Voltage Sag
What is a Voltage Sag? Voltage sag is a power quality problem: ►A voltage sag is a momentary (Short duration) under voltage condition usually caused by power system fault. ►This is the most important power quality variation for many industrial customers. ►A voltage sag is defined as a decrease in rms voltage for duration from 0.5 cycles to 1 min and typical magnitudes are between 0.1 to 0.9 p.u.
Example of Voltage Sag
Does these voltage sag represent PQ problems?
Only if they cause equipment to misoperation or fail
Example of Voltage Sag
Does these voltage sag represent PQ problems?
Only if they cause equipment to misoperation or fail
Source of Voltage Sag
Voltage Sags are caused by faults on the power system, generally on the utility side.
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Source of Voltage Sag
Voltage Sags are caused by faults on the power system, generally on the utility side: same and parallel feeder.
Voltage Sag Caused by Faults
Faults on Parallel Circuit 46%
Faults on Transmission System 31%
Faults on Own Circuit 23%
Parallell circui Paralle circuits ts or radial feeders
Characteristics of Voltage Sag
►Voltage sags are typically characterized by the minimum
rms voltage and duration during sag.
Voltage Sag Caused by Short Circuit
►During short circuits, bus voltages throughout the supply network are depressed, severities of which are dependent of the distance from each bus to the point where the short circuit occurs. ►After clearance of the fault by the protective system the voltage returns to their new steady state values. ►Part of the circuit that is cleared will suffer supply disruption or Blackout. ►Thus, a short circuit will cause voltage sags throughout the system but cause blackout to a small portion of the network.
Number of Phases Involved
Sags and Interruptions Below 0.9 Per Unit Three-phase 17%
One-phase 66%
Two-phase 17%
Causes of Voltage Sags in Industrial Plant
►Voltage sags can be caused by, switching oh heavy loads or starting of large induction motors. ►An induction motor will draw six to ten times its full load current during starting. This lagging current causes a voltage drop across the impedance of the system. ►If the current magnitude is large relative to the system available fault current, the resulting voltage sag can be significant.
Effects of Voltage Sag
►In general, voltage sag effects: Motor load to stall * Digital devices to reset because of loss of data * Equipment damage or failure * Materials spoilage * Loss production due to downtime * Additional labour costs * Product reworks * Product quality impacts * Impacts on customer relations ( e.g. late delivery, lost sales) * Cost of investigation into problems.
Voltage Sag Due to Source of Motor
Estimating Voltage Sag Performance ►Followings are the general procedure to estimate voltage sag, to assure compatibility between supply system and facility operation. ►Determine the number and characteristics of voltage sag that results from X-mission systems faults. ►Determine the number and characteristics of voltage sag that results from Distribution systems faults. ►Determine the equipment sensitivity to voltage sag for actual performance Of the production process based on voltage sag performance of 1 & 2. ►Evaluate the economics of different solutions that could be improve the performance either on the supply system or within the customer facility. ►Area of Vulnerability * Equipment sensitivity of voltage sags * Transmission system voltage sag performance evaluation * Distribution system voltage sag performance evaluation.
Area of Vulnerability
►The concept of an area of vulnerability has been developed to help evaluate the likelihood of sensitive equipment being subjected to voltage lower than its minimum sag ride-through capability.
►An area of vulnerability is determined by the total circuit miles of exposure to faults that can cause voltage magnitude at an end-user facility to drop below the equipment minimum voltage sag ride-through capability ►Loads will be subjected both X-mission and distribution. ►Voltage sag is determined by combination of vulnerability area and expected fault performance
Area of Vulnerability
Equipment Sensitivity to Voltage Sag
►Equipment within an end-user facility may have different sensitivity based on specific types of loads, control setting and application
►Equi pment sensitivity to onl y magnitude of a voltage sag : under voltage relay, process control, motor drive control and automatic machines ► Equipment sensitivi ty on both magnitu de and dur ation of a voltage sag : All equipment that uses electronic power supplies ► Equipment sensitivi ty to character istics other than magnitu de and dur ation : Phase unbalance, transient oscillation occur during disturbances
Transmission System Sag Performance Evaluation
► figure book 53
Distribution System Sag Performance Evaluation
►Customers that are supplied from distribution are impacted faults from both X-mission and distribution system.
►These are the following information that need to compute voltage sag performance on distribution system: Number of feeder supplied from the substation * Average feeder length * Average feeder reactance * Short circuit equivalent reactance's at the substation * Feeder reactor, if any * Average feeder fault performance on 3LG and SLG in per fault per mile per month.
►Faults on parallel Feeder, E Parallel (V s )=N 1 x E p1 + N 3 x E p3 ►Faults on same feeder, E Same (V s )=N 1 x E p1 + N 3 x E p3
Distribution System Sag Performance Evaluation
Distribution System Sag Performance Evaluation
Fundamental Principles of Protection Utility, end-user and equipment manufacturer have done several solution to reduce the number and the severity of voltage sag and to reduce the sensitivity of equipment to voltage sag incorporating ride-through capability into the equipment.
►Equipment manufacturer should have voltage sag ride-through capability curve available to their customer, so that an initial evaluation of the equipment can be performed. ►The company procuring new equipment should establish a procedure that rates the importance of the equipment ride-though capability. ►Equipment should at least be able to ride-though voltage sag with a minimum voltage of 70%.
Fundamental Principles of Protection
Solutions at the End-User Level
►Different technologies are evaluated to determine the optimum solution for improving overall voltage sag performance and reliability. ►Protection for small loads (less than 5 kVA): protection for equipment control, small, Individual machine and single phase load ►Protection for individual equipment or groups of equipments up to about 300 KVA ►Protection for large groups of loads or whole facility at the low voltage level ►Protection at the medium voltage level or on the supply system
Types of Voltage sag Mitigating Instrument
►To ride through a voltage sag requires some form of stored
energy at the critical load side. This can be achieved by using power conditioning devices such as, - Ferroresonant transformer - Magnetic synthesizers - Active series compensator - UPS systems - Motor generator set - Dynamic voltage restorer
- Static transfer switch - Flywheel energy storage systems - Superconductor magnetic energy storage (SMES) device
Ferroresonant Transformers
►Also known as constant-voltage transformers (CVT) and can handle most voltage sag conditions for constant, low power loads. It is basically a 1:1 transformer which are excited high on their saturation curves thereby providing an output voltage which is not significantly affected by input voltage variations. ►Example: With the CVT, the process controller can ride through a voltage sag down to 30% of nominal, as opposed to 82% without one. ►CVT should be sized significantly larger than the load because as loading is increased, the ride-through capability is reduced.
Ferroresonant Transformers
Magnetic synthesizer
►Magnetic synthesizers use a similar operating principle to CVT except they are three phase devices and take advantage of the three phase magnetics to provide improved voltage support and regulation for three phase loads. ►Sized in the range of 15 to 200 kVA. ►Applied for large computer-based loads. ►Its operation comprises of energy transfer, line isolation and energy storage. Energy transfer and line isolation are accomplished through the use of nonlinear chokes. AC output waveforms are built by combining voltage pulses from saturated transformers. The waveform energy is then stored in the saturated transformer and capacitors as current and voltage. This energy storage enables the output of a clean waveform with little harmonic distortion.
Magnetic synthesizer
Active series compensator ►A electronic device that can boost the voltage by injecting a voltage in series with the remaining voltage during voltage sag condition
UPS System
►On-line UPS ►Standby UPS ►Hybrid UPS
►On-line UPS: The load is fed through the UPS. The incoming ac power is rectified into dc power, which charges a bank of batteries. This dc power is inverted back to ac power , to feed the load. If the incoming ac power fails, the inverter is fed from the batteries and continues to supply the load.
UPS System
►Standby (Off-line UPS: A standby UPS is termed as off-line UPS since the normal line power is used to power the equipment until a disturbance is detected and a switch transfers the load to the battery-backed inverter. A transfer time of 4 ms would ensure continuity of operation for the critical load. Usually applied for protection of small computer loads.
UPS System
►Hybrid UPS: The hybrid UPS utilizes a voltage regulator on the UPS output to provide regulation to the load and momentary ride-through when the transfer from normal to UPS supply is made.
Motor_Generator Set
►A motor powered by the line drives a generator that powers the Load. Flywheels on the same shaft provide greater inertia to increase ride-through time. When the line suffers a disturbance, the inertia of the machines and the flywheels maintains the power supply for several seconds.
►loss associated with machines ►Noise and maintenance ►Frequency and voltage drop during sag as the machine slow.
Superconducting Magnetic Energy Storage (SMES) Device
►The SMES based system has several advantage over battery based UPS # Smaller footprint than batteries for same energy storage and delivery capability # Energy delivered to the protected system very quickly # Unlimited discharge and charge duty cycles, can do thousands of time with Degrading Superconducting magnet
Dynamic Voltage Restorer
►DVR is a voltage compensating device that connected in series with a source and a load in a distribution system, in which it consists of an inverter, transformer and dc capacitor. ► A DVR is designed to inject a compensating voltage into a distribution line through an injecting transformer so as to restore the load voltage to an acceptable level during the period of voltage sags.
Dynamic Voltage Restorer
►The basic idea in the operation of a DVR is to inject a controlled voltage generated by an inverter in series to the bus voltage by means of an injecting transformer. A dc capacitor Bank which acts as an energy storage device, provides a regulated dc voltage source. An inverter regulates this dc voltage and converts its into a synchronous ac voltage of controllable amplitude and phase angle.
Dynamic Voltage Restorer
Static Transfer Switch
►Static transfer switch is a device that ensures high-quality power supply to sensitive load by transferring load from faulted bus to a healthy one. It is usually designed using thyristors as solid-state breaker because of their higher current capacities to withstand load side fault levels. ►The function of static switch is to maintain ac electrical power
to a critical load by switching between two independent power sources.
Static Transfer Switch
►It consists of two sets of three phase solid-state static switches, one for the main feeder and the other one for the back-up feeder. Each switch is arranged with anti-parallel thyristors which allow fast transfer of power from the main feeder that is affected by a disturbance to the backup feeder.
Levels of Voltage Sags Cause Equipment Problems?
Indices for Voltage sag performance should be based on the sensitivity of equipment to voltage sags
►The eequipment ride-through characteristics to represent power quality are as follows: CBEMA Curve ITIC Curve SEMI 2844 Curve Actual Equipment Sensitivity Characteristics
RMS Variations – CBEMA
(Computer & Business Equipment Manufacturers Association)
ITIC Curve
(Information Technology Industry Council)
RMS Variations- ITIC
SEMI F-47 Curve
Benchmarking Voltage Sag Performance
►Basis for evaluating ongoing system performance ►Basis for evaluating economics of power quality improvement options ►Basis for implementing PQ-based contracts ►Basis for attracting customers that are concerned about PQ levels
Benchmarking Voltage Dip Performance
►Determine expected performance from fault statistics and system electrical characteristics. Th is establishes benchmark levels f or expected per f ormance.
► Monitor performance with a systems monitoring approach. Th is tr acks actual per f ormance against the expected per f ormance levels.
System Average RMS Variation Frequency Index - SAFRI(x)
SARFI(x) for a specific location is the number of voltage sags per year with a minimum voltage below x Country USA
SARFI
<70%Sag
50
17
Europe Cable Combined
34 (100)
11 44
South Africa
153
43
Voltage Swell
A short duration, temporary voltage rise with duration between 0.5 to 30 cycles and with typical magnitudes between 1.1 and 1.8 per unit.
►It is a high voltage condition on one or more phases. Voltage swell Is due to rapid load reduction, utility switching etc. The effect will be hardware damage.
Causes and Effect of Voltage Swell
Swell ►What : High voltage condition
on one or more phases
►Why : Rapid load reduction, utility switching, etc. ►Effect: Hardware damage by Breaking down insulation
Evaluating the Economics of Different Ride-through Alternatives
The economic evaluation procedure to find the best option for improving voltage sag performance consist the following steps: ►Characterise the system power quality performance ►Estimate the costs associated with the power quality variations ►Characterise the solution alternatives in terms of costs and effectiveness ►Perform the comparative economics analysis
Estimating the Costs for the Voltage Sag Events
The cost of a power quality disturbance can be captured preliminary through three major categories ►Product related loss i.e. loss of product and materials, loss of production capacity, disposal charge and increase inventory requirement ►Labour related loss i.e. idled employees, overtime, cleanup and repair ►Ancillary cot i.e. damaged equipments, lost opportunity cost and penalties due to shipping delays
Estimating the Costs for the Voltage Sag Events
Characterizing the Cost for Solution Alternatives
and
Effectiveness
The solution of cost and effectiveness includes initial procurement and installation expenses, operating and maintenance expenses and Any disposal and salvage value consideration
