During sag, RMS voltage dips with 10

Fig. 1: Waveform explaining voltage sag.

Sources of Voltage Sag

1. Voltage sag and its interruption generally occur due to short ciicuit faults on the system. An unexpected increase in load yields an unexpected fall in current or voltage. This sudden increase in the load may cause voltage sag in

• Motors
• Switching operation
• Abrupt faults

2. As starting of the motor requires more current during start than at running, thus starting of motor is one of the reasons for the voltage sag (Fig. 2).

Fig. 2: Voltage sag due to motor starting

3. When line-to-ground fault occurs, there will be voltage sag until a protective switchgear operates.

4. Some unwanted accidents in power lines like flickering; lightning may cause line to ground faults, which in turn cause voltage sag.

5. When loads changes often, if causes voltage sags.

6. The transformer’s connection is another reason for voltage sag.

Causes of Voltage Sag (or) Effect of Voltage Sag on Various Equipment

The voltage sag primarily occurs due to:

Fig. 3: Effect of voltage sag on various equipment.

Starting of motors

Initially, a motor requires a large current during its start. The current ranges between 80 to 100

Traiisformer energizing model

Abrupt energizing of load tends to reduce the voltage. If the source is competent to run with this high load, then the voltage can return back to its initial voltage.

Single line to ground faults

The fault currents cause voltage sag in the power line.

Utility system

For any unwanted reason if the circuit breaker is tripped off, then the feeder from the distributing side is temporally cut off. The remaining feeder lines, will treat this as voltage sag and spread this to the consumer on the other lines. The degree of voltage lag may vary according to the supplied line voltage.

Equipment faults

If the system fails due to voltage imbalance like overvoltage/Undervoltage, the equipment installed in the substation will be observed on other feeder lines across the system.

Poor Weather condition

Lightening and thunder cause noteworthy voltage sag. The lightening caused due to voltage sag attack the power line and then to the ground, which form a line to ground (L-G) fault.

Severe Breezing

Heavy breeze can cut off the tree, which then fall on the power lines. As the branch of the tree touches the power line, it creates line to ground faults.

Pollution

In coastal areas, over a period of time, the accumulation of salt on the power line causes voltage sag.

Vehicles

Due to heavy traffic, vehicle collides with the utility pole, which tends the wire to touch. So the protective switchgear functions, which create voltage sag.

Construction of building

For constructing new buildings, the underground is dugged. While digging the underground cable gets damaged, which causes voltage sag.

Arc Furnaces

They operate with capacitor banks and harmonic filter. The capacitor hank causes the voltage to rise. To mitigate the voltage sag, the capacitor bank should be turned on else they are switched off.

Service system

Voltage sag can occur in both transmission and distribution voltages. The voltage sag with higher voltage tends to enter the utility system and gets spread to the other transmitters through transformers.

Inside Plants

Voltage sags are sometimes created inside the industrial plants even though utility is not involved. These are mainly due to the initial start of motors.

The post What is Voltage Sag? Definition, Sources & Causes appeared first on ElectricalWorkbook.

The good quality of power at the generation, transmission, distribution, and utilization of AC electrical power, describes the term electric power quality. There are a number of reasons for the poor quality of the AC supply systems such as lightening, flashover, equipment failure, and faults.

Power quality problems due to the current drawn from the AC mains are poor power factor, reactive power burden, harmonic currents, unbalanced currents, and harmonic currents generated by some nonlinear loads..

Any deviation in the voltage or current from the ideal value is a power quality disturbance. As per The IEEE definition, Power quality is the concept of powering and grounding sensitive equipment in a manner that is suitable to the operation of that equipment.  

Power quality has gained its importance in recent years as it plays a vital role in supplying electricity effectively to its end users. Power becomes an important resource at the global level and hence it is important to keep its quality at various level of abstraction. Hence, power quality has increased prominence and has importance in terms of electronic equipments. Both electric utilities and end users of electric power are mounting day by day and are more worried about the worth of electric power.

The term power quality has to turn into one of the most important catchphrases in the power industry since 1980s. The term power quality is rather vague and may be associated with dependability by electric utilities.

Definition of Power Quality

1. Power quality is a term that is different to different people.
2. Power quality generally refers to quality of voltage supply.
3. To a utility, it means supply of adequate and reliable power.
4. To a customer, it means adequate, uninterrupted power which does not affect the life of equipments.
5. To the manufacturer, it means the quality and tolerance of voltage and current parameters that is within the range of parameters for which he has manufactured and tested the products.

Importance of Power Quality

1. Power quality is significant because:
2. Customer pays for good quality power, if power quality is poor, it breach of trust.
3. Poor quality power damages consumer’s equipments and affects equipments life.
4. Increase of system losses.
5. Bad power quality causes severe health hazards.
6. Poor voltage and high current with harmonics causes heating and high losses.
7. The reliability of power supply s affected due to relay operations, frequent faults and equipment failure.

Symptoms of Poor Power Quality

The symptoms of poor power quality are:

1. Excessive reactive power.
2. Flickering of lights.
3. Computers shuuing down.
4. Overheating of wires, motors and transformers.
5. Process control devices act unpredictably.
6. Overloaded generators.
7. Supply interruption.

Effects of Poor Power Quality on Economy

1. If high harmonics are developed by the load, the rotating machine in generation side and consumer side get overheated.
2. Industries like rolling mills, paper mills, textile mills get affected badly as the interruption produced is too high.
3. If there is frequent abnormalities, the cost incurred towards the equipment goes high
4. Poor quality increases, more losses.

Power Quality Issues

Some power quality issues as follows

Voltage Sag: A momentary voltage decrease in rms voltage for durations of 0.5 cycle to 1 min. The voltage sag waveform is shown in Fig. 1.

Fig.1: Voltage sag, voltage swell, and outage.

Voltage swells: A momentary voltage rise in rms voltage for durations of 0.5 cycle to 1 min. The voltage swell waveform is shown in Fig. 1.

Outage: When voltage zero for a few seconds to several hours. The voltage outage waveform is shown in Fig. 1.

Over voltage: A steady state voltage having a value greater than the nominal voltage for a time greater than 1 min.

Under voltage: A steady state voltage having a value less than the nominal voltage for a time greater than 1 min.

Transients: A transient can be a unidirectional impulse or a damped oscillatory wave for an extremely short duration as compared to voltage sag and swell. The transient waveform is shown in Fig. 2.

Fig.2: Transient voltage (oscillatory)

Harmonic:

Harmonics are sinusoidal voltage/current having frequencies that are integer multiple of Fundamental frequency. The harmonic distortion waveform is shown in Fig. 3. Harmonics in the system can:

• Make relays maloperate.
• Increase loss in capacitances, noises.
• Telephonic cross talks.
• Can cause increase in resonance

Fig.3: Harmonic distortion

Power Quality Standards

Due to the poor power quality, the customers facing a number of problems such as equipment failure, mal-operation, interruption in the process, and loss of production. Thus, several standards have been developed, modified, recommended, to improve the power quantify. Table 1 shows a list of some standards on power quality.

Table 1 List of some standards and its Description.

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