voltage sag

POWER QUALITY

VOLTAGE SAG

• A voltage magnitude event with a

magnitude between 10% and 90% of the

nominal RMS voltage and duration

between 0.5 cycles and one minute. [ieee

std. 1159].

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tage

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Voltage Sag

Voltaje sag duration

Voltage sag magnitude

CLASSIFICATION OF VOLTAGE SAG

MULTI PHASE SAGS AND SINGLE PHASE SAGS

• SINGLE PHASE SAGS

• The most common voltage sags, over 70%, are single phase events which are typically due to a phase to ground fault occurring somewhere on the system. This phase to ground fault appears as a single phase voltage sag on other feeders from the same substation. Typical causes are lightning strikes, tree branches, animal contact etc. It is not uncommon to see single phase voltage sags to 30% of nominal voltage or even lower in industrial plants.

• PHASE TO PHASE SAGS

• 2 phase, phase to phase sags may be caused by tree branches, adverse weather, animals or vehicle collision with utility poles. The two phase voltage sag will typically appear on other feeders from the same substation.

• 3 PHASE SAGS

• Symmetrical 3 phase sags account for less than 20% of all sag events and are caused either by switching or tripping of a 3 phase circuit breaker, switch or recloser which will create a 3 phase voltage sag on other lines fed from the same substation.

• 3 phase sags will also be caused by starting large motors but this type of event typically causes voltage sags to approximately 80% of nominal voltage and are usually confined to an industrial plant or its immediate neighbours

WHERE DO VOLTAGE SAGS OCCUR?

1.UTILITY SYSTEMS• Voltage sags can occur on utility

systems both at distribution voltages and transmission voltages. voltage sags which occur at higher voltages will normally spread through a utility system and will be transmitted to lower voltage systems via transformers

WHERE DO VOLTAGE SAGS OCCUR?

2. INSIDE INDUSTRIAL PLANTS

• Voltage sags can be created within an industrial complex without any influence from the utility system. These sags are typically caused by starting large motors or by electrical faults inside the facility.

CAUSES OF VOLTAGE SAGS

UTILITY SYSTEMS

• OPERATION OF RECLOSERS AND CIRCUIT BREAKERS

• If for any reason a sub-station circuit breaker or a recloser is tripped, then the line which it is feeding will be temporarily disconnected. All other feeder lines from the same substation system will see this disconnection event as a voltage sag which will spread to consumers on these other lines (see fig). The depth of the voltage sag at the consumer’s site will vary depending on the supply line voltage and the distance from the fault.

• EQUIPMENT FAILURE

• If electrical equipment fails due to overloading, cable faults etc, protective equipment will operate at the sub-station and voltage sags will be seen on other feeder lines across the utility system

• BAD WEATHER

• Thunderstorms and lightning strikes cause a significant number of voltage sags. If lightning strikes a power line and continues to ground, this creates a line to ground fault. The line to ground fault in turn creates a voltage sag and this reduced voltage can be seen over a wide area. Note that the lightning strike to ground causes voltage sags on all other lines. Circuit breakers and Reclosers operate more frequently in poor weather conditions.

• High winds can blow tree branches into power lines. As the tree branch strikes the line, a line to ground fault occurs which creates a voltage sag. If the line protection system does not operate immediately, a series of sags will occur if the branch repeatedly touches the power line. Broken branches landing on power lines cause phase to phase and phase to ground faults

• Snow and ice build up on power line insulators can cause flash-over, either phase to ground or phase to phase. Similarly snow or ice falling from one line can cause it to rebound and strike another line. These events cause voltage sags to spread through other feeders on the system

• POLLUTION

• Salt spray build up on power line insulators over time in coastal areas can cause flash over especially in stormy weather. Dust in arid inland areas can cause similar problems. As circuit protector devices operate, voltage sags appear on other feeders

• VEHICLE PROBLEMS

• Utility power lines frequently run alongside public roads. Vehicles occasionally collide with utility poles causing lines to touch, protective devices trip and voltage sags occur.

• ANIMALS & BIRDS

• Animals particularly squirrels, snakes occasionally find there way onto power lines or transformers and can cause a short circuit either phase to phase or phase to ground. large birds, geese and swans, fly into power lines and cause similar faults. while the creature rarely survives, the protective circuit breaker operates and a voltage sag is created on other feeders

• CONSTRUCTION ACTIVITY

• Even when all power lines are underground, digging foundations for new building construction can result in damage to underground power lines and create voltage sags

• TRANSFER OF LOADS FROM ONE POWER SOURCE TO ANOTHER

• Most facilities contain emergency generators to maintain power to critical loads in case of an emergency. Sudden application and rejection of loads to a generator could create significant voltage sags or swells

• During power transfer from the utility to the generator, frequency deviations occur along with voltage changes. The generator frequency can fluctuate as much as ±5 Hz for a brief duration during this time. It is once again important to ensure that sensitive loads can perform satisfactorily within this frequency tolerance for the duration of the disturbance

• INDUSTRIAL PLANTS

• Voltage sags can be caused within an industrial facility or a group of facilities by the starting of large electric motors either individually or in groups. The large current inrush on starting can cause voltage sags in the local or adjacent areas even if the utility line voltage remains at a constant nominal value

• INDUCTION MOTORS

• Draw starting currents ranging between 600 and 800% of their nominal full load currents. The current starts at the high value and tapers off to the normal running current in about 2 to 8 sec, based on the motor design and load inertia. Depending on the instant at which the voltage is applied to the motor, the current can be highly asymmetrical

• ARC FURNACES

• Arc furnaces operate by imposing a short circuit in a batch of metal and then drawing an arc, which produces temperatures in excess of 10,000°c, which melt the metal batch. Arc furnaces employ large inductors to stabilize the current due to the arc. Thousands of amperes are drawn during the initial few seconds of the process.

• Once the arc becomes stable, the current draw becomes more uniform. Due to the nature of the current drawn by the arc furnace, which is extremely nonlinear, large harmonic currents are also produced. Severe voltage sags are common in power lines that supply large arc furnaces.

• furnaces are operated in conjunction with large capacitor banks and harmonic filters to improve the power factor and also to filter the harmonic frequency currents so they do not unduly affect other power users sharing the same power lines

• It is not uncommon to see arc furnaces supplied from dedicated utility power lines try to minimize their impact on other power users. The presence of large capacitance in an electrical system can result in voltage rise due to the leading reactive power demands of the capacitors, unless they are adequately canceled by the lagging reactive power required by the loads. This is why capacitor banks, whether for power factor correction or harmonic current filtration, are switched on when the furnace is brought on line and switched off when the arc furnace is off line.

CHARACTERISTICS OF VOLTAGE SAG

• Magnitude of the sag

• Duration of the sag

• Balanced or unbalanced

• Phase-angle jump

• Missing voltage

• Point at which sag initiated ..

Estimate the voltage sag magnitude

• The magnitude of voltage sag determined from RMS voltage.

• The magnitude of the sag is considered as the residual voltage or remaining voltage during the event

• RMS value during the sag is not completely constant and that the voltage does not immediately recover after the fault.

• There are various ways of obtaining the sag magnitude from the RMS voltages.

• Most power quality monitors take the lowest value obtained during the event. As sags normally have a constant RMS value during the deep part of the sag, using the lowest value is an acceptable approximation

• In the case of a three phase system,

• Voltage sag can also be characterized by the minimum RMS -voltage during the sag if the sag is symmetrical i.e. equally deep in all three phases

• If the sag is unsymmetrical, i.e. the sag is not equally deep in all three phases, the phase with the lowest remaining voltage is used to characterize the sag

• The magnitude of voltage sags at a certain point in the system depends

1. The type and the resistance of the fault

2. The distance to the fault

3. The system configuration

The calculation of the sag magnitude for a fault somewhere within a radial distribution

system

• ZS is the source impedance at the PCC and ZF is the impedance between the PCC and the fault

• The voltage sag at the PCC equals the voltage at the equipment terminals

• Assume that the pre-event voltage is exactly 1 pu, thus E= 1.

• N is the number of samples per cycle

• Vi is the sampled voltage

• K is the instant at which RMS voltage is estimated

• RMS value is calculated from previous samples of voltage- post estimation

• One cycle window algorithm: RMS values are estimated with one cycle of instantaneous values

• Half cycle window algorithm: choose instantaneous values over a half cycle

• More sensitive and faster response than other

SAG DURATION

• The duration of voltage sag is mainly determined by the fault–clearing time.

• The actual duration of a sag is normally longer than the fault-clearing time.

• The duration of a voltage sag is the amount of time during which the voltage magnitude is below threshold is typically chosen as 90% of the nominal voltage magnitude

• For three phase system, consider the three RMS values to find the duration

• The voltage sag starts when at least one of the RMS voltages drops below the sag-starting threshold. The sag ends when all three voltages have recovered above the sag-ending threshold

• The commonly used definition of sag duration is the number of cycles during which the RMS voltage is below a given threshold.

• This threshold will be somewhat different for each monitor. But typical values are around 90% of the nominal voltage.

• A power quality monitor will typically calculate the RMS value once every cycle

• Post-fault sag will affect the sag duration.

• When the fault is cleared, the voltage does not recover immediately. This is mainly due to the reenergizing and reacceleration of induction motor load

• This post-fault sag can last several seconds, much longer than the actual sag

• Magnitude-duration plot is a common tool used to show the quality of supply at a certain location or the average quality of supply of a number of locations as the fault clearing time depends on the type of transmission distribution system

• Faults in transmission systems are cleared faster than faults in distribution systems. In transmission systems, the critical fault-clearing time is rather small

• Fast protection and fast circuit breakers are essential

• Distance protection or differential protection, both of which allow for fast clearing of the fault

• The protection schemes used should have the ability to clear a fault within one half-cycle

MONITORING AND MITIGATION TECHNIQUES

INTRODUCTION

• Voltage sags are most costly of all power quality disturbances.

• Lead to disruption of manufacturing processes due to equipment being unable to operate correctly at the reduced voltage levels.

• Industrial equipment such as variable speed drives and some control systems are particularly sensitive to voltage sags.

• In many manufacturing processes, loss of only a few vital pieces of equipment may lead to a full shut down of production leading to significant financial losses.

• For some processes which are thermally sensitive a significant loss of material as well as the time taken to clean up and restart the process must also be considered.

1.Ferroresonant transformers• FERRORESONANT transformers are designed to

achieve regulation with non-linear operation. They provide line regulation, reduce harmonics, and are current limiting.

• Also known as Constant Voltage Transformers(CVT)

• Operates in the saturation region of the transformer B-H curve

REGION OF OPERATION

• A 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

• The magnetic shunt provides a path for the imbalanced flux of the primary and secondary by allowing a portion of the primary flux to return to the primary winding without coupling the secondary. At the same time, it allows the secondary flux to return to the secondary winding without coupling the primary.

• OPERATION:

• When a voltage is applied to the primary winding the secondary voltage increases as the primary voltage increases. As the primary voltage increases the secondary voltage continues to increase up to a point of discontinuity, or secondary resonance, where an abrupt increase, about 20 %, in secondary voltage occurs. The resonance effect immediately increases the secondary flux density and causes saturation of that portion of the core. This partial core saturation is the key to the magnetic design of the ferroresonant transformer.

• The voltage induced in the capacitor winding by the primary flux causes a capacitive current to flow. The flux due this current is in phase with the primary flux. This flux addition occurs in the secondary portion of the core. The increased flux saturates the portion of the core on the secondary winding only. The primary portion of the core is operating below saturation or below the “knee” of the magnetization curve.

• FERRORESONANT TRANSFORMERS are inherently self-protected against short circuits, and are able to supply large surge currents if required because of the large amount of energy stored in the secondary circuit.

• Ferroresonant transformers are simple and relatively maintenance free devices which can be very effective for small loads.

• Ferroresonant transformers are available in sizes up to around 25 KVA

• Voltage sags down to 30 % retained voltage can be mitigated through the use of ferroresonant transformers.

• Typically ferroresonant transformer regulators can maintain secondary voltage to within ±0.5% for changes in the primary voltages of ±20%

• The disadvantages of a ferroresonant transformer are:

• Frequency sensitive.

• Temperature sensitive.

• External magnetic field may require shielding for sensitive component.

• Ferroresonant transformers are generally not suitable for loads with high inrush currents such as direct-on-line motors

STATIC TRANSFER SWITCH

• For facilities with a dual supply, one possible method of voltage sag mitigation is through the use of a automatic static transfer switch.

• Upon detection of a voltage sag, these devices can transfer the load from the normal supply feeder to the alternative supply feeder within half a cycle.

• Conventional transfer switches will switch from the primary supply to a backup supply in seconds.

• Fast transfer switches that use vacuum breaker technology are available that can transfer in about 2 electrical cycles. This can be fast enough to protect many sensitive loads.

• Static switches use power electronic switches to accomplish the transfer within about a quarter of an electrical cycle

VOLTAGE REGULATOR

VOLTAGE REGULATOR• Voltage regulators are devices that can maintain a constant

voltage (within tolerance) for voltage changes of predetermined limits above and below the nominal value.

• A switching voltage regulator maintains constant output voltage by switching the taps of an autotransformer in response to changes in the system voltage

• The electronic switch responds to a signal from the voltage-sensing circuitry and switches to the tap connection necessary to maintain the output voltage constant.

• The switching is typically accomplished within half of a cycle, which is within the ride-through capability of most sensitive devices.

UNINTERRUPTIBLE POWER SUPPLIES (UPS)

• UPS mitigate voltage sags by supplying the load using stored energy.

• Upon detection of a voltage sag, the load is transferred from the mains supply to the ups. Obviously, the capacity of load that can be supplied is directly proportional to the amount of energy storage available.

• Ups systems have the advantage that they can mitigate all voltage sags including outages for significant periods of time (depending on the size of the ups).

• 3 CONFIGURATIONS

• ONLINE UPS

• OFFLINE/STANDBY UPS

• HYBRID UPS

ONLINE UPS

• The load is always fed through the UPS. The incoming ac power is rectified into dc power, which charges a bank of batteries. This dc power is then inverted back into 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.

• However, the on-line operation increases the losses and may be unnecessary for protection of many loads.

OFFLINE/STANDBY UPS

• A standby power supply is sometimes termed 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. The transfer time from the normal source to the battery-backed inverter is important.

• 8 ms is the lower limit on interruption through for power-conscious manufacturers. Therefore a transfer time of 4 ms would ensure continuity of operation for the critical load.

• A standby power supply does not typically provide any transient protection or voltage regulation as does an on-line ups. This is the most common configuration for commodity UPS units available at retail stores for protection of small computer loads.

• UPS specifications include kilo-voltampere capacity, dynamic and static voltage regulation, harmonic distortion of the input current and output voltage, surge protection, and noise attenuation. The specifications should indicate, or the supplier should furnish, the test conditions under which the specifications are valid.

HYBRID UPS

• Similar in design to the standby 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

FLY WHEEL AND MOTOR- GENERATOR SETS• Flywheel systems use the energy stored in the inertia of a rotating

flywheel to mitigate voltage sags.

• A flywheel is coupled in series with a motor and a generator which in turn is connected in series with the load.

• The flywheel is accelerated to a very high speed and when a voltage sag occurs, the rotational energy of the decelerating flywheel is utilised to supply the load.

• Flywheel storage systems are effective for mitigation of all voltage sags including interruptions and can supply the load for a significant period of time (up to several seconds depending on the size of the flywheel).

• Flywheels have maintenance and reliability advantages over other energy storage systems such as batteries. However, if large energy storage capacities are required, flywheels must be large and are heavy. The configuration has high losses during normal operation.

• In this configuration, the motor which drives the flywheel is connected through a variable speed drive. This connection arrangement results in better starting characteristics for the flywheel and efficiency gains for the motor.

• Connection of the ac generator to a voltage source converter increases the amount of energy that can be extracted from the flywheel due to the fact that the converter is able to produce a constant dc voltage, which may then be used directly or converted back to ac voltage, over a wide speed range.

SAG PROOFING TRANSFORMERS

• Known as voltage sag compensators

• A multi-winding transformer connected in series with the load

• These devices use static switches to change the transformer turns ratio to compensate for the voltage sag

• Sag proofing transformers are effective for voltage sags to approximately 40 % retained voltage

• ADVANTAGE:

• Maintenance free and do not have the problems associated with energy storage components

• DISADVANTAGE:

• Sag proofing transformers are only available for relatively small loads of up to approximately 5 kVA.

• With the transformer connected in series, the system also adds to losses and any failure of the transformer will lead to an immediate loss of supply.

UTILITY EFFORTS IN MITIGATION OF VOLTAGE SAGS

• REDUCE THE NUMBER OF FAULTS

• Limiting the number of faults is an effective way not only to reduce the number of faults but also to reduce the frequencies of short and long term interruptions

FAULT PREVENTIVE ACTION includes

• Tree trimming policies

• Addition of lightning arresters

• Proper insulators

• Addition of animal guards

• Considerable reduction of faults can be achieved by replacing overhead lines by underground cables which are less affected by bad weather

• REDUCE THE FAULT CLEARING TIME

• The modern static circuit breakers available are able to clear the fault within a half cycle at power frequencies ensuring that no voltage sag can last longer

• Redesign existing systems to achieve faster fault clearing time

• SYSTEM DESIGN AND CONFIGURATION

• By proper changes in the design and configuration we can achieve reduction in voltage sag and other problems

INTERRUPTIONS

• SHORT INTERRUPTIONS

• LONG INTERRUPTIONS

SHORT INTERRUPTIONS

• Total interruption of electrical supply for duration from few milliseconds to one or two seconds

• Causes:

• Opening and reclosing of protective device to decommission the faulty part

• Insulation failure, insulator flashover, lightning

LONG INTERRUPTIONS

• Total interruption of electrical power supply for a duration greater than one or two seconds

• Causes:

• Equipment failure in power system network

• Storms and objects(trees, vehicles etc)

• Striking lines, poles

• Fire

• Bad coordination of protective device

MOMENTARY POWER INTERRUPTIONS

• Lasts no longer than few seconds

• Causes:

• Lightning strikes

• Fallen branches

• Animals coming into contact with power lines

• Transfer of load from one source to another

• Advanced electronic devices are more sensitive to disturbances

• How to minimize momentary interruptions

• Taller trees should be planted at a minimum distance of 30feets away from power lines

• Medium sized trees should be planted atleast 15 feet away from power lines

• Care should be taken if small sized trees are planted near the power lines

• Vulnerable equipments are

• Digital clocks

• VCR

• Microwave ovens

• Stereos, TV

• Computers

POWER OUTAGES

Total interruption of electrical supply

Utilities have installed protection devices that briefly interrupts power to allow time for a disturbance to dissipate

If lightning strikes the power line, large voltage is induced into the power lines. The protection equipment momentarily interrupts power, allowing time for the surge to dissipate

• Types of power outages:

• A transient fault is a momentary loss of power typically caused by a temporary fault on a power line. Power is automatically restored once the fault is cleared

• A blackout refers to the total loss of power to an area and is the most severs form of power outage that can occur. It is difficult to recover from it quickly

CAUSES:

• Ice storms, lightning, wind, utility equipment failure

VULNERABLE EQUIPMENT:

• All electrical equipments

EFFECTS:

Complete disruption of operation

Solutions: Transient voltage surge suppression, UPS