This document discusses voltage sags and interruptions in power systems. It defines voltage sags as short-duration reductions in voltage caused by faults or large load starting. Interruptions are defined as reductions in voltage or current below 10% of nominal lasting up to 60 seconds. Sources of sags and interruptions include faults on the utility system and large motor starting. The document discusses estimating sag performance, equipment sensitivity, and mitigation methods such as motor-generator sets and static transfer switches.
This document discusses voltage sags and interruptions in power systems. It defines voltage sags as short-duration reductions in voltage caused by faults or large load starting. Interruptions are defined as reductions in voltage or current below 10% of nominal lasting up to 60 seconds. Sources of sags and interruptions include faults on the utility system and large motor starting. The document discusses estimating sag performance, equipment sensitivity, and mitigation methods such as motor-generator sets and static transfer switches.
This document discusses voltage sags and interruptions in power systems. It defines voltage sags as short-duration reductions in voltage caused by faults or large load starting. Interruptions are defined as reductions in voltage or current below 10% of nominal lasting up to 60 seconds. Sources of sags and interruptions include faults on the utility system and large motor starting. The document discusses estimating sag performance, equipment sensitivity, and mitigation methods such as motor-generator sets and static transfer switches.
This document discusses voltage sags and interruptions in power systems. It defines voltage sags as short-duration reductions in voltage caused by faults or large load starting. Interruptions are defined as reductions in voltage or current below 10% of nominal lasting up to 60 seconds. Sources of sags and interruptions include faults on the utility system and large motor starting. The document discusses estimating sag performance, equipment sensitivity, and mitigation methods such as motor-generator sets and static transfer switches.
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Unit - 2
VOLTAGE SAGS AND INTERRUPTIONS
Introduction • Voltage sags and interruptions are related power quality problems. • Both are usually the result of faults in the power system and switching actions to isolate the faulted sections. • They are characterized by rms voltage variations outside the normal operating range of voltages. Voltage sag • 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. Interruption • An interruption is defined as a reduction in line-voltage or current to less than 10 percent of nominal, not exceeding 60 seconds in length. • Interruptions can be a result of control malfunction, faults, or improper breaker tripping. Sources of sags and interruptions • Voltage sags and interruptions are generally caused by faults (short circuits) on the utility • system. Sources of sags – Due to faults • A voltage sag is an event where the line rms voltage decreases from the nominal line voltage for a short period of time. • This type of variation can occur if a large load on the line experiences a line-to-ground fault, such as a short in a three-phase motor or a fault in a utility or plant feeder. Sources of sags – Due to motor starting US System Estimating Voltage Sag Performance • It is important to understand the expected voltage sag performance of the supply system so that facilities can be designed and equipment specifications developed to assure the optimum operation of production facilities. • The following is a general procedure for working with industrial customers to assure compatibility between the supply system characteristics and the facility operation: 1. Determine the number and characteristics of voltage sags that result from transmission system faults. 2. Determine the number and characteristics of voltage sags that result from distribution system faults. 3. 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 1 and 2. 4. Evaluate the economics of different solutions that could improve the performance, either on the supply system (fewer voltage sags) or within the customer facility. 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. 2. Equipment Sensitivity to Voltage Sags • Equipment within an end-user facility may have different sensitivity to voltage sags. • Equipment sensitivity to voltage sags is very dependent on the specific load type, control settings, and applications. • The most commonly used characteristics are the duration and magnitude of the sag. Other less commonly used characteristics include phase shift and unbalance, missing voltage, three phase voltage unbalance during the sag event, and the point-in-the- wave at which the sag initiates and terminates. • Generally, equipment sensitivity to voltage sags can be divided into three categories: 1. Equipment sensitive to only the magnitude of voltage sag - This group includes devices such as under voltage relays, process controls, motor drive controls,6 and many types of automated machines 2. Equipment sensitive to both the magnitude and duration of a voltage sag - This group includes virtually all equipment that uses electronic power supplies 3. Equipment sensitive to characteristics other than magnitude and duration - Some devices are affected by other sag characteristics such as the phase unbalance during the sag event, the point-in-the wave at which the sag is initiated, or any transient oscillations occurring during the disturbance. These characteristics are more subtle than magnitude and duration, and their impacts are much more difficult to generalize. 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. • The magnitude of the lowest secondary voltage depends on how the equipment is connected: • Equipment connected line-to-line would experience a minimum voltage of 33 percent. • Equipment connected line-to-neutral would experience a minimum voltage of 58 percent. 4. Utility Distribution System Sag Performance Evaluation • Customers that are supplied at distribution voltage levels are impacted by faults on both the transmission system and the distribution system. • The overall voltage sag performance at an end-user facility is the total of the expected voltage sag performance from the transmission and distribution systems. • The critical information needed to compute voltage sag performance can be summarized as follows: 1. Number of feeders supplied from the substation. 2. Average feeder length. 3. Average feeder reactance. 4. Short-circuit equivalent reactance at the substation. Motor-Starting Sags • Motors have the undesirable effect of drawing several times their full load current while starting. • This large current will, by flowing through system impedances, cause a voltage sag which may dim lights, cause contactors to drop out, and disrupt sensitive equipment. • The situation is made worse by an extremely poor starting displacement factor usually in the range of 15 to 30 percent. The time required for the motor to accelerate to rated speed increases with the magnitude of the sag, and an excessive sag may prevent the motor from starting successfully. 1. Motor-Starting Methods 1. Single step 2. Autotransformer 3. Part winding 4. Resistance and reactance starter 5. Delta wye starter • Energizing the motor in a single step (full-voltage starting) provides low cost and allows the most rapid acceleration. It is the preferred method unless the resulting voltage sag or mechanical stress is excessive. • Autotransformer starters have two autotransformers connected in open delta. Taps provide a motor voltage of 80, 65, or 50 percent of system voltage during start-up. Line current and starting torque vary with the square of the voltage applied to the motor, so the 50 percent tap will deliver only 25 percent of the full-voltage starting current and torque. The lowest tap which will supply the required starting torque is selected. Resistance and reactance starters • Resistance and reactance starters initially insert an impedance in series with the motor. After a time delay, this impedance is shorted out. Starting resistors may be shorted out over several steps; starting reactors are shorted out in a single step. Line current and starting torque vary directly with the voltage applied to the motor, so for a given starting voltage, these starters draw more current from the line than with autotransformer starters, but provide higher starting torque. Reactors are typically provided with 50, 45, and 37.5 percent taps. Part-winding starters • Part-winding starters are attractive for use with dual-rated motors (220/440 V or 230/460 V). The stator of a dual-rated motor consists of two windings connected in parallel at the lower voltage rating, or in series at the higher voltage rating. When operated with a part winding starter at the lower voltage rating, only one winding is energized initially, limiting starting current and starting torque to 50 percent of the values seen when both windings are energized simultaneously. • Delta-wye starters connect the stator in wye for starting and then, after a time delay, reconnect the windings in delta. The wye connection reduces the starting voltage to 57 percent of the system line-line voltage; starting current and starting torque is reduced to 33 percent of their values for full-voltage start. 2. Estimating the Sag Severity During Full-Voltage Starting • The following data will be required for the simulation: 1. Parameter values for the standard induction motor equivalent circuit: R1, X1, R2, X2, and XM. 2. Number of motor poles and rated rpm (or slip). 3. WK2 (inertia constant) values for the motor and the motor load. 4. Torque versus speed characteristic for the motor load. Mitigation Methods 1. Motor - generator set 2. Ferroresonance transformer 3. Electric tap changer 4. Uninterrupted power supply 5. Static transfer switch 6. Series – connected voltage source converter 7. Shunt connected back – up source 8. Superconducting magnetic energy storage (SMES) devices 1. Motor Generator Set • Motor-generator (M-G) sets come in a wide variety of sizes and configurations. This is a mature technology that is still useful for isolating critical loads from sags and interruptions on the power system. • This concept is very simple • This arrangement may also be used to separate sensitive loads from other classes of disturbances such as harmonic distortion and switching transients • M-G sets have disadvantages for some types of loads: • There are losses associated with the machines, although they are not necessarily larger than those in other technologies described here. • Noise and maintenance may be issues with some installations. • The frequency and voltage drop during interruptions as the machine slows. This may not work well with some loads. • Another type of M-G set uses a special synchronous generator called a written-pole motor that can produce a constant 60-Hz frequency as the machine slows. • It is able to supply a constant output by continually changing the polarity of the rotor’s field poles. • Thus, each revolution can have a different number of poles than the last one. Constant output is maintained as long as the rotor is spinning at speeds between 3150 and 3600 revolutions per minute (rpm). • The rotor weight typically generates enough inertia to keep it spinning fast enough to produce 60 Hz for 15 s under full load. Another means of compensating for the frequency and voltage drop while energy is being extracted is to rectify the output of the generator and feed it back into an inverter. This allows more energy to be extracted, but also introduces losses and cost. 2. Ferroresonance Transformer • Ferroresonant transformers, also called constant- voltage transformers (CVTs), can handle most voltage sag conditions. • CVTs are especially attractive for constant, low power loads, Variable loads, especially with high inrush currents, present more of a problem for CVTs because of the tuned circuit on the output. • Ferroresonant transformers are basically 1:1 transformers which are excited high on their saturation curves, thereby providing an output voltage which is not significantly affected by input voltage variations. • A basic ferroresonant transformer consists of a core, a primary winding, two secondary windings (one for the load and one for the capacitor) and a magnetic shunt that separates the primary and secondary windings. 3. Electric Tap Changer • The actual tap-changer which is used in the Indian Railways locos, it will better to understand in general what tap- changers are. • Broadly tap-changers can be divided into two categories- namely off-load and on-load. • Off-load tap-changers cannot be operated while current is flowing in the circuit. Off-load tap changers are used mainly for non-critical applications where a momentary interruption in the current can be tolerated. Hence, such tap-changers have no use in traction duty. • In traction only On-Load Tap-Changers (OLTC) are used. They are capable of changing the taps rapidly without interrupting the flow of current. 4. Uninterrupted Power Supply • Uninterruptible Power System, Continuous Power Supply (CPS) or a battery backup is a device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when utility power is not available. • There are two distinct types of UPS: off-line and line-interactive (also called on-line). • An off-line UPS remains idle until a power failure occurs, and then switches from utility power to its own power source, almost instantaneously. • An on-line UPS continuously powers the protected load from its reserves, while simultaneously replenishing the reserves from the AC power. • The on-line type of UPS, in addition to providing protection against complete failure of the utility supply, provides protection against all common power problems, and for this reason it is also known as a power conditioner and a line conditioner. Hybrid UPS 5. Static Transfer Switch • The Static Transfer Switch provides break- before-make switching between two independent AC power sources for uninterrupted power to sensitive electronic equipment. • When used with redundant AC power sources, the switch permits maintenance without shutting down critical equipment. • Features • 0.25 cycle maximum transfers between AC power sources; • Manual and automatic transfers; • Selectable preferred input sources; • AccuVar TVSS for both AC inputs; 6. Series connected voltage source converter • A device and a method for controlling the flow of electric power in a transmission line carrying alternating current. • first voltage source converter is connected to the transmission line at a first point • second voltage source converter is connected to the transmission line at a second point. • Further, the first and second voltage source converters have their direct current sides connected to a common capacitor unit. 7. Shunt connected back - up source • A shunt connected energy stabilizing system with isolation switching for providing stored energy to loads or to a utility or industrial electrical distribution system or source of electrical power. • An energy backup and recovery system stores energy in a superconducting magnet and releases the energy to a real power/reactive power (VARs) generator which in turn delivers energy to either the loads or to both the loads and the source of electrical power. 8. Superconducting magnetic energy storage (SMES) devices • An SMES device can be used to alleviate voltage sags and brief interruptions. • The energy storage in an SMES-based system is provided by the electric energy stored in the current flowing in a superconducting magnet. • Through voltage regulator and inverter banks, this energy can be injected into the protected electrical system in less than 1 cycle to compensate for the missing voltage during a voltage sag event. • The SMES-based system has several advantages over battery-based UPS systems: 1. SMES-based systems have a much smaller footprint than batteries 2. The stored energy can be delivered to the protected system more quickly. 3. The SMES system has virtually unlimited discharge and charge duty cycles. Active Series Compensators • Device that can boost the voltage by injecting a voltage in series with the remaining voltage during a voltage sag condition. These are referred to as active series compensation devices. • They are available in size ranges from small single-phase devices (1 to 5 kVA) to very large devices that can be applied on the medium- voltage systems (2 MVA and larger). • When a disturbance to the input voltage is detected, a fast switch opens and the power is supplied through the series-connected electronics. • This circuit adds or subtracts a voltage signal to the input voltage so that the output voltage remains within a specified tolerance during the disturbance.