Case 1: THDI I1 I2 I3 > 3% when inverter output is low, no load
1. Project information and issue summary:
- On 01/22/2022 THDI is high > 3% in early morning (6AM to 9AM) and late afternoon (15PM to 18PM). From 10AM to 14PM, THDI is less then 3%.
- Number of inverter: 08 pcs SUN2000-100KTL-M1
- Customer confirm no reactive power compensation device.
- The load is very low - no use (approximately 0 at this moment 1 pump and fans for cooling 0.4/22kV transformer )
- There is no enery storage converter.
- SLD is attached, measurement device is Power Meter Schneider PM5310
- Measurement value:
o At 15:05PM (01/22/2022), Iavg = 319.8A, THDImax = 1.26%
o At 17:21PM (01/22/2022), Iavg = 23.94A, THDImax = 11.8%
- Measurement value: R&D check all inverter log files, THDI of L1/L2/L3 always less than 1%
2. Explaination:
- Based on SUN2000-100KTL-M1 datasheet, Thdi < 3% following IEC61727 standard
- Following IEC61727 standard, total harmonic current distortion shall be less than 5% at rated inverter output. Huawei inverter can adapt to less than 3% at rated inverter output.
- Inverter 100KTL rated current is 144A@400V. For 1 inverter in 1 phase, I1inv_max_harnomic < 144A*3% = 4.32A/1 phase. For 8 inverter connected parallel: I8inv_max_harmonic = 4.32A*8 = 34.56A /1 phase
- At site, maximum Thdi measured by meter Schneider PM 5300 is 11.774%.
Imeter_harmonic= Iavg * 11.774% = 23.94A * 11.774% = 2.82(A)
Conclusion: Imeter_harmonic < I8inv_max_harmonic and Thdi8inv = 2.82/(8*144)*100%=0.24%, so the inverters are working normal in this case.
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Case 2: THDI increase > 3% when inverter output is high, no load
1. Project information and issue summary:
DC power: 2.15 MWp
Inverters: 16 units of SUN 2000 100 KTL in parallel
Transformer: 1800 KVA
All the power is step up to 22kV and sent to the local grid (no loads)
Location: Hai Phong, Vietnam
On 03/08/2021, This system has a very high THD on current, with the following details (measuring point is the AC combiner cabinet in LV before the transformer):
- When the system runs at very low power <10% the THD I is around 2%
- As soon as the sun increase, (around 50%) of the power, the THD increases to 16%-18% >20%, it seems that the THD I, grows together with the system power
- When disconnecting 4-5 inverters, the total THD returns lower than 3% (we tried to disconnect different groups of inverters and the result is the same, we we could not identify any "faulty" inverter)
- Customer performed the test for 3 days and around 50% of the load and we recorded THD always way beyond the threshold (with an average of 14%), the 3rd and 5th harmonics are the one above the limit
- when the system is running at more than 50% of the capacity it causes high overheating of the LV busbars and high vibrations (the LV cabinet is duly designed for more than 3200A)
- -
- Some information of the grounding system as it may influence the harmonics.
- The PV panels are arranged on a single roof divided in 4 groups or 525 kWp each, all the panels in the group are bonded to the mounting structures and then connected to the inverter station by a 16sqmm copper PE cable, so we have 4 PE cables 16 sqmm coming from different groups of panels on the roof and bonded to the inverter PE terminal on the frame, this terminal is also bonded to a grounding pit nearby the inverter station with Resistance < 1 Ohm. All the inverters frames are bonded in parallel with a 16 sqmm PE cable.
- The inverters are all about 4 meters distant from the step up station, this station consists of a LV AC combiner cabinet 3200A and a transformer 1800 KVA, 0.4/22kV DY type. The LV cabinet is bonded to the Transformer and also to the inverter loop with a 16x 35 sqmm PE cables and have one more grounding pit right below the transformer
- All the cables insulation, rotation and continuity have been checked several times to look for mistakes in the installation but it is normal.
2. Analysis:
- Based on SUN2000-100KTL-M1 datasheet, Thdi < 3% following IEC61727 standard.
- Following IEC61727 standard, total harmonic current distortion shall be less than 5% at rated inverter output. Huawei inverter can adapt to less than 3% at rated inverter output.
- Inverter 100KTL rated current is 144A@400V. For 1 inverter in 1 phase, I1inv_max_harnomic < 144A*3% = 4.32A/1 phase. For 16 inverter connected parallel: I16inv_max_harmonic = 4.32A*16 = 69.12A /1 phase
- At site, maximum Thdi measured by Flicker is 23.05% at I1 = 794.05A at 13:05PM (inverter output is high).
Imeter_harmonic < I16inv_max_harmonic and Thdi16nv = 183/(16*144)*100%=7.9% > 3%
So, Need to investigate further.
3. Troubleshooting:
- Measure THDI of 01 inverter at a time, do it 16 times for 16 inverters to check whether THDI of each inverter is abnormal or not.
- Then measure THDI when 2 inverter on-grid, 3 inverter on-grid, … 16 inverter on-grid.
We have measurement table:
From 1 inverter to 12 inverters are on-grid, THDI is normal.
From 13 inverters to 16 inverters are on-grid, THDI is abnormal.
Analyzed the logs and current harmonic data of 16 inverters. The THDI recorded by the software was lower than 1%. The following figure shows an example of one inverter.
The current harmonic recorded in the log is actually on the inverter side. The field instrument tests the low-voltage side (low-voltage cabinet) of the box-type transformer. Currently, the entire PV system is powered by no load， according to the feedback picture, the inverter is installed in the factory building. It is suspected that other devices (such as frequency converters) are connected to the low-voltage side of the box-type transformer. Other devices generate harmonic components due to the impact of the power, which causes the harmonics to exceed the threshold at the measurement point. Sort out all the devices connected to the low-voltage cabinet of the box-type transformer again
Customer have investigated other possible sources of harmonics in the low voltage part:
a. Small inverter for LED lights (DISCONNECTED)
b. Current transformer for measuring equipment and Protection Relays might be not wired well (under investigation)
c. The transformer has Harmonics Barrier, it might not be properly grounded or sized and cause resonance (under investigation)
4. Conclusion:
For this case customer inspected the Transformer of the client and found a possible faulty grounding connection and customer have repaired grounding connection after that (Ground inslulation < 4Ohm, based on Vietnam standard).
After that we tested again and the harmonics seem lower, we will wait the result of the test early next week
Customer just tested again after connecting the PE cable of the transformer, the result got harmonics within the limits. So it was not Inverter fault.
