Overview On Power Quality Problems Engineering Essay

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As we have mentioned earlier we will be selecting the best time so that collected data can be characterized in best way and for that purpose we will first analyze the data and look for the best time period. Howell buildings was first taken as our first case study and we noticed it for the period of 9am - 5pm as it is a Commercial building and in particularly on Tuesday, 26th October had the highest loading pattern for the whole week. We used similar technique for Kilmorey Hall as well but since it is a residential building so we have analyzed the data on different time periods for which the analyzer was installed. All the data that we have collected, it was evaluated based on the limits proposed by International Standards (European, IEEE). Result related to different parameters such as overvoltage, Harmonics, active and reactive power consumption etc were analyzed in detail and according to that we need to rectify them by using appropriate correction technique.

Chapter No. 1 Introduction

Overview on Power Quality Problems:

Power is the result of Voltage and Current. The Power network has an ability to control the voltage but it cannot control the current because it varies accordingly to customer's demand. We know that power delivered to commercial and residential buildings around the world are connected to 240 volts (single phase)/415 volts (3 phase) AC supply that is coming from the secondary side of delta/star connected transformer. If we disintegrate the supplied power and observe it on oscilloscope then we can find that the power supplied is a smooth symmetrical sine wave which is varying at 50Hz. As in recent time due the quality of power has reduced because wide range of non-linear load is attached to a network. Recent studies have shown that we are facing two main problems regarding the supply of electricity i.e. Power Quality & Reliability.

Studies around the world have shown that due to poor quality of power, producer and consumer are paying $billion/year. ERPI sponsored studies have shown that US economy losing approximately $104 -$164 Billion/year in terms of outages and another $15-$24 Billion/year because of Power quality issues. Similarly European commerce and industry pay €10 Billion/year due to power quality issues [ [1] ].

Above figure no. 1 is showing the difference of opinion between the customer and Utility Company related to power quality issues. This survey clearly shows that both groups censure that 60% of power quality issues are because of natural events.

Extensive power quality issues are causing blue screens as sudden voltage fluctuation damage the IDE controller of drive. It also causes our systems to hang for a while during normal working [ [2] ]. These computer device although takes very small amount of power but when we are considering an office building such as Howell-Tower A where we have approximately 500 computers then it can cause serious power quality issues to the feeder which is supplying power to the building.

Recent studies in this field have shown that proper monitoring can forestall next problems that could harm of equipments or premature deterioration of the electrical parts such as transformers, breakers and OHL/ Cable. If we have poor power quality then power consumption will also increase due to power losses. In later chapter we'll cover various power quality issues in depth along with their causes and correction techniques and then we are going to analyze our results against them.

Aims and Objectives:

As we have explained earlier that reason for carrying out this research is because we are facing increasingly power quality issues in both commercial and domestic sector. As we have explained above that because of poor quality we are paying huge amount of money either in terms of cash or loss in productivity. To overcome these problems we first have to obtain the data from our desire facility from a particular point in the network from where we can observe all the incomings and outgoing parameters of the network. To analyse the state of our network we will be using Power quality analyzer which will be installed at point common coupling (PCC). Our main aim is to analyse the data obtained from these buildings and if there are any power quality issue in any of the measured parameters then we must look for their rectification techniques. Recommendations made in this project are totally up to owner's own decision whether to install them or not.

Dissertation Outline:

Outline for the Dissertation to achieve our goals is very simple and it is defined as follows;

Chapter No. 1: In this chapter we gave a brief introduction of our project which explains the need of doing this project. It also explains Aim & Objectives related and a brief description of the entire chapters that we have to cover during this dissertation.

Chapter No. 2: In this chapter we will give detail description of all power quality issues that appear in any network. We will cover the different techniques that are commonly used to rectify these individual problems. More over we will be discussing the methodology that is normally followed in these types of projects. This chapter will also give a brief introduction of the different standards that are practiced around the globe and some detailed information through chart of different parameters against which we are going to analyze our results.

Chapter No. 3: In this chapter we will be analysing the parameters statistically which were obtained from these buildings. We will cover two case studies of residential and commercial buildings which are locate in Brunel University. Reason for analysing two building will give us a brief idea of load pattern that is attached with these types of buildings and by doing so we can then provide solutions to same problem that exist in other buildings within the same vicinity. Chapter No. 4: In this chapter we will be give conclusive suggestion and some amendment in the networks depending upon the nature of problem observed in the network.

Chapter No. 2 Literature Survey

Power Quality:

Power quality can be stated as follows;

"Power Quality refers to a wide variety of electromagnetic phenomena that characterize the voltage & current at a given time & at a given location on the power system [ [3] ]."

Type of Power Quality Problem:

It is a common understanding that power quality issues are not only because of voltage variation or harmonics. There are no. of issues that can lead to poor power quality issue such as Harmonics, Voltage or Frequency variation, etc. These power quality issues are divided into various categories depending on their behaviour such as Transients, Small/ Large Voltage disturbances, Waveform distortion, Voltage Fluctuation or Frequency variation. Some of these problems are very intense but they rarely occur in the power network, but some of them are not that dangerous to the network. These problems will be studied with different International standards such as IEEE 1159, EN-50160 and if any anomaly is found, rectification techniques will be suggested to correct them.

Transients

The statue transients have been utilized in the analysis of power system variations for a tenacious period. Transients are sharp and squab continuance disturbances which are caused by a really rapid alter in the steady-state of voltage or current that can be either Unidirectional or bi-directional in polarity. Another word that is commonly used in Power network is surge. As a power network engineer we consider that these are caused by lightning strike for which we install surge arrestor in our network for protection [ [4] ]. Transients are single spikes or oscillatory having high magnitude for a very short period of time typically 50 ns-1 ms [ [5] ].

These terms are further classified as follows which also reflects their shape of Current or Voltage transients;

Impulsive:

An Impusive transients are an abrupt high peak events in the steady state condition of voltage/current which is uni-directional in either polarity. Impulsive transients itself is categorized depending on the speed at which they occur in the network and they are mentioned in the following table [1];

Impulsive transients are normally defined by their rise-decay times, which is easily visible from its spectrum analysis. Consider an example of an Impulsive transient (1.2 * 50) µs 2000-volt (V), transient nominally rises from zero to its extreme value of 2000 V in just 1.2 µs and then decay to half of its peak value in 50 µs [5].

Mostly these impulsive transient occurs in network from very close vicinity such as lightning strike[ [6] ], switching of inductive loads, Utility fault clearing or an Electrostatic discharge which enter into the power network thus interrupting smooth supply of power. Best solution to overcome this issue is to use Surge protective device (SPD) or Transient Voltage surge Suppressors (TVSS) [ [7] ] [ [8] ].

Oscillatory:

Figure : Oscillatory TransientAn Oscillatory transients is similar to Impulsive transients but it occurs in positive and negative polarity values as shown in figure no. 3. Oscillatory transients itself is categorized depending on the frequency at which they occur in the network and they are mentioned in the following table [2];

An oscillatory transient occurs when we disconnect any inductive/capacitive loads in form of Motor/ Capacitor bank and in result it resists the change. Common solution to overcome this problem is to add line reactors/ chokes that reduce these transients to manageable intensity [7].

Small Voltage Disturbance

This category of power quality issue comprises of voltage interruption, Sags and swells. These short terms variation mostly defined on their waveform and their time [6]. These small voltage variations occur because of faults at the utility company/customers end, starting of large motor Loads by the customer on the disturbed network, faults on other lines of the power network [ [9] ].

Primary source of sag in the network were studied very carefully and found out that they occur because of short circuit at any point in power network. These Short circuit causes abrupt increase in the current which can cause severe voltage drop in the impedance of power network [ [10] ]. Most of the faults are rapidly cleared, we can have much better response time for transmission lines (60-150 ms), but the fault clearing on distribution networks could be slower such as MV side is 0.5-2s and LV side is dependent on the characteristics of fuse [ [11] ].

Sag

Sag (Dip as defined by IEC) is sudden decrease in rms voltages/currents between 0.1-0.9 p.u for duration of 0.5 cycle -1 min. Figure 4 shows typical waveform of sag.

In area of power network, Sag which is not formally defined but it is used to describe short-duration voltage decrease for very long time which is recognized and utilized by utilities, equipment manufacturing companies, and end users [5]. There are various reasons which are connected Voltage sags such as faults in network or it is caused by high starting torque motors which are used in Lifts. Studies have shown that 80% of sag exists for about 3 cycles until the substation breaker energize and the fault clearing time is usually reliant on the level of fault and type of protection provided which can vary from 3 to 30 cycles. If the current level is large, relative to the existing fault current in the system at that point, the resulting voltage sag can be significant [6]. Common problems that are observe because of Sag are equipment tripping, dimming of light, failure of IDE controller of drives. We can correct this issue by using power conditioners, regulators or UPS systems across our devices.

Swell

Swell is sudden increase in rms Voltage/current between 1.1 p.u - 1.8 p.u at the power frequency for durations from 0.5 cycles to 1 min. This disturbance is also described by IEEE C62.41-1991[ [12] ].

Similar to Sags, Swells are also related with system fault conditions in the power network, but they are not as frequent as voltage sags. One way that a swell can occur is from the temporary voltage rise on the unfaulted phases during an SLG (Single line to ground) fault [5]. Swells can appear when we switch off a large load Motor or a large capacitor bank is energized. Similar problem of sensitive equipment failure, loss of data, and increase in the intensity of light are observed. Solutions are likely to be similar to sag which includes power conditioners, ferroresonant "Control" Transformers or use of Uninterrupted power supply (UPS) [8].

Interruption

Interruption occurs when suddenly the voltage supply falls below 0.1p.u for less than 1 Minuit. The interruptions are considered by their period since the voltage magnitude is always less than 10% of nominal. Typically interruptions are observed when we have system faults on utility protective devices when they fail to reclose and these protective devices cause momentary or temporary interruption of supply [5].

Following table is showing the typical types of short duration voltage problem depending on the magnitude and their duration length [5].

Large Voltage Disturbance

Long voltage disturbance are also categories depending upon their nature and typical duration typically greater than 1 Minuit. IEEE defined them under categories of Overvoltage, Undervoltage and sustained interruption. Overvoltage and under voltage doesn't appear because of the faults but because of continuous load variation on the network [5]. Typical categories, duration and voltage magnitude is mentioned in table no 4.

Undervoltage:

Undervoltage defined according to IEEE 1159 that when voltage level drops to less than 0.9p.u at the power frequency for duration longer than 1 Minuit. This problem causes over heating in equipment as equipment starts to draw more current which can cause excessive wear of its parts [6]. Undervoltage is a constant problem caused by a number of factors beyond the end user's control. Utility companies try to maintain voltage levels delivered to customers at ±5%. However, natural factors like weather, irregular high demand can cause the utility voltage to fall within a permissible ±10% range [ [13] ]. The term brownout is often used to describe sustained periods of undervoltage initiated as a particular strategy by utility dispatch company to reduce power demand [5]. Normally we use undervoltage lockout (UVLO) device to overcome this problem as these protection system simply disconnect the equipment from main supply if the voltage drops from a certain level [ [14] ].

Overvoltage:

Overvoltage is defined by the IEC 60071-1 that any voltage including one phase conductor and earth or between phase conductors having a peak value higher than the related peak of the highest voltage for equipment [ [15] ]. When an overvoltage appears, the rms voltage increased to 1.1-1.2p.u.

This phenomenon occurs when suddenly a very large load is disconnected or we turn on capacitor bank for reactive compensation in the network. Similarly we have overvoltage when we have wrong tap settings for the transformers [6]. Overvoltage can have internal and external origin. In internal origin they can be generated mainly because of unexpected loads reduction, self-excited generators, and faults on line. Similarly external origin can take place because of atmospheric activity such as electrostatic or electromagnetic induction of lines [12].

Overvoltage is dangerous from sensitive equipments as it causes overheating which can damage the equipment at large, tripping of protective devices and interruption in the operation of equipment every time the fault appears across the network [12]. To protect our system and equipment from overvoltage, we can use either preventive protections (protection relays, Switching impulse limiting Circuit breakers) or repressive protections (dischargers, Surge arresters) devices.

Sustained Interruption:

Sustained interruption is defined as Outage in IEEE standard 100-1992 which is a caused by the failure of any component in the network. Voltage interruption over 1 Minuit is considered as sustained interruption. These interruptions could be natural or planned outage for the maintenance of network. These interruptions require manual restoration so power is supplied to customers [6].

We can use local stand-by generator or UPS to overcome this problem.

Categories

Typical Duration

Typical Voltage Magnitude

Sustained interruption

Greater than 1 Min

0.0p.u

Under voltage

Greater than 1 Min

0.8-0.9p.u

Over voltage

Greater than 1 Min

1.1-1.2p.u

Table : Categories for Long duration disturbance [5]

Waveform Distortion

DC offset:

DC offset is often introduced into the AC distribution system upon the failure of a rectifier or AC/DC power supply. These failures can cause DC to traverse the AC line and add unwanted current to our online devices. Overheating and saturation of transformers can also occur, creating additional waveform distortion and instability.

Noise and Notching:

Noise is an unwanted voltage or current that is superimposed on the network voltage or current waveform. Noise can be generated by power electronic devices, arc welders, switching power supplies etc. Poor grounded sites make the systems more susceptible to noise and can cause data errors, hard disk and component failures, as well as video display distortion. Notching is a periodic voltage disturbance which appears in the network when we use equipment such as variable speed drives, light dimmers or arc welders under normal operation. Usual consequences are system halts, data losses and data transmission errors.

Harmonic distortion:

Harmonic distortion is the corruption of the fundamental sine wave at frequencies that are multiples of the fundamental. (e.g., 150Hz is the 3rd harmonic of a 50Hz fundamental frequency; 3x50=150). Examples of non-linear loads are:

Industrial equipment (welding equipments, arc & induction furnaces, rectifiers etc.).

Variable-speed drives for asynchronous or DC motors.

Uninterruptible Power Supply.

Office equipment (computers, photocopy machines, fax machines, etc.).

Home appliances (television sets, micro-wave ovens, fluorescent lighting).

Certain devices involving magnetic saturation (transformers)

The increase in losses reduce the capacity of the system, including conductors, transformers, and motors. The increased loading generates heat and accelerates the aging of power equipment, like transformers and motors. Other cost of impacts is harmonics which includes noise and vibration, reduction in motor torque, decreased power factor, decreased performance of television sets and relays, and inaccurate readings from induction watt-hour meters. The increased loading from harmonic currents also accelerates the aging of utility transformers and generators. In fact, utilities typically derange their transformers and generators up to 25% because of the additional heating from harmonics. Some utilities are setting harmonic limits for their customers based on IEEE Standard 519-1992[ [16] ].

Voltage Fluctuation

Voltage fluctuation is defined as a rapid change in voltage within the allowable limits of voltage magnitude of 0.95-1.05 p.u of nominal voltage [6]. A waveform may show voltage flicker if its waveform magnitude is modulated at extremely low frequencies i.e. less than 25 Hz, which the human eye can notice light flicker if the frequency in the range of 6-8Hz [6]. Devices like electric arc furnaces and welders can create continuous and rapid changes in load current which is the source of voltage fluctuations. Voltage fluctuations shown in figure no. 11 can cause fluorescent lights to flicker rapidly. This blinking of lights is often referred to as "flicker" [ [17] ]. It can cause people to suffer headaches and cause mental stressed. It can also cause sensitive equipment to breakdown. The measurement technique suggests that the lamp-eye-brain creates a transfer function and produces a fundamental unit called short-term flicker sensation (Pst) and the other measure is known as long-term flicker sensation (Plt) [5]. A typical solution to this problem involves the use of costly but effective static VAR controllers (SVCs) that direct the voltage fluctuation frequency by calculating the amount of reactive power being delivered to the arc furnace [ [18] ].

Frequency Variation

Power frequency variation is defined as the deviation of power system's fundamental frequency from it given nominal value i.e. 50 Hz which is defined in EN50160/2006. This frequency is directly proportional to the rotational speed of the generator installed in the network. At any instant the frequency is dependent on the balance between the load and the capacity of the available generation. When this dynamic balance changes, small changes in frequency occur. The size of the frequency change and its duration is dependent on the load and how the generation system responds to the variation in demand. Under normal operating conditions, the mean value of the fundamental frequency measured over 10 seconds stays within the following range for interconnected systems having synchronous connection [12]:

50Hz ±1% (i.e. 49.5-50.5 Hz) for 99_5% of the year.

50Hz +4% - 6% (i.e. 47-52 Hz) for 100% of the time.

Frequency variations usually occurs when there are faults on the bulk power transmission system, a large load disconnects or a large source of generation going off-line. Frequency variations that affect the operation of rotating machinery are very rare in modern interconnected power systems. Frequency variations occur when such equipment is powered by an island generator which is isolated from the utility system. In such cases, governor reaction towards an unexpected load changes may not be enough to regulate within the narrow bandwidth for those equipment which are very sensitive to frequency variations [6].

Power Quality Evaluation procedure:

Power quality problems include a wide range of diverse event, as defined in preceding topic. Each of these events may have a range of diverse reasons and diverse way outs that can be exercised to improve the power quality and equipment operation. However, it is handy to look at the common steps that are correlated with examining many of these problems, particularly if the utility supply system and the customer facility are included. Figure no. 13; below furnish some universal steps that are often obligatory in a power quality analysis, along with the major concerns that must be tackled at each step.

The common practice must also consider whether the evaluation involves an old power quality problem or one that could be an outcome from a new plan or from recommended modifications to the system. Measurements will play a significant part for more or less any power quality concern. This is the most important method of distinguishing the problem before the existing system that is being assessed. When carrying out the measurements, it is essential to record consequences of the power quality differences at the same time so that problems can be compared with likely reasons. Results should be assessed using system's point of view, and both the cost-effective and the technical constraints should be examined. Possible way outs are categorized at all points of the system from utility supply to end users equipment. The optimal solution is dependent on the nature of problem, the number of end users being affected, and the possible way outs. Figure 14; below give a graphical interpretation of how the responsibility between the number of disturbance and location of disturbance [5] [18].

Power Quality Standards:

Power quality standards are needed in the power quality industry because they must know how they can transfer power to their customers within certain levels. Similarly equipment manufacturing companies should have certain rules so that their equipment couldn't pollute the network with various problems. The power quality industry recognizes that power quality standards are critical to the viability of the industry. So, stakeholders in the power quality industry have laid down several power quality standards in recent years. They understand that the increased use of sensitive equipment, increased application of nonlinear devices to improve energy efficiency, the advent of deregulation and the increasingly complex and interconnected power system all contribute to the need for power quality standards. These Standards lay down voltage and current limits that sensitive equipment can accept from electrical disturbances. Utilities require standards that set limits on different parameter of their power systems, so it can bear harmonics generated by their customers having nonlinear loads. [18] [ [19] ].

Power Quality Standards Organizations:

The organizations responsible for expanding power quality standards in the United States are IEEE (Institute of Electrical and Electronics Engineers) and ANSI (American National Standards Institute). Other than United States, the organization which is responsible for international power quality standards is IEC (International Electro technical Commission) [18].

Institute of Electrical and Electronic Engineers (IEEE)

The IEEE was founded in 1963 from two organizations: the American Institute of Electrical Engineers (AIEE) and the Institute of Radio Engineers (IRE). It has contributed in the improvement of electrical industry standards of all kinds, including power quality. Its members mainly focus on resolving particular power quality problems. IEEE power quality standards deal mainly with the power quality issues at the point of common coupling (the point where the utility ties to its end user) [18].

American National Standards Institute (ANSI)

This Institute does not develop standards, but assists standards development by qualified groups, like the IEEE. It is the only United States representative and it two major international standards organizations are ISO (International Organization for Standardization) and IEC (International Electro technical Commission).

International Elect technical Commission (IEC)

The beginning of the IEC took place in 1890 at the Electrical Exposition Conference. IEC power quality standards groups are feared mostly about standards that will improve international trade. IEC refer to power quality standards as electromagnetic compatibility (EMC) standards which shows that IEC's main apprehension is the accordance of end-user equipment with the utility's electrical supply system. The IEC has accepted many EMC standards which are replica of IEEE standard and which has created confusion in the power quality industry. As a result, some experts have tried to correlate the IEC with the IEEE standards. Meanwhile, the user of power quality standards have become familiar with both standards and worked out that which standard will meet their requirements [18] [ [20] ].

EN 50160 give a brief idea about the main parameter i.e. voltage, Flickers, inter-harmonics and their permissible deviation ranges at the customer's point of common coupling in public low voltage (LV) and medium voltage (MV) electricity distribution systems, under normal operating conditions. In this context, LV means that the phase to phase nominal rms voltage must not exceed 1000 V and MV means that the phase-to-phase nominal rms value should be in 1 kV - 35 kV. As we know that EN50160 deals with the supply voltage and gives only general limits, while the EMC standards deals with the utility voltage, according to IEC-038. It is because these voltages are due to voltage drops in the installation where interruptions originate from the network. Because of this, in various standards of the EN 61000 series equipment current is the main factor, while the load current is not important to EN 50160. Following table give the brief comparison between EN50160 and EN61000 series;

Power Quality Correction Techniques:

Often the Utilities act as a agent for transmitting harmonics, transients, Long and short term voltage sag, or flicker from one place to another. In this condition, it is not realistic to move the sensitive equipment to a place that is not connected to the system transmitting the power quality problem. To overcome this problem, a survey is required for the site which includes power quality monitoring as well, to determine the source and how it is spreading in the network. One of the most convenient methods to overcome this issue is the installation of power conditioning equipment. Following figure no. 15 gives a very brief idea how disturbance penetrate in the network and where the power conditioning barriers should be installed [18].

Power conditioning generally related with voltage conditioning because most poor quality issues are related to voltage quality problems. The majority of voltage conditioning devices condition or adjust the voltage magnitude or frequency. They usually reduce the transient effect and keep the voltages in steady condition or keep sensitive equipment apart from the interruption. For example, surge suppressors helps in limiting the transient voltage amplitude, and regulators keep the voltage within specified nominal voltage (Dugan, 1996).

In spite of various advantages such as providing isolation from the disturbance, they have certain drawback as well for providing only inadequate amount of energy for an inadequate time. Power conditioning equipment is sometimes introduced as mitigation equipment. The equipment can be divided into following categories [18] [ [21] ].

Surge suppressors

Surge suppressors are used to provide shield to the sensitive equipment from voltage surges or lightning strikes on the power system. They are act as shock absorbers on power systems. There location on the network determines their role for example on utility side of the meter; they act as surge or lightning arresters. If they are installed on the end-user side they act as TVSS (transient voltage surge suppressors). They protect the equipment by diverting transients caused by different element to ground. Utilities identify the points in the network and list down the equipment they wish to protect, like transformers and substation equipment. Similarly end-users identify the point within their facility and then install TVSSs between the power outlet and sensitive equipment, for example computers, Fridge and freezer or at the main power Distribution Board (DB) (Dugan, 1996).

Noise filters

Noise originates from transient surges caused by lightning or switching on the utility power system. It also can come from the operation of motors, laser printers, transformers or any loose connections. The basic reason of having filters is to prevent these unnecessary frequencies from entering sensitive equipment. This is done by using different variation in the arrangement of inductors and capacitors. Inductors produce impedance that reduces fairly to the magnitude of the frequency. Reason for using inductors and capacitors in a range of arrangements is to reduce and diverts voltages and currents of various frequencies. Mainly Noise filters are considered to be low-pass filters. Inductors in noise filter only allows power having fundamental frequency of 50Hz. Similarly capacitor which is in parallel with the inductors redirects the high frequencies to ground.

Isolation transformers

Isolation transformer is a very common power-conditioning device. The basic advantage of having them in the network is not only protecting sensitive equipment from transients but also restrict harmonics from entering in the network which is generated by end-user's nonlinear equipment. Shielded isolation transformers are frequently used in combination with surge suppressors. They do not normalize the voltage or save equipment from voltage sags.

Low-voltage line reactors

Line-voltage regulators are specially designed transformers that regulate the output voltage constant when the input voltage changes. The basic principle of working of reactor is similar to the transformer that is used on the utility's transmission and distribution system to prevent long-duration voltage disturbances. If we want to transmit power over long distances by keeping economical factor under consideration then Power transformers are used for this purpose. Voltage regulators are installed locally so they can make some small adjustments in voltage to maintain the voltage constant. Typical types of line reactors are tap changers, constant-voltage transformers (CVTs) or buck-boost regulators to keep the voltage constant.

Static VAR compensators (SVCs)

Static VAR compensator is another type of power conditioning device which use an arrangement of capacitors and reactors to control the voltage rapidly. They are preferred over old-style synchronous condensers which were too expensive to operate, and maintain. Utilities are using SVCs on their system at high-voltage side which maintains the voltage from sagging when the fault appears on the network. SVCs are also used to tackle flicker problems which are caused by electric arc furnaces. These variations caused by electric arcs produce fluctuation in the line voltage. Normally Industrial plants use SVCs to decrease voltage flicker (Baggini, 2008 pg- 156).

Uninterruptible power supplies (UPSs)

UPS is also considered as power conditioning device. It maintains the voltage by supplying a constant voltage even for the period of a voltage dip (sag) or any interruption/outage. It supplies a constant voltage using static or rotary source. In UPS units we have a battery which is continuously charged by the main source of power and when there is an interruption, power is supplied through the battery. UPS contains essential components which can be connected in different configurations: on line, off line, and line interactive. Mainly a UPS system consists of the battery, an inverter and a rectifier. The battery can be lead acid; inverter is a solid-state device such as thyristors that convert dc to ac.

Harmonic filters

Harmonic filters are used by the utility companies on their distribution networks, while end users use them in their facilities so the harmonic don't make their electrical equipment to overheat which causes pre-mature aging. Harmonic filters work on the principle that inductors and when the capacitors are arranged together, it will either block harmonics or shunt them to ground. There are many types of harmonic filter (Baggini, 2008. pg- 204,236).

Passive filters

Passive harmonic filters use static inductors and capacitors. Static inductors and capacitors don't vary their inductance and capacitance values. They are designed to handle particular harmonics and don't respond to frequency changes. They are often connected to electrical devices that cause harmonics, such as variable-speed drives and fluorescent lights. Harmonic filters sometimes are referred to as traps or chokes [12].

Active filters

Active harmonic filters are also known as active power line conditioners (APLCs). They differ from passive filters in a way that they condition the harmonic currents rather than blocking or diverting them. Active harmonic filters use bridge inverters and rectifiers to monitor the harmonic currents and generate counter harmonic currents to cancel out the harmonics generated by the load. They also control sags and swells by reducing the voltage harmonics [12]. Following table gives a brief power conditioning methods;

Power Consumption in buildings

In this section we will discuss briefly regarding the type of loads attached to buildings such as residential, commercial, and institutional buildings. Various case studies have shown that the building sector is the main energy consumer between the three energy- consuming sectors: transportation, industry and buildings. Overall energy demand in the building sector has been swelling at an average rate of 3.5% per year since 1970 (DOE, 2006). Form different studies we have understood that Urban buildings typically have higher levels of energy consumption than buildings in rural areas.

We are consuming energy in our buildings in different purposes such as space heating, water heating, lighting, kitchen appliances, and office equipments. From statistical data of different cases studies we have figured out the lighting is the major source of energy consumption (30%-40%) in commercial buildings ahead of any other purpose, but this is quiet opposite in case of residential buildings where lighting energy consumption is less than that of space heating/cooling and water heating in residential buildings. Different studies have proved that heating is the major energy consumer in the Europe domestic and commercial building sectors followed by lighting. Other main consumers are cooking appliances. The IEA (IEA 2006) estimated that 1133 TWh of electricity was consumed in the world by commercial lighting in 2005 which was 19% of total generated electricity (IEA 2006) [17].

Mainly we differentiate our loads in Passive and Active. In Active loads the amount of power consumed is delivered in response to the load demand. Commonly this category includes any type of electronic equipment which has switch-mode power supplies. In residential or commercial building we have loads such as computers, printers, fax machines, TV etc. In these types of loads, the main factor we have is the power consumption. In electronic equipment we have a very strict relation between the voltage and efficiency of equipment because slight changes in the consumption can damage the sensitive components such as capacitors, ICs or even transformers where losses will rapidly increase on slight changes in the voltage.

Similarly as we know that passive loads are resistive loads. In this type of load the amount of power consumed (for a constant value of load) is directly proportional to supply voltage (Vs). So an increase of 10% in a supply voltage of 230V will result in an increase of power consumption of 21% [ [22] ], even where resistance changes or load is partly inductive or capacitive, this is still generally true. Examples are boilers, cookers, heaters, incandescent lights and kettles etc.

Power Factor

Electricity is simply described as distributing power from one point to another. This relation is for delivering power for direct current. The condition becomes more difficult when alternating current is used to transfer power as the instantaneous value of voltage / current continually changes. Power delivered is given in terms of Watts, and given by the following equation [ [23] ];

Pdc=EI

Power factor relates the active power P [Watts (W)] to the apparent power S [volt-ampere (VA)]. Similarly electric components of the utility distribution systems are designed on a kVA basis; more over their design ensure that they can carry rated current at a certain voltage without excessive temperature increase. As we know that active power does useful work and it works oppositely in case of reactive and harmonic powers. This increase system losses and absorb system capacity, but they are necessary to provide magnetic fields or nonlinear currents. Apparent power limits the capacity of our electrical system. Power factor is quantity that describes how efficiently our electrical distribution system is working. When the value of power factor is less than unity it means that full system capacity is not available for useful work. Power factor also depends on the type of load connected to the network [ [24] ].

Linear Loads

Electric Loads that draw sinusoidal currents from the power system, i.e. the current wave shape is similar to voltage wave shape, are identified as linear loads. Earlier we had relatively higher percentage of linear loads. Typically equipments which are considered to be linear loads are induction motors, incandescent lighting; and heating elements. A linear load consumes AC electric power directly to complete its function. Following figure explains the relationship between Phasor (S), Active (P), and Reactive (Q) power.

Instantaneous voltage and current are represented in terms of vt and it then;

v (t) = Vmax Sin (t) and i(t) = Imax * Sin(t+θ)

Then Instantaneous power (p) can be defined as follows;

p = v(t) * i(t) = Vmax * Imax * Sin ( t) Sin ( t + θ)

Therefore the Average power is given by;

P = Vmax * Imax * cos (θ)

P = Vrms * Irms * cos (θ)

Power factor (pf) in linear systems is defined by cos (θ)

Apparent power (KVA) S = Vrms * Irms

Active power (KW) P = S * Cos (θ)

Reactive power (KVAR) Q = S * Sin (θ)

Nonlinear Loads

We can define nonlinear loads which draw AC electricity power directly and often they convert AC power into DC current using some kind of rectifier before they use it to perform their work. In these loads the waveform of current differs from the wave form of voltage. As we know that most of the manufacturing companies are focusing on energy efficient equipment so the use of non-liner loads have relatively increased. Mainly non-linear load includes computers or computer controlled equipment, temperature-controlled furnaces and heating elements and adjustable-speed motor drives. Following equations gives relation of Active, Apparent and Reactive power;

S = P2+Q2+D2)

Where;

S = Apparent power in volt-amperes (KVA)

P = Active power in (KW)

Q = Reactive power in volt-amperes (KVAR)

D = Distortion power in (KVA)

Above figure is often called a power triangle, which shows the relationships between the three types of power defined above. Reactive power is orthogonal to active power and is shown as positive for lagging current [19].

Power Factor = =

From above equation we can see that PF is a ratio, the maximum value is 1 and if the value is less than 1 then it is understood that the active power is less than the apparent power. Linear loads can have PF around 0.98 while non-linear load's PF can be as low as 0.65 because of power is consumed by frequencies except fundamental frequency.

Effects of Power Quality Problems

The effects of power quality problems are countless and most power quality problems apparent on an end-user's electrical equipment. These signs include overheating of motor, tripping of adjustable speed drives, computer shutdown. The effects of power quality problems can be best understood by looking at the various types of loads that are affected by power quality problems. Most power quality problems on computers are caused by voltage variations such as by voltage sags and outages causing the electronic timer to shut down. Telephones will experience noise induced by adjacent electrical equipment. The frequent shutdown of an adjustable-speed drive is usually an indication of excessive harmonics and manufacturing facilities experience regular shutdowns due to voltage sags.

Methods of Power Factor Correction

There are no. of power factor correction techniques, each of them have their own advantages and disadvantages [ [25] ].

Increased system capability and capacity.

Reduced losses in energy distribution.

Improved voltage due to reduced line losses.

Large overheads may be avoided due to increased capacity.

Reduced operating costs under constant load.

Static Correction

In static correction, capacitors are directly connected with the load such as induction machine. Reason for doing so is to save the cost of installing switching equipment that simply switched in/out the capacitor as the machine is switched on/off [27].

Switched Capacitor Banks

In this process we install switching capacitor banks between the electrical bus and neutral to provide the reactive power compensation. The bank automatically adjusts them according to best possible power factor. This switching gear is very expensive because of its nature. In this process we must include those reactors which are not properly tuned to avoid or reduce harmonics [27].

Synchronous Condensers

Synchronous condensers are large synchronous machines which are not connected to any load. They provide power factor correction of large industrial plants when operated either under or overexciting condition to deliver or absorb VAR's [27].

Chapter No. 3: Case Studies on Power Quality & Energy Management.

In previous chapter we have discussed different power quality problems and its reasons in detail. In this chapter we are going to analyse the data which we have obtained from these buildings. The data we are going to analyse is from commercial and residential buildings and it is obtained from the PCC (Point of common coupling). Commercial building includes the office building i.e. Howell building and Tower 'A' which is more an academic building consisting of Labs and different types of lecture theatres. Similarly the residential building includes a Kilmorey Halls where student are living as university has provided them on campus accommodation. The power quality analyser was installed in these building's PCC for about 5 working days and one week with resolution of 8 and 17 minutes respectively.

Before going in to the details and analysing the data we must define that which type of load is attached with these particular buildings. In general when it comes to offices and academics building we have non- linear load which is the main source of power quality issues in the network in terms of generating harmonics. Non-Linear type of loads majorly consists of computers, printers, photocopiers, and other small equipment which most of them are single phase. In commercial building we are analysing will have around 250 or more computer. This huge no. of computer can cause two significant problems as a result of harmonics. First of them is the voltage variations that is generated because of the finite impedance of Lines and second is the overheating of neutral line [ [26] ]. We have to eliminate this problem because this problem will not only disturb the building itself but also those buildings which are located nearby. Now it is a very common practice that we always install neutral lines almost 1.5 times of the ones we are using for the current carrying lines in each phase. Similarly when we are considering the residential building, we have more no. of non- linear loads in form of computers, TVs, Kitchen appliance (Heaters, Boilers, Electric stoves etc.) More over there are more no. of lightening (CFL) as compare to commercial buildings.

As we have mentioned different governing bodies in chapter 2, which have established power quality standard. These standards have limits in which both power supplier and customers should remain to ensure smooth and constant supply of power. We will be analysing and comparing results in accordance with EN-50160 such as measurements of line - phase voltage, line- phase currents, real and reactive power consumption, power factor, flickering and total harmonics distortion.

Commercial Buildings (Tower 'A'-Howell building) at Brunel University:

In this part of the chapter we are going to analyse the results we have obtained from the PCC of Tower A and Howell building. These result will be analyse according to EN-50160.

Frequency:

As we know that frequency of any power network is dependent on the load profile that is attached with a network. Frequency will vary if there is surplus or deficit in generation in respect of demand. If the frequency of any network is changing too much and exceeding the limits defined in the EN-50160 then system may cripple or there will be serious problems in power quality.

From above graphical representation, we can observe that frequency is varying because of change in load which is normal. Here we must note that frequency variation is well in range of ±1% (49.5 Hz-50.5 Hz) as described in the EN-50160. The graph also represents that the pattern is almost same throughout the time scale which confirms that there is no serious power quality problems because of this factor.

Current waveform:

In this task we have to analyse the waveform of the current from all three phases and neutral line. This will give us an idea that how and when the maximum current is drawn from the network. Moreover by using the data of unbalance we can check that if the percentage of unbalance is high then we must equally distributed the load in each phase to avoid any power quality issue.

Here Figure no. 21 is showing the load current for 3 days and it is clear from the figure that we have a same pattern of load for given time period. In particular we observe that load current and unbalance current increases typically from early morning i.e. 8:00 Am and maintain this profile till late afternoon i.e. 4:00 Pm. As we all know Howell building is office building and the defined time is office timing, so we can expect this pattern of load. Figure 21 also show that the load is not equally distributed because most of the load is single phase. This considers as poor practice that we overload one phase and at the same time other phase is lightly loaded. Due to this we have high current in neutral which can not only increase the losses but possible chances of overheating the equipments. The current in different phases varies from 100A to maximum value of 450A during different time scales.

In figure no. 22, we have particularly taken readings from 26th October as it is considered to be the busiest day due to load current. The maximum current is observed Maximum current at 2:35 Pm in A1 i.e. 472 Amps and similarly minimum current was observed at Midnight i.e. 126Amps in A2. Similarly neutral current was also very high during this day and we observed maximum reading of 111Amps. Reason of high neutral current is unbalance loading of phases and mostly the loads attached with this building is non-linear Load in form of large no. of switch mode power supplies are used in different equipments for example computers, printers, speed drive which are installed mechanical labs lathe machines etc. Because of these machines we have unbalances and voltage distortion. We have also noticed that during night time we are still having considerable neutral current which is because of this non-linear load. It is common observation that most of the staff use to of leaving their computers on sleep mode which still draws current to keep the important parts of computer running so it can restart within no time. Following figure No. 22 gives an overview of the typical day of the Howell building for 24 hours. As we can see that load started to rises from 9:00 am in the morning and by 9:30 am the load reached it maximum value. Usually most of the staff of the university comes into their offices and turn on their PC and other printing devices. At the same time we can see that neutral current started to rise at the same time. We can see that around 2:35pm, we observed the peak load current of 472Amps. Similarly as the office closing time i.e. 5:00 pm, we can see that load current starts to drop but still load current is around 250Amps because most of the PhDs staff keeps on working outside office hours till late night, moreover at night some of the lighting is necessary to maintain enough light so people working during night time don't face any difficulty.

Similarly in Figure no. 23, we can get an idea of unbalance between the 3 phases which shows that load is not equally distributed among them.

The current which is introduce by the harmonics in the system in neutral can be more than 100% of phase current and few case studies have shown presence of neutral current between 150%-200% because of high 3rd harmonics current.

In order to avoid the effects of 3rd order harmonics we must put them in groups and regulations should be imposed on their harmonics level with special restriction on negative and 3rd harmonics.

Line Voltage waveform analysis:

Voltage profile is always considered one of the most important areas in power quality analysis. As we have already mentioned above that we are following EN-50160 standard, and it is stated that variation in Low and medium voltages levels must be with in ±10% for 95% of the week. We must also be sure that we are not always providing voltage near to limits which is not good for customer's equipment.

Figure no. 24 above is showing the voltage profile for 5 working days of Howell building. This voltage profile is showing that voltage is within permissible range as the minimum voltage of 425.7 Volts was observed on 25th October U2. Similarly the maximum voltage of 445.5 Volts was observed on 27th October on U3.

During our analysis of current waveform, we observed that we usually have high load demand from 9:00 am - 5:00 pm. In figure 25 below we have observed that line voltage keeps on fluctuating between 438 - 445 Volts during high demand condition (circled), but the voltages remained in considerable range ±10% which is defined in EN-50160.

Phase Voltage waveform analysis:

After considering the Line voltages we must also analyse the phase voltage. Following figure no. 26 gives us a brief idea of voltage level for 5 days. As we know that as we are following EN50160, it states that voltage should be in ±10% i.e. 216-264 volts if we assume the nominal voltage of 240 volts. From the figure no. 26 below we can see that voltage is varying from 245V to 257.5 V which is within range of permissible percentage.

Here we must observe in figure no. 27 which is voltage trend for 26th of October that the voltage is not dropping beyond 245.7 V which is still above than nominal voltage. It is a common understanding in electrical that power delivered to any load is govern by the following equation;

p = v(t) * i(t) = Vmax * Imax * Sin ( t) Sin ( t + θ)

Here it is clear that if we supply excessive voltage to our equipment, it will consume more power which in return will overheat the instrument. This increase the probability that lifespan of different component in the instrument will decrease which in result causing it to mal function or may damage it completely. Sending voltages above than the nominal voltage is deliberately practiced by the Distribution Network Operator (DNOs). As we know that if the voltage is increase than the current decreases which decreases the I2R losses in the lines.

Here in figure no. 28, we have voltage unbalance between the 3 phases. It is clear from the figure that unbalance is not more than 0.3% which is well within range defined in EN-50160 of 2% for 95% of the week in both LV and MV supply voltage.

As we have also discussed in chapter 2, that flicker are also caused by the rapid changes in voltages. They usually occur because of the Intermitted loads, which tends to operate for a very short period of time. These loads can be of any type such as lifts, heavy equipment machines (Installed in Labs of tower A). When these loads operate we suddenly get rapid change in voltage and the same is observed in our data. As figure no. 29 & 30 shows that the magnitude of flickers is less than 0.4% for Pst and less than 0.3% of Plt which is well within range described in EN-50160 & EN-61000-2-2 which is Pst≤1.0 and Plt≤0.8.

Real & Reactive Power analysis:

As we have explained in chapter 2 about real and reactive power in detail so here we have analysed the data obtained from this building. We would like to mention that loads attached to the building are not resistive loads which mean that load will have capacitive and inductive component as well for example SMPS( Switch mode power supplies) used for the PC. Due to this we will have all components in of power in our result but we are considering real power consumption. After analysing the data which has span of 5 day we found out that it has moreover similar pattern of power consumption. Power consumption on 26th October, was considered to study the behaviour of power consumption. It was also observed that on this day, the power consumption was on a higher side as compare to other days. Figure no. 31, shows typical power consumption of a working day where we can see that most of the power is consumed during the office timings i.e. 9:30am - 3:00pm. The power consumption in evening and early morning is because of the lighting and the PhD staffs works late in their offices and when they leave the place most of them leave their computers on sleep mode which still consumes power. As we have discussed during current waveform analysis that Phase 1 is slightly more loaded than other 2 phases, so it is visible by the power consumption in Phase 1.

For same we have also analysed the reactive power consumption is very well balanced throughout the day as shown in the figure no.32. From this we have also noticed that during night time after 10:30pm reactive power is negative which shows that capacitor installed were over compensating for inductive loads.

Power Factor analysis:

As we have explained in chapter 2 that power factor plays an important role in power generation. If our power factor is not near to unity, then we are wasting some of the generated energy. Following figure no. 33 is giving us a brief idea of power factor in each phase.

In figure 31, we can see that power factor of phase 1 & 3 are much stable as compare to phase 2 which has really poor power factor. There may be several reasons behind this for example; most of non-linear load is attached to this particular phase which is causing harmonics which is affecting the power factor of this phase. To overcome this problem we must divide the load equally or install the power factor correction plant for this particular phase so that we can keep the power factor near to unity.

Total Harmonics Distortion analysis:

As we have already explained about the Harmonic distortion in chapter 2, here we are going to analyse the data obtained from this building. Mostly we observe harmonics up to 40th level but during analysing the result we pay more attention to the first few harmonics i.e. 3rd, 5th, 7th which can put significant effect on the quality of power if not controlled. As we know that Non-Linear load is the source of harmonic current which causes distorted current waveform and when this current passes the system it causes the voltage distortion.

As power system engineers', we know that harmonics in our system can pose serious threat to the quality of power. Some of the common problems that can occur in the system are as follows;

The simultaneous use of capacitive and inductive devices in distribution networks results in parallel or series resonance manifested by very high or very low impedance values respectively. The variations in impedance modify the current and voltage in the distribution network.

The active power transmitted to a load is a function of the fundamental component I1 of the current. When the current drawn by the load contains harmonics, the rms value of the current, Irms, is greater than the fundamental I1.

Derating of the generators by 10% if we have non-linear load up to 30%.

Distortion in supply voltage can disturb the operation of sensitive equipment such as computer, protection relays operating for sensitive equipments and cause distortion in telephone signal.

Size of neutral conductor must be appropriate as current in neutral will approximately equal or greater than the phase current. Similarly the service life of equipment reduces at large such as 32.5% for single phase, 18% for three phase and 5% for transformers.

Data that we obtained from this building contains large no. of Non-linear loads in form of SMPS which are installed in shape of computers, printers and photocopying machines. In this section we are going to analyze both voltage and current harmonics in accordance toEN-50160.

In figure no. 34 above, graphical representation of the voltage harmonic distortion is given. We can observe from the given data that Total Harmonic distortion was high during the night time. As we know that during October sunset earlier and the lighting off the buildings automatically turns on from 5:00 pm. Since most of the lighting is CFL which is the major contrib

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