**Power Quality and Voltage Sag Indices in Electrical Power Systems**

 Alexis Polycarpou *Frederick University Cyprus* 

## **1. Introduction**

In modern electrical power systems, electricity is produced at generating stations, transmitted through a high voltage network, and finally distributed to consumers. Due to the rapid increase in power demand, electric power systems have developed extensively during the 20th century, resulting in today's power industry probably being the largest and most complex industry in the world. Electricity is one of the key elements of any economy, industrialized society or country. A modern power system should provide reliable and uninterrupted services to its customers at a rated voltage and frequency within constrained variation limits. If the supply quality suffers a reduction and is outside those constrained limits, sensitive equipment might trip, and any motors connected on the system might stall.

The electrical system should not only be able to provide cheap, safe and secure energy to the consumer, but also to compensate for the continually changing load demand. During that process the quality of power could be distorted by faults on the system, or by the switching of heavy loads within the customers facilities. In the early days of power systems, distortion did not impose severe problems towards end-users or utilities. Engineers first raised the issue in the late 1980s when they discovered that the majority of total equipment interruptions were due to power quality disturbances. Highly interconnected transmission and distribution lines have highlighted the previously small issues in power quality due to the wide propagation of power quality disturbances in the system. The reliability of power systems has improved due to the growth of interconnections between utilities.

In the modern industrial world, many electronic and electrical control devices are part of automated processes in order to increase energy efficiency and productivity. However, these control devices are characterized by extreme sensitivity in power quality variations, which has led to growing concern over the quality of the power supplied to the customer.

According to the IEEE defined standard (IEEE Std. 1100, 1999), power quality is "The concept of powering and grounding electronic equipment in a manner suitable to the operation of that equipment and compatible with the premise wiring system and other connected equipment". Some authors use the term 'voltage quality' and others use 'quality of supply' to refer to the same issue of power quality. Others use the term 'clean power' to refer to an intolerable disturbance free supply. Power quality is defined and documented in established standards as reliability, steady state voltage controls and harmonics. Voltage sag is defined as a short reduction in voltage magnitude for a duration of time, and is

Power Quality and Voltage Sag Indices in Electrical Power Systems 141

and failure. Overvoltages with duration of 0.5 cycle to 1 min are called voltage swells. A swell is typically of a magnitude between 1.1 and 1.8 pu and is usually associated with

The increasing application of power electronic devices like adjustable speed drives, uninterruptible power supplies and inverters, raises increasing concerns about harmonic distortion in the power system. These devices can not only cause harmonics in the system but are also very sensitive to voltage-distorted signals. The presence of harmonics in the system could also cause several unwanted effects in the system including excessive

The maximum total harmonic distortion which is acceptable on the utility system is 5.0% at 2.3-69kV, 2.5% at 69-138kV and 1.5% at higher than 138kV voltage levels (IEEE Std. 1250,

This type of power quality disturbance is caused by unequal distribution of loads amongst the three phases. At three-phase distribution level, unsymmetrical loads at industrial units and untransposed lines can result in voltage imbalance. Voltage imbalance is of extreme importance for three-phase equipment such as transformers, motors and rectifiers, for which it results in overheating due to a high negative sequence current flowing into the equipment. The asymmetry can also have an adverse effect on the performance of

For many years, electricity companies have used sustained interruption indices as indicators describing the quality and reliability of the services they provide. In order to compare power quality in different networks, regulators need to have common, standardised quality indices. The number of these indices should be kept at a minimum, easy to assess, and be representative of the disturbance they characterise. This section briefly discusses various voltage sag indices proposed by electrical association organisations and indices suggested by recent researchers. These indices are used to characterise any voltage sag, according to the individual index point of view. The procedure to evaluate the quality of supply, reference to non-rectangular events and equipment compatibility issues are also

Any available voltage sag index can be classified within the following three categories

a. Single-event index: a parameter indicating the severity of a voltage or current event, or otherwise describing the event. Each type of event has a specific single-event index. b. Single-site index: a parameter indicating the voltage or current quality or a certain

c. System index: a parameter indicating the voltage or current quality or a certain aspect

transformer heating or overloading and failure of power factor correcting capacitors.

single line to ground faults where voltages of non-faulted phases rise.

**2.4 Harmonic issues** 

**2.5 Voltage imbalance** 

converters, as it results in the production of harmonic.

aspect of voltage or current quality at a specific site.

of voltage or current quality for a whole or part of a power system.

**3. Voltage sag statistical indices** 

1995).

discussed.

(Bollen, 2000).

**3.1 Types of indices** 

considered to be the most common power quality issue. The economic impact of power quality on a utility is of great importance. Living in a world where making money is the major objective of electricity companies, quality is often overlooked. Thus a need for specific, easy to assess and well-defined performance criteria for use worldwide encouraged the definition and establishment of Power Quality indices.

In the first section of this chapter Power quality disturbances, such as Voltage sag, Interruption, transient overvoltage, swell, Harmonic issues and voltage imbalance, are Introduced. Statistical voltage sag Indices used for characterization and assessment of power quality are then presented. These indices classify within the following categories: Single event, Site indices and System indices. Furthermore the development and verification of Mathematical voltage sag indices is presented, applicable for power quality improvement through optimization techniques. The impact on the voltage profile of heavy induction motor load switching is predicted, and the possibility to mitigate potential power quality violations before they occur is created. Finally the chapter conclusions are presented, highlighting the importance of the statistical indices and how the mathematical indices could further enhance the power quality of an electric power system.
