Power quality issues due to different kinds of faults. Essay

Power quality issues due to different kinds of faults.

Muzafar Ahmad Shah

Department of Electrical Engineering

Lovely Professional University

Punjab, India

Email: [email protected]

Abstract—Power quality is one the major concerns in

day to day life. Proper functioning of the power system

and the equipments connected to it depends upon the

power quality. Power quality is mainly concerned with

the voltage and frequency variations in the power system.

Quality of power or the power quality can be adversely

influenced by the different faults that occur frequently

in the power systems. The paper provides an analysis

of the power quality issues due to these faults. Faults

usually lead to the voltage sag on the consumer bus. Hence

a method is proposed for the quantification of voltage

sag. The method to be chosen should be technically and

economically acceptable. This method will provide a design

which is beneficial for the utilities as well as consumer

engineers to analyze the faults and come up with an

effective and optimum solution for these issues.

Keywords—power quality, voltage sag, distribution sys-

tems, distributed system protection.

I. INTRODUCTION

A number of events such as interruptions, outages",

surges, swell and sag are responsible which can affect

the power quality of a particular bus. In the distribution

power systems, it has been experienced that these

problems of power quality are mainly originating

from faults in the power system, which eventually

leads to voltage sag, interruption and outage. This

paper provides the work regarding the voltage sag",

its prediction, magnitude, its rate and occurrence of

duration.

Power quality issues have been studied in recent

times and is becoming the interesting field for the

research purposes. Already there have been a number

of field studies which have been presented, discussing

these issues. One of the documented reports present the

work regarding the voltage sag, swell, interruption and

surge frequency [1]. The work presents the volt-time

characteristics of these events which nowadays is

commonly is known as CBEMA curve [2]. CBEMA

curve is used as the computer for the understanding

of these characteristics. Information about the power

quality can be clearly presented by the use of CBEMA

curve which plots the measurement data on the volt-time

curve. It is therefore clear that this field is an emerging

filed of interest which has already drawn a lot of

attention in the recent times.

Fig. 1. CBEMA Curve - Steady-State Portion

Site surveys are done to address power quality

issues [3]. The main aim of study is the identification

of problem and achieving the required solution. Power

supply must be coordinated with the end use consumer

equipment. Lack of proper coordination can lead to

the increase in the outage costs and/or equipment

costs from the consumer side. Generally, greater

the ride-trough capability, larger are the equipment

costs. It is not economically viable to provide this

ride-though capability against all these issues of sag

and interruptions [4]. As such, knowledge regarding

the sag probability needs to be studied very accurately

for a wide range of voltage sag and its duration. This

approach is of utmost importance, particularly for the

integrated loads. This is because such loads constitute a

variety of equipments which can range from induction

motors to high power loads and process controlling

computers. As such this system must be properly

coordinated. Many cases have been that unnecessary

powering off of these lines results in a huge economic

loss. So far no practical have been designed which can

truly deal with these types of power quality issues.

Fig. 2. Waveform showing Voltage Sag

There is a need of accurate and quantified knowledge

about these voltage sags which can help us for the

designing of equipments [5]. Hence a method needs to

be developed which can address such issues and can

also provide the platform for the information exchange

between the consumers, utilities and equipment

designers. The analytical approach used in this paper

solves these issues. This methodology can give the

system performance in terms of volt-time curve, which

will eventually help us to predict the performance and

to design the equipments accordingly.

II. DISTRIBUTION SYSTEM FAULT

PERFORMANCE

From the consumer point of view, the fault or the

abnormalities in the system can be divided into four

categories viz outages, interruptions, swell and sag.

Swell and voltage can be due to some temporary faults

occurred in the distribution system and do not seen

in the direct load supplying path. Interruptions, which

comes in the direct path occurs due to the action of

breaker or a recloser, which interrupt a temporary fault

or restored the supply. Outages are a result of some

permanent fault which comes directly in the consumer

feeding path. As for the overhead distributed system",

the temporary faults occur more frequently as compared

to the permanent faults. Therefore it is obvious that a

consumer suffers more of the remote faults than the

direct faults. Therefore, sags",swells and interruptions will

be more in number as compared to the outages [6].

But because of the severity of this issue, it has become

necessary to address this event quickly.

The effect of system faults on sags, swells, and

interruptions, however, has received less attention. While

sags in particular are more numerous than outages",

their effect has historically been of lesser consequence

for many loads and for many consumers. Recently",

however, sags have become increasingly important, for

two reasons. First, most modern electronic loads are

sensitive to the voltage sags and interruptions which

occur due to these faults. Secondly, increasing levels of

industrial automation has led to a significant number of

installations where the disruption caused by these faults

has a cascading effect and high costs [7]. While the

emergence of this problem has been reported, general

techniques for analyzing and avoiding these problems

have not been developed. In this paper, a method is

proposed for quantifying the rate at which distribution

system faults will cause power quality events at a given

consumer location.

The electric power distribution system will consist of

one or more sources of energy, one or more substa-

tion transformers, and a combination of overhead and

underground lines. The most common cause of major

power disturbances on these systems are from faults

on the overhead lines [8]. The effect of overhead line

faults on the power quality will be quantified in the

proposed procedure. It is assumed in this paper that the

fault protection equipment functions normally during the

faults. The effects of faults on transformers, underground

line segments and buswork can be treated in a manner

similar to that proposed in this paper.

The system is divided into zones of protection, with

faults in each zone of protection to be sensed by a

group of protective relays. Fault duration times within a

zone, then, will be a function of these relays as well as

of the network impedances. For a circuit segment within

a given zone, a fault along that segment will produce

a fault current If, which depends on the location of

the fault and on the fault impedance. The fault current

will in turn cause a relay to operate, with the relay

timing depending on the current level. The fault clearing

time is then the sensing time plus the circuit breaker

operating time. For temporary faults which are not in

a direct path between the source and the consumer

location, fault clearing will lead to a restoration of

normal voltage, and the customer will experience a sag

or possibly a swell [9]. For faults in the same zone

of protection as the consumer (or faults in the direct

source path on radial systems), the fault clearing will

be followed by an interruption, which will last until

the circuit breaker recloses. For temporary faults, the

reclosing will be successful, and the system will be

restored to normal. If the fault is permanent, however",

the reclosing of the circuit breaker will be followed by

either a fuse operation or one or more additional circuit

breaker or recloser operations.

Fig. 3. One line diagram of study distribution system

It is assumed that multiple faults do not occur simul-

taneously. While the study system is radial with time-

overcurrent protection, the method proposed in this paper

is not limited to either radial system or to this type

of protection scheme. The method can be applied to

any system for which knowledge of fault current levels

and relay operating characteristics are available. Also",

the method is readily extended to include sectionalizing

fuse operation in cases where it is expected that these

fuses will clear temporary faults or in studies involving

multiple sags due to permanent faults.

III. SAG LEVEL PREDICTION

For many consumers fed from the distribution system",

the largest number of distribution system induced power

quality events will be sags which occur on the customer

bus as a result of temporary faults in remote parts of

the system. A fault on a given remote line segment will

be characterized by its location and its fault resistance.

By neglecting the effect of system loading on the fault

performance, the radial distribution system can then be

represented by sequence equivalent circuits as shown in

Figure 4. In Figure 4, node c represents the consumer

location and node f represents the fault location. Node

s represents the point of common coupling between the

fault location and the consumer’s node. The sequence

networks can be connected and solved for each of the

various fault types using standard symmetrical compo-

nent techniques. From these solutions, the fault current

levels at the protective device location and the depth of

the voltage sag at the consumer’s node are determined for

a particular fault type, fault location and fault resistance.

The overall study involves multiple solutions of Figure 4

for all fault types at every system location and throughout

the ranges of the fault resistances. In order to accomplish

these calculations, the range and distribution of the

expected fault resistance is needed. The variation in fault

location and fault resistance can be handled by a suitable

number of discrete values of each of these parameters.

In this study, 40 fault points were considered on each

line segment, and five fault resistance values were used

for each type of fault.

The footprint of Figure 5 is therefore valid for any

of the three-phase voltages at the consumer’s bus. Un-

balanced faults will lead to unbalanced voltages at the

consumer’s bus, so that the presentation of results is

not as straightforward. In particular, the nature of the

equipment that may be affected by the fault is important",

as different equipment will respond differently to un-

balanced voltages. Single-phase equipment will respond

only to the voltage of the phase from which it is fed. If

the consumer’s concern centers on one piece of single-

phase equipment, then the voltage of a particular phase

should be plotted on the graphs. If the concern is about a

group of single-phase loads which are balanced between

phases, then the concern would be for the minimum

phase voltage during a fault. The transformer connection

between the distribution system and the load bus is also

important, as discussed in Reference 6. While outside

the scope of this paper, it is worth noting that significant

benefit can be obtained by connecting the single-phase

powered control equipment for a process in the best

manner.

A footprint for all types of faults can be developed by

Fig. 4. Simplified sequence diagrams used for calculating voltage

sag

superimposing the results of the fault studies to present a

composite view. Figure 6 shows the plot of the minimum

phase voltage versus time curve for all fault types on

the same line segment. The ranges of fault resistance

assumed for the various fault types are given in Table 3.

Other types of voltage—time plots (for example, positive

sequence voltage) can be readily plotted but are omitted

for space purposes. The resulting histogram for the

system of Figure I is shown in Figure 7, where faults

causing sags greater than 10% are considered. Figure 7

is a plot of the expected value of voltage sag occurrence

over a 0.05 p.u. interval of positive sequence voltage

Fig. 5. Node 2 sag characteristics due to three-phase fault on line

segments 10-12

Fig. 6. Node 2 minimum phase voltage sag characteristics due to all

faults on line segment 10—12

and over an 0.05 interval of the logarithm of time. A

significant percentage of the voltage sags due to remote

faults will not create significant disturbances at the

consumer location. In the case considered here, 40 sags

per year are expected to result from temporary faults on

the feeders fed by circuit breakers cb2 and cb3. Analysis

of the data presented in Figure 7 shows that of these 40

faults, 23.7 on average are expected to produce voltage

sags of greater than 10% at node 2, while 16.3 faults will

produce sags of less than 10% at node 2. It is apparent

that knowledge of the number of faults expected on the

remote system cannot be used to predict the number

of significant sags that the consumer will experience.

The voltage sags experienced by the consumer are a

complex function of fault type, fault location and fault

resistance, and are also influenced by the type of sag

to which the consumer’s equipment is susceptible. The

full evaluation of fault effects outlined in this section is

needed to predict the number of significant disturbing

events per year due to distribution system faults, as well

as the voltage—time signature of the expected sags.

IV. TEMPORARY FAULTS DIRECTLY IN THE

SOURCE PATH

The effect of temporary faults remote from the con-

sumer location were considered in the previous section.

When temporary faults occur directly on the line feeding

the consumer, the consumer will experience a voltage

sag followed by a full outage. Therefore, every fault in

this area will cause significant disruption, and the total

number of events can be predicted readily. The outage

will persist until the circuit breaker or recloser closes

back in, when the full voltage will be restored. While

many designs will provide ride-through capability only

for remote faults [10], there will be some instances where

ride-through of source path faults will be investigated.

Fig. 7. Expected sag rate for positive sequence voltage at node 2

due to all temporary faults on the other two feeders.

The fault duration can be of similar length to the

reclosing interval. In cases where partial ride-through

of direct faults is desired, an effective approach would

be to provide ride-through of single phase to ground

faults only. In this case, a substantial level of positive

sequence voltage will be present during the fault, and

the designer will need separate information regarding

the fault period and the reclosing period. If this is the

design strategy, particular attention should also be paid

to the transformer connections and the single-phase load

connection. In cases where ride-through is required for

any temporary fault in this area, a pessimistic assumption

for three-phase faults would be to consider the voltage

to be zero from the fault initiation through to the breaker

reclosing. This assumption can be overly pessimistic for

single-phase or high resistance faults, however.

V. CONCLUSION

This paper presents a method for the determination

of voltage sag rates at a customer location. The method

provides voltage level, duration and rate of occurrence

information for faults on the system. The voltage sag

probability data developed by the proposed technique

can be readily used by facility designers to develop

minimum cost approaches to the design of the ride-

through capability during remote faults.

While the study system is radial with time-overcurrent

protection, the method proposed in this paper is not lim-

ited to either radial systems or to this type of protection

scheme. It can be expected that different types of lines

with differing protection strategies will offer significantly

different voltage sag curves. The method proposed in this

paper will provide consumers with a practical method to

make comparisons between various power distribution

systems.

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