There Are Many Issues Still to Be Resolved Such as Islanding Issue

8/2/2019 There Are Many Issues Still to Be Resolved Such as Islanding Issue

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There are many issues still to be resolved such as islanding issue, interconnection

of many PV system in a connected area, cost evaluation of grid interconnected PV

system and so on.

The main areas of concern are

Voltage level control

Power quality, including harmonics, power factor and DC injection

The real risk associated with islanding

Rate of change of conditions and flicker

Although they are not fully competitive with conventional power generation but

they attract interest because

They are clean power source

Quite amd having no movning parts

Can be installed near the point of power demand

Offer high reliability, long life and low maintenance

Offer ownership of a power source to individual and companies

In addition they offer following advantage over standalone system

Do not require expensive battery storage, as grid provide the backp source

Can be integrated into buildings and other structures thus saving on cladding

costs and land requirement.

Technical areas to be concerned in context of achieving sfe and practical

interconnection of single phase PV inverter generator are:

Protection

Operation and safety

Power quality

Commissioning and acceptance testing

Protection:

Protection of people

Protection of electrical network


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Protection of the PV inverter system

Islanding phenomenon refers to the condition which can exists whereby part

of the network has become electrically isolated from the main network and all

the loads within that network are being supported by the generatorsembedded within the network.

Power quality is not the only issue which is raised by the islanding. The debate

surrounding the risks of islanding is normally asoociated with MW scale

generators. With these large generators the energy stored by virtue of inertial

mass of the rotor shaft means that the consequences of an out of sync circuit

breaker reclosure on a multi MW generator set is a major problem. This

resulted in the generator shaft being torn out of its bearing, allowing it to

make contacts with the generator stator and thereby completely destroyingthe generator. An out of sync reclosure is still an event that could effect the

grid connected PV inverters, but the lack of mechanical inertia within the

inverter should mean that the inverter is in the better position to survive such

event without damage to itself.

Operation & Safety:

Firstly there is a requirement for a manual isolation switch which is easily

accessible and lockable. This switch should isolate the PV inverter output fromboth domestic circuit and the grid supply to allow maintenance to be done to either.

Second requirement is for the automatic protection for islanding and disconnect the

the equipment under any fault condition ( PV array failure, inverter failure, inverter

power transistor failure, overunder voltage, over/under frequency, islanding etc.)

Power quality:

It includes Harmonics, PF, flicker, EMC and DC injection.

Harmonics:

Harmonics are generated by the nonlinear loads. Harmonics can be viewed as a

form of electrical pollution on the distribution network since they provide no

useful benefits but their existence can interfere with the correct operation of

certain types of systems connected to the network.


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Power Factor:

Power factor is a technical term defining the relative displacement in time between

the mains voltage waveform and current waveform. Real Power relates to the

current component at peak while reactive power relates to the current component at

zero.Flicker:

It is normally associated with the nonlinear or pulsed loads, such as welding

equipments. Ficker is associated with lighting and its caused by repetitive, periodic

voltage fluctuations.

DC Injection:

The Islanding phenomenon, probability of occurrence, detection methods, Impact

of islanding and mitigation.

Multiple PV system and their effect on power quality and power system design andoperation.

[1]

Harmonic problems within the installation

There are several common problem areas caused by harmonics: -

_ Problems caused by harmonic currents:

_ overloading of neutrals

_ overheating of transformers_ nuisance tripping of circuit breakers

_ over-stressing of power factor correction capacitors

_ skin effect

_ Problems caused by harmonic voltages:

_ voltage distortion

_ induction motors

_ zero-crossing noise

_ Problems caused when harmonic currents reach the supply

Each of these areas is discussed briefly in the following sections.

Problems caused by harmonic currents

Neutral conductor over-heatingIn a three-phase system the voltage waveform from each phase to the neutral star

point is displaced by 120 so that, when each phase is equally loaded, the

combined current in the neutral is zero. When the loads are not balanced only the

net out of balance current flows in the neutral. In the past, installers (with the


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approval of the standards authorities) have taken advantage of this fact by

installing half-sizedneutral conductors. However, although the fundamentalcurrents cancel out, the harmonic currents do not - in fact those that are an odd

multiple of three times the fundamental, the triple-N harmonics, add in the

neutral.

Figure 12 shows the effect. In this diagram the phase currents, shown at the top,

are introduced at

120

intervals. The third harmonic of each phase is identical, being three times the

frequency and one-third of a (fundamental) cycle offset. The effective third

harmonic neutral current is shown at the bottom. In this case, 70 % third harmonic

current in each phase results in 210 % current in the neutral.

Case studies in commercial buildings generally show neutral currents between 150

% and 210 % of the phase currents, often in a half-sizedconductor! There is someconfusion as to how designers should deal with this issue. The simple solution,

where singlecored cables are used, is to install a double sized neutral, either as two

separate conductors or as one single large conductor. The situation where multi-cored cables are used is not so simple. The ratings of multicore cables (for example

as given in IEC 603645-523 Table 52 and BS 7671 Appendix 4) assume that the

load is balanced and the neutral conductor carries no current, in other words, only

three of the four or five cores carry current and generate heat. Since the cable

current carrying capacity is determined solely by the amount of heat that it can

dissipate at the maximum permitted temperature, it follows that cables carrying


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triple-N currents must be de-rated. In the example illustrated above, the cable is

carrying five units of currentthree in the phases and two in the neutralwhile

it was rated for three units. It should be de-rated to about 60 % of the normal

rating. IEC 60364-5-523 Annex C (Informative) suggests a range of de-rating

factors according to the triple-N harmonic current present. Figure 13 shows

derating factor against triple-N harmonic content for the de-rating described in IEC

60364-5-523 Annex C and for the thermal method used above.

The regulatory position is under discussion at present and it is likely that new

requirements and guidance notes will be introduced into national wiring codes in

the near future.

Effects on transformersTransformers are affected in two ways by harmonics. Firstly, the eddy current

losses, normally about 10 % of the loss at full load, increase with the square of the

harmonic number. In practice, for a fully loaded transformer supplying a load

comprising IT equipment the total transformer losses would be twice as high asfor an equivalent linear load. This results in a much higher operating temperature

and a shorter life. In fact, under these circumstances the lifetime would reduce

from around 40 years to more like 40 days! Fortunately, few transformers are fully

loaded, but the effect must be taken into account when selecting plant. The second

effect concerns the triple-N harmonics. When reflected back to a delta winding

they are all in phase, so the triple-N harmonic currents circulate in the winding.

The triple-N harmonics are effectively absorbed in the winding and do not

propagate onto the supply, so delta wound transformers are useful as isolating

transformers. Note that all other, non triple-N, harmonics pass through. The

circulating current has to be taken into account when rating the transformer.

A detailed discussion on rating transformers for harmonic currents can be found in

a later section of the Guide.

Nuisance tripping of circuit breakersResidual current circuit breakers (RCCB) operate by summing the current in the

phase and neutral conductors and, if the result is not within the rated limit,

disconnecting the power from the load. Nuisance tripping can occur in the presence

of harmonics for two reasons. Firstly, the RCCB, being an electromechanical

device, may not sum the higher frequency components correctly and therefore trips

erroneously. Secondly, the kind of equipment that generates harmonics alsogenerates switching noise that must be filtered at the equipment power connection.

The filters normally used for this purpose have a capacitor from line and neutral to

ground, and so leak a small current to earth. This current is limited by standards to

less than 3.5 mA, and is 0.4 0.6 0.8 1.0 0 10 20 30 40 50 60 70 % third harmonic

Cable derating factor Thermal IEC Figure 13 - Cable derating for triple-Nharmonics usually much lower, but when equipment is connected to one circuit the


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leakage current can be sufficient to trip the RCCB. The situation is easily

overcome by providing more circuits, each supplying fewer loads. A later section

of this Guide covers the problem of high earth leakage in greater detail.

Nuisance tripping of miniature circuit breakers (MCB) is usually caused

because the current flowing in the circuit is higher than that expected from

calculation or simple measurement due to the presence of harmonic currents.

Most portable measuring instruments do not measure true RMS values and can

underestimate non-sinusoidal currents by 40 %. True RMS measurement is

discussed in Section 3.2.2.

Over-stressing of power factor correction capacitors

Power factor correction capacitors are provided in order to draw a current with a

leading phase angle to offset lagging current drawn by an inductive load such

as induction motors. Figure 14 shows the effective equivalent circuit for a PFC

capacitor with a non-linear load. The impedance of the PFC capacitor reduces

as frequency rises, while the source impedance is generally inductive andincreases with frequency. The capacitor is therefore likely to carry quite high

harmonic currents and, unless it has been specifically designed to handle them,

damage can result. A potentially more serious problem is that

the capacitor and the stray inductance of the supply system can resonate at or near

one of the harmonic frequencies (which, of course, occur at 100 Hz intervals).

When this happens very large voltages and currents can be generated, often

leading to the catastrophic failure of the capacitor system. Resonance can be

avoided by adding an inductance in series with the capacitor such that the

combination is just inductive at the lowest significant harmonic. This solution also

limits the harmonic current that can flow in the capacitor. The physical size of the

inductor can be a problem, especially when low order harmonics are present.

Skin effectAlternating current tends to flow on the outer surface of a conductor. This is

known as skin effect and is more pronounced at high frequencies. Skin effect is

normally ignored because it has very little effect at power supply frequencies but

above about 350 Hz, i.e. the seventh harmonic and above, skin effect will

become significant, causing additional loss and heating. Where harmonic currents

are present, designers should take skin effect into account and de-rate cables

accordingly. Multiple cable cores or laminated busbars can be used to helpovercome this problem. Note also that the mounting systems of busbars must

be designed to avoid mechanical resonance at harmonic frequencies. Design

guidance on both these issues is given in CDA Publication 22, Copper forBusbars. Problems caused by harmonic voltages Because the supply has source

impedance, harmonic load currents give rise to harmonic voltage distortion on


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the voltage waveform (this is the origin of flat topping). There are two elementsto the impedance: that of the internal cabling from the point of common coupling

(PCC), and that inherent in the supply at the PCC, e.g. the local supply

transformer. The former is illustrated in Figure 15. The distorted load current

drawn by the non-linear load causes a distorted voltage drop in the cable

impedance. The resultant distorted voltage waveform is applied to all other loads

connected to the same circuit, causing harmonic currents to flow in them - even if

they are linear loads. The solution is to separate circuits supplying harmonic

generating loads from those supplying loads which are sensitive to harmonics, as

shown in Figure 16. Here separate circuits feed the linear and non-linear loads

from the point of common coupling, so that the voltage distortion caused by the

non-linear load does not affect the linear load.

When considering the magnitude of harmonic voltage distortion it should be

remembered that, when the load is transferred to a UPS or standby generator

during a power failure, the source impedance and the resulting voltage distortionwill be much higher. Where local transformers are installed, they should be

selected to have sufficiently low output impedance and to have sufficient capacity

to withstand the additional heating, in other words, by selecting an appropriately

oversized transformer. Note that it is not appropriate to select a transformer design

in which the increase in capacity is achieved simply by forced coolingsuch a unit

will run at higher internal temperatures and have a reduced service life. Forced

cooling should be reserved for emergency use only and never relied upon for

normal running.

Induction MotorsHarmonic voltage distortion causes increased eddy current losses in motors in the

same way as in transformers. However, additional losses arise due to the

generation of harmonic fields in the stator, each of which is trying to rotate the

motor at a different speed either forwards or backwards. High frequency

currents induced in the rotor further increase losses.

Where harmonic voltage distortion is present motors should be de-rated to take

account of the additional losses.

Zero-crossing noiseMany electronic controllers detect the point at which the supply voltage crosses

zero volts to determine when loads should be turned on. This is done becauseswitching reactive loads at zero voltage does not generate transients, so reducing

electromagnetic interference (EMI) and stress on the semiconductor

switching devices. When harmonics or transients are present on the supply the rate

of change of voltage at the crossing becomes faster and more difficult to identify,

leading to erratic operation. There may in fact be several zero-crossings per half

cycle.


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Harmonic problems affecting the supply When a harmonic current is drawn from

the supply it gives rise to a harmonic voltage drop proportional to the source

impedance at the point of common coupling (PCC) and the current. Since the

supply network is generally inductive, the source impedance is higher at higher

frequencies. Of course, the voltage at the PCC is already distorted by the harmonic

currents drawn by other consumers and by the distortion inherent in transformers,

and each consumer makes an additional contribution. Clearly, customers cannot be

allowed to add pollution to the system to the detriment of other users, so in most

countries the electrical supply industry has established regulations limiting the

magnitude of harmonic current that can be drawn. Many of these codes are based

on the UK Electricity Associations G5/3 issued in 1975, recently replaced by G5/4(2001). This standard is discussed in detail elsewhere in this Guide.

Harmonic mitigation measuresThe measures available to control the magnitude of harmonic current drawn are

discussed in detail in later sections of this Guide. In this section a brief overview isgiven in generic terms. Mitigation methods fall broadly into three groups; passive

filters, isolation and harmonic reduction transformers and active solutions. Each

approach has advantages and disadvantages, so there is no single best solution. It is

very easy to spend a great deal of money on an inappropriate and ineffective

solution; the moral is to carry out a thorough surveytools suitable for this

purpose are described elsewhere in this Guide.

Passive filtersPassive filters are used to provide a low impedance path for harmonic currents so

that they flow in the filter and not the supply (Figure 17). The filter may be

designed for a single harmonic or for a broad band depending on requirements.

Sometimes it is necessary to design a more complex filter to increase the series

impedance at harmonic frequencies and so reduce the proportion of current that

flows back onto the supply, as shown in Figure 18. Simple series band stop filters

are sometimes proposed, either in the phase or in the neutral. A series filter is

intended to block harmonic currents rather than provide a controlled path for them

so there is a large harmonic voltage drop across it. This harmonic voltage appears

across the supply on the load side. Since the supply voltage is heavily distorted it is

no longer within the standards for which equipment was designed and warranted.

Some equipment is relatively insensitive to this distortion, but some is verysensitive. Series filters can be useful in certain circumstances, but should be

carefully applied; they cannot be recommended as a general purpose solution.

Isolation transformersAs mentioned previously, triple-N currents circulate in the delta windings of

transformers. Although this is a problem for transformer manufacturers and

specifiers - the extra load has to be taken into accountit is beneficial to systems


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designers because it isolates triple-N harmonics from the supply.


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The same effect can be obtained by using a zig-zag wound transformer. Zig-zagtransformers are star configuration auto transformers with a particular phase

relationship between the windings that are connected in shunt with the supply.

Active FiltersThe solutions mentioned so far have been suited only to particular harmonics, the

isolating transformer being useful only for triple-N harmonics and passive filters

only for their designed harmonic frequency. In some installations the harmonic

content is less predictable. In many IT installations, for example, the

equipment mix and location is constantly changing so that the harmonic culture is

also constantly changing. A convenient solution is the active filter or active

conditioner. As shown in Figure 20, the active filter is a shunt device. A currenttransformer measures the harmonic content of the load current, and controls a

current generator to produce an exact replica that is fed back onto the supply on the

next cycle. Since the harmonic current is sourced from the active conditioner, only

fundamental current is drawn from the supply. In practice, harmonic current

magnitudes are reduced by 90 % and, because the source impedance at harmonic

frequencies is reduced, voltage distortion is reduced.

[3]


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The same effect can be obtained by using a zig-zag wound transformer. Zig-zag

transformers are star configuration auto transformers with a particular phaserelationship between the windings that are connected in shunt with the supply.

Active FiltersThe solutions mentioned so far have been suited only to particular harmonics, the

isolating transformer being useful only for triple-N harmonics and passive filters

only for their designed harmonic frequency. In some installations the harmonic

content is less predictable. In many IT installations, for example, the equipment

mix and location is constantly changing so that the harmonic culture is also

constantly changing. A convenient solution is the active filter or active conditioner.

As shown in Figure 20, the active filter is a shunt device. A current transformermeasures the harmonic content of the load current, and controls a current

generator to produce an exact replica that is fed back onto the supply on the next

cycle. Since the harmonic current is sourced from the active conditioner, only

fundamental current is drawn from the supply. In practice, harmonic current

magnitudes are reduced by 90 % and, because the source impedance at harmonic

frequencies is reduced, voltage distortion is reduced.

[4]Grid connection issues (


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Off > 5 min after disturbanceNegligible EM interferenceComponent ground, GFI, disconnects Metering: where is PVconnected

Util. side: no direct benefit to customer, (utility owns PV)Cust. side: complicated (offset load); different financial arrangementsAnswer: net metering (>35 states); not all meters allow 2-wayELEG 628 Gridconnected systems 2008 11Grid connection issues (


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used to perform all the required control tasks. But, in the two-stage system, a DC-DC

converter precedes the inverter and the control tasks are divided among the twoconverters. Two-stage systems provide higher flexibility in control as compared to

single-stage systems, but at the expense of additional cost and reduction in the reliability

of the system [35]. During the last decade, a large number of inverter and DC-DC

converter topologies for PV systems were proposed [35]- [39], almost saturating theresearch in this area.

1. Maximum power point tracking (MPPT)One of the main tasks of PCUs is to control the output voltage or current of the PVarray to generate maximum possible power at a certain irradiance and temperature.

There are many techniques that can be used for this purpose [40]- [42] with the

Perturb-and-Observe and Incremental Conductance techniques being the mostpopular ones. A recent study [43] presented a qualitative comparison between 19

different MPPT techniques to serve as a guideline for choosing a suitable technique.

Partial shading of PV arrays is considered one of the main challenges that face

MPPT techniques. In this case, there might exist multiple local maxima, but only one

global maximum power point, as illustrated in Figure 2-9. In this case, the task of thePCU is to identify and operate at the global MPP. The research in this field is activeand several

studies have focused on developing new MPPT techniques and PCUtopologies that can perform this task

2. Control of the injected currentPCUs should control the sinusoidal current injected into the grid to have the samefrequency as the grid and a phase shift with the voltage at the point of connection

within the permissible limits. Moreover, the harmonic contents of the current should

be within the limits specified in the standards. The research in this field is mainly

concerned with applying advanced control techniques to control the quality ofinjected power and the power factor at the grid interface [47]- [49].

3. Islanding detection and protectionIslanding is defined as a condition in which a portion of the utility system containing

both loads and distributed resources remains energized while isolated from the rest ofthe utility system [50]. Most of the standards require that PCUs of PV systemsshould cease

injection of power into the grid under specific abnormal operating

conditions of the grid including those leading to islanding [50] [51]. Islanding detection methodscan be classified into three categories [52]: 1) Communicationbased

methods that depend on transmitting signals between the PV system and the

grid to identify an islanding condition, 2) Passive methods that depend on monitoringa certain parameter and comparing it with a threshold value, and 3) Active methods

that depend on imposing an abnormal condition on the grid such as injecting

harmonic current with a specific order at the point of connection with the grid. Most

of the recent studies have focused on assessing and comparing different islandingtechniques as well as developing new methods with minimized non-detection zones

[53]- [56].

4. Voltage amplificationUsually, the voltage level of PV systems requires to be boosted to match the grid

voltage and to decrease the power losses. This task can be performed using step-up

DC-DC converters or multilevel inverters. Three-level inverters can be used for this


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purpose as they provide a good tradeoff between performance and cost in highvoltage

and high-power systems [57].

5. Additional functionsThe control of PCUs can be designed to perform additional tasks such as power

factor correction [58], harmonics filtering [59], reactive power control [60], and

operating with an energy storage device and/or a dispatchable energy source such asdiesel generator as an uninterruptible power supply [61].

2.4 Impacts of PV Systems on the GridGrid-connected PV systems are usually installed to enhance the performance of the

electric network by reducing the power losses and improving the voltage profile of the

network. However, this is not always the case as these systems might impose severalnegative impacts on the network, especially if their penetration level is high. Such

negative impacts include power and voltage fluctuation problems, harmonic distortion,

malfunctioning of protective devices and overloading and underloading of feeders.

3.5 Potential Problems Associated with Grid-connected PV

SystemsDespite all the benefits introduced by PV systems to electric utilities, these systems mightlead to some operational problems. One of the main factors that lead to such problems isthe fluctuations of the output power of PV systems due to the variations in the solar

irradiance caused by the movement of clouds. Such fluctuations lead to several

operational problems and make the output power forecast of PV systems a hard task. Inaddition, the high cost of these systems limits the possible solutions that can be adopted

by electric utilities to reduce the severity of the operational problems that might arise due

to these fluctuations The negative impacts of Grid-connected PV systems on the network

operation did notreceive much attention until lately, after the noticeable increase in installation of these

systems. The work done in this area can be classified under three main categories: 1)

impacts on the generation side, 2) impacts on the transmission and sub-transmission

networks, and 3) impacts on the distribution networks. However, before discussing thepossible negative impacts of installing PV systems, it is important to present an overview

of the source of power fluctuations in these systems and discuss the data required for

analyzing the impact of these fluctuations.

3.5.1 Fluctuation of output power of PV systemsFluctuation of the solar irradiance due to passage of clouds over a PV array is the main

reason behind the fluctuation of the output power of PV systems. There are 10 reportedcloud patterns, with cumulus clouds (puffy clouds looking like large cotton balls) and

squall lines (a solid line of black clouds) causing the largest variations in the output

power of PV systems [73]. Squall lines can cause the output power of a PV system to fall

to zero, and thus, they lead to the worst-case scenario for the operation of the system.However, squall lines are predictable, and thus, the periods of time during which the PV

system will be out of service can be predicted [73]. On the other hand, cumulus clouds

result in lower loss of the PV power, but they cause the output of the PV system tofluctuate more frequently as the irradiance fluctuates due to the passage of such clouds

[73]. The time period of fluctuations can range from few minutes to hours depending on

the wind speed, the type and size of passing clouds, and the area covered by and topology


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of the PV system.

The most severe fluctuations in the output power of PV systems usually occur atmaximum irradiance level around noon. This period usually coincides with the off-peak

loading period of the electric network, and thus, the operating penetration level of the PV

system is greatest. The severity of PV power fluctuations on the electric network is

governed by several factors, such as:1. Type of clouds,

2. Penetration level,

3. Size of PV system,4. Location of the PV system,

5. Topology of the PV system, and

6. Topology of the electric network.

3.5.2 Irradiance data required for studying the impact of PV systemsThe time resolution of the irradiance data, required for studying the fluctuations of the

output power of PV systems, should be match for the main goal of the study as it plays an

important role in the accuracy of the results.

In general, the solar irradiance can be separated into two components [74]: 1)deterministic component defined by the daily, monthly and yearly climate at a certain

location, and 2) stochastic component that comprises fluctuations around thedeterministic component and is defined by the daily weather. In cases where the expected

output energy of a PV system is to be estimated over a period of time, either the

deterministic component of the irradiance [74] or the hourly irradiance data can be used[75] [76]. On the other hand, if it is required to study the performance of PV systems and

their impacts on the electric network, then the time resolution of the irradiance data

should be high enough to include the short-term or sub-hourly fluctuations of the

irradiance (fluctuations within one hour) [77]. Moreover, irradiance data with high timeresolution (e.g., 10-min. resolution) can lead to better prediction accuracy because the

auto-correlation coefficients will have higher positive values as compared to thoseobtained for the data with 1-hour time resolution [78].In the following subsections, the possible negative impacts of the PV systems are

discussed. A summary of these impacts is presented in Figure 3-1.


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3.5.3 Impact of PV systems on the generation sideSevere fluctuations in the output power of large PV systems might affect the generation

in electric utilities. This is mainly due to the fact that the utilities have to follow thesefluctuations in order to compensate for any rise and fall in the generation of PV systems.

Hence, the generating units that are scheduled to operate during the generation period of

PV systems should have ramping rate capabilities that are suitable for the fluctuations ofthese systems. Moreover, the power fluctuations from the PV system make it difficult to

predict the output power of these systems, and thus, to consider them when scheduling the

generating units in the network. Most of the studies performed in this area have

addressed this problem and tried to provide some operational solutions that can beadopted by utilities. For example, the studies presented in [79] and [80] discuss the

impact of installing large, centralized PV plants on the operation of thermal generation

units. The aim of these studies is to identify the penetration level of PV systems that will

not lead to generation control problems due to passing clouds. Both studies conclude thatthe ramping rate capability of the utility is the main factor that controls the penetration

level of PV systems. However, the analysis performed in both studies considered the

worst-case scenarios only, without providing any details about the frequency and periodsduring which these scenarios might occur. The work in [81] introduces some factors that


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can affect the economical and operational values of PV systems for large-scale

applications. Some of these factors are the generation mix, maintenance schedules,ramping rates, fuel costs, spinning reserve requirements, PV power fluctuations, and

geographical diversification of PV systems. The study suggested some solutions that can

be applied to the cases where the severity of the changes in the output power of the PV

system is beyond the ramping capacity of the system. These solutions include: 1)increasing scheduled tie-line power, 2) bringing more generating units online to increase

the overall ramping capacity of the system, and 3) decreasing the output power of the PV

system. A rule-based dispatch algorithm was presented in [82] to take into account theproblems that might arise due to the fluctuations in the power of a 100-MW PV plant

during the dispatch period. The method is based on predicting the solar irradiance every

10 minutes and assumes that not all the PV power is injected into the grid. A set of rulesare proposed to provide operational plans based on the power production of the PV

system. These rules depend mainly on the time of the year and the type of electric utility

under study.

In general, the generation side of an electric utility can be affected by the PV system if

the penetration level of the PV system is comparable to the size of the generating units.However, systems with such large sizes are not expected to be widely installed in the near future

due to the high cost of PV systems. Thus, studying the impacts on the generationside does not seem to be crucial at the time being.

3.5.4 Impact on transmission and sub-transmission networksPV systems might cause problems in the transmission and sub-transmission networks iftheir sizes are large enough to affect these networks. The problems arise mainly due to

power fluctuations of these systems which might lead to: 1) power swings in lines, 2)

power reversal, 3) over and under loading in some lines, and 4) unacceptable voltage

fluctuations in some cases [83]. The effect of large PV systems on the voltage levels andthe stability of transmission systems after fault conditions was studied in [84]. The results

show that replacement of conventional generating units with large PV units affects thevoltage levels of the system during normal operating conditions. During fault conditions,rotors of some of the conventional generators present in the network might swing at

higher magnitudes due to the existence of PV units. Moreover, at very high penetration

levels of PV systems, voltage collapse might occur. In these studies, the sizes of the PV

systems required to cause the aforementioned problems were assumed to range from 700MW to 1500 MW. According to the current market prices of PV systems, such sizes are

not expected to be installed soon. Hence, studying the impact of PV systems on

transmission and sub-transmission networks does not seem to be important for electricutilities at the time being.

3.5.5 Impact on distribution networks

The impacts of PV systems on the performance of distribution networks are currently oneof the main issues for electric utilities. This is because the size and location of the

installed PV systems mainly influence these networks. The operational problems

introduced by PV systems are similar to those imposed by distributed generators that

produce constant active power, such as diesel generators and fuel cells. These problemsarise mainly due to the installation of generators at the customer side in a feeder designed

for unidirectional power flow. They include malfunctioning of protective relays, voltage

regulation problems, reverse power flow, as well as overloading or underloading of some


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feeders. Other problems arise due to the use of interfacing electronics that lead to

harmonic distortion and parallel and series resonances if a large number of inverters areinstalled in a certain area. Moreover, the fluctuation of the output power of PV systems

adds to the problems faced by the system operator and can deteriorate the power quality

of the network.

The impact of small PV systems installed on rooftops of houses has received the attentionof many researchers during the last few years. This is mainly due to the increase in

installation of these systems due to the incentives provided by governments to residential

customers. Typical ratings for PV systems installed on rooftops of houses range from 1to50 kW.

The issue of harmonic distortion introduced by the power conditioning units used in

small PV systems was the main focus of the studies presented in [85]- [87]. All casestudies showed harmonic distortions far below the limits specified in the standards. This

is mainly because of the great advances made in the inverters technology. However, the

filter capacitors of the interfacing inverters might lead to resonances with the electric

network if a large number of PV systems are installed in a certain neighborhood [88] [89].

The impact of installing small PV systems on the voltage profile of different distributionnetwork topologies was studied in [32]. The results showed that the acceptable voltage

limits were exceeded for all networks when the size of each PV system was 200% of theload of the household. The study assumed that PV systems were installed at every node in

the network, which might not be a realistic assumption. The results of a real case study

presented in [90] indicate the presence of slow transients in the voltage of a mediumvoltagedistribution feeder corresponding to the frequency of fluctuations of the output

power of small PV systems installed on rooftops. Moreover, it was concluded that the

presence of PV systems in the network might reduce the lifetime of transformer tap

changers due to the increase in their operation. Other studies analyzed the impact of smallPV systems on the voltage profile of a low-voltage grid [91]- [93]. However, these studies

did not consider the fluctuations of the irradiance in the analysis.

In general, small PV systems installed on rooftops and facades of buildings might not

impose serious problems on the distribution network. This is mainly because the size ofthese systems requires high concentration in a small area in order to be able to affect the

performance of the network. Such situation is not likely to happen often, as the current

trends show that small PV systems are usually dispersed over a large area. Suchdispersion reduces the impact of fluctuations as the combined irradiance profile over the

complete area is more smooth than that over the individual systems [8].

Only few studies have focused on the impact of large centralized PV systems on thedistribution network. For example, the study in [94] illustrates that the improper choice of

the location of large PV systems can affect the security of the network. Such problem

becomes more severe if the generation of the PV system matches the peak loading of the

electric network as this might increase the loading of some lines that are already heavilyloaded. Thus, to check the network security, the study considered the scenario when the

maximum output power of the PV system matched the peak loading conditions of the

network. However, the overall performance of the network, including voltage profilesand power losses, was not evaluated because no other scenarios were included.

Moreover, no information was provided about how often or when the case of peak

matching might occur.


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In [95], the impact of installing a 5-MW PV system on the voltage regulation and

overcurrent protection of a real distribution feeder was studied. The study shows that thePV system might cause the voltage to reach unacceptable levels during certain periods.

On the other hand, the overcurrent protection was not affected by the operation of the PV

system, as the inverter of the system seized to operate as soon as the fault was detected.

The main advantage of this study is in the fact that a real case is analyzed where thecorrective devices used for voltage regulation (transformer LTCs and shunt capacitors)

and protection were included. However, the conclusions drawn are based on simulating

the output power of the PV system over a five-day period only and with time resolutionof 1 hour. Hence, the variations of loading during different seasons and the sub-hourly

fluctuations are not considered in this study, even though they are essential for proper

evaluation of the performance of the network.The impact of increasing the penetration level of PV systems on the network losses was

analyzed in [96]. However, the analysis did not investigate the impacts of the PV system

on other performance parameters such as the voltage profile of the network and power

flow in the lines. To perform such a study, the power fluctuations of the PV output power

should be simulated accurately. Thus, the hourly irradiance data used in the analysis of[96] might not be appropriate for this case.

From the above discussions, it can be concluded that large centralized PV systems shouldbe the main concern when studying the impacts of PV systems on the performance of

distribution networks. Upon studying these impacts, it is important to consider the

fluctuations in the output power of the PV system as it constitutes an inherentcharacteristic for these systems. Moreover, to obtain accurate results, it is important to

examine the performance of the network for an extended period of time in order to

consider different possible patterns of generations from the PV system and loading

conditions of the feeder under study. To consider these aspects, it is essential to use amethod that can manipulate the available data efficiently to be able to provide realistic

evaluations about the performance of the network.

major advances in power electronicstechnology using integrationas a keytechnique to achieve major improvements in:

Performance/quality Reliability Cost

There Are Many Issues Still to Be Resolved Such as Islanding Issue

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