STUDY ON 2.5MW SOLAR INVERTER AND SUPRA HARMONICS

The proof of the pudding is in the eating. It is the same when commissioning solar plants.

We have a few experiences such as follows:

• ?One 1 MW solar plant installed at a airport in south India had a issue of leading power factor of just 0.5 pf. The installer was just not able to resolve this serious Issue.
• One 500 Kw? solar Plant installed at a manufacturing facility when a solar Plant was installed with other sources like DG set had interoperability issues, and the installer of the solar plant was not able to resolve it.
• One grid tied Inverter 3MW manufactured by leading MNC brand feeding directly to the grid we observed current THD above 3% (According to their catalogue) was above 3% and more at operating at less than 50% of its capacity (less irradiation) it was about 10%. The High frequency harmonics Voltages at 20 to 50Khz was about 30 to 40 V (Refer to the recordings) These supraharmonics can be a serious issue if it is made to operate with other Industrial electrical Power system. Since this installation was injecting directly to the grid, and the grid being a infinite impedance source the issue is not serious.
• Our study of the catalogue of the Globally reputed manufacturer shows that the catalogue hides more than it reveals. Hence, it is more critical to ask the right questions and seek clarification on such issues.???

These are some of the serious issues confronted by some users of solar power. Hence becomes necessary to undertake a load flow study and analysis for designing and implementation. The following issues need to be carefully considered.

1. The solar Inverter to be installed whether a centralized of multi string. Which would be the ideal in terms of optimizing solar energy ,electrical efficiency and reliability. Each has its own advantages and disadvantages.
2. The selection of the solar inverter and technology adopted will be very critical for successful operation of Inverter.
3. The anti islanding protection.
4. Whether the Inverter is interactive and smart inverter with function of supplying reactive Power or reactive Power.
5. Whether effective grounding needed to be implemented for Ground Fault over voltage (GFOV) condition and grounding of common mode noise generated by the Inverter.
6. The output of the inverter is sinusoidal and complies with IEEE-1547 -2018. The power quality standard and for seamless integration with grid.

There are many technologies and techniques adopted by various manufacturer. With advent of better Switching devices like the Silicon carbide (SIC) mosfets and IGBT with high density switching and low ripple content, making it possible to switch at higher frequencies, to minimize voltage and current switching harmonics.

Multilevel Inverter with multilevel switching at different switching DC levels enables the Inverter to draw current with minimum current distortion.

Some of the techniques used are a. clamped diode b. flying capacitor and c. Cascaded H bridge Multilevel Inverter (CHBMLI)

Some of the unique feature of these technologies are:

1. Smaller common mode voltage avoiding noise.
2. Input current Multilevel frequency Inverters with low switching frequency. Improving switching efficiency.
3. Fault tolerance of multilevel Inverters are much better.
4. Some smart inverter integrate Selective Harmonic cancellation.
5. The intent and scope of this article is to exercise caution when deciding on the Solar Inverters.

?We are sharing some of the recordings for your study.

PLANT 1 - 2.5MW SOLAR POWER PLANT

SUPRAHARMONICS(abnormal Levels) FROM 2KHZ TO 9KHZ RANGE

SUPRAHARMONICS FROM 9KHZ TO 150KHZ RANGE IN PERCENTILE

CURRENT HARMONICS THD IN PLANT 1 SOLAR:

CURRENT TDD:

PLANT 2: 1MW SOLAR POWER PLANT

SUPRAHARMONICS FROM 9KHZ TO 150KHZ RANGE:

CURRENT HARMONICS THD IN PLANT 2 SOLAR:

CURRENT TDD:

While analysing we witnessed burning of cables at the edges (is it supra Harmonics?)

PLANT 3: 1MW SOLAR POWER PLANT:

SUPRAHARMONICS FROM 9KHZ TO 150KHZ RANGE:

CURRENT HARMONICS THD IN PLANT 3 SOLAR:

CURRENT TDD:

TDD variation depending on Irradiation.

TDD LIMITS AS PER IEEE 519 2014 ON HARMONICS:

FROM ABOVE OBSERVATION

IL = 26.394A

ISC = 26.3KA (from the transformer specification)

HENCE, ISC/IL = 26300/26.39 = 996.58

ISC/IL RATIO = 996.58, ACCORDING IEEE 519-2014 CURRENT HARMONICS LIMIT MUST COMPLIANCE <15% ITDD.

SOME OBSERVATION OF THE SOLAR PLANT:

1.???? We observed even Harmonics of 10th, 12th of about 23%THD Max. These harmonics predominant below 150A. However, these are not issues as per IEEE-519 2014. It is within the Compliance of the TDD limit of 15%.

2.???? Though The IEEE-519 standard indicates harmonics above 2500 Hz may be included, it is unclear if it is mandatory. However high frequency harmonics are always an issue where reliability of the Solar? Inverter? and PV panel are concerned

3.???? In Plant 1 we find that high frequency are abnormal in one phase (Blue). But the Plant 2 and 3 the High frequency is abnormal. Refer to the harmonic graphs.

4.???? There is a lot of ground noise of 60% to 70% also called common mode noise, and this may affect the PV panels Life and reliability of Solar Inverter.

5.???? On perusal of the Solar Inverter Specification, we observed the Current Harmonics of <3%. However in 1st solar Plant the Average THD I is as much as 10% to 12% at 50% load, 150% to 300% at <25% load. The THD I is 4 to 5% at 90 % to 100% load, in 2nd and 3rd at 50% load the THD I varies from 7% to 9%.

6.???? The ground voltage is observed to be 40 to 50 V. This is can cause a leakage circulating current affecting the solar plant reliability.

OUR RECOMMENDATION FOR IMPROVEMENT:

1.???? To reduce Harmonics and Performance of Inverters various switching PWM techniques have been used. They are called Multilevel Inverter with switching at different DC levels to ensure there is no distortion of the Input sinusoidal Waveforms of current and Voltage. This is possible because of advancement of switching devices like SIC (silicon carbide) IGBT and employing soft switching techniques. The Cascaded H Bridge 5 level Inverter is considered to be the best topography.? In terms of electrical efficiency, reliability and minimum distortion of Input waveforms.

2.???? Grounding: Employing Grounding transformer at the injection Point will create a low impedance path for the common mode noise will improve the reliability of solar installation.

Additionally NGR (grounding resistor) and grounding reactor for protecting the solar inverter can be used for protection? against transients and for providing tripping mechanism In the event of Ground fault.

3.???? EMC protection of the emissions towards the PV panels and prevent the premature aging of PV panels because of high frequency leakage currents, for meeting the international EMC regulation of EN 61000-6-3.

4.???? ?Use of shielded cables and grounding them will help in reducing emissions.

5.???? Employing LC filters can mitigate low frequency harmonics (2000Hz to 5000z).

?INSTRUMENT USED:

FOLLOW US?WAVEFORMS

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OUR RANGE OF PRODUCTS:

1. WAVEFORMS TERMINATOR?- ADVANCE PASSIVE HARMONICS FILTER.
2. WAVEFORMS HYBRID HMT?- HARMONICS MITIGATION TRANSFORMER.
3. WAVEFORMS SINEWAVE FILTER?- SINE FILTER FOR OUTPUT SIDE OF VFD.
4. WAVEFORMS TRIPLEN ELIMINATOR?- FOR 3RD,9TH, 15TH..... HARMONICS (Neutral Harmonics Eliminator)
5. SOCOMEC UPS.
6. CVT, STABILIZER, LED LIGHTING & INVERTER.