Power System Security Economy Availability

Published Date: 02 Nov 2017

Introduction

<intro> In a power system security, economy, availability and

quality are the main objectives. Last three of which, directly

affect the customer. But in the recent years due to the rapid

development in the electronics and digital computers the nature

of the load has changed from mostly conventional such as motors

and filament bulbs to mostly non-linear loads that use switch

mode power supplies (SMPS), electric drives and compact

fluorescent lamps (CFL). That unfortunately for the power system

has made the quality issue more serious.

Power quality is anything that affects the voltage, current and

frequency of power being supplied to the customers. Constant

voltage is the prime requirement of the customer because if the

voltage is lower than the tolerable limits it will cause over

heating of the equipment and less illuminating power to the

lighting load. If it is higher than the limit it cause material

insulation break down, reduces the life of lighting load etc.

Lightning (transient over voltages), switching over voltages (i.e

capacitor switching, disconnection of lines), short circuit

faults (such as voltage sags) and short interruptions are the

main causes for voltage deviations which lead to permanent damage

of the equipment.

Power system frequency is related to the balance between power

generation and the load. When this balance changes, small change

in frequency occurs. The frequency variations that go beyond

acceptable limits for normal steady state operation of power

system are normally caused by fault on the transmission lines,

large portion of load being disconnected, or a large source of

generation being isolated. Drop in frequency could result high

magnetizing currents in induction motors and transformers,

causing problem of overheating and saturation. Off nominal

frequency will cause damage to turbine and generator due to high

vibration of turbine blades which causes protection to trip out.

Therefore it is essential requirement to maintain frequency of

the system within the tolerable limits.

Nowadays due to lot of harmonic injection in to the power system

by the load that are also more sensitive in nature, the use of

custom power devices/custom controllers (electronics based) to

maintain power quality has become essential. As custom power

controllers are used for current interruptions and voltage

regulations, their utilization in the industry saves its

equipment from voltage dips and interruptions which lead to loss

of production. Minimize total harmonic distortion (THD) and

improve the load power factor that minimizes the losses in the

network.

This chapter will present an overview of major power quality

issues at the distribution level: voltage disturbances,

harmonics, transients, unbalance and flickers. Moreover different

solutions for mitigation of these issues by using custom power

devices along with some case studies will also be presented.

1.1 Power quality Issues

Power quality issues occur due to different type of electrical

disturbances that depends on amplitude and/or frequency. These

disturbances are divided into short, medium or long term

depending upon their duration. The power quality disturbances

that most often occur at the distribution level are discussed

below.

1.1.1 Voltage Disturbances

Voltage disturbances are characterized by their duration and

depth. Duration is the length of time for which the voltage

remains below a threshold. The depth is characterized by the

retained voltage, which is the voltage that persists during the

voltage disturbance, as opposed to the voltage decrease.

IEEE definition of voltage sag is described as a sudden and short

duration reduction in RMS value of the voltage at the point of

electrical system between 0.1 to 0.9 pu with duration from 0.5

cycles to 1 minute.

It is known as a voltage swell if there is an increase between

1.1 pu and 1.8 pu in RMS voltage at the power frequency for

durations from 0.5 cycles to 1 minute.

In case, the voltage is completely lost or is less than 0.1 pu in

one or more phase conductors for a period between 0.5 cycles and

1 minute than it is known as interruption.

Any disturbance that persists for less than 0.5 cycles is

considered a transient phenomenon while the one that is longer

than 1 minute could be overvoltage, undervoltage or sustained

interruptions depending upon the RMS voltage magnitude greater

than 1.1 pu, less than 0.9 pu or completely lost or less than 0.1

pu respectively. These voltage disturbances are categorized in

the Figure [fig:VoltDisturb] with respect to RMS voltage and time

duration of the disturbance.Short duration voltage disturbances

mostly depend upon the location of the fault and also on the

condition of the power system. Figure [VoltDip] and [VoltIntp]

show some fault events [1].

[float Figure:

[Figure 1.1:

Categorization of Voltage disturbances.

]<fig:VoltDisturb>

]

Voltage sags occur a lot in a power system thatâ€™s why they are

considered the most severe disturbance to industrial equipment.

Disturbance of only 10% sag for duration of 100ms can affect the

quality of papers in paper mill [3]. In case of semiconductor

industry, voltage sag of 75% (of the nominal voltage) with

duration shorter than 100ms results in material loss in the range

of thousands of U.S dollars [2]. The cause of voltage sag could

be due to different events in the power system like short circuit

faults in the transmission and distribution system, starting of

large Induction motor or transformer energizing can also result

in shallow dips [4].

[float Figure:

[Sub-Figure a:

]

[float Figure:

[Sub-Figure b:

]

[Figure 1.2:

(a) Voltage Dip and (b) Voltage Interruption events.

]

[float Figure:

[Figure 1.3:

Faults on parallel feeder causing voltage sag

]<feeder>

]

In case of three phase voltage sag due to a fault at the point of

common coupling (PCC) in a radial system, as shown in Figure [feeder]

. The Voltage divider model can be used for the calculation of

voltage sag magnitude. In this case voltage \overline{E}g

during

fault can be expressed as

\overline{E}_{g}=\frac{\overline{Z}_{f}}{\overline{Z}_{g}+\overline{Z}_{f}}*\overline{E}_{s}

Where \overline{Z}_{g}

is the impedance of the grid, \overline{Z}_{f}

is the impedance between the PCC and the fault including fault

and line impedances and \overline{E}_{s}

is the supply

voltage.Voltage sag is also related to the changes in voltage

phase angle. This change in phase angle is also called as phase

angle jump (i.e the phase angle between during sag and pre-sag

voltages) and is obtained by taking argument of the complex of

voltage Eg [5].

Long duration voltage variations are mostly due to inadequate

voltage regulation, equipment malfunction or failure or switching

of large loads in a power system. Figure [Overvoltage] presents

an overvoltage caused by a transformer failure. Figure [sustainedIntp]

presents a sustained interruption due to the operation of an

upline circuit breaker. These disturbances are often permanent

and require human intervention for system restoration [6].

[float Figure:

[Sub-Figure a:

]

[float Figure:

[Sub-Figure b:

]

[Figure 1.4:

(a) Ovevoltage due to transformer failure (b) Sustained

Interruption due to mulfunction of equipment.

]

1.1.2 Harmonics

According to the IEEE definition harmonics are sinusoidal

voltages or currents having frequencies that are integer

multiples of the frequency at which the supply system is designed

to operate (termed the fundamental frequency; usually 50 Hz or 60

Hz).

Both the voltage and current harmonics are measured in terms of

total harmonic distortion (THD) at the PCC, which is the point

where other customers can be connected with the electric utility.

And the quoted values are explicitly specified as voltage and

current values. Conventionally, current distortion measurements

are suffixed with â€˜Iâ€™, e.g. 30 % THDI, and voltage distortion

suffixed with â€˜Vâ€™, e.g. 5 % THDV. Figure 7 shows a fundamental

sinewave with 60% 3rd and 40% 5th harmonics.

[float Figure:

<H1>[Figure 1.5:

Fundamental with 3rd and 5th harmonics.

]

When talking about harmonics in power installations it is the

current harmonics that are of more concern because the harmonics

originate as currents and most of these waveform distortions are

due to these currents [8.1]. Because of which striker limits are

imposed by electrical utilities [7] .When harmonics currents from

the different sources produce a drop across the impedance

network; they distort the voltage at that point. Another harmonic

phenomenon is due to the shunt or series resonance in the network

that can cause a voltage magnification [8].

But the most important reason that is also a concern for the

future electrical utility is the way the loads draw current from

the utility, which introduces harmonics in the system these are

non-linear loads. Figure [nonlinear loads] shows the current

drawn by different non-linear loads. These currents result in

distorted voltages that further affect the other loads in the

system. As the number of harmonic producing loads has increased

over the years, it has become increasingly necessary to address

their influence when making any additions or changes to an

installation [9].

[float Figure:

<nonlinear loads>[Figure 1.6:

Harmonic currents of different non-linear loads.

]

Normally the power system distribution is able to absorb the

harmonic produced by the loads that satisfy the IEEE Std 519-1992

limits at the PCC [10]. But it is the combined effect of many

loads at the PCC that could cause the harmonic distortion of main

supply voltage, which can lead to energy losses due to unwanted

current in the system. As clearly visible in the Figure 7 and 8,

the sum of the fundamental, 3rd and 5th harmonic that gives high

peaks that are phase dependent, which may result in defective

operation of regulating devices, malfunction of protective relays

or circuit breaker and other control systems. High harmonics

amplitude may also overload the power distribution network thus

reducing its power transfer capability and overheat the neutral

conductor because of skin and proximity effects, which increase

with frequency and dielectric breakdown may occur causing it to

burn out [11- 13].

1.1.3 Voltage Unbalance

In simple terms, voltage unbalance is a voltage variation in a

power system in which the voltage magnitudes are not equal or the

phase angle differences between them is not 120 degree. In

reality this is impossible to achieve in a power system.

Before going into any formal definition of unbalance system we

first have to know how to quantify an unbalance voltage or

current in a three-phase system. This is done by mathematically

breaking down the unbalance system in to three symmetrical

balanced. That are the positive-sequence, negative-sequence and

zero-sequence components indicated by subscripts 1, 2, 0

respectively, shown in the Figure [unbalance]. They are

calculated using matrix transformations of the three-phase

voltage or current phasors.

\begin{bmatrix}U_{0}\\

U_{1}\\

U_{2}

\end{bmatrix}=\frac{1}{3}\begin{bmatrix}1 & 1 & 1\\

1 & a^{2} & a\\

1 & a & a^{2}

\end{bmatrix}\begin{bmatrix}U_{a}\\

U_{b}\\

U_{c}

\end{bmatrix}

Where the rotation operator a is given by: a=e^{j120^{o}}

.

[float Figure:

<unbalance>[Figure 1.7:

Positive, negative and zero sequence components

]

For a perfectly balanced system both negative and zero sequence

systems would be absent. These sequence systems can be given some

physical interpretation. The direction of rotation of a

three-phase induction motor when applied with a negative sequence

set of voltages is opposite to what is obtained when the positive

sequence voltages are applied. Having no phase displacement

between the three voltages in the zero sequence system, when

applied to a three-phase induction motor, it will not rotate at

all as there will be no rotating magnetic field.

According to the IEEE definition of voltage unbalance its defined

as the ratio of the negative or zero sequence component to the

positive sequence component.

This is also known as negative sequence unbalance factor (NSUF).

And it is considered to give the true unbalance in the system.\%\, NSUF=\frac{U_{2}}{U_{1}}*100

Where U_{1}

and U_{2}

are positive and negative sequence

components of three-phase line voltages can be calculated using

equation above.

According to The National Electrical Manufacturers Association

(NEMA) the voltage unbalance is as follows:

VoltageUnbalance\:=\frac{Maximum\, deviation\, from\, the\, mean\, of\,\left\{ Uab,Ubc,Uca\right\} }{Mean\, of\,\{U_{ab},U_{bc},U_{ca}\}}

Line-neutral voltages should not be used with equations (1) and

(2) as the zero sequence components can give incorrect results.

Although both definition give different result but the analysis

in [17] shows that its not significant uptill 5% voltage

unbalance.

The main cause of voltage unbalance is the lack of symmetry in

the loads at the LV distribution network. Due to large

single-phase loads like arc furnaces and welders connected at the

distribution network that makes the system unbalance. Also due to

the rapidly increase in the small distributed generations like PV

panels that are connected to low voltage distribution network

through a single phase power electronics converter. These small

generations have high impedance and low short circuit power that

result in a large unbalance voltages. More over faulty power

factor correction capacitor banks, and open delta or wye

transformers also contribute to unbalance voltage. Abnormal

system conditions that typically include phase-to-ground,

phase-to-phase and open conductor faults also cause phase

unbalance.

The most common victim of unbalanced voltages is the electric

motor that happens to be the work horse of the modern industry.

Unbalance voltage in an induction motor will reduce its

efficiency and cause heating in the windings that will reduce the

life of insulation and hence the motor life [21]. Even the NEMA

premium efficiency motors are built for voltage unbalance of 1%

[20].Furthermore it also decreases the capacity of the cables and

transformer in power system due to negative sequence. In case of

transformer if the delta configuration is used the zero sequence

will circulate and cause heating [18]. AC variable speed drives

and converters normally draw non-linear current that injects

harmonics in to supply due to the unbalance voltage these

harmonics increases because the current becomes more non-linear

[19].

1.1.4 Transients

In IEEE Std 100-1992 another synonymous used for transients is

surge. It is defined as "a transient wave of current, potential,

or power in an electric circuit." Based on the wave shape of the

transient current or voltage the transients can be classified

into two main categories, impulsive and oscillatory transients.

Impulsive Transient is one of the two types of transient

disturbance that may enter the power system. It is defined by

IEEE as a sudden, nonâ€“power frequency change in the steady-state

condition of voltage, current, or both that is unidirectional in

polarity â€“ either primarily positive or negative.

It is normally a single, very high impulse like lightning.

Currents produced from a lightning strike can go to several

thousand amps in about 2-3us

. Impulsive transients are generally

described by their rise and decay times. They can also be

characterized by their spectral content. Impulsive transients are

not usually transmitted far from the source of where they enter

the power system. However, in some cases, they may propagate for

some distance along distribution utility lines. Also, it may

considerably have different characteristics when viewed from

different parts of the electrical system. In addition, the high

frequencies involved allow damping of the impulsive transients

through the resistive component of the system.

Electrostatic Discharge is another form of an impulsive

transient. Most of us are familiar with this, since we may have

already experienced such when touching an object like door knob

or another person, after walking across a carpeted floor. The

sudden release of charge can damage sensitive electronics. This

is the main reason why technicians use wrist straps when

servicing electronic equipment.

The effects of transients on a power system depend on the

amplitude of the transient and its frequency. In the case of

impulsive transients, its amplitude is the main cause of

problems. The damage caused by a transient can be immediate (i.e.

lightning strike). It can also be gradual as in the case of

low-amplitude transients, which slowly degrade equipment

insulation making it prone to short circuit [22].

Oscillatory Transient is described as a sudden, nonâ€“power

frequency change in the steady-state condition of voltage,

current, or both that has both positive and negative polarity

values (bidirectional).

In other words, the instantaneous voltage or current value of an

oscillatory transient varies its polarity quickly. It is

described by its spectral content or predominant frequency,

magnitude and duration. The oscillatory transient is subdivided

into three classes. These are based on selected frequency ranges,

which correspond with common types of power system oscillatory

transient phenomena. It should also be noted that the frequency

of the oscillation gives a trace to the origin of the disturbance

[25].

According to the classification given in IEEE Standard 1159-1995,

an oscillatory transient with a primary frequency component less

than 5 kHz, and duration from 0.3 ms to 50 ms, is considered a

low frequency transient [27]. These transients are normally

encountered on subtransmission and distribution systems that

could originate primarily due to capacitor bank energization.

Electric distribution utilities use capacitor banks to improve

power factor, as well as lower system losses. For better results,

capacitor banks have to be switched in and out of the system to

match with changes in the load profile. However, capacitor bank

energization yields an oscillatory voltage transient with

frequencies between 300 and 900 Hz [23-24], Figure shows this

effect.

Also, oscillatory transients with fundamental frequencies less

than 300 Hz can be observed on the distribution system due to

transformer energization and ferroresonance. In addition, series

capacitors may also produce this transient type when the system

resonance causes the magnification of low frequency components in

the transformer inrush current or when unusual conditions lead to

ferroresonance.

An oscillatory transient with a predominant frequency component

between 5 and 500 kHz and a duration measured in tens of

microseconds is termed a medium frequency transient. Back-to-back

capacitor switching is a typical example of these transients

[26]. It occurs when a capacitor bank is switch in close

electrical proximity to another capacitor bank that is already

energized, which sees the de energized bank as a low impedance

path. Other causes of this type of transient includes cable

switching and as a system response to an impulsive transient.

Oscillatory transients with a predominant frequency component

greater than 500 kHz and a typical duration in microseconds are

considered high frequency transients. This type of transients are

linked with power electronics and switching events such as line

or cable energization. Power electronics, like the SMPS in

computers, generate oscillatory voltage transients that repeat

several times the system frequency. Usually, they are also the

result of a local system response to an impulsive transient.

1.1.5 Voltage Flickers

[float Figure:

[Sub-Figure a:

]

[float Figure:

[Sub-Figure b:

]

[Figure 1.8:

(a)Voltage flicker due to an arc furnace and (b) transient due to

capacitor switching.

]

A random or repetitive variation in the RMS voltage between 0.9

pu and 1.1 pu, which cause rapid visible changes of light level

in lighting equipment is known as flicker. The term flicker is

derived from the impact of the voltage fluctuation on lighting

intensity. Voltage flicker can be separated into two types:

cyclic and noncyclic. Cyclic flicker is a result of periodic

voltage fluctuations in the system voltage, with noncyclic

referring to occasional voltage fluctuations.

The main cause of voltage changes is the time variability of the

reactive power component of fluctuating loads. In general,

voltage flicker occurs on relatively weak systems with a low

short-circuit capacity. So the loads with a high rate of change

of line current in a short time can result in a flicker [28]. Arc

furnaces are the most common cause of voltage fluctuations on the

transmission and distribution system, see Figure [flicker]. Also

note that small power loads such as starting of induction motors,

welders, boilers, pumps and compressors, cranes, elevators etc.

can also be the sources of flicker [28.1]. Capacitor switching

and on-load transformer tap changers that can change the

inductive component of the source impedance are also a cause of

flickers [34]. Variations in generation capacity like, in wind

turbines can also have an effect. In some cases, voltage

fluctuations can be caused by low frequency voltage

inter-harmonics [33].

Voltage fluctuations have an effect on wide range of equipment

and devices that are commonly used in industry. In an induction

motor these fluctuations can cause changes in torque and slip and

in a worst case scenario that may lead to excessive vibrations

that will reduce the mechanical strength of the motor [31]. In

phase-controlled rectifiers with dc-side parameter control

usually it is a reduction of power factor and the generation of

non-characteristic harmonics and inter-harmonics. In an inverter

during drive braking commutation failure could occur due to

flickers [32]. In order to protect equipment from unexpected

failures and damages the voltage fluctuation measurements are

required to determine actual load emission levels for comparison

with limit values given in [29-30].

1.2 Mitigation of Power Quality Issues

For the mitigation of the power quality issues whether itâ€™s the

customer or the power utility the economic factor is very

important. Most often no one is interested in investing until the

power quality issue becomes serious. In order to save huge

investment latter the best solution is to manage power quality

from the very beginning at the equipment level rather than at the

PCC.

For mitigation of PQ issues custom power devices are used. These

devices include both the passive and power electronics devices

that are able to react in real time to the state of the

distribution system and adjusting itself to maintain the required

level of power quality. The decision to choose between different

custom power controllers is based on particular PQ issue, size of

sensitive load, geographic location and the condition of the

electrical network based on this data a cost analysis is done to

find the most suitable solution. Nowadays due to more sensitive

nature of loads use of custom power devices that are electronics

based to maintain power quality has become essential. The key

technology that has made custom power devices possible is the

solid-state switch like the gate turn-off thyristor (GTO), the

insulated gate bipolar transistor (IGBT), and the integrated gate

commutated thyristor (IGCT). These devices have the operational

capabilities suitable for high power applications at a cost that

makes them economically possible for distribution power levels.

In this section the passive and power electronic based custom

power devices for voltage sags that are regarded as one of the

most harmful power quality (PQ) disturbances due to their costly

impact on industrial processes, harmonics that are rapidly

increasing due to the use of non-linear loads and the voltage

unbalance that affects the electric motor that is the work horse

of the modern industry are discussed.

1.2.1 Passive Mitigation Devices

In the following section, the passive custom power for voltage

dip, harmonics and unbalance mitigation as shown in Figure [passiveMit]

, are discussed briefly:

[float Figure:

<passiveMit>[Figure 1.9:

Passive mitigation devices for voltage sag, harmonics and

unbalance.

]

1.2.1.1 Voltage Dip Mitigation

Ferro- resonant Transformer

The purpose of the Ferro-resonant transformer is to provide

constant output voltage despite of changing in input voltage and

load. Sometimes these transformers are also called as Ferros or

CVTS (constant voltage transformers) [37]. In actual design of

this transformer as shown in Figure [FRTa] the capacitor is

connected to the secondary winding of the transformer to set the

operating point above the knee of the saturation curve, Figure [FRTb]

. These kinds of transformers are only used for low-power;

constant loads because variable loads can cause problems, due to

the presence of tuned circuit on the output.

[float Figure:

[Sub-Figure a:

]

[float Figure:

[Sub-Figure b:

]

[Figure 1.10:

(a) Single line diagram of Ferro-resonant transformer and (b) the

saturation curve of transformer.

]

Static Var Regulator (SVR)

The first static tap-changer in the world was used in the field

operation in Norway in 1986 by ABB components [38]. It is used to

avoid voltage sag. In this kind of the transformer thyristor

based tape changer can be mounted on its secondary winding for

the sensitive loads to change its turn ratio according to

variations in the input voltage [39]. The Figure [SVR] shows the

diagram of transformer with electronic tap-changer.

[float Figure:

<SVR>[Figure 1.11:

Single line diagram of Transformer with electronic tap-changer.

]

The secondary winding feeds the load and its voltage regulation

is accomplished by connecting and disconnecting different

sections of the same winding by fast static switches in the

steps. This fast switching of winding sections also results in

transients because of change in winding inductance which is the

drawback of this technique. Also note that this technique

involves static switches so it can also be put in power

electronics based voltage sag mitigation devices.

Rotating machines (Motor-Generator set)

Motor-Generator set consists of motor supplied by grid, a

synchronous generator supplying the load and the flywheel; all

are connected at a common axis. Three-phase diagram of

motor-Generator set with flywheel is shown in Figure [MGset].

When motor rotates the rotational energy will be stored in the

flywheel which is used to maintain voltage regulation during

disturbances. This scheme has high efficiency and low initial

cost but it can only be used in industrial environment due its

size, noise and maintenance requirements.

[float Figure:

[Figure 1.12:

Three phase diagram of Motor-Generator set with flywheel.

]

<MGset>

]

1.2.1.2 Harmonic mitigation

Phase Shifting Transformer

Phase shifting transformers are the effective way to cancel out

the 5th and 7th harmonic. This method is clear and fundamentally

easy to understand. The principal is to take harmonics generated

from separate sources, shift one source of harmonics 180Â° with

respect to the other and then combine them together; this will

result in cancellation. When load currents are not matched, the

harmonics can be partially cancelled. It may be a single

transformer with two separate windings (ie: delta and wye) as

shown in Figure [PshiftingTran].

[float Figure:

[Sub-Figure a:

]

[float Figure:

[Sub-Figure b:

]

<PshiftingTran>[Figure 1.13:

(a) Three phase diagram of Dyd transformer and (b) single line

diagram of tuned filter.

]

Tuned Harmonic filter

These harmonic filters are used to reduce the harmonic distortion

in the supply system. A tuned harmonic filter is a device with

two basic elements an inductor and a capacitor. These reactive

elements are connected in series to form a tuned LC circuit. The

tuned harmonic filter is connected as a shunt device to the power

system as shown in the Figure [Tuned filter] below.

The tuned harmonic filter is a resonant circuit at the tuning

frequency so its impedance is very low for the tuned harmonic.

Because of the low impedance at the tuned harmonic frequency the

tuned filter now becomes the source of the tuned frequency

harmonic energy demanded by the loads, rather than the utility.

The filter impedance at the tuning harmonic behavior is like a

resistor; below the tuning frequency it has a capacitive

behavior, while the impedance above the tuning frequency has an

inductive behavior. Due to the capacitive behavior at below the

tuning frequency, the filter improves the displacement power

factor. At the tuning frequency the filter acts like a very low

resistance, and a great amount of harmonic current at this

frequency flows through the filter and the total harmonic current

distortion in the upstream system decreases. And the harmonic

currents flow between the filter and loads. This decrease in the

total harmonic current distortion improves the distortion power

factor and thus the total power factor [35].

Low Pass Filter

Low pass harmonic filters are popular due to their ability to

attenuate all harmonic frequencies and achieving low levels of

residual harmonic distortion. There are several circuit

configurations available for the low pass filters. Typically, low

pass filter configuration includes one or more series elements

plus a set of tuned shunt elements as shown in Figure [LPF].

[float Figure:

[Figure 1.14:

Single line diagram of Low pass filter.

]

<LPF>

]

There are three parts input side, the shunt part and output the

load side. The purpose of input stage is to isolate the LPF from

other harmonics sources connected to the same power source and to

prevent power system resonance. This stage also protects the load

and filter capacitors against transients. The output stage

contains a precise amount of impedance that minimizes the amount

of harmonics produced by the load. It also reduces the harmonic

burden placed on the shunt stage, and prevents resonance between

the shunt stage and the load. The shunt stage absorbs residual

harmonics remaining after both the input and output stages have

played their roles in reducing load harmonics (primarily 5th and

7th). Our basic three stage filter design achieves the lowest

possible harmonic distortion levels while preventing power system

resonance. The low pass filter forms a hybrid combination of

series and shunt elements that can be applied without performing

system harmonic analysis.

IGBT Based fast switching harmonics filter

This harmonic mitigation technique is useful in situations

involving dynamic loads with rapidly changing demands for

reactive power. These filters are switched very rapidly IN and

OUT of the circuit using IGBTâ€™s instead of contactors. This type

of filter is also capable of soft switching the capacitors

suppressing voltage spike. It can be switched, without

discharging the capacitors, at switching rates up to 60 times per

second. The main advantages of this filter are the capability to

switch without transients and to respond in real time, to

dynamically changing load conditions. The performance of the fast

switched filter is similar to the performance that can be

expected from a typical tuned filter, a total harmonic current

distortion from 3 to 12% [35].

1.2.1.3 Unbalance Mitigation

Scott-transformer

This transformer transforms three phase power to two-phase power.

It consists of two single phase transformer with a special

winding ratio. One of the two single phase transformers has a

middle-tapped winding on its primary side, and a single winding

on its secondary side. Figure shows the scott connection scheme.

They are connected in such a way that at the output, a two-phase

orthogonal voltage system is generated allowing the connection of

two single-phase systems. This setup draws a balanced three-phase

power from the grid.

[float Figure:

[Sub-Figure a:

]

[float Figure:

[Sub-Figure b:

]

[Figure 1.15:

(a) Schematic diagram of scott-transformer and (b) the single

line diagram of steinmetz transformer for induction furnace.

]

Steinmetz-transformer

It is a three-phase transformer with an extra power balancing

load, consisting of a capacitor and an inductor that are rated

proportional to the single phase load, Figure shows the steinmetz

transformer for load balancing in a large induction furnace load.

When the reactive power rating of both the inductor and the

capacitor is equal to the active power rating of the load

(divided by root 3) then the three-phase grid sees a balanced

load. The three-phase rated power of the transformer is equal to

the single-phase active power of the load. This balancing is only

perfect for loads with an active power equal to the value used to

design the system. The detail calculation using steinmetz

transformer can be found in [36].

1.2.2 Power Electronics based mitigation devices

At the heart of the custom power devices that are power

electronics is a three-phase voltage source inverter. The

inverter is controlled by a digital system that constantly

monitors the distribution line and controllers the output

according to some control algorithm. The inverter is connected to

the distribution line via a filter that removes the harmonics

injected by the inverter and a transformer for isolation. The

inverter itself could be a 2 level PWM or multi-level inverter.

The custom power devices that can inject active power need an

energy storage that could be capacitors, batteries, flywheel or

superconducting magnetic energy storage (SMES). Table [ActiveDevices]

shows some of the power electronic based custom power devices

for mitigating voltage dips, interruptions, harmonics, unbalance

and flicker. These devices are briefly discussed in the following

section.

[float Table:

[Table 1.1:

Summary of mitigation capability of various power electronics

based custom power devices.

]

+------------+--------------+---------------+------------+--------------------+---------+

+------------+--------------+---------------+------------+--------------------+---------+

| STS | x | x | | | |

+------------+--------------+---------------+------------+--------------------+---------+

| DVR | x | | | x | x |

+------------+--------------+---------------+------------+--------------------+---------+

| SVC | | | x | x | x |

+------------+--------------+---------------+------------+--------------------+---------+

| UPS | x | x | x | x | x |

+------------+--------------+---------------+------------+--------------------+---------+

| D-STATCOM | x* | x* | x | x | x |

+------------+--------------+---------------+------------+--------------------+---------+

[footnote:

* Only if energy storage is present.

]

]

1.2.2.1 Static Transfer Switch (STS)

The static transfer switch (STS) is an electrical device which

allows the instantaneous transfer of the load from preferred

source to an alternative healthy source in case of the voltage

disturbance. This means that if one power source fails STS

switches to the back-up power source quickly in such a way that

load never realizes any disturbance. Figure [STS]shows single

line diagram of Static Transfer Switch (STS).

[float Figure:

[Figure 1.16:

Single line diagram of STS.

]

<STS>

]

Normally, static switch on the primary source is fired regularly,

while the other one is off. It is generally used to mitigate

voltage sags and interruptions in the distribution system but it

cannot protect against the sag originating in the transmission

system.

1.2.2.2 Uninterruptible Power Supply (UPS)

The main purpose of uninterruptible power is to provide

uninterruptible, reliable, and high quality power to the loads.

UPS consists of rectifier which is supplied by grid, battery and

the inverter which supplies the load. Figure [UPS] shows

three-phase diagram of UPS. The rectifier is used to convert ac

voltage into dc which supplies power to the inverter as well as

battery bank to keep it charged. In normal operating conditions,

battery gets charged and power is supplied to the inverter by

rectifier. In case of an outage, battery bank supplies the power

to the load [40]. Depending upon the storage capacity of the

battery, it can supply the load for minutes or even hours. UPS is

a low power application device and is used in medical equipment,

data storage and computer system, emergency equipment,

telecommunications and online management systems [41]. For

higher-power loads the costs associated with losses due to the

two conversions and maintenance of the batteries become too high

and, therefore, a three-phase, high power UPS is not economically

feasible.

[float Figure:

[Figure 1.17:

Three phase diagram of UPS.

]

<UPS>

]

1.2.2.3 Shunt connected Voltage source Converter (D-STATCOM)

The Distribution Static Compensator (D-STATCOM) is a shunt

connected voltage source converter based static compensator which

is used for voltage regulation at the PCC and reactive power

control by injecting controlled amount of current \underline{i}_{r}(t)

of desired amplitude, frequency and phase into the grid. The

typical configuration of a STATCOM is shown in Figure [Dstatcom1]

. This device consists of a VSC, an injection transformer, an AC

filter and a DC-link capacitor. The line impedance has a

resistance R_{g}

and inductance L_{g}

. The grid voltage and

current are denoted by \underline{e}_{s}(t)

and \underline{i}_{g}(t)

, respectively. The voltage at the PCC, which is also equal to

the load voltage, is denoted by \underline{e}_{g}(t)

and the

load current by \underline{i}_{l}(t)

. The inductance and

resistance of the AC filter reactor are denoted by R_{r}

and L_{r}

, respectively.

[float Figure:

[Figure 1.18:

Single line diagram of shunt connected VSC.

]

]

By injecting a controllable current, a STATCOM can limit voltage

fluctuation leading to flicker [42] and cancel harmonic currents

absorbed by the load, thus operating as an active filter [43]. In

both cases, the principle is to inject a current with same

amplitude and opposite phase as the undesired components in the

load current, so that they are cancelled in the grid current.

These mitigation actions can be accomplished by only injecting

reactive power.

[float Figure:

[Figure 1.19:

Phasor diagram of D-STATCOM

]

]

A STATCOM can also be used for voltage dip mitigation if energy

storage can also be mounted on the DC link to allow active power

injection into the AC grid. In this case, the device has to

inject a current in the grid which results in an increased

voltage amplitude at the PCC, as shown in the phasor diagram in

Figure [Dstatcom2]. The voltage phasor at PCC is denoted by \overline{E}_{g}

, \overline{Z}_{g}

is the line impedance, \overline{E}_{s,dip}

is the grid voltage phasor during the dip and \psi

is the

phase-angle jump of the dip.

From the diagram it is possible to understand that when the

shunt-connected VSC is used to mitigate voltage dips, it is

necessary to provide energy storage for injection of active power

in order to avoid phase-angle jumps of the load voltage. If only

reactive power is injected, it is possible to maintain the load

voltage amplitude E_{g}

to the pre-fault conditions but not its

phase [41.1]. Therefore, the voltage dip mitigation capability of

a shunt-connected VSC depends on the rating of the energy storage

and on the rating in current of the VSC. One drawback of using

D-STATCOM for voltage dip mitigation is a high rating of voltage

source converter. To restore the load voltage to the pre-fault

conditions (without introducing phase-jump), the following

condition must be fulfilled

\overline{E}_{g}=\overline{E}_{s,dip}+\overline{Z}_{g}\overline{I}_{r}

Active and the reactive power injected by the device can be

calculated in per unit as

P_{inj}=\frac{cos\varphi}{Z_{l}}-\frac{E_{s}(E_{s,dip}cos(\varphi_{g}-\varphi_{s})-cos(\varphi_{g}-\varphi_{s}+\psi))}{E_{s,dip}Z_{g}}

Q_{inj}=-\frac{sin\varphi_{l}}{Z_{l}}+\frac{E_{s}(E_{s,dip}cos(\varphi_{g}-\varphi_{s})-sin(\varphi_{g}-\varphi_{s}+\psi))}{E_{s,dip}Z_{g}}

where the souce voltage, the line impedence and the load

impedance are expressed as \overline{E}_{s}=E_{s}e^{j\varphi_{s}}

, \overline{Z}_{l}=Z_{l}e^{j\varphi_{l}}

and \overline{Z}_{g}=Z_{g}e^{j\varphi_{g}}

, respectively.

1.2.2.4 Dynamic Voltage Restorer (DVR)

Dynamic Voltage restorer (DVR) is series connected voltage source

converter based compensator which is used to inject voltage \underline{e}_{c}(t)

of controllable amplitude and phase angle between the PCC and

the load in series with the grid voltage through injection

transformer to compensate for voltage dips. It is connected in

series with a distribution feeder and is used to generate or

absorb active and reactive power at its ac terminals. The first

DVR was installed for rug manufacturing industry in North

Carolina [44]. Another was used in Australia for large dairy food

processing plant [44]. Figures [DVR1] and [DVR2] show the single

line and simplified diagram of the Dynamic voltage restorer

respectively. It is used to maintain load voltage \underline{e}_{l}(t)

to the pre-fault condition by injecting missing voltage of

appropriate amplitude and phase.

[float Figure:

[Figure 1.20:

Single line Diagram of DVR.

]

<DVR1>

]

[float Figure:

<DVR2>

[Figure 1.21:

Simplefied diagram of DVR.

]

[float Figure:

<DVR3>

[Figure 1.22:

Phasor diagram of DVR during voltage sag mitigation.

]

Figure [DVR3] shows the phasor diagram of series injection

principle during voltage dip mitigation, \overline{E}_{l}

is the

phasor of pre-fault load voltage, \overline{E_{c}}

is the phasor

of injected voltage by the device, \overline{I_{l}}

is the

phasor of load current, \varphi

is the phase displacement from

the load current and load voltage, \overline{E}_{g,dip}

is the

dip in the amplitude of the grid voltage and \psi

is phase angle

jump. The DVR injects reactive power for compensation of voltage

amplitude but it injects active power for correcting phase angle

jump as well. Injection of active power by DVR is related to

energy storage. DVR injecting larger amount of active power

requires bigger size of energy storage leading to more expensive

scheme rather than injecting less amount of active power. The

size of energy storage can be reduced by optimization of energy

storage by using the techniques in [45]. Assuming the load

voltage and current in pre-fault conditions is equal to 1 pu, the

injected power by the device during voltage dip mitigation is

equal to

\overline{S}_{inj}=\overline{E}_{c}\overline{I}_{l}^{*}=(\overline{E}_{l}-\overline{E}_{g,dip})\overline{I}_{l}^{*}=(1-E_{g,dip}e^{j\psi})e^{j\varphi}

The euler identity can be written as e^{j\varphi}=cos\varphi+jsin\varphi

, applying to equation 2.2 we get

\overline{S}_{inj}=cos\varphi+jsin\varphi-E_{g,dip}cos(\varphi+\psi)-jE_{g,dip}sin(\varphi+\psi)

\overline{S}_{inj}=(cos\varphi-(E_{g,dip}cos(\varphi+\psi))+j(sin\varphi-E_{g,dip}sin(\varphi+\psi))

Power absorbed by the load will be given by

\overline{S}_{load}=P_{load}+jQ_{load}=\overline{E}_{l}\overline{I}_{l}^{*}=e^{j\varphi}=cos\varphi+jsin\varphi

Therefore the active and reactive power injected by the DVR are

given by

P_{inj}=(1-\frac{E_{g,dip}cos(\varphi+\psi)}{cos\varphi}P_{load}

Q_{inj}=(1-\frac{E_{g,dip}sin(\varphi+\psi)}{sin\varphi}Q_{load}

The main components of DVR are:

Voltage source converter (VSC) Generally pulse-width modulated

voltage source converter is used because of simplicity and good

response. It is used to generate desired voltage to be injected

for the compensation. The basic function of VSC is to convert DC

voltage supplied by the energy storage into AC voltage and vice

versa so it can be said that it is a converter through which

power flow is reversible. When power flow is from DC to AC it

said to be in inverter mode and when power flow is from AC to DC

it is in rectifier mode. The valves in converter are usually

IGBTs, but some DVR manufacturers also use IGCTs [46-47].

Series Injection Transformer The main purpose of injection

transformer is to increase the voltage supplied by LC filter and

to inject the missing voltage of the system at the load bus. For

three-phase DVR, three single-phase transformers are used for

this purpose [48]. The high voltage side of the transformer is

connected in series to the line, while DVR power circuit is

connected to the low voltage side. The primary winding can be

connected in either star or in delta with the converter side.

When three single-phase converters are used, a connection of

three windings of transformer will be in wye to realize a

four-wire configuration in order to inject a zero-sequence

voltage into the line. A delta connection increases the injected

voltage (in pu) with respect to the converter output voltage by a

factor of âˆš3 and have an advantage of blocking zero sequence

currents that may circulate in the feeder. To operate the

injection transformer properly into the DVR, MVA rating, turns

ratio, the primary winding voltage and current ratings and short

circuit values of transformer are required [48].

Energy storage An energy storage device is normally connected to

the DC bus of the converter to provide the required energy for

the compensation. Commercially available DVRs use large

capacitors banks for the storage of energy [49]. This is the most

expensive part of DVR .In normal operating condition it is

charged through grid voltage and in case of disturbance it

supplies energy to compensate for load voltage.

Passive filter The nonlinear characteristics of semiconductor

devices cause distorted wave forms associated with high frequency

harmonics at the output of the inverter. To reduce these

harmonics filter unit is used. Although PWM technique is used in

the converter with high switching frequency which generates a

voltage wave form with very low content of the harmonics, a

second-order LC filter is used for further reduction of the

harmonics in the injected voltage.

1.2.2.5 Static var compensators (SVCs)

The purpose of a compensator is to measure adequate electric

quantities of the load and generate in the compensator such

currents, that the resultant load: compensator â€“ compensated

load, as seen from the supply network, was symmetrical, and the

fundamental harmonic reactive current drawn from the network did

not exceed the value permitted in the supply conditions.

Generally, static compensators are the systems, which comprise

reactors and/or capacitors controlled by means of semiconductor

circuits.

Thyristor Controlled Reactor (TCR)

The TCR operates as a gradually variable reactor, shown in Figure

. Itâ€™s just a reactor connected in shunt through a thyristor that

can be controlled by continuously changing the firing angle \alpha

, which changes the susceptance of the TCR, Figure shows the V-I

characteristics of TCR. In the V-I plane, it means "jumping" from

one characteristic to the other â€“ the whole range between \alpha=0

and \alpha=90

can be covered [6].

[float Figure:

[Sub-Figure a:

]

[float Figure:

[Sub-Figure b:

]

[Figure 1.23:

(a) Single line diagram of TCR and (b) the characteristic curve

of TCR.

]

Fixed capacitor/thyristor controlled reactor (FC/TCR)

The simplest way to realize continuous voltage regulation both in

inductive and capacitive range is putting a capacitor in parallel

with a TCR known as FC-TCR as shown in Figure . When TCR is off,

we have only FC and when TCR is fully on, half of its current is

taken by fixed capacitor. The range of regulation is always 1 pu,

but now we regulate both E>Vref

and E<Vref

(both positive and

negative currents), as can be seen in Figure . But the drawback

of this configuration is the losses at zero current.

[float Figure:

[Sub-Figure a:

]

[float Figure:

[Sub-Figure b:

]

[Figure 1.24:

(a) Single line diagram of FC/TCR and (b) the characteristic

curve of FC/TCR.

]

FC/TCRs are employed where control requirements demand finer

resolution that is not possible or economical with switched

capacitor steps. For example, in applications where the short

circuit ratio is low, very small compensation steps are needed to

reduce flicker. A large number of small binary-switched capacitor

steps would be required, increasing the cost and complexity of a

solution that only used a fixed capacitor array. The FC/TCR

combination can provide a more effective and lower cost solution

when finer resolution is necessary.

Thyristor switched capacitor (TSC)

The TSC is used to inject reactive power into the grid. Similar

to the TCR, it is constituted by an AC switch connected in series

with a fixed capacitor. A small inductor is needed to limit the

di/dt during switching operation. Using one TSC gives discrete

variation of reactance with two operating points for each voltage

value (TSC ON or OFF). Splitting the same total capacitance into

more TSCs gives more operating points and higher controllability

but increases cost. Also it is not possible to change the

susceptance continuously to realize constant voltage. When the

TSC is started, a resistor in series with the capacitors can

ensure that they are charged slowly, thereby avoiding high inrush

currents and system disturbances. After the capacitors are

initially charged, a contactor can automatically bypass the

resistor.

[float Figure:

[Sub-Figure a:

]

[float Figure:

[Sub-Figure b:

]

[Figure 1.25:

(a) Single line diagram of TSC and (b) the characteristic curve

of TSC.

]

Thyristor controlled reactor/thyristor switched

capacitor(TCR/TSC)

[float Figure:

[Figure 1.26:

Characteristic curve of TCR/TSC.

]

]

With a combined TCR/TSC compensator, continuously variable

reactive power is obtained throughout the complete control range

as well as full control of both the inductive and the capacitive

parts of the compensator. This is a very advantageous feature

permitting optimum performance during large disturbances in the

power system. Also with the capacitor able to switch on/off by

thyristors (TSC) the same resulting characteristic can be

obtained with half the size of the reactor. Itâ€™s a good solution

when the voltage variations around Vref are not big and when the

same regulation range is needed in both inductive and capacitive

regions. But the cost of the switches and the complexity of the

algorithm for transient-free switching of the capacitor are few

drawbacks for this solution.

1.3 Case Studies

This section presents some case studies where custom power

controllers were installed to mitigate a power quality problem.

Each case study presents a problem and the solution chosen along

with the results after the mitigation.

1.3.1 Case Study1: Voltage Flicker mitigation using D-Statcom at

Seattle Iron & Metals Corporation steel recycling facility in

Seattle.

Problem

In the year 1999 the Seattle Iron & Metals Corporation was a new

steel recycling facility in Seattle, Washington. They have a huge

4000 hp shredder motor load that will be operating at this new

facility. The distribution company Seattle city light has two

substations to supply the plant, the South Substation and

Duwamish Substation at 26.4 kV distribution network. But bothe

feeders during the operation of the shredder motor gives voltage

flicker at the PCC. The Figure shows the plant single line

diagram with source impedance Z1, feeder impedance Z2 and the

transformer impedance Zt.

[float Figure:

[Figure 1.27:

System Configuration

]

Solution

The company provided the load fluctuation profile, shown in

figure along with the flicker limitation as specified by IEEE std

1250-1995. The shredder motor specifications along with the max

torque limit of the motor are also given in []. Only the voltage

fluctuations caused by the shredder motor are considered in this

study. The PCC for evaluating the voltage fluctuation is on the

4.16 kV bus on the low voltage side of the system supply

transformer (5 MVA,26.4kV / 4.116kV transformer), i.e., point C

in Figure 5. Most effective way is to mitigating the problem at

the source so that it doesnâ€™t affect the other loads in the

distribution network. The measurement of voltage fluctuation is

made to detect the 4.16kV bus through a PT, and the rms value of

line-to-line voltage of one phase is measured with a voltage

transducer, which can be seen in Figure 8.

[float Table:

[Table 1.2:

Load Fluctuation Profile and Flicker Limit

]

+-----+-----------------+---------------+------------------------+-----------------------------+

|

+-----+-----------------+---------------+------------------------+-----------------------------+

| 1 | 200% load | 2*FLA | 1 hit/hour | 2.7% |

+-----+-----------------+---------------+------------------------+----------------

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