Solar PV Switch-Disconnector 1500VDC 400A CLBDPHUS-400 is widely used in 1500VDC solar photovoltaic systems
Although most utility-scale PV developers are struggling to choose string or centralized inverters, is “another photovoltaic architecture: medium voltage direct current power plant (MVDC)” ready to consider?
As First Solar envisioned on the 2016 Analyst Day, the MVDC power station architecture replaces the DC combiner box with a DC-DC converter to increase the string voltage from 1500V DC to 5kV DC to 20kV DC. Higher DC voltage will reduce the current, and can be transmitted to the nearest substation for longer distances, where a considerable MVDC grid-connected inverter converts back to AC power for grid transmission, eliminating the current PV array required for medium voltage AC power Field inverter, transformer and switchgear (MVAC) method. For example, the inverter station of the Siemens MVDC PLUS solution provides power variants from 30MW to 150MW, covering the best points of many utility-scale photovoltaic power plants!
Of course, MVDC also eliminates the AC trenching and wiring of the photovoltaic power station, thereby reducing the initial cost and installation workload. Then, more photovoltaic modules can be installed on the land once reserved for cables, inverters, transformers and switchgear. Other benefits touted by First Solar include preparing for storage on the MVDC bus and providing powerful grid capacity through the MVDC grid-connected inverter in the substation. More interesting details appear in the key First Solar patent application published by the World Intellectual Property Organization (WIPO):
In the event of a grid failure or power outage, the MVDC architecture supports a black start, and the grid power can be restored without relying on an external power supply. The island operation mode is also supported in microgrid applications. In both cases, self-starting of the DC-DC converter or sufficient stored energy and MVDC grid-connected inverter are necessary.
Most of the patents involve DC-DC converters and their interaction with photovoltaic power plant systems, including string maximum power point tracking (MPPT), tracker power and control, DC storage connection, and local controller functions that coordinate with the power plant controller.
After listening to his lecture at the Next Generation Photovoltaic Center, I asked Dr. Mahesh Morjaria, Vice President of First Solar Photovoltaic System, about the current status of utility-scale photovoltaic MVDC. Dr. Morjaria said:
“Some core technologies, especially DC-DC converters that upgrade low PV voltages (such as 1.5 kV) to MVDC, have not yet been fully developed and commercialized. The main value story of MVDC is the potential compared with current utility-scale photovoltaic power plant designs. The capital cost and labor saving of the company. Compared with the AC collection system, the loss of the collection system is also expected to be reduced.”
Siemens Transmission Solutions answered my questions about the MVDC PLUS solution and its potential use in MVDC PV applications. Siemens launched MVDC PLUS in October 2017 and emphasized that its intended use is as a power transmission and distribution grid solution for MVAC utility-scale photovoltaics. Mr. Mirko Duesel, Director of Transmission Solutions of Siemens Energy Management Department, said:
“Yes, MVDC PLUS is ready to connect large-scale photovoltaic power plants up to 150MW and transmit power through DC. Especially for long distances, it is beneficial to use DC technology in the voltage range of 20 to 50 kVdc. This is mainly suitable for passing DC The link connects remote areas (such as large photovoltaic power plants) to the “bridging distance” use case of the power distribution or transmission grid.
Currently, photovoltaic power plants operate at a low voltage level of 1500V and need to be coupled to MVDC PLUS through an AC step-up transformer to adapt to the applicable voltage level. ”
“At the same time, Siemens is studying direct DC-DC boost conversion to reduce power loss. Usually this is possible, but in order to reduce loss, the DC voltage level depends on the distance of the DC link and the power that needs to be transmitted. Therefore, Case investigation is necessary.
According to an article from PV-Tech earlier in 2015, GE looked at 20-30MW solar inverters when outlining its next-generation technology plans:
According to Vlatkovic, General Electric’s largest inverter is currently about 4MW, but under medium voltage, larger inverters are possible. “We hope to expand it to 20-30MW blocks,” he said.
Rumor has it that GE Power Conversion has built a medium-voltage PV test power plant for MVDC inverters (possibly from a third party) and low-voltage solar arrays powered by DC-DC converters. Catherine O’Brien’s keynote speech at GE Global Research at the U.S. Department of Energy’s SunShot Initiative 2016 Power Electronics Symposium further echoed the development direction of MVDC.
GE Power declined my request for an interview on the status of MVDC PV. A GE Power spokesperson said:
“We are in a constantly changing environment, and solar energy has always been one of the fastest growing areas in today’s energy landscape. We are constantly adjusting our strategy to meet market conditions and customer requirements. We are currently researching, evaluating and developing what is most suitable for us. MVDC is one of the plans for the benefit of customers.”
As part of the overall super grid research, the Fraunhofer Institute for Solar Energy Systems ISE and First Solar have a similar vision, namely a new MVDC-based photovoltaic power plant structure, as shown in the figure below. However, Fraunhofer ISE predicts that the string voltage will increase to a range of +/-1.5kV DC to +/-3.5kV DC as the input of the DC-DC converter. Double-glass bifacial photovoltaic modules are very suitable for higher string voltage conditions, although the entire bill of materials requires certification.
As a medium voltage power electronics demonstrator, Fraunhofer ISE has developed a 30kW DC-DC converter using a 10kV silicon carbide (SiC) MOSFET with a switching frequency of 16 kHz. Under 3.5kV DC input voltage and 8.5kV DC output voltage, it achieves 98.5% operating efficiency at rated power. Fraunhofer ISE pointed out that with two (2) such DC-DC converters grounded symmetrical configuration, a 17kV DC output voltage can be achieved. A higher voltage is required to connect the MVDC grid-connected inverter. Using the new 15 kV silicon carbide MOSFET, a single system can achieve an output voltage of more than 10 kV, and a symmetrical system can achieve an output voltage of more than 20 kV. This is high enough for a three-phase medium voltage inverter with an AC output voltage of 10 kV.
The second Fraunhofer ISE development project to be completed is a three-phase medium voltage inverter with 20 kV DC link voltage and 10 kV AC output voltage. The demonstrator uses a SiC transistor with a high blocking voltage of 15 kV and a switching frequency of 16 kHz. Due to the special switching mode, the ripple frequency of the inductor is 32 kHz, so the inverter can use extremely small inductors.
On the market, the string optimizer provided by companies such as Ampt and Alencon Systems is the closest commercial product to the required boost DC-DC converter, but it is insufficient in terms of output voltage. Alencon has developed a 2500V DC output string power optimizer and transmitter (SPOT) with galvanic isolation to provide a fixed voltage to Alencon’s central inverter based on 2.5MW grid inverter package (GrIP) modules. Alencon has developed the patented harmonic neutralization technology of the Grip module in cooperation with the SunShot Initiative of the US Department of Energy, and is commercializing the central inverter. Hanan Fishman, President of Alencon Systems said:
“In Alencon’s case, the increased voltage boost is not a technical limitation, but a limitation that has not yet become the actual demand of the end user.”
Recently, very similar to Ampt, Alencon has targeted its string optimizer for photovoltaic power plant transformation, photovoltaic power plant optimization, DC coupled storage, and microgrid applications. These applications do not require a DC output exceeding 1500V. In many cases, DC storage and microgrids require step-down DC-DC converters.
Therefore, the utility-scale photovoltaic MVDC is gradually making progress, but it is still in the development and testing stage. Is it necessary for a conglomerate like Siemens AG, GE or ABB Group to complete the entire MVDC PV product, or does it require a tangent company or start-up company to “step up” the development of medium voltage DC-DC converters, focusing on subverting the utility-scale PV market ? The future of MVDC PV will be to optimize the DC/DC power ratio.
Solar PV Switch-Disconnector 1500VDC 400A CLBDPHUS-400 is widely used in 1500VDC solar photovoltaic systems
Post time: Aug-17-2021
