ELECTRIC, WITH AN EDGE

Pad Mounted Distribution Transformer for the PV Inverter Market

In this paper, the authors will comprehensively analyze the development process of the pad mounted distribution transformer for high capacity PV inverters in the North American market according to the four main sections: external structure of the machine, principle of the electrical system, low-voltage cabinet, and oil-immersed three phase pad mounted transformer. The development process of the Pad mounted distribution transformer for PV inverter for the North American market is analyzed.

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Pad mounted distribution transformer for PV inverter

At present, the global fossil fuel reserves are depleting tremendously, and countries around the world are gradually tightening their own strategic reserves of coal, crude oil, natural gas, and so on. In the 20th century, many university scholars and industry experts have been discussing how to efficiently convert clean energy into electricity. Clean energy refers to green energy, that is, energy that does not emit pollutants and can be used directly in production and life.

Clean energy currently includes solar energy, wind energy, hydrogen energy, ocean energy, water energy, nuclear energy and other nearly ten categories, and solar energy because of its low threshold of energy access, the overall simplicity of the power generation and conversion system, a short return on investment cycle, and many other advantages, has become one of the world’s highest proportion of clean energy supply.

In May 2022, the International Energy Agency (IEA) released “Renewable Energy Market Update -Outlook for 2022 and 2023”, which mentioned that the global solar and other renewable energy power generation capacity increase in 2021 hit a new record, and with governments increasingly looking for energy security and climate benefits of utilizing renewable energy, it is expected that this will be the first time in the world that renewable energy will be utilized. This record is expected to grow further in 2022 as governments increasingly look for energy security and climate benefits from utilizing renewable energy. The report also mentions that the global new renewable power generation capacity has reached a record 295GW in 2021, and the unprecedented growth in new installations is mainly driven by solar PV installations in China, Europe and North America, etc. It is expected that the global solar power installations will continue to expand in 2022~2023.

In summary, it can be seen that in the next few years, new energy power generation technology led by solar PV will continue to develop worldwide, and both the overall installed scale and stand-alone capacity of PV power generation will continue to increase. In order to better adapt to the challenges of the international market in the future, the development of high-capacity pad mounted distribution transformer products based on the North American PV inverter market has taken on a more realistic significance.

Liquid-Filled Radial Feed Pad Mounted Transformer

Rated Power:75~2500 kva No-load Loss:180~2680W On-load Loss:1250~27786W

Liquid-Filled Loop Feed Pad Mounted Transformer

Primary Voltage Ratings: 34.5-19.92/13.8-7.957/13.2-7.62/12.47-7.2 or others

Small Substation Transformer

Primary Voltage Ratings: 2.4kV/4.16kV/4.8kV/6.9kV/7.2kV/ 12kV/12.47kV/13.2kV/13.8kV/ 23kV/34.5kV/44kV TYPE: Oil immersed BIL: 95

Single Phase Pad mounted transformer

Rated Power:15~167 kva No-load Loss:50~350W On-load Loss:195~1410W

PV inverter market demand and Pad mounted distribution transformer for PV inverter in the program

The site uses photovoltaic solar panels to generate electricity, which enters the low voltage side of the three phase pad mounted transformer through the inverter, and is then converted into voltage by the three phase pad mounted transformer and output from the high voltage side into the ring network. The developed PV inverter pad mounted distribution transformer not only takes into account the work of the inverter, but also the control tasks of the energy storage modules in the field.

The three phase pad mounted transformer has a capacity of 4500 kVA, a voltage ratio of 34.5 kV/0.8 kV, five-stage no-load voltage regulation, aluminum windings for both HV and LV, a rated operating frequency of 60 Hz, a coupling group labeled Dy1, a vegetable oil cooling medium, a cooling method KNAN, and an average winding temperature rise of ≤ 65 Kilowatt-hours. The average temperature rise of the windings is ≤65K.

The low voltage pad mounted distribution transformer needs to have both docking reservation with the inverter and control circuit reservation. In addition to providing power supply branches for the protection, measurement and control components in the low voltage room, the control branch for photovoltaic energy storage and the PLC circuit branch for inverter monitoring are also required to be reserved for on-site use.

Pad mounted distribution transformer for PV inverter - Housing design

(1) R & D target customers for the North American market, products designed and manufactured in China need to be shipped according to the export process, if the development of China’s conventional PV inverters in the form of the structure of the integrated machine, although you can make the whole machine inside the various parts of the components of the layout space is more adequate, but the problem brought about by the process of shipment pad mounted distribution Although it can make the space of each component inside the PV inverter more adequate, it brings the problem of the qualified structural strength of the pad mounted distribution transformer during transportation and the increase of the cost of export packaging.

Pad mounted distribution transformer for PV inverter packaging

The direct use of containers as pad mounted distribution transformer shell is the most economical way, for R & D and manufacturing enterprises can save a lot of unnecessary expenditure. However, although the use of fully enclosed freight containers can fully utilize their advantages in terms of structural strength, it is difficult to achieve effective heat dissipation after the pad mounted distribution transformer is put into operation. The final solution is to dismantle the corrugated baffles around the container, and keep only the load-bearing frame as the shell of the pad mounted distribution transformer, as shown in Fig. 1.

Effective view of the modified pad-mounted distribution transformer enclosure

(2) Adopting variant container as the shell of pad-mounted distribution transformer, and adopting international ocean-going standard container ship transportation for transportation. After loading, pad mounted distribution transformer has standard container shell, its fixed way can be fixed according to the standard container ship stacking structure.

According to a large international container production and transportation group for international shipping standard container stacking management requirements, loaded container stacking stacking type for 5-4-3-2 trapezoidal layer by layer to reduce the stacking height stacking, and with nylon chemical fiber fastening belt from the side of the cross-fixed to prevent the sea transport process of bumps and rollover, as shown in Figure 2.

Schematic diagram of stacking and fastening effect of loaded containers

pad-mounted transformer diagram system principle design

The whole machine primary The pad-mounted transformer diagram system is shown in Fig. 3.

pad-mounted transformer diagram

(1) Starting from the proposed demand point, the primary and secondary systems of the pad mounted distribution transformer not only have to take charge of the internal temperature and humidity control, lighting and exhaust fan measurement and protection circuits, but also need to provide the energy storage control circuit and the inverter monitoring PLC circuit. Since the working voltage level of the inverter and its monitoring PLC circuit is the same as that of the low voltage side of the three phase pad mounted transformer, both of which are 0.8kV, it is necessary to use it as a branch circuit 1; the remaining energy storage control circuit and the internal control and protection circuits can be controlled by the small dry transformer for pad-mounted distribution transformerthree phase pad mounted transformer. The remaining energy storage control circuits and internal control and protection circuits can be supplied through the small dry transformer used for control to adjust the voltage on the low-voltage side of the pad-mounted distribution transformerthree phase pad-mounted transformer to form another branch 2.

(2) Branch 1 is where the DC power output from the PV panels is converted into AC power by the inverter and fed into the low voltage side of the three phase pad mounted transformer at 0.8kV, and then boosted by the three phase pad mounted transformer to 34.5kV before being fed into the power supply loop. Since the inverter contains frame circuit breaker and branch circuit breaker for protection, the low voltage side of the pad mounted distribution transformer only needs to be connected to the copper row of the inverter, and there is no need for additional protection.

(3) The branch circuit 2 needs to be connected in parallel from the low voltage side of the three phase pad mounted transformer of the pad-mounted distribution transformer, and then through the two control circuits which control the adjustment of the three phase pad mounted transformer to 0.48kV and the energy storage control circuit and the 0.48kV control circuit of the pad mounted transformer respectively. and 0.24kV internal control circuits, respectively. This branch circuit is protected against short-circuit and overload currents by means of frame load switches and fuses. The inverter monitoring PLC circuit is directly connected to the back of the circuit and is protected by a set of fuses.

(4) The energy storage control loop turns the 0.8kV voltage into 0.48kV output through a three-phase control three phase pad mounted transformer, and the control loop turns the 0.8kV voltage into 0.24kV output through a single-phase control three phase pad mounted transformer. The energy storage and control circuits each have a set of fuses for secondary protection.

(5) The high voltage side of the system does not require additional design, because the American combined three phase pad mounted transformer has its own internal fuse and high voltage oil-immersed load switch, which can effectively protect the power transmission path between the high voltage inlet and outlet lines and the three phase pad mounted transformer body.

Pad mounted distribution transformer for PV inverter- Low-voltage cabinet design

The layout of the low voltage cabinet is shown in Figure 4.

Pad-mounted transformer schematic - low voltage cabinet

1) The shell of the low voltage cabinet is made of 2.0mm cold rolled steel plate, and the base is made of 10# channel steel or 4mm steel plate bending. Surface spraying adopts the combination of epoxy primer with a thickness of 80μm and polyurethane topcoat with a thickness of 60μm, which can play a good anti-corrosion effect.

(2) The internal arrangement is divided into two areas according to the system principle design and primary system diagram. The left half of the area is for the three phase pad mounted transformer to lead directly from the low-voltage side to the external copper rows for the use of the inverter circuit.

The right half of the area is the secondary wiring area for energy storage + control circuit, the right half of the area is divided into the upper, middle and lower layers, the upper layer is set up for the isolation load switch, the middle layer is the secondary wiring area for the control circuit, and the lower layer is placed with two dry three phase pad mounted transformer for control.

(3) The cabinet door of the low-voltage cabinet is a double-door with lever lock. The air outlet is reserved above the two side panels of the cabinet for heat exchange of the fan in the low-voltage cabinet.

Design of three phase pad mounted transformer

(1) three phase pad mounted transformer core material using 070 grade high-performance cold-rolled silicon steel sheet, using 45 ° full six-stage seams in the form of step stacking stacked, the core is not set inside the oil channel. Stacking coefficient of 0.97, elliptical cross-section, the total number of levels is divided into 16 levels. Low-voltage upper clamp is made of 8.0mm thick non-magnetic steel plate, which can effectively reduce the eddy current loss of low-voltage upper clamp due to the proximity of low-voltage leads. High-voltage upper clamps, high-voltage lower clamps and low-voltage lower clamps are made of 16# channel steel Q235A. Upper and lower clamps and the core are set between the clip insulation.

(2) The low-voltage winding is wound with 2.2mm aluminum foil, and epoxy DMD is used as the insulating material between the layers of aluminum foil, which can withstand higher winding heat temperature. The inlet and outlet ends are led out by welded aluminum rows. The high-voltage winding is a three-section cylinder type, the wire material is paper-coated aluminum wire, and diamond-dot adhesive paper is used as the interlayer insulation between each layer of the winding, and the inter-section insulation is made by laminating the paper circle with laminated wood. A 22mm main air duct is set between the high and low voltage windings.

(3) The oil tank is made of 8.0mm hot rolled steel plate welded, with 7 sets of chip radiators in the direction of the long axis to provide the necessary heat dissipation for the whole three phase pad mounted transformer. Both sides of the short axis for the high-voltage casing and low-pressure casing respectively, high-voltage casing with the ring network with the use of a one-piece plug head casing 6, V-shaped layout, with the corresponding, as shown in Figure 5. Low-voltage casing for the porcelain bottle casing 3, horizontal arrangement.

pad-mounted transformer diagram - high voltage side

(4) The low-voltage leads are connected by 120mm×14mm aluminum rows, and the aluminum rows are connected to the low-voltage casing by copper-aluminum sheets. The high-voltage phase wires are connected with 25mm2 copper stranded wires; the high-voltage tap wires are connected with 16mm2 copper stranded wires, and the phase wires and tap wires are insulated with 3mm paper on one side. The HV phase wires are connected to the HV casing after being led out from the winding and passing through backup fuses, plug-in fuses and oil-immersed load switches.

According to the recommendation of the fuse manufacturer, it is determined that the insertion fuse is two 40A type in parallel for each phase, and the backup fuse is two 100A type in parallel for each phase.

three phase pad mounted transformer

(5) There is no special structure inside the high-voltage room, which is directly connected to the high-voltage side of the three phase pad mounted transformer by flange. The three phase pad mounted transformer is shown in Fig. 6, and the pad mounted distribution transformer is shown in Fig. 7.

transformer for PV inverter

Simulation test of pad mounted distribution transformer for PV inverter

Key mechanical simulation analysis

  • The static mechanical strength of the improved container pad mounted distribution transformer shell is assessed according to the limiting working condition of ten-layer container stacking height. Through the means of finite element mechanical simulation on the improved container pad mounted distribution transformer shell under the limit working condition stress analysis. The maximum stress is 232.33MPa, which does not exceed the yield limit value of Q235 material. It is proved that the strength of the structure meets the requirements of international marine container stacking. The stress diagram is shown in Fig. 8.
Deformation and stress diagram of the shell under the ten-stage stacking condition.
  • The actual total weight of three phase pad mounted transformer after production is 10500kg, and the same self-gravitational acceleration is additionally added to the three phase pad mounted transformer to analyze the mechanical simulation of the limiting working condition of the three phase pad mounted transformer with four lifting mixes and only two diagonal lifting mixes. The maximum stresses of the two sets of diagonal mounted transformers are 169.52MPa and 208.02MPa respectively, which are less than the yield stress limit value of 235MPa of the fabricated materials. This proves that the strength of the mounted transformers meets the requirements. The deformation cloud diagram of the lifting mix is shown in Fig. 9.
Cloud diagram of deformation of pad mounted distribution transformer
  • The seismic strength analysis of the pad mounted distribution transformer is carried out. After external vibration is applied, the maximum stress generated is 210.06MPa, which is less than the yield stress limit value of 235MPa of the manufacturing material. It can be assumed that the structural strength of the whole machine meets the requirements and no plastic deformation will occur when vibration occurs. The deformation cloud diagram of the machine is shown in Fig. 10.
Cloud diagram of seismic performance of pad mounted distribution transformer

Test results of key performance parameters of pad-mounted transformer specifications

The measured results of the key performance parameters of the three phase pad-mounted transformer ecifications are shown in Table 1.

Table 1 Comparison of measured and standardized values of key performance parameters.

Test item

Standard value

Measured value

Data Conclusion

No-load loss/W

3300

3205

 

 

Qualified

No-load current/%

0.6

0.15

Load loss (75℃)/W

32480

29870

Short circuit impedance (75℃)/%

7.0

7.02

Load loss (85℃)/W

32480

30391

Short circuit impedance (85℃)/%

7.0

7.02

Winding temperature rise (HV)/K

65

51.1

Winding temperature rise (LV)/K

65

63.9

Temperature rise of oil surface/K

55

51.8

 

3-Phase Pad mounted transformer Specification

kVA

HV

LV

Total weigh

oil weight

 

75 kVA

25000Delta

 480GrdY/277

1,400Kg

540KG

 

100 kVA

24900GrdY/14400

600V

729KG

138KG

 

150 kVA

14400V

 600GrdY/347

2400kg

750L

 

200 kVA

13800V

380GrdY/220

2000kg

250kg

 

225 kVA

4160Delta

347GrdY/600V

2500KG

850L

 

300 kVA

34500V

208GrdY/120

2300kg

1025L

 

500 kVA

24940DELTA×12470DELTA

208GrdY/120

3050kg

1170L

 

625 kVA

13,800Delta

480GrdY/277V

2000KG

520KG

 

750 kVA

 24940GrdY/14400

800GrdY/462

3100Kg

920L

 

1,000 kVA

13,200DELTA

 480GrdY/277

8,360lbs

310GAL

 

1,250 kVA

 25000V

416V

4000kg

1250L

 

1,500 kVA

 4160GRDY/2400

 600GrdY/347

4300kg

1400L

 

1,700 kVA

13,800Delta

480GrdY/277V

4600Kg

1050L

 

2,000 kVA

 4160DELTA

 415GrdY/240

4600Kg

1280L

 

2,500 kVA

25000DELTA

600GrdY/347

5850Kg

1680L

 

2,550 kVA

 12470DELTA

 415GrdY/240

5800Kg

1750L

 

2600 kVA

34500DELTA

 416GrdY/240

6350Kg

1940L

 

2800 kVA

21000V

480V

13669lb

489gal

 

3000 kVA

13800V

415V

6700Kg

1885L

 

4000 kVA

34,500DELTA

12,470DELTA

9,500Kg

3560L

 

5000 kVA

 22860GrdY/13200

4160GrdY/2400

9100Kg

2435L

 

7000 kVA

27,600DELTA

13,800DELTA

11600kg

2820L

 

7500 kVA

249400DELTA

4.16kv Wye

12000 Kg

3000KG

 

10000 kVA

24900GrdY/14400

600V

15000Kg

3000L

 

Conclusion

In this paper, the authors start from the background of research and development, combined with examples and related technical parameters, the pad mounted distribution transformer shell, system principle, low voltage cabinet and three phase pad mounted transformer design process has been more in-depth analysis. The mechanical simulation results of the key parts and the test data of the final samples are further verified, which provide a reference for the design and development of high-capacity pad mounted distribution transformer products for the North American market.

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About Bin Dong

Hello, I am Bin, General manager of Daelim which is a leading transformer manufacturer. If you have problems when you are looking for the equipment, what you need to do is tell us.

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