Power quality issues due to high penetration of rooftop solar pv in low voltage distribution networks a case study

Increased penetration of rooftop solar PV is causing undesirable technical impacts on the distribution networks. Several urban distribution transformers in Sri Lanka are exceeding fifty percent of the solar PV over the transformer capacity, which shed a green light to assess the cumulative effect of the rooftop solar PVs. In this study, a distribution feeder with high solar PV penetration has been selected and power quality issues such as harmonics, over voltage and DC injection are analyzed under various conditions. Effects of nonlinear loads were also assessed to create an accurate representation of the existing network. The simulation results identify whether the selected system satisfies the statutory limits imposed by various global regulations concerned with power quality.31931 00 ©2019 IEEE Power Quality Issues Due to High Penetration o.

Power Quality Issues Due to High Penetration of Rooftop Solar PV in Low Voltage Distribution

Networks: A Case Study

H H H De Silva

Department of Electrical Engineering,

University of Moratuwa, Sri Lanka

ORCID:

https://orcid.org/0000-0002-4359-1643

D K J S Jayamaha Department of Electrical Engineering, University of Moratuwa, Sri Lanka ORCID:

https://orcid.org/0000-0002-6803-8107

N W A Lidula Department of Electrical Engineering, University of Moratuwa, Sri Lanka ORCID:

https://orcid.org/0000-0003-3556-4693 Abstract—Increased penetration of rooftop solar PV is

causing undesirable technical impacts on the distribution

networks Several urban distribution transformers in Sri

Lanka are exceeding fifty percent of the solar PV over the

transformer capacity, which shed a green light to assess the

cumulative effect of the rooftop solar PVs In this study, a

distribution feeder with high solar PV penetration has been

selected and power quality issues such as harmonics, over

voltage and DC injection are analyzed under various

conditions Effects of non-linear loads were also assessed to

create an accurate representation of the existing network The

simulation results identify whether the selected system satisfies

the statutory limits imposed by various global regulations

concerned with power quality

Keywords— DC Injection, harmonics, low voltage network,

over voltage, power quality, rooftop solar PV

I INTRODUCTION With the popularity of the national promotional projects

for roof-top solar PV in Sri Lanka, electricity consumers

showed a tendency of increased installation of roof-top solar

PVs Ceylon Electricity Board (CEB), the electrical power

generation, transmission and distribution authority of Sri

Lanka has published that 70 MW of rooftop solar were

installed by the end of 2017 [1] However, unplanned

interconnection of distributed generators (DGs) could lead to

technical impacts on power system reliability, power quality

and stability

Particularly, power quality issues arising from increased

integration of solar PVs to the utility grid have gained

significant research attention The power electronic converter

that interfaces the solar PV to the system is the main factor

that affects the power quality of the utility network

Thyristor-based, line-commutated inverters are considered

undesirable on the power system due to the generated

harmonic currents [2] To achieve better control and to

address the harmonic issue, the inverter technology evolved

to pulse width modulation (PWM) technology, which

resulted in a better interface system to the solar PV [2] Once

a roof-top solar PV system is installed, it will be

commissioned if and only if all the system parameters are

within statutory limits imposed by the standards such as [3]

and [4] However, cumulative effects are not particularly

evaluated at the commissioning level and therefore, the

impact of increased integration of rooftop solar PV to the

local network is found a timely necessity and has been

evaluated by several research studies

In distribution level, power quality issues in literature

mainly focus on voltage issue In many such studies, PV

inverters are modeled as constant PQ sources, which is

sufficient for analyzing the voltage profile [5-7] In [8], detailed modelling of the PV system is used to analyze the voltage levels and losses along the feeder Futuristic analysis based on a stochastic approach to increase the solar penetration levels is also covered in [8-10] However, the studies reveal that voltage levels have not exceeded the statutory limits due to the existing solar PV penetration levels [6,8,9,11,12] In harmonic analysis related to distribution networks, total voltage harmonic distortion (VTHD) and total current harmonic distortion (ITHD) are observed under different conditions in [13,14] The aggregated effect of multiple single phase inverters and three phase inverters has been analyzed separately in [15] However, according to [11] and [14], with the considered solar PV penetration levels, recorded VTHD levels were within the statutory limits while ITHD levels were violated

in many cases Several studies have considered solar PV as the main or only means of harmonics injection, while the effects of non-linear loads being omitted [9-14] In studies, which assess the harmonic injection by multiple solar PV inverters, a finite pattern on harmonics is not quite observable due to the primary and secondary emissions of the solar PV inverters [13] The resultant harmonics on the system may either attenuate or enhance the individual harmonic content

In this paper, both solar PV and non-linear loads are modeled in detail to replicate the accurate picture of the distribution feeder In analyzing current harmonics at the system level, total demand distortion (TDD) is preferred over ITHD [2] and used in this paper in contrast to other studies [5-8,10-14] Total demand provides a common reference for measuring current distortion compared to the fundamental current component DC injection is equally important due to its impact on the transformers [16,17] However, DC injection due to solar PV has not been recognized in the reported power quality studies, despite its position in regulatory standards [4, 16] The paper is organized as follows In Section II, power quality issues due to increased rooftop solar PV and the limitations are presented In Section III, the details of the modeled network are presented Section

IV presents the results of the study and a qualitative analysis

on the results The effects of solar irradiance and loading condition are the main variables and are presented under six different cases Finally, section V draws the conclusion

II POWER QUALITY ISSUES DUE TO INCREASED SOLAR

PVINTEGRATION The power quality issues associated with solar PV co-relates with the intermittent nature of the PV generation and the effects from the power electronics interface Possible

power quality issues and their causes related to solar PV are

listed in Table I [10]

A Harmonic Limitation Requirements and Standards

THD

The THD definition in (1) is derived from IEEE 519

standard [3] where, Mh is the RMS value of the hth harmonic

component of the quantity M, which can be either voltage or

current [2,3], and M1 is the fundamental value of the quantity

M concerned In a system-level study, TDD is more

appropriate to evaluate the effect of current harmonics at

varying load conditions [2] It is a general practice to check

the current harmonic levels with the base as the inverter rated

current, which is known as total rated distortion (TRD) [18]

However, this study employs maximum demand for the

feeder at considered cases as the base to calculate the TDD

In the TDD definition given in equation (2), Ih is the hth

harmonic current and IL is the maximum demand current at

the PCC

As defined in IEEE 519 standard, the recommended

limits for LV VTHD and TDD are given in Table II As per

the guidelines in [3], since the PV inverter falls into the

power generation equipment category, the TDD limits are

considered from the category Isc/IL<20, hence the

requirements as in Table II Isc is the maximum short circuit

current at the PCC

TABLE I P OWER Q UALITY I SSUES R ELATED TO S OLAR PV

Flicker • Variations of the solar PV generation [5]

Slow voltage

variations • Power flow variations • Apparent motion of the position of the sun, change in

cloud cover, Shading effects

Fast voltage

variations

• High capacity solar PV

• Tracking systems

• Intermittency of the power generation due to change in

cloud cover, climatic conditions

• Variations in the reactive power [10]

Over- voltage • Solar generation (or any DG) increases voltage at

terminals of the generator

Voltage

Unbalance • Uneven interconnection of single phase inverters

Low order

harmonics

• Harmonic network impedance affected by solar PV

inverters [10]

• Primary emissions determined by the control algorithm

of the inverter

• Secondary harmonics caused by the background

distortion and the input impedance of the inverter [10]

Supra

harmonics • Residues from the switching in inverters • Connection, disconnection of neighbouring sources

impact on primary emissions

• Neighbouring harmonic sources causes secondary

emissions [10]

DC offset • Inverters are sources of DC injection [17]

TABLE II H ARMONIC L EVELS [3]

TDD 3 h 11 11 h 17 17 h 23 23 h 35 23 h 35

For the harmonic measurements, standards do not specify whether line-line or line-neutral currents/voltages to be used Based on vectorial analysis, line-line harmonic voltages could range between 0 to 1.15 times measured between line-neutral harmonic voltages at a given frequency These two extreme cases correspond to an angle between two vectors of 0º and 180º respectively [19] However, field measurements

at typical LV distribution sites show that these extreme cases are not observed and the harmonic currents/voltages measured in line-line are very similar to those measured in line-ground Hence in this study, line - ground voltages/ currents are chosen for the harmonic analysis The harmonic current limits indicate the minimum quality of current waveform the customer can inject into the grid at the PCC

At the same time, the utility is responsible for providing a clean low distorted voltage to the customer In addition to harmonic distortion it is worthwhile to analyze the other power quality requirements related to solar PV integration

B Other Power Quality Requirements and Standards Over Voltage: The LV standards specify a ± 6% of the nominal voltage to be maintained [20,21] Specifically, for DG, temporary over voltage (TOV) limits are specified in [4] to be maintained at the PCC However, this study will not focus on the TOV limits, but will consider the 6% threshold

DC Injection: According to IEEE 929-2000, the PV system shall not inject DC current greater than 0.5% of the rated inverter current into the utility interface under any operating condition [22] This limit is highly country-specific, unlike the harmonic limits Some countries have imposed an absolute current as the maximum limit; for example, Australian standards specify a limit of 5 mA as the maximum permitted DC injection, and in Germany it is 1 A [23] The maximum allowable limit in Sri Lanka at the inverter PCC is 1% [21] In this study, a representation of the absolute current at the system is presented High DC injection to the grid is harmful due to possible saturation of the transformer

Voltage Unbalance: The unbalance voltage under operating conditions for a period of 1 week of the 10-minute RMS value of the negative sequence voltage unbalance factor (VUBF) shall be 2% A VUBF of 3%

is allowed occasionally [24]

Flicker: For LV distribution systems, the compatibility levels for flicker absolute short term - Pst and long-term flicker - Plt indices are 1 and 0.8 respectively [4,25] In [26], flicker measurements for 3.5 kW grid connected inverters have not displayed any violations Assessing the flicker levels in high capacity inverters might be a good future study as the low capacity inverter would not cause any significant voltage variation if the grid is sufficiently stiff

III MODELLING THE NETWORK High solar PV penetrated feeder (68% penetration from the maximum demand) of Lanka Electricity Company (LECO, one of the distribution level authorities) network is selected for the study The network is modeled in MATLAB/SIMULINK, and smart meter data recorded in May 2018 are used in the model The details of the feeder are shown in Table III

TABLE III N ETWORK D ETAILS

Transformer 250 kVA 11 kV/400 V

Present maximum

demand

192 kVA / 352 customers Conductor type ABC : 3x70 mm 2 + 54 mm 2

R = 0.443 /km , X = 0.26 mH/km Selected feeder No 2 from 3 feeders /82 customers

Feeder length 400m

Maximum feeder

demand

70 kVA (67 kW+ 21 kVar) Solar customers 9 (48 kW)

A Solar PV Modelling

Detailed single phase and three phase solar PV systems

were developed in MATLAB/ SIMULINK representing the

Yingli Solar (China) YL260p-35b A study conducted on

the solar assessment in Sri Lanka has shown that the

maximum irradiance can reach up to 1000 Wm-2 [27]

Therefore, considering the hourly insolation variation, three

scenarios are considered:

1 100% irradiance at daytime (1000 Wm-2)

2 50% irradiance at daytime due to shading effects

(500 Wm-22)

3 0% irradiance at nighttime

Temperature was kept constant throughout the

simulation at 40 ºC

B InverterModelling

Both 3 phase and single-phase solar PV inverters were

employed in the simulation Single phase inverters are of 3

kW, 5kW capacities and the three phase inverters are of 6

kW, 10 kW capacities The inverter is modeled as a detailed

switching model to create an accurate model of the system

Single phase Inverter model:

Single phase full bridge IGBT inverter

2 level Unipolar PWM generator 20 kHz

MPPT Tracking – Perturb and observe algorithm

Active power controller: single phase dq current control

Double loop PWM controlling

Three phase Inverter model:

3 phase universal bridge with IGBT/ Diode

DC boost converter (350- 700 VDC)

DC link pulse generator 5 kHz

Double loop PWM controlling

Active power controlling: dq current control

C Load Modelling

Energy consumption at each pole of the selected feeder is

taken for the calculation of loads Data is collected in every

15-minute period and the average load consumed at each

pole during different times in the day is considered based on

the load curve of SL Three-time ranges in the load profile

were selected based on the demand as (i) 12 am to 4 am (ii)

10 am to 4 pm (iii) 6 pm to 10 pm Since the effect of solar

penetration has to be studied, two different loadings: as high

loading (above 90 % of the maximum feeder loading) and

low loading (below 30 % of the maximum feeder loading)

were considered in the daytime The load variations along

the feeder are shown in Fig 1 The poles of the feeder were

reduced using the full feeder reduction method in [28] The

feeder was reduced to 13 poles as shown in Fig 2 and,

different scenarios simulated are shown in Fig 3 Scenario 1

Fig 1 Feeder loading at 4 different times considered

Fig 2 Modeled network in MATLAB/ SIMULINK

Fig 3 Scenarios and cases considered in this study considers 100% linear loads while Scenario 2 considering 50% of the loads to be non-linear

Non-linear loads such as computers, fluorescent lamps, induction motors with variable speed drives (VSDs) and the recent trends of using inverter-based motors and compressors

in domestic appliances like air conditioners, refrigerators, and the washing machines are also major concerns on the power quality of the distribution network According to [29]

up to 69% of the modern domestic load may now be comprised of non-linear loads Due to the unavailability of data for the penetration level of non-linear loads in the considered network, 50% of loads are considered to be non-linear, and modeled accordingly A resistive load is connected across a diode bridge to mimic the non-linear load [30].

D Measurements The fast Fourier transform (FFT) analyzer in MATLAB was used for the harmonic calculations The THD, DC, offset, I1 were directly obtained from the simulation TDD is defined as in equation (3) and the THD value was obtained from the FFT analyzer in MATLAB [31] Here, I1 is the fundamental current and IL is the peak demand current At

each case, peak current measured for over 30 minutes was

considered as the IL

(3)

The single phase loads and the single phase solar PV

generation are located in random phases of each pole

replicating the unplanned connections This adds a voltage

unbalance to the system Hence, the maximum demand

current at any given phase is considered and, the maximum

demand currents for the Cases 01-06 are 31 A, 86 A, 86 A,

28 A, 28 A and 96 A respectively

IV RESULTS AND DISCUSSION

As the first step, the THD and TDD levels at the metering

PCC of each inverter were measured and noted that the

levels were within the accepted limits [3] However, the

scope of this study is to analyze the system-level impacts

Therefore, when each inverter is operated alone, the THD

and TDD levels at the corresponding pole were measured

and presented in Table IV It can be observed from Table IV,

that the highest harmonic distortion levels are due to the

three phase inverters The three phase inverters do not

generate triplen harmonics under balance loading [2] and, a

rise of THD levels by single phase inverters compared to the

three phase inverters is expected However, due to the

three-phase unbalanced loads, the effect of triplens will be

significant [33] It can also be noted that the 3 kW single

phase inverter has significant distortion levels compared to 5

kW single phase inverters Lower the power rating of an

inverter, higher distortion level is caused to the power

system [33,34] When system-level impact of individual

inverters is considered, they present VTHD variation of 0%

- 0.39% and ITHD variation of 0.01% - 0.82% and the DC

offset ranges from 0% - 0.06%

When the cumulative effect is considered, with the

presence of multiple harmonic generation sources, the

resultant harmonic levels might either attenuate or enhance

[2,15,32]

A Voltage Variation Along the Feeder

To analyze the voltage variation along the feeder, the

transformer secondary voltage was set to 235 V according to

the measurement data obtained As shown in Fig 4, under

the existing conditions, no voltage violation is observed in

this selected feeder having 68% solar PV penetration

Voltage variation along the Feeder under Scenario 1 (in

Fig 3 - 100% linear loads) is presented in Fig 4(a) and

same under Scenario 2 (in Fig 3 - 50% non-linear loads) is

presented in Fig 4(b) A similar variation is observable

under both scenarios

According to Fig 4(a) and (b), voltages have risen at the

poles having solar PV connected lines when there is solar

generation This agrees with the observations in [10], and is

observable, when comparing Cases 01 and 06 (no solar

generation) with Cases 02 – 05 (having the solar

generation) Typical voltage drop along the line due to the

conductor impedance can be observed in Cases 01 and 06

In the Cases 02 – 05, specially under high solar generation,

voltage rise is observable up to pole 8, where the solar PV

connection terminates Then, the voltage drops along the

line due to the line losses A higher increase in voltage

compared to the other cases is observable in Case 04, where

TABLE IV M AXIMUM L EVELS O F P OWER Q UALITY I NDICATORS

W HEN E ACH I NVERTER IS O PERATED A LONE

Config 1ph 1ph 1ph 3ph 1ph 3ph 3ph 1ph VTHD % 0 0.09 0 0.39 0 0.39 0.33 0.06 TDD% 0.01 0.07 0.01 0.13 0.01 0.13 0.82 0.46

DC (%) 9e-4 0.053 0.003 0.04 0.002 0.04 0.06 0.04

Fig 4 Voltage variation along the Feeder (a) Scenario 1, (b) Scenario 2

Fig 5 VTHD along the Feeder (a) Scenario 1, (b) Scenario 2

there is maximum solar irradiance with low load demand If the PV generation exceeds, in feeder loads as can be observed in Case 04, power flows back from feeder to the upstream network which causes the voltage rise [35] In this case maximum of 4 V voltage rise is observed at pole 8

B Voltage Total Harmonic Distortion- VTHD

It is observed that the VTHD does not exceed the limit

of 8% as indicated in Table 2 However, individual harmonic levels might exceed the limits, but were not analyzed in this study

When the individual inverter operation is compared against all the inverters in operation significant voltage distortion is expected Analyzing the effect of multiple inverters on system power quality is a complex study

because, many factors, such as inverter control, location of

the inverter and the strength of the grid could contribute

Even though a cumulative effect is expected, in some

instances the harmonics can also be attenuated due to the

secondary emissions For low order harmonics, the multiple

inverters might increase the harmonic levels if the grid is

weak [32] Having all the solar inverters being connected,

the VTHD along the Feeder under Scenario 1 and Scenario

2 (in Fig 3) are presented in Fig 5 (a) and (b) respectively

The VTHD levels have significantly increased when

non-linear loads in the system are considered As clean

undistorted voltage waveform supplied at the transformer by

the utility is unlikely to get distorted at its LV side, under

both scenarios, the voltage distortion at the end of the feeder

is comparatively higher than the transformer LV side

In Scenario 1, maximum recorded THD is 0.9 % and in

the Cases 01 and 06, the THD is zero due to the absence of

solar PV generation The impact of solar irradiance level on

the THD can also be observed in Fig 5(a) At all the poles,

given a 50% increase of irradiance, the THD has a slight

increase of around 0.1 - 0.5% This is also verified by the

study done in [36], which states 1.5% - 2.2% change of

THD for instantaneous fluctuations of irradiance However,

this study has focused only on a step change of irradiance

and the THD is considered only at the steady state in

accordance with the definition of THD [2] The rise of

VTHD at the feeder end is significant in low loading

conditions with high PV generation (Case 04) under

Scenario 1 (case with 100% linear loads)

As can be seen in Fig 5(b), under Scenario 2 (case with

50% non-linear loads), VTHD levels have significantly

increased in high loading conditions with high PV

generation (Case 02) However, the results do not reveal a

clear relation between the VTHD and the loading condition

and PV generation under Scenario 2, due to the effects of

non-linear loads

C Total Demand Distortion -TDD

The variation of TDD under scenario 1 and 2 along the

feeder are shown in the Fig 6 (a) and (b) respectively

Unlike in voltage variation and VTHD levels, TDD in both

scenarios is insignificant or marginal to the end of the feeder

from the Pole 6, but TDD limits are violated up to the pole 5

from the transformer [3] This is in the agreement with the

general tendency of harmonic current flow, which is from

harmonic source towards the transformer [2] As in Fig

6(a), under scenario 1, at pole 3, TDD is observed to be

18.8% The higher solar penetration at pole 4 has increased

the TDD at the pole 3

Although it is expected the high irradiance to lower the

current harmonics [36], it is not observed in this study The

two irradiance variation levels (50% and 100%) considered

are not sufficient to represent this fact In [36] a clear ITHD

variation can be observed when the irradiance levels are

changed from 10% - 50% In Scenario 2, in the night peak

(Case 06) higher current distortion is observed indicating the

distortion levels by the non-linear loads In the transformer

end it is high as 19% and at the feeder-end it is around 3%,

which is still higher compared to the other cases The

harmonic current towards the transformer LV side can be

clearly observed in the Scenario 2 (Fig 6(b)) as well

Individual harmonic evaluation must be carried out for a

better representation of the harmonic propagation

Fig 6 TDD along the Feeder (a) Scenario 1, (b) Scenario 2

Fig 7 DC Injection along the Feeder (a) Scenario 1, (b) Scenario 2

D DC Offset The study of DC current injection is important due to the following facts [2,11]: (i) the flow of harmonic currents is towards the transformer, and (ii) DC current can lead to transformer core saturation The allowable limit of DC current injection is 0.5% at the PCC of the inverter [22] However, in this study, the absolute DC offset at the poles are presented

The DC injection along the feeder under Scenarios 1 and

2 are shown in Fig 7 (a) and (b) respectively In Fig 7 (a), a

DC offset of more than 0.1 A is observed upto pole 7, which

is highly undesirable For example, at the inverter PCC (5

kW, 1 phase) a 0.5 A of DC offset will create a DC injection

of 2.3% The effect of high solar irradiance has affected the rise of DC offset irrespective of the load non-linearity At the feeder end, the DC current is low due to absence of solar

PV at the end of feeder

V CONCLUSION This paper has presented a quantitative analysis of power quality issues arising with high penetration of rooftop solar

PV in the low voltage distribution network A case study was carried out for a selected distribution feeder with 68%

of rooftop solar PV penetration The system was developed with detailed three phase and single phase inverter models, linear and non-linear loads, to represent the existing system The voltage variation, THD, TDD, and DC offset along the feeder are the power quality indicators used to analyze the

system-level impact introduced with the high solar PV

penetration The effects on the power quality indicators

under varying conditions; irradiance level, loading level and

the time of the day are discussed

Although, THD and DC injection levels do not violate

the statutory limits, the voltages towards the end of the

feeder are marginal under the present solar PV penetration

level of 68% The TDD values exceed the specified limits at

several cases considered in this study Furthermore, all the

power quality indicators present a significant increase when

the system is equipped with non-linear loads, indicating

negative impacts on the system

The cumulative effect on the system power quality by

the non-linear loads, which constitutes a considerable

percentage in the domestic network, has to be assessed in

detail In addition, the effect of increased rooftop-top solar

PV on system power quality in the upstream system, i.e the

MV side of the transformer, requires further investigation

ACKNOWLEDGMENT Authors would like to acknowledge the financial support

provided by the University of Moratuwa, Sri Lanka under

the research grant SRC/CAP/2018/1

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