tool used to show the quality of supply at a certain location or the average quality of supply of number of locations
Between the grid and process
Grid
Very expensive
Not always possible
- Faster times may be achieved for short circuits on transmission lines (60-150 ms)
- Slower times for fault clearing on distribution circuits (for MV about 05 to 2 s and for LV , depending on the fuse characteristic)
- Slower times for fault clearing in remote distribution fault where the asg magnitude is shallow and high
- Electric motors draw a large inrush current during starting leading exending the duration for the voltage dip (longer time)
In the manufacturing process
Cheapest
No suitable equipment readily available
Install protective measures between sensitive process and the grid
Reduction of the Fault Clearance Time
Modification of the Supply System Configuration
Reduction of The Number of Faults
Voltage Stabilizers
Improvement in Equipment Immunity
Magnitude of the voltage sag can be determined using rms voltage monitors or calculation methods for calculation knowledge of network impedance ,fault impedance (if any) and fault location are required
voltage signal recorded (monitor)
Theoretical
for computer based analysis by matrix calculations (meshes systems)also theoretical
for the case of zero-impedance short circuit the system represented by a single-phase equivalent circuit
rms can be calculated using the following equation referring to sampled time-domain voltages
Transmission Network
Distribution Network
or using analayser containing algorithm
One also half cycle rms voltage for the voltage sag shown
Install line arresters
Regular insulator washing
Adjust tower footing resistance
for power system voltage magnitude can be calculated by using voltage divider model
Regular tree trimming
Install animal guards
Install arresters
Significantly limit duration of voltage dip
the result plot should illustrates sag magnitude in pu and distance to the fualt in Km
during fault based on Thevenin's superposition therm and node impedance matrix
Use current-limiting fuses
capable of clearing fault in a very short time
Increase number of substations and busbars
Install current-limiting reactors
Install generators close to sensitive loads
Supply sensitive customers' busbars from several substations
Energy Storage System
No-energy Storage Capabilty
To supply critical load during disturbance
Particularly sensitive equipment only
Example
UPS
SMES
Flywheel
Only to reduce effect of voltage dips
Cheaper cost
Example
Constant voltage transformer,CVT
Static fast transfer switching,SFTS
Static generators
Procedures
Importance of Voltage Sag Immunity
Acquire equipment sensitivity information
Determine the potential effect
Acquire system operation information
Reliable operation of sophisticated system
Able to ride through typical voltage sags
Selecting appropriate sag immunity spec when purchasing equipments
- Voltage dips at points O1,O2 or O3 for short circuit at the point SC
- The nearer the short-circuit location to the considered point, the lower is the residual voltage. on the other hand, systems nearer to supply source can experience lesser voltage reduction during disturbance
A short circuit on the transmission system is likely to result in a voltage dip that is observed over a very wide area, even at a distance of some hundreds of kilometers. A short circuit in a distribution circuit has a much smaller field of influence. The severity of the disturbance will be moderated considerably by neighboring circuits.
The depth of avoltage dip depends on the kind of short circuit and connection of winding of the transformer(s) between the short corcuit's location and the condidered point of the system
in practice. loads that are sensitive to voltage dips (power converters,adjustable speed drives,motors, control equipments ,else)often connected line to line in industrial installations so they subjected line to line voltage dips
The phasing of the short circuit or other disturbance, the cause of a dip, the connection methods of the primary and secondary transformer windings are significant factors of the negative impact of the disturbance. For instance, considering a step-down transformer connected as Dyn or Dy, a single line-to-neutral fault on the primary side (initially a voltage dip of 0 V residual voltage on one phase) results in voltage dips on the secondary side in two phases, each 58 %
sags in short duration variation
Instantaneous
Momentary
Temporary
Typical duration : 0.5-30 cycles
Typical voltage magnitude : 0.1-0.9 pu
Typical duration : 30 cycles-3 s
Typical duration : 3 s-1 min
Typical voltage magnitude : 01- 0.9 pu
Typical voltage magnitude : 0.1-0.9 pu
The change of phase angle called phase angle jump it causes the shift in zero crossing of the instantaneous voltage
V.sag indices
Area of vulnerability
Equipment sensitivity to voltage sag
Transmission system voltage sag performance evaluation
Utility distribution system sag performance evaluation
Area of vulnerability can define the equipment voltage sag immunity or minimum voltage sag ride through capability.
Sources
Area of vulnerability is determined by the total circuit miles of exposure to fault
Sudden loss of heavy load
Voltage rise on healthy phase during unbalanced fault
Motor contactors having a minimum voltage sag ride-through capability of 0.5 pu would have tripped out when a fault causing a voltage sag with duration of more than 1 cycle occurs within the area of vulnerability. Faults outside this area will not cause the voltage to drop below 0.5 which end user voltage drop below 0.5pu.
Effects
1.Equipment sensitive to only the magnitude of voltage sag
2.Equipment sensitive to both the magnitude and duration of a voltage sag
3.Equipment sensitive to characteristics other than magnitude and duration
Types
Momentory: It has a magnitude of (1.1 to 1.8 pu) for (30 cycle to 3 sec) duration
Temporary: It has a magnitude of (1.1 to 1.8 pu) for (3 sec to 1 min) duration
Instantaneous: It has a magnitude of (1.1 to 1.8 pu) for (0.5 to 30 cycle) duration
Unwanted tripping of equipment
Fault on parallel feeders
Fault on same feeder
For each equipment it is important to determine how long it operates after interruption by performing a simple test.
The same test can be operated for different voltage magnitude ( eg, 90%, 80%,.....,10% of V rated )
Equipment will stop within one second ( Desktop computers ) and other longer (eg, lap-tops for hours).
Connecting point >>> voltage tolerance curve
Electrical equipment work best under rated V, and it will stop operating if V= 0 for a certain period.
Equipment have different voltage - tolerance curve
Electronic component damage
Voltage Sag indices 9
Limits
Voltage Sag indices 10
Voltage Sag indices 11
Voltage Sag indices 13
The immunity of equipment to voltage swells is given
by the upper limit of the ITIC Curve
Assessment
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The trigger level should be set a 110% of the nominal voltage (230 V for phase‐to‐neutral
LV).
Mitigation
Mitigation of the sources of the voltage swell
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Interruption V< 0.1pu
Sags 0.1<V<0.9 pu
Voltage Sag indices 12
Swells 1.1<V<1.8pu
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Fault conditions
The energization of large loads which require high starting current
Intermittent loose connections in power wiring
8 disturbances have been recorded during 61 days of measurement (at the assumed threshold value X), then after calculation to a standardized one-month period (30 days) SARFI for a given location will be SARFI90 = (8/61) x 30 = 3.93.
Voltage Sag indices 14
Voltage Sag indices 15
Voltage Sag indices 16
Description of the Phenomena
SARFIx: average number of specified rms variation measurement event that occurred over the assessment period per customer served, where the specified disturbances are those with the magnitude less than x for sags and more than x for swells.
• 𝑆𝐴𝑅𝐹𝐼𝑥=σ𝑁𝑖 𝑁𝑇
Short duration voltage variations are caused by
The System Average RMS (Variation) Frequency Index or SARFIx in short is the number of sags per year a customer on the average would have experienced, with remaining voltage is less than x percent of the declared voltage.
Voltage Sag indices 17
Voltage Sag indices 18
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Indices are needed for standardization of terminology.
• Indices should be compatibility-based.
• Indices should allow for general planning or specific evaluation.
application of surge suppression on load equipment
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CBEMA Curve
STANDARD 1
Standards for voltage sags or dips use reliability indices to set voltage sag limits (IEEE uses the term sag or momentary interruption, but IEC uses the term dip or short time interruption to refer to the same phenomenon)
SEMI-F47-0706 Curve
Voltage sags are typically the most important power quality variation affecting industrial and commercial customers
STANDARD 2
Apart from the IEC 61000-2-8 which is a guide on voltage sags and short interruptions on public electric power supply systems, the other standards on voltage sags deal only with the equipment sensitivity on voltage sags
Standards on equipment immunity began as an industry standard which is the CBEMA and ITIC
Following this the IEC develops a standard on
equipment compatibility the IEC 6100-4-11
Another industry which is the Semiconductor
Equipment and Material Industry SEMI, develop their standards particularly the SEMI F 47 in the early 2000
STANDARD 3
Computer Business Equipment Manufacturers Association (CBEMA) used to describe the tolerance between the voltage and duration that computers or other electrical control devices can operate without failure
IEC 61000-4-11 standard and SEMI F47 Standard
STANDARD 4
Information Technology Industry Council (introduced a curve based on the CBEMA)- digitized version of the CBEMA curve-provides guidelines related to voltage tolerance limits within which information technology could function properly
ITIC Curve
STANDARD 5
SEMI-F47-0706 Curve- CBEMA curve does not define voltage sag immunity for semiconductor manufacturing tools effectively- Thus, SEMI introduces standard that overcome this weakness-It is for the "Specification for Semicon. Processing Equipment Voltage Sag Immunity" in order to meet semicon. industry needs.
STANDARD 6
IEC 61000-4-11 and IEC 61000-4-34-introduced by IEC-for the testing and the measuring techniques for voltage sags, short interruptions and voltage variation immunity tests- The 2 key variable parameters are sag depth and duration
STANDARD 7
The standards accommodated the differences by having three different classes of testing
CLASS 1
CLASS 2
CLASS 3
CLASS X
series dynamic voltage restorer, DVR
shunt static VAR compensator, SVC
series-shunt unified power quality conditioner
Install on both supplier or customer side and connected between distributed supply source and sensitive loads
Overheating and equipment damage
Voltage-tolerance curve was introduced by Thomas Key for reliability of power supply to military installation, become well-known when computer business equipment manufacturers association (CBEMA) used them.
-
CBEMA curve was replaced by the ITIC curve, as recommended by the information technology industry council
applies to protected supplies and compatibility levels lower than public network levels.
applies to point of common coupling (PCC for consumer system) and PCC in the industrial environment in general.
applies to PCC in the industrial environments. It has higher compatibility levels than those of class 2 for some disturbance phenomena.
user defined - incase of SEMI F47-0706, the test points are defined in the SEMI F47 Standard.
observations - We see that the rms voltage does not immediately drop to a lower value but takes one cycle for the transition.- We also see that the rms value during the sag isnot completely constant and that the voltage does not immediately recover after the fault.
the sag magnitude increases for increasing distance to the fault and for increasing fault level. We also see that faults at tens of kilometers distance may still cause a severe sag.
The sag magnitude can be influences also by the cross section of the line or cable ** also fault levels
can culculate the voltage sag at any node due to a fault at any other node
In Non-Radial Systems
theoretical
Local Generators.(with and without)
mitigates voltage sags - The generator increases the fault level at the distribution bus, which mitigates voltage sags due to faults on the distribution feeders. This especially holds for a weak system.
For a strong system, the fault level cannot be increased much without the risk of exceeding the maximum-allowable short-circuit current of the switchgear.
An overview of the fault-clearing time of various protective devices:-
• current-limiting fuses: less than one cycle
• expulsion fuses: 10-1000 ms
• distance relay with fast breaker: 50-100 ms
• distance relay in zone 1: 100-200 ms
• distance relay in zone 2: 200-500 ms
• differential relay: 100-300 ms
• overcurrent relay: 200-2000 ms
caused by : faults, motor starting or utility protective equipment