DC CURRENT INJECTION INTO THE NETWORK FROM PV GRID INVERTERS

V. Salas1, E. Olas1 , M. Alonso2 , F. Chenlo2 and A. Barrado1Electronic Technology Department, Universidad Carlos III de Madrid1, Legans (Madrid), SPAIN

Photovoltaic Solar Energy CIEMAT2, Madrid, SPAIN

ABSTRACT

The present paper is focused on the study of the DC current injection for low voltage small grid-connected PV systems, which is one power quality requirements by the utility companies. For this aim, the existing status of guide-lines and regulations in six selected countries where the development in the grid PV sector has evolved rapidly over the last decade, (Japan, Germany, USA, Australia,Spain and United Kingdom) has been viewed, according to the dc current injection into the grid. Furthermore, a grid-connected system installed in Spain has been used to perform measures about their possible DC current injec-tion into the grid. Thus, twelve single-phase inverters (ac-cording to the transformer options: 50 Hz LF transformers, HF transformers or transformer-less) from the Europeanmarket have been tested. Many groups of measurements were made, under different conditions. The results show that in all cases there is any DC current injection, even if a LF transformer inverter is used.

INTRODUCTION

Currently the management of energy sources repre-sents a fundamental problem for the development and prosperity of any community. As a result, there exist two major problems: the energy sources and the ambient pollution from the residues from conventional sources.

Taking this into consideration, it is necessary to opti-mize energy resources as with using alternative energy sources. The main characteristics of such sources include their renewability and small contamination contribution. Photovoltaic solar energy is in this category and its use also has increased notably in industry over the past few years.

Common distributed generators, PV generators par-ticularly, are increasingly being connected to utility grids to contribute electrical power to the utility grid to meet power demands and to meet electric consumers demands for alternate sources of power.

GRID PHOTOVOLTAIC ELEMENTS: PHOTO-VOLTAIC INVERTER

Figure 1 shows the main structure of the photovoltaic system, which consists of the photovoltaic generator, inverter, ac-filter and utility grid. The integration of PV systems to electricity networks is covered in the standard [1].

Figure 1 Grid photovoltaic elements

The DC voltage at PV generator is converted to a sinu-soidal AC current waveform at the output of the switch converter, inverter, in order that may be connected and synchronized to the utility network. According to state of the art, inverters can be divided into three groups, in agreement with transformer options: 50 Hz LF transform-ers, HF transformers and transformerless. A review of the state of the art inverter has been also carried out. Thus, three hundred and seventy nine have been viewed, from which the 76 % are 10 kW below and 24 % up of 10 kW.

Table 1 According to the dc current injection into the grid, existing status of guidelines and regulations in six selected countries, in alphabetic order: Australia, Germany, Japan, Spain, United Kingdom and United States

Country Standard Max DC current

permitted with trans-

former

Max DC cur-rent permitted without trans-

former

Australia AS 4777.2 5 mA 5 mAGermany DIN VDE

126- 1000 mA

Japan Technical Guideline for the Grid Interconnec-tion

1 % inverter rated power

1 % inverter rated power

Spain RD 1663/2000

- -

United Kingdom

ER G83/1 - 5 mA

USA IEEE 929-2000

0.5 % rated power in-

verter

0.5 % rated power inverter

23711-4244-0016-3/06/$20.00 2006 IEEE

By the other hand, one of the most important technical issues of the grid connection of generation plants is the power quality. For any grid-connected system, voltage and frequency regulation, harmonic distortion in the operating load range, power factor, protection and operation criteria in the event of a grid failure consideration are important. However, in this paper special attention has been shown in the DC current injection. Because, a dc current fed from the customer's side into the grid can disturb the regular operation of the upstream distribution transformer. It can shift the transformers operating point and might causesaturation. This would result in high primary current peaks, which might trip the input fuse and thus cause a power outage to that specific section of the grid. It would further-more cause increased harmonics.

However, the grid-interface criteria vary with the utility companies and have yet to be standardised internation-ally. Then, according to the dc current injection into the grid, existing status of guidelines and regulations in six selected countries, in alphabetic order, Australia [2], Ger-many [3], Japan [4], Spain [5], United Kingdom [6] andUnited States [7], where the development in the grid PV sector has evolved rapidly over the last decade, have been viewed, Table 1.

Table 2 Inverters tested in the experimentation

Inverter type Company Nominal Power

(W)

Nominal Current

(A)LF transformer

A 2500 10B 4600 22C 5000 33D 2200 10

HF transformerF 2500 10.9G 3000 14.9H 3000 13I 2600 12

TransformerlessJ 3300 14.3K 2300 14.3L 2300 10.9M 2500 13

SOURCES OF DC INJECTION

There are numerous potential sources of direct current, namely: power supplies computer, network faults, geo-magnetic phenomena, cycloconverters, lighting cir-cuits/dimmers, embedded generators, AC and DC drivesand PV grid inverters.

Measurements have been performanced of some sources. For examples, measurements were taken from computer power supplies, [8], monitoring the DC levels.

Thus, from a Laptop 0.04 A DC (7.7% of rms current) was measured as well as 0.03 A DC (11.2 % of rms current) from a Desktop PC. Also, the reference [9] reports 0.34 A DC (0.53 % of rms current) from a fluorescent lighting load. However, up now measurements of DC current injec-tion from PV grid inverter have not been made. Theoreti-cally, two of the three types of inverter inverters, HF trans-former and transformerless inverters, are candidates to DC current injection.

EXPERIMENTAL RESULTS

A photovoltaic grid-connected system installed in the CIEMAT (Madrid 40 23 N, Spain) has been used to per-form measures. This installation was connected to an acquisition data system. This system received values frommeteorological variables and electric signals on the input and output of the inverter. Such values have been meas-ured by means of the Yokogawa PZ4000 modular power analyzer.

To carry out this study, twelve 50 Hz single-phase in-verters have been selected and tested, around 3 kW, from European market. In the Table 2 is shown their most im-portant characteristics.

Three groups of measurements were made: firstly, measurements for all inverters, under different dc operat-ing voltage, using function in harmonic mode of power analyzer, were made. Then, only the cero harmonic was taken. Secondly, in the transformerless and HF inverters a toroidal LF transformer, in accordance with IEC 61558[10], was put into outside between the inverter and the grid connection, observing the possible influence of the trans-former into the dc current. And thirdly, measurements, using function in the normal mode of power analyzer, Idcsimple average, were taken and compared with the meas-urements using the function in harmonic mode.

In the Figures 2 to 6 are shown some results more representatives obtained in this study for every type of inverter.

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0 200 400 600 800 1000 1200 1400 1600

AC Power (W)

DC

cur

rent

in A

C s

ide

(A)

23/08/05

Figure 2 DC current in AC side (A) gathered from a PV LF transformer inverter (type A), taken in harmonic mode

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From the Figures 2, 3 and 4 can be observed that there are some differentiates appreciable for the DC cur-rent measured among inverters for the three groups. Thus, the maximum value is greater in transformerless inverters than HF transformer and LF transformer inverters, respec-tively. Thus, there is DC current non zero when a LF trans-former inverter is used. To probe if that DC current came from inverter or was existing in the network the only DC current network was measured. Values non zero were met.

0

0.01

0.02

0.03

0.04

0.05

0.06

0 500 1000 1500 2000 2500AC Power (W)

DC

cur

rent

in A

C s

ide

(A)

27/07/05

Figure 3 DC current in AC side (A) gathered from a PV HF transformer inverter (type E), taken in harmonic mode

In a second group of measures taken, when a LF toroidal transformer was inserted between inverter and network in a HF transformer and transformerless inverter (previously measured without transformer), is shown in Figures 5 and 6 that the DC current decreases with re-spect to the same measures without transformer. There-fore, the results, then, are similar than LF transformer inverter.

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0 200 400 600 800 1000 1200 1400

AC Power (W)

DC

cur

rent

in A

C s

ide

(A)

8/2/2005

Figure 4 DC current in AC side (A) gathered from a PV transformerless inverter (type E), taken in harmonic mode

And finally, the measures using function in the normal measurement mode, Idc simple average, were similar to harmonic measurement mode.

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0 500 1000 1500 2000

AC Power (W)

DC

cur

rent

in A

C s

ide

(A)

HF transformer Inverter + LF transformerHF transformer Inverter

9/2/2005

Figure 5 DC current in AC side (A) gathered from a PV HF transformer inverter adding an LF toroidal transformer, meas-ured in harmonic mode

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0 200 400 600 800 1000 1200 1400

AC Power (W)

DC

cur

rent

in A

C s

ide

(A)

Transformerless Inverter + LF transformerTransformerless Inverter + LF transformer

8/2/2005

Figure 6 DC current in AC side (A) gathered from a PV transformerless inverter adding an LF toroidal transformer, measured in harmonic mode

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CONCLUSIONS

Based on the results presented the following conclu-sions may be stated. From said legislation it has been possible to determine that discrepancies exist among the standards which are applied in the different countries. For example, in three of the countries analyzed (the United States, Japan and Australia), limitations with respect to the injection of the DC are imposed for inverters with a trans-former.

With respect to measures taken from different invert-ers have been observed that there are some differentiates appreciable for the DC current measured among inverters for the three groups.

Also, there is DC current non zero when a LF trans-former inverter is tested. It might probe that there is a current existing in the network that varies with respect the time.

Finally, when a LF toroidal transformer was inserted between inverter and network in DC/AC converters without LF transformer (previously measured without transformer), is shown that the DC current decreases with respect to the same measures without transformer. Although there is DC current non zero. It might probe that there is a DC current injection to the network.

REFERENCES

[1] IEC 61727: 1996 Photovoltaic (Pv) Systems. Charac-teristics of the utility interface.

[2] AS 4777.2, Grid connection of energy systems via inverters Part 2: Inverter requirements. Australia. 2002

[3] DIN VDE 0126-1-1 Automatic disconnection device between a generator and the public low-voltage grid, 1999.

[4] JISC 8980:1997 Power conditioner for small photo-voltaic power generating system.

[5] Royal Decree 1663/2000, dated September 29th, on the connection of photovoltaic installations to the low volt-age network.

[6] ER G83/1Recommendations for the connection of small-scale embedded generators (up to 16 a per phase) in parallel with public low-voltage distribution networks.Engineering Recommendation, United Kingdom, Septem-ber 2003.

[7] IEEE 929-2000, IEEE Recommended Practice for Utility Interface of Photovoltaic (PV) Systems, 3 April, 2000.

[8] Industry Consultation on Grid Connection of Small PV systems, ETSU S/P2/00332/REP, Halcrow Gilbert, 2000

[9] The BSRIA Power Quality Guide, Application Guide AG2/2000, Pearson,C.C, Uthayanan, V.

[10] IEC 61558-1: 1997, Safety of power transformers, power supplies, reactors and similar products - Part 1: General requirements and tests.

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