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Topic 2: Short Duration Voltage Variation (Group 3: Islam, Hisyam, Amad…

- - - - 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.
    - - Utility system level
        
        Serve as a general quality metric to be used by the power provider for proactive planning and maintenance
      - Individual customer level
        
        Serve as a specific metric of compatibility between customer processes and the electrical environment
    - - Magnitude-Duration Characterization of RMS Voltage Variations
    - - 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
    - - SARFI related index and curve
        SARFI index
    - - ✓ 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
    - - 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
    - - 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.
    - - 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
    - - 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.
    - - Indices are needed for standardization of terminology.
        
        • Indices should be compatibility-based.
        
        • Indices should allow for general planning or specific evaluation.
  - - - Area of vulnerability can define the equipment voltage sag immunity or minimum voltage sag ride through capability.
      - Area of vulnerability is determined by the total circuit miles of exposure to 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.
    - - 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
    - - Fault on parallel feeders
      - Fault on same feeder
- - - - Between the grid and process
        
        Install protective measures between sensitive process and the grid
      - Grid
        
        Very expensive
        
        Not always possible
      - In the manufacturing process
        
        Cheapest
        
        No suitable equipment readily available
    - - Reduction of the Fault Clearance Time
        
        Significantly limit duration of voltage dip
        
        Use current-limiting fuses
        
        capable of clearing fault in a very short time
      - Modification of the Supply System Configuration
        
        Increase number of substations and busbars
        
        Install current-limiting reactors
        
        Install generators close to sensitive loads
        
        Supply sensitive customers' busbars from several substations
      - Reduction of The Number of Faults
        
        Transmission Network
        
        Install line arresters
        
        Regular insulator washing
        
        Adjust tower footing resistance
        
        Distribution Network
        
        Regular tree trimming
        
        Install animal guards
        
        Install arresters
      - Voltage Stabilizers
        
        Energy Storage System
        
        To supply critical load during disturbance
        
        Particularly sensitive equipment only
        
        Example
        
        UPS
        
        SMES
        
        Flywheel
        
        No-energy Storage Capabilty
        
        Only to reduce effect of voltage dips
        
        Cheaper cost
        
        Example
        
        Constant voltage transformer,CVT
        
        Static fast transfer switching,SFTS
        
        Static generators
        
        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
      - Improvement in Equipment Immunity
        
        Procedures
        
        Acquire equipment sensitivity information
        
        Determine the potential effect
        
        Acquire system operation information
        
        Importance of Voltage Sag Immunity
        
        Reliable operation of sophisticated system
        
        Able to ride through typical voltage sags
        
        Selecting appropriate sag immunity spec when purchasing equipments
  - - - Sudden loss of heavy load
      - Voltage rise on healthy phase during unbalanced fault
    - - Unwanted tripping of equipment
      - Electronic component damage
      - Overheating and equipment damage
    - - 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
    - - The immunity of equipment to voltage swells is given
        by the upper limit of the ITIC Curve
    - - The trigger level should be set a 110% of the nominal voltage (230 V for phase‐to‐neutral
        LV).
    - - Mitigation of the sources of the voltage swell
      - application of surge suppression on load equipment
- - - - 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
  - - - 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)
      - Voltage sags are typically the most important power quality variation affecting industrial and commercial customers
    - - 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
    - - 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
    - - 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
    - - 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.
    - - 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
    - - The standards accommodated the differences by having three different classes of testing
        
        CLASS 1
        
        applies to protected supplies and compatibility levels lower than public network levels.
        
        CLASS 2
        
        applies to point of common coupling (PCC for consumer system) and PCC in the industrial environment in general.
        
        CLASS 3
        
        applies to PCC in the industrial environments. It has higher compatibility levels than those of class 2 for some disturbance phenomena.
        
        CLASS X
        
        user defined - incase of SEMI F47-0706, the test points are defined in the SEMI F47 Standard.
- - - - 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
      - 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)
      - 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
        
        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
    - - 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)
        
        rms can be calculated using the following equation referring to sampled time-domain voltages
        
        or using analayser containing algorithm
        One also half cycle rms voltage for the voltage sag shown
        
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      - Theoretical
        
        for power system voltage magnitude can be calculated by using voltage divider model
        
        the result plot should illustrates sag magnitude in pu and distance to the fualt in Km
        
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      - for computer based analysis by matrix calculations (meshes systems)also theoretical
        
        during fault based on Thevenin's superposition therm and node impedance matrix
        
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      - for the case of zero-impedance short circuit the system represented by a single-phase equivalent circuit
        
        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 %
      - 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.
    - - 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
        
        The change of phase angle called phase angle jump it causes the shift in zero crossing of the instantaneous voltage
      - sags in short duration variation
        
        Instantaneous
        
        Typical duration : 0.5-30 cycles
        
        Typical voltage magnitude : 0.1-0.9 pu
        
        Momentary
        
        Typical duration : 30 cycles-3 s
        
        Typical voltage magnitude : 01- 0.9 pu
        
        Temporary
        
        Typical duration : 3 s-1 min
        
        Typical voltage magnitude : 0.1-0.9 pu
        
        caused by : faults, motor starting or utility protective equipment
  - - - 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.
    - - Interruption V< 0.1pu
      - Sags 0.1<V<0.9 pu
      - Swells 1.1<V<1.8pu
      - Fault conditions
        
        The energization of large loads which require high starting current
        
        Intermittent loose connections in power wiring