Accommodating High PV Penetration on the Distribution System of Kinmen Island

doi:10.4236/epe.2013.54B041

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Energy and Power E ngineering, 2013, 5, 209-214

doi:10.4236/epe.2013.54B041 Published Online July 2013 (http://www.scirp .o rg/journal/epe)

Accommodating High PV Penetration on the Distribution

System of Kinmen Island

Yuan-Kang Wu1, Shao-Hong Tsai2, Ming-Yan Zou1

1Department of Electrical Engineering, National Chung-Cheng University

2Department of Electrical Engineering, Hwa Hsia Institute of Technology

Email: [email protected], [email protected]

Received February, 2013

ABSTRACT

A lar ge-scale PV installation has a profound effect on the p ower quality and operation of the grid. Therefore, as instal-

lations of PV systems increase, changes to both technologies and interconnectio n standards are needed to address these

high PV pe netrat ion systems. This paper investigates the main technical i mpacts of large-scale PV generations on power

grids and associated interconnection standards. A case study b y us ing t he PSS /E so ftware in the Kinme n po wer syst em

was impleme nted to study the steady-state and transient char a c te r istics for the high penetr ation PV systems.

Keywords: PV; Penetra tion; Interconnectio n S tandard; PSS/E; Kinmen

1. Introduction

There is an increasing introduction of renewable energy

generation into po wer grids, like wind power and photo-

voltaic (PV) generations, which causes control and oper-

ation problems to the utilities since the net wor ks have not

been pre-designed to take into account these distributed

or intermittent generations. The electricity from PV is

difficult to predict and mainly dependent on weather

conditions. Furthermore, they are usually connected to

grid through power electronic converter, and this also

presents a significant difference from conventional ge-

nerators. As the penetration of the solar PV plant in-

creases, its impact on the stability and operation of the

grid needs to be well understood. Because of the inter-

mittency of the output from solar plants, these impacts

might be significant, especially when the solar irradiance

drops from 100% to 20% in a minute, causing numerous

problems in a high penetration scenario. Additionally,

since traditional feeders are commonly designed for radi-

al unid irectional power flows, it is expected that some of

the most significant impacts occur for large PV penetra-

tion levels.

The main technical impacts of large scale PV genera-

tions on power grid are: voltage, current profiles, power

quality, protection, electric losses, power factor, power

balancing, reliability, protection and operability of the

system. Solar PV generation impacts can be of steady

state or dynamic in nature. These impacts vary in severi-

ty as a function of the degree o f penetration and location

of solar PV distributed gene rat ion. Fo r inst ance, PV ge n-

eration is genera lly at its optimal arou nd moon, and ge n-

erally this is a period of light load condition; therefore,

power export through the distribution feeder and the

substation back to the system becomes possible. The as-

sociated r everse po wer flow will te nd to ra ise the volta ge

on the distribution feeder. Additionally, introducing PV

at the load side would reduce the load demand and in

turn leads to reduced losses and improved voltage pro-

files on the feeder. This is an important benefit from a

capacity planning perspective since it allows utilities to

defer capital investments.

Typical system integration problems in large PV sys-

tems include [1]: no dispatch capability of PV solar

farms without storage, ultra-fast ramping requirements

(400 – 1000 MW/min), reactive power management of

feeders are not designed with high PV production, power

quality, especially voltage fluctuations, flicker and har-

monics may be out of IEEE-519 and other standards.

Several research works [2] indicate that the impact is

lower in the case of distributed PVs compared to concen-

trated PVs with the same penetration level, and a larger

grid with slow-response generators experiences larger

variations in system.

In the USA, the department of energy (DOE) solar

energy conducted a competitive Funding Opportunity

Announcement, and six competitively selected projects

have been completed [3]. For example, several projects

used actual distribution feeders as the basis for model

development and testing high PV penetration; according

to the research results, PV inverter capabilities may be

useful in mitigating system impacts. Intelligent inverters

are expected to p rovide significant benefit to fut ure elec-

Y.-K. WU ET AL.

210

tric distribution system operations. As PV penetration

rises, new functionality from the inverter would need to

be incorporated. Advanced PV inverters should have the

following features: reactive power control, low voltage

ride through, dynamic control, and works like a tradi-

tional synchro nous generator .

Some of the projects demonstrated how the high pene-

tration of PV will affect operations of an electric distri-

butio n gr id . O the r i mpo r tant re sea r ch it e ms i n t he se DOE

projects include the evaluation of the effects of energy

storage in combination with distributed PV generation,

the benefits from high resolution data collection, the im-

pact of PV system integra tion on p ower syste m relia bility

indices, the impact of cloud movement on power quality,

the impact of feeder configuration and the PV location on

the maximum PV penetration.

This paper investigates several potential problems as-

sociated with high penetration levels of grid-tied PV.

Additionally, interconnection codes or standards for PV

generation and major requirements will be studied. Fi-

nally, a case study by using the PSS/E software in the

Kinmen power system was implemented to study the

steady-state and transient characteristics for the high pe-

netration PV systems.

2. Potential Problems Associated with High

Penetration Levels of Grid-tied PV

The International Energy Agency (IEA) has produced a

series of reports on Task V of the PV power systems

(PVPS). The demonstration projects issued by the IEA

have revealed that voltage rise is one of the primary

concerned problems [4]. The study in [5] also indicates

that PV penetration is limited to approximately 33% by

voltage rise issues in the UK. A study from Japan [6]

suggested that voltage rise is the most serious problem

and would limit penetration levels between 5% and 20%.

Any injection of power into a distribution system would

cause a voltage rise at the point of connection, and this

must be considered by network operators.

Voltage dips due to cloud transients might be an issue

at high PV penetration. Cloud cover and morning fog

require fast ramping and power balancing. Several re-

search works [7] examined cloud transient effects if the

PV were deployed as a central-station plant, and the re-

sults indicate that the maximum tolerable system level

penetration le vel of PV wo uld be limited by the tra nsient

following capabilities (ramp rates) of the conventional

generators. Additionall y, slo w volta ge reg ulatio n a nd fast

voltage regulation in high penetration PV scenarios

should also be addressed. To mitigate the effects of PV

output variability, energy storage systems are another

option, in which the PV inverter control system is inte-

grated into the system. The storage system will generate

or absorb energy as the PV power fluctuates according to

the available solar resource.

In terms of power quality, the results of several re-

search works reveal that the PV contribution to voltage

distortion is far les s than t he c ontributio ns made b y many

customer loads [8]. Therefore, harmonics would not be a

problem as long as the inverter has a well design.

3. Interconnection Codes and Standards

Grid connection guidelines are a major subject with re-

gard to PV gene rat ion. The connection rules and technic-

al requirements that differ from region to region make it

more complicated. IEC has been developing many stan-

dards re lated to individual DG te chnologie s includi ng P V

generators. These guidelines would serve as input to de-

velop a more general connecting guideline for all types

of distributed generation systems and their interactions

with the grid. In 2000, the IEEE published IEEE Stan-

dard 929 IEEE Recommended Practice for Utility Inter-

face of Photovoltaic (PV) Systems up to 10 kW [9]. Thi s

standard was updated from IEEE Standard 929-1988 by

incorporating developments in the PV industry and by

coordinating with UL1741 1999; meanwhile, the IEEE

Standards Coordinating Committee 21 on Fuels, Photo-

voltaics, Dispersed Generation, and Energy Storage

formed a working group to develop the IEEE Standard

for Interconnecting Distributed Resources with Electric

Power Systems, or IEEE Standard 1547 [10]. It provides

a uniform standard for interconnection of distributed re-

sources with electric power systems, and also provides

requirements relevant to performance, operation, testing,

safety and maintenance of the interconnection. In Europe,

every country has its own technical requirements. How-

ever, major requirements in these interconnection codes

for distributed generators or PV generation include gen-

eral requirements and specifications, safety and protec-

tion requirements, and power quality requirements.

For general requirements and specifications, most

standards use a power size to limit the scope. IEEE 929

uses 10 kW and limits the technology to PV systems,

while IEEE 1547 uses 10 MVA and attempts to cover all

technologies. In addition, different PV interconnection

standards specify different requirements on voltage vari-

ation, system frequency, synchronization, and voltage

unbalance. As fo r safety and protection requirements, PV

generators must detect and respond to abnormal condi-

tions occurring on the power distribution systems. For

instance, in almost all technical requirements and stan-

dards, unexpected islanding operation is not wanted. PV

units must be disconnected as soon as possible when the

mai n grid is not energized. All interconnected PV plants

must have a readily accessible, lockable, visible-break

isolation switch s hall be located between the DG unit and

the gri d. T hi s d isconnectio n switc h must ac cessible to the

distribution co mpany. In addition, after grid fault, the

Y.-K. WU ET AL.

211

reconnection of the DG should be automatic once the

grid voltage and frequency have returned to and main-

taine d normal r anges.

PV generators should provide electric power to the

grid with acceptable power quality. Generally, the re-

quirements of interconnection standards include the lim-

its on ha rmonic, DC current injectio n, flicker, and power

factor. PV units using power electronic converters may

inject harmonic currents in the network. The level of

harmonics produced by the PV sho uld not cause a ny dis -

turbances on the distribution system. Many interconnec-

tion codes require specific values for different order

harmonics and the total harmonic distortion. Flicker can

be the result of fast variations of power output of gene-

rators or rapid changes in load current like arc furnaces

or induction motor starting leading to significant voltage

changes on the feeder. In order to reduce or avoid flicker,

many countries require the maximum installed power is

several times smaller than the level of the short-circuit

capacity at the power common coupling (PCC) point.

The PV unit shall not create objectionable or observable

flicker for other customers on the power system. DC in-

jection is a problem because the increased DC voltage

has the potential to increase saturation of magnetic com-

ponents, such as cores of distribution transformers. This

saturation, in turn, causes increased power system distor-

tion. Following IEEE 1547, DC currents injected by DG

must be smaller tha n 0.5% of its rated current at the con-

necti on po int.

4. Dynamic Model of PV Arrays

4.1. Modeling of PV Arrays

The physics based relationship between the output cur-

rent and vol tage of the PV cell is given as [11]:

( )

qV RI

nKT

ph Osh

V RI

II IeR



=−− −





(1)

where, I is output current from the cell, V is output vol-

tage of the cell, Iph is the generated current from photo-

voltaic actio n, Io is the reverse saturation current, q is the

charge of an electron, k is B oltzmann’s con stant, T is the

ambient temper a ture and n is the ideality factor.

4.2. Dynamic PV Model in PSS/E

The PV model was built using the user defined model

integration feature of PSS/E. The PSS/E Solar PV Unit

dynamic stability model was developed to simulate per-

formance of a photovoltaic (PV) plant connected to the

grid via a power converter. The model is based on the

generic type 4 wind model, with the added ability to si-

mulate output changes due to solar irradiation. The PV

Generic Model comprises the following modules: power

converter/generator module, electrical control module,

linear model of a panel's output curve, and linear solar

irradiance profile.

5. Case Study

The Kinmen power system (Figure 1) is chosen as the

test system to integrate PV plants in PSS/E. Before add-

ing any PV plants to the system, the load flow and dy-

namic study have been conducted in this work. Then, a

few case studies were performed where PV plants are

integrated at different buses (1101, 1104 and 1105) and

the effect of the changes in solar irradiation and loss of

PV plants for various penetration levels was observed.

5.1. Effect of Sudden Change of Solar Irradiance

on System Stability

PV power is characterized as an intermittent source of

energy as it is largely dependent on the environmental

conditions. The solar irradiance can have 70~80% drop

in a minute. The drop out of a large PV power in such a

short time could have significant i mpact on the system.

To investigate the impact on the system due to sharp

changes in the level of radiation, the solar irradiance has

been reduced from 1000 W/m2 to 0 W /m2 using different

ramping down rate. The objective is to find out the criti-

cal rate beyond which the system might become unstable

or require some load shedding action. Figure 2 shows

the rate at which output power was changed due to the

drop in solar irradiance at different rate, where PV pene-

tration is 30%. While the change in power for PV is

sho wn in F ig ure 2, t he cha ng e in the s yste m fr equency i s

shown in Figure 3. In this work, various scenarios with

different ramping down rates on radiation and PV pene-

tration have been simulated and the results are shown in

Table 1. It was found that in 30% PV penetration, the

frequency of the PV connected bus for the case of solar

Figure 1 . One-line diagram of the Kin men power s ystem.

Y.-K. WU ET AL.

212

Figure 2 . PV out put pow er with 3 0 % PV pe netr atio n under

diff erent ramping down rate.

Figure 3. Frequency response with 30% PV penetration in

the Kinmen syst em.

Table 1. Frequency response under various scenarios.

Penetration 5 s 10 s 30 s 60 s

5% 59.68 Hz 59.81 Hz 59.93 Hz 59.95 Hz

10% 59. 35 Hz 59.62 Hz 59.85 Hz 59.89 Hz

20% 58. 73 Hz 59.26 Hz 59.71 Hz 59.79 Hz

30% 58. 10 Hz 58.89 Hz 59.57 Hz 59.68 Hz

irradiation change (from 1000 W/m2 to 0 W/m2) in 5s

drops to 58.1 Hz which is above the limit for a

low-frequency load-shedding rela y in the Kinmen power

system to operate. If the PV penetration is continuously

increased, we can find the critical PV penetration at

which the frequency response indicated that the

load-frequency load-shedding relay started to operate.

The critical PV penetration in this study case under the

solar irradiance drop of 1000 W/m2 in 5 s is 42.5%.

5.2. Effect of Tripping all of PV Generation

In this case, the expected worst case scenarios are dem-

onstrated by assuming PV generator tripping. The PV

penetration under 5%, 10%, 20% and 30% is simulated

respectively. It is assumed that all of the PV generation

are tripped out at time t = 2 s. Fig ure 4 and Figure 5,

show the system frequency and the voltage at bus 2201

respectively when all of the PV generators are tripped.

Fro m Figures 4 and 5, it can be observed that penetration

level of PV power has significant impact on the system

frequency and voltage. With the increase of penetration

level, the severity of impact on system stability also in-

creases. A trip of the PV generation creates momentary

oscillations in the frequency. In Figure 4, the magnitude

of the frequency perturbation is influenced by the pene-

trat ion leve l. The higher the pene tratio n, the higher is the

magnitude of oscillation. For instance, for 30% penetra-

tion level, the system frequency goes down to 57.63 Hz.

Figure 4. Frequency response in the Kinmen system if PV

generati on trips of fline.

Figure 5. Transient voltage at bus 2201 in the Kinmen sys-

tem if PV generation trips offlin e.

Y.-K. WU ET AL.

213

If the PV penetration is continuously increased, we can

find the critical PV penetration at which the frequency

response indicated that the load-frequency load-shedding

relay started to operate. The critical PV penetration in

this study case (all of PV trip o ffline) is 34%.

5.3. Effect of a Three-phase Short Circuit Fault

Another worst case scenario, a three phase short circuit

fault, is applied at bus 2201 where the PV plant is con-

nected. The fault was applied through ver y small imped-

ance and lasted for 0.2 s. The solar irradiance is assumed

to remain constant at 1000 W/m2 and therefore, the out-

put po wer fro m the P V plant was at its max imum.

Due to the fault, the voltage at bus 2203 goes down to

0.058 pu until the fa ult is cle ar ed . As sho wn in Fig ure 6,

when the fault was cleared, the voltage curve shows

some transient with a maximum to a value of 1.11 p.u.

volt. However, the oscillation damped down shortly and

goes back to a normal value. Figure 7 shows the system

Figure 6. Voltage respo nse i n the K inmen s yste m if a three-

phase short circuit fa ult appe a rs.

Figure 7. Frequency response in the Kinmen system if a

three-pha se shor t ci rcuit fault appears.

frequency response. The frequency shows oscillatio n and

varies between 61.06 Hz and 59.58 Hz under 5% pene-

tration, which does not trigger a series of loads shedding

relays. If the PV penetration increases, the oscillation

amplitude reduces.

5.4. Effect of PV on Steady-state Voltage and

System Loss

Load current flowing through line impedances and dis-

tribution transformers causes voltage drops along the

feeder. Introducing distributed PV reduces the current

being drawn from the substation and improves the vol-

tage drop. The performance of grid at the point of con-

nection for PV generators is an important factor for in-

creasing the PV penetration. In this work, two types of

PV generation configurations, i.e., concentrated and dis-

tributed, ar e considered in the simulatio n stud ies. Fo r the

concentrated configuration, all of the PV generators are

installed on bus 1101. For the distributed configuration,

PV plants are integrated at different buses (1101, 1104

and 1105). Figure 8 shows the simulation results for the

voltage on bus 1101. It is found from Figure 8 that whe n

the PV penetration increases, then the voltage on bus

1101 increases. Furthermore, the type of concentrated PV

generation would cause larger voltage rise.

This work also investigates the system losses under

different PV configurations. According to the simulation

results, as 20% and 30% penetration is discussed, the

Figure 8. Voltage characteristic under different PV pene-

tration and two ty pes of PV generat ion configu r ations.

Table 2. S y st em loss under different PV penetration and

configura tions.

Penetration

(%) Distributed Type

(MW) Concentrated Type

(MW)

5 0.04 0.04

10 0.04 0.04

20 0.03 0.04

30 0.03 0.04

Y.-K. WU ET AL.

214

system power loss with distributed configuration type is

lower than that with concentrated one. Additionally, the

system power loss is reduced when the PV penetration is

increased under the distributed configuration. As shown

in Table 2.

6. Conclusions

As the integration of large and commercial scale PV on

the distribution level, such systems can have considera-

ble effect on the operation and protection of the system.

This work investigates the main technical impacts of

large scale PV generations on power grids and associated

interconnection standards. Additionally, a case study by

using the PSS/E software in the Kinmen power system

was implemented to study the steady-state and transient

characteristic s for t he hig h pe netratio n PV s ystems . Fro m

the simulation results, several system disturbances, such

as change of solar irradiance, tripping of PV generation,

and three-phase short circuit fault would affect the max-

imum PV penetratio n as a grid . Mor eo ver, PV generation

penetration and configuration types would affect system

voltage and power losses.

7. Acknowledgements

The authors grate fully ac kno wledge the fi nancia l support

by Bureau of Energy, Ministry of Economic Affairs,

R.O.C. under the Project "Development of Key Control

Technologies for Distributed Energy System", and the

National Science Council, Department of Executive,

R.O.C. under the grant no. NSC-101-2221-E-146-009.

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