•A voltage sag is a shortduration (typically 0.5 to 30 cycles) reduction in rms voltage caused by faults on the power system and the starting of large loads, such as motors. •Momentary interruptions (typically no more than 2 to 5 s) cause a complete loss of voltage and are a common result of the actions taken by utilities to clear transient faults on their systems. •Sustained interruptions of longer than 1 min are generally due to permanent faults.
Chapter 02Votage Sags and Interruptions GV: Nguyễn Hữu Phúc Chapter 02Votage Sags and Interruptions •A voltage sag is a short-duration (typically 0.5 to 30 cycles) reduction in rms voltage caused by faults on the power system and the starting of large loads, such as motors •Momentary interruptions (typically no more than to s) cause a complete loss of voltage and are a common result of the actions taken by utilities to clear transient faults on their systems •Sustained interruptions of longer than are generally due to permanent faults Utilities have been faced with rising numbers of complaints about the quality of power due to sags and interruptions There are a number of reasons for this, with the most important being that customers in all sectors (residential, commercial, and industrial) have more sensitive loads Voltage sag due to a single line-to-ground fault 3.1 Sources of Sags and Interruptions: Voltage sags and interruptions are generally caused by faults (short circuits) on the utility system •If there is a fault on the same feeder, the customer will experience a voltage sag during the fault followed by an interruption when the breaker opens to clear the fault •If the fault is temporary in nature, a reclosing operation on the breaker should be successful and the interruption will only be temporary •It will usually require about or cycles for the breaker to operate, during which time a voltage sag occurs • The breaker will remain open for typically a minimum of 12 cycles up to s depending on utility reclosing practices •Sensitive equipment will almost surely trip during this interruption Fault locations on the utility power system •Note that to clear the fault shown on the transmission system, both breakers A and B must operate •Transmission breakers will typically clear a fault in or cycles In this case there are two lines supplying the distribution substation and only one has a fault • •Therefore, customers supplied from the substation should expect to see only a sag and not an interruption •The distribution fault on feeder may be cleared either by the lateral fuse or the breaker, depending on the utility’s fuse saving practice Example of fault locations that caused misoperation of sensitive production equipment at an industrial facility (the example system had multiple overhead distribution feeders and an extensive overhead transmission system supplying the substation) Voltage sag due to a short-circuit fault on a parallel utility feeder •Figure clearly shows the voltage sag prior to fault clearing and the subsequent two fast recloser operations •The reclose time (the time the recloser was open) was a little more than s, a very common time for a utility line recloser •Apparently, the fault— perhaps, a tree branch— was not cleared completely by the first operation, forcing a second •The system was restored after the second operation Utility short-circuit fault event with two fast trip operations of utility line recloser 3.2 Estimating Voltage Sag Performance The following is a general procedure for working with industrial customers to assure compatibility between the supply system characteristics and the facility operation: Determine the number and characteristics of voltage sags that result from transmission system faults Determine the number and characteristics of voltage sags that result from distribution system faults (for facilities that are supplied from distribution systems) Determine the equipment sensitivity to voltage sags This will determine the actual performance of the production process based on voltage sag performance calculated in steps and Evaluate the economics of different solutions that could improve the performance, either on the supply system (fewer voltage sags) or within the customer facility (better immunity) 3.2.1 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 voltage sag ride-through capability The latter term is defined as the minimum voltage magnitude a piece of equipment can withstand or tolerate without misoperation or failure This is also known as the equipment voltage sag immunity or susceptibility limit An area of vulnerability is determined by the total circuit miles of exposure to faults that can cause voltage magnitudes at an end-user facility to drop below the equipment minimum voltage sag ride-through capability Figure shows an example of an area of vulnerability diagram for motor contactor and adjustable-speed-drive loads at an end-user facility served from the distribution system The loads will be subject to faults on both the transmission system and the distribution system 3.2.2 Equipment sensitivity to voltage sags Equipment sensitivity to voltage sags can be divided into three categories: ■ Equipment sensitive to only the magnitude of a voltage sag ■ Equipment sensitive to both the magnitude and duration of a voltage sag This group includes virtually all equipment that uses electronic power supplies ■ Equipment sensitive to characteristics other than magnitude and duration => Illustration of an area of vulnerability Some devices are affected by other sag characteristics such as the phase unbalance during the sag event, the pointin-the wave at which the sag is initiated, or any transient oscillations occurring during the disturbance 3.2.3 Transmission system sag performance evaluation •The voltage sag performance for a given customer facility will depend on whether the customer is supplied from the transmission system or from the distribution system •For a customer supplied from the transmission system, the voltage sag performance will depend on only the transmission system fault performance •On the other hand, for a customer supplied from the distribution system, the voltage sag performance will depend on •the fault performance on both the transmission and distribution systems Typical equipment voltage sag ridethrough capability curves Case - Installation High Voltage compartment (26kV) Injection Transformer Case – Polyethylene Plant •115kV Utility supply •8 -10 process upsets per year, most caused by lightning and birds •Plant restart and recoveries in some cases took several days •Lost production and damage to equipment were they the largest cause of loss Solution - Polyethylene • Customer originally requested 60MVA AVC protection but this was not cost effective • Protected Extruders, Cutters and agitators only • AVCs each 2MVA at 480V installed - 2007 • To date averted potential shutdowns • Customer reported full project payback in first events (Less than months) Case - Solar Cell Manufacturer • 34.5kV Utility Supply • Customer reported on average 18 process upsets per year prior to installation of AVC • Estimated Cost per event $125-250K Solution – Solar Cell Manufacturer • Installed 1600 KVA, 30% correction AVC, in 2002 (Indoor Unit) • 24 events recorded in first year of operation, AVC functioned as designed and protected the connected equipment • The Customer, subsequently, installed more units • Currently, 10 more units are on order each at 1.5MVA for installation in December, 2008 Case - Solar Cell Manufacturer 1600 kVA, 2000 Amp, 480V AVC rated for 30% correction Dimensions: 128” x 48” x 85”H Case - High Speed Printer • Incoming supply at 26.4 kV • Densely treed plus high lightening area • On average, process failed more than 40 times per year • Failure resulted in Late delivery of product to customer • Printer Maintenance costs over $100K/year Solution – High Speed Printer • Installed 3000 kVA, 30% AVC – 2005 • Corrected all voltage sag events that occurred in 30 Months of operation • Printing Press Maintenance costs virtually eliminated following installation of AVC • Customer reported project payback was achieved in months Case - High Speed Printer 3000 kVA, 480V AVC rated for 30% The World’s largest power protected site - 190MVA semiconductor application, Million ft2 Semiconductor FAB,130 x 1,500kVA AVCs, All units 208V Container Crane, 100% Regenerating Application 18 - 400kVA AVCs Conclusions • Voltage Sag is Considered the Most Costly Power Quality Problem • Mitigation Equipment Including AVC are Readily Available to Address the Voltage Sag on a Plant Wide or at Process specific level • Conduct Power Quality Study to Determine any Existing System Abnormalities and when Installing new Equipment • On-Line Monitoring Equipment are readily Available for Power Quality Measurements www.ieee.ca ANY QUESTIONS, PLEASE ! Thank You ... sensitive loads Voltage sag due to a single line-to-ground fault 3.1 Sources of Sags and Interruptions: Voltage sags and interruptions are generally caused by faults (short circuits) on the utility...Chapter 0 2Votage Sags and Interruptions •A voltage sag is a short-duration (typically 0.5 to 30 cycles) reduction in rms voltage caused by faults on the power system and the starting of... characteristics and the facility operation: Determine the number and characteristics of voltage sags that result from transmission system faults Determine the number and characteristics of voltage sags