impact of high level penetration of photovoltaics on power system

IMPACT OF INCREASED PENETRATION OF

PHOTOVOLTAICS ON POWER SYSTEMS

PRESENTED BY: MUWAFFAQ USMAN

M.TECH POWER SYSTEMS ROLL N0:6

OUTLINE• INTRODUCTION •WHAT IS PENETRATION? • VOLTAGE PROFILE • POWER FACTOR• HARMONICS • SYSTEM STABILITY• PROTECTION ISSUES• REMEDIAL MEASURES• CONCLUSION 08-04-2016 07EE6264 SEMINAR: IMPACT OF INCREASED PENETRATION OF PV ON POWER SYSTEMS 2

INTRODUCTION• A rapid growth is observed in the installation of solar photovoltaic

systems and would increase in the upcoming years. • The development of power electronic devices & fast acting controllers

made it possible to integrate solar PV systems to existing grids at distribution levels. • With the decreasing prices of PV panels, coupled with government

financial incentives, has promoted the used of solar plants.

307EE6264 SEMINAR: IMPACT OF INCREASED PENETRATION OF PV ON POWER SYSTEMS08-04-2016

• Many of these grids were designed to operate with conventional power houses at one end.• Structural changes in power systems result in new concerns regarding

the reliable and secure operation of systems with high penetration of renewable energy.• Integration of Photovoltaics has both positive as well as negative

impacts on the power grid.

08-04-2016 5

WHAT IS PENETRATION?

• Penetration is defined as the ratio of nameplate PV power rating to the maximum load seen on the distribution feeder [4].

• PV penetration (%) = [1].

ADVANTAGES OF PV PENETRATION

• Less pollution• Low maintenance• Minimization of transmission loss• Reduce the stress on the power system

VOLTAGE PROFILE

• In high or low penetration conditions, this results in over voltages and results in changes in power flow.• Increased penetration can cause improper selection of tap setting for the

distribution transformer. • Excess power generated by solar PV during low demand period must export

the active power to grid which results in over voltages and reverse power flow.

VOLTAGE PROFILE WITH AND WIHTOUT PV

08-04-2016 11

CONT.• High penetration of PV can result in voltage instability• Intermitted nature of PV is the reason for voltage fluctuations in grid

connected PV.• Passing of clouds, angle of incidences also play a major role in driving

the system to instability by means of voltage fluctuations.• The voltage regulation becomes difficult during high level penetration

due to tap setting of the distribution transformer.

VOLTAGE FLUCTUATION • Voltage quality is affected by intermittency of PV power output in

distribution systems• Climatic changes can create irradiance fluctuations either for a short

period or long periods.• Voltage problems of system that has solar PV can be characterized as

voltage rise, voltage unbalance and flickers in the network.

VOLTAGE UNBALANCE • Voltage unbalance is a consequence of line voltages not being equal.• Caused by uneven distribution of single-phase loads on a three-phase

system.• PV sources also contribute to unbalance as they generate power on

only one phase. • Unbalance of 2.5% to 3% or greater is unacceptably high Calculated as:

max( , , )100 %a b cunb

dV dV dVVV

POWER FACTOR

POWER FACTOR• Grid connected PV systems operate at unit power factor & the power

produced by the PV is active power.• Active power for the demand is provided by PV and reactive power is

provided by the grid.• In effect this reduces the power factor, as the grid is then supplying

less active power, but the same amount of reactive power.• The PV inverter can be made to provide reactive power, improving the

voltage regulation and power factor. Unfortunately this can interfere with most anti islanding schemes.

HARMONICS

HARMONICS• Penetration of PV systems into the distribution grid causes harmonic

distortion of current and voltage waves.• Its due to the conversion of DC current in order to synchronize with

the AC supply by utilizing an inverter.• PV inverters are the main source to inject current harmonics to the

system.• Produced harmonics can cause series and parallel resonance,

overheating equipment, misoperation of protection devices.• Harmonic filters can be used to overcome this.

DC BIAS• When the inverter is not properly designed, there is a chance that it

will introduce DC bias into the system• DC bias will increase transformer core loss as it will cause the heating

of windings and saturation of transformer core.

STABILITY

SYSTEM STABILITY• Depending on PV penetration level, a portion of the generation is

transferred to the locations closer to the loads which may alter the amount of reactive power supplied to the load.• High PV penetration results in replacing large scale generating units

with the PV systems can limit the availability of reactive power.• This is because most of the PV systems provide active power only.• Increased PV penetration will also result in reduced inertia within the

system which can cause potential rotor angle stability problems.

STEADY STATE STABILITY• Ability of the system to maintain a steady state equilibrium while

satisfying system constrains.• High PV penetration levels can result in variations of bus voltage

magnitudes.• Steady state voltages increases as the PV penetration increases. • Increase in PV penetration, reactive power export decreases while

reactive power generated by the generators increase.

TRANSIENT STABILITY• Ability of power system to maintain synchronism during large

disturbances.• Disturbances range from equipment, line outages to could cover

incase of PV systems.• System inertia plays a key role in transient stability.• High PV penetration results in low inertia of system and can lead to

potential transient stability issues.

CONT.• Power system tend to have larger oscillations during post fault

transients when PV systems are present.• Thus power system is more perturbed with higher PV penetration

levels.• Power systems employed with PV shows increased voltage dips during

the transients.

Relative rotor angle of generators with and without PV during fault.2608-04-2016

CONT.• Dependent on the weather conditions, the output of PV systems can

fluctuate due to cloud transients etc.• As the amount of PV penetration increases the voltage fluctuations

increase.• DG solar PV systems can trip simultaneously due to a single system

event such as extreme voltage dip condition.• Modern PV inverters are equipped with Low Voltage Ride Through

Capability (LVRT) which improves transient stability.

PROTECTION ISSUES

• The protection issues of grid integrated solar PV systems are1. Change of short circuit levels 2. Reverse power flow3. Islanding

1. CHANGE OF SHORT CIRCUIT LEVEL• High penetration of PV systems result in higher fault current

compared to systems without PV.• This is because during a fault PV systems continue to inject current

into the feeder until islanding condition is detected and breakers are open.• This leads to further damage of the grid causing conductor damage or

damage to transformer.

2. REVERSE POWER FLOW

• Power flow is unidirectional in radial distribution systems, hence the protection schemes are designed accordingly.• DG connected can cause alteration in the coordination of the relay

due to reversal in power.• This is also explained in the voltage profile.

3. ISLANDING • The higher the PV penetration into the distribution grid, the greater

the chances of islanding which the PV continues to supply local loads even after supply loads even after utility fault.• If the protection relays do not detect islanding, then the inverters may

remain online and pose a serious threat to equipment and service personnel.

REMEDIAL MEASURES

• Increased PV penetration has positive and negative effects on the power system.• However above a particular level of penetration its observed to have

significant detrimental effects on the power system.• Higher PV penetration causes voltage rise, stability, harmonics, poor

power factor and protection issues.• These effects are not significant for lower penetration levels.

3408-04-2016 07EE6264 SEMINAR: IMPACT OF INCREASED PENETRATION OF PV ON POWER SYSTEMS

CONT.• Thus solar PV systems can be operated at safe penetration levels.• Penetration limit stems from the technical ability of the grid to

reliability integrate distributed solar capacity.• At present there is no perfect rule for determining the penetration

limits.• Penetration limit is dependent on factors 1. Load2. Consumer behavior3. Climate etc.

CONT.• Penetration limits can be found by simulations.• At present PV penetration issues are of concern for European nations

like Germany, Italy, Greece and others.• Penetration level varies from region to region.• In India CEA has approved for a penetration level of 10-15%.

CONCLUSION

CONCLUSION • Use of grid integrated PV systems are increasing rapidly.• PV penetration can have both positive and negative effects on the

grid.• The detrimental effects of grid integration can be avoided limiting the

penetration levels within safe limits.• There is no definite rule to determine the safe penetration level.• Safe penetration level is dependent on many factors of the grid and

varies from region to region.

REFERENCES1. Eftekharnejad, S; Vittal, V. ; Heydt, G.T. ; Keel, B.Loehr, J., “ Impact of Increased

Penetration of Photovoltaic Generation on Power Systems” IEEE transactions on power system, May 2015, pages 893-901

2. Hasheminamin, M. Agelidis, V.G. ; Salehi, V. ; Teodorescu, R.Agelidis, V.G. ; Salehi, V. ; Teodorescu, R. ; Hredzak, B.; “ Index-Based Assessment of Voltage Rise and Reverse Power Flow Phenomenon in Distribution Feeder Under High PV Penetration ” IEEE Journal of photovoltaics, June 2015, pages 1158-1168

3. Cheng, D.; Mather, B.A. ; Seguin, R. ; Hambrick, J. Mather, B.A. ; Seguin, R. ; Hambrick, J. ; Broadwater, R.P ; “ Photovoltaic (PV) Impact Assessment for Very High Penetration Levels “ IEEE Journal of photovoltaics, October 2015, pages 295-300

4. Baran, M.E.Hooshyar, H. ; Zhan Shen ; Huang, A. “Accommodating High PV Penetration on Distribution Feeders” IEEE transactions on smart grid , June 2014, pages 1039-1046

5. Shichao Liu, P. X. Liu , Xiaoyu Wang “Stochastic Small-signal stability Analysis of grid connected photovoltaic systems ” IEEE transactions of industrial electronics, January 2016

THANK YOU

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