Why Fixed String Voltage Is the Key to Unlocking TerraMax™ Inverter Efficiency in 1500 Vdc, Large-Scale Solar
Table of contents
- Background on PV Plant Design Philosophy
- Fixed String Voltage Addresses Large-Scale PV Challenges
- What is Fixed String Voltage and Why Does It Matter?
- Overview of TerraMax Product Family
- Introducing SolarEdge H-Series Optimizers: Designed for 1500 Vdc Systems
- H-Series vs. Other SolarEdge Optimizers: What’s the Difference?
- What is Fixed String Voltage and Why Does It Matter?
- How Does an Optimized System Work?
- Efficiency and PV Module Characteristics
- What are the Technical Advantages of Buck-Only Optimizers?
- How do DC-to-DC Converters Create Fixed String Voltage?
- Maximizing Design Flexibility and Balance-of-System (BoS) Savings with Fixed String Voltage
- What is String Stretching with Fixed String Voltage for TerraMax Inverters?
- Trunk Bus Systems
- Mixing Module Types
- In a nutshell: Why Fixed String Voltage is a Game-Changer
- Other Key Benefits
- The Winning Combination for Large-Scale Solar Installations
- FAQs
Background on PV Plant Design Philosophy
Designers of large-scale PV power plants have been having the Centralized-versus-Distributed configuration debate for over a decade. Central inverters dominated the ground mount space until the mid-2010s when string inverters were first introduced for these types of systems. Central inverters offered the cheapest CAPEX and highest power density but reduced reliability of the plant and required OEM-trained service technicians to perform basic maintenance services.
String inverters, on the other hand, offered extremely easy O&M and high redundancy of the overall plant, but cost more to install and required racking. String inverters at the time had multiple inputs which meant they needed to be installed out in the array and took the place of the DPV combiner boxes. The net effect of this is that the AC collection system would cover the long distance to get back to a centralized AC combiner and medium-voltage step-up transformer. This trade off leads to higher AC cabling costs: the AC voltage was 600 - 800 Vac and required 3 conductors plus a ground compared to a DC collection system where DC voltages are 1000 - 1350 Vdc and only require 2 conductors plus a ground.
Fixed String Voltage Addresses Large-Scale PV Challenges
Large-scale commercial and utility-scale solar projects face several challenges: fluctuating DC voltage, inefficient inverter operation, and limitations on string length and layout flexibility. These issues can lead to energy losses, increased balance-of-system (BoS) costs, and constrained project design. Fixed string voltage technology addresses these pain points by ensuring consistent, optimized voltage levels across the system, maximizing performance and long-term return on investment.
What is Fixed String Voltage and Why Does It Matter?
Fixed string voltage refers to a system architecture where power optimizers regulate and maintain a consistent output voltage from each string — typically around 1250 Vdc, regardless of module mismatch or environmental variation. In 1500 Vdc systems, SolarEdge H-Series power optimizers maintain each string at a nominal voltage — typically 1250 Vdc — allowing the inverter to operate at its highest efficiency point consistently. We’ll talk more about how Fixed String Voltage is created later on in the blog.
Overview of TerraMax Product Family
When SolarEdge launched the TerraMax product family, we combined the best of both distributed and centralized architectures. The TerraMax product family consists of optimized inverters that are 250-330 kW single-input, fixed string voltage and are used with the H-Series 1500 Vdc optimizers.
In a nutshell, the TerraMax inverter handles the DC-AC inversion, grid operability and processing of data sent to a SCADA system and/or SolarEdge's monitoring system. The H-Series optimizer performs module-level MPPT, buck-only voltage regulation, and data collection.
Introducing SolarEdge H-Series Optimizers: Designed for 1500 Vdc Systems
There are two optimizers within the H-Series family: the H1300 and H1500. Both are designed for 1500 Vdc system operation and for use with the TerraMax inverters. The Power Optimizers are intended to be used in a "2-to-1" stringing configuration, meaning that two PV modules are strung in series and connected to the optimizer. The H1500 was specifically designed to accommodate G12 large-format solar cells with higher output current and power than traditional modules with M10 and M6 photovoltaic cells, which are smaller format cells.
Key Highlights:
- H1300 and H1500 support 1500 Vdc
- 2-in-series input per Power Optimizer
- Buck-only DC-DC conversion for higher efficiency (~99%)
H-Series vs. Other SolarEdge Optimizers: What’s the Difference?
Key Technical Details
How Does an Optimized System Work?
Let's get down to the basics. As you’re probably aware, solar modules output DC voltage and current whereas our grid is built on AC voltage and current so to convert DC power to AC power, you need a power conversion system, or inverter. In simplified terms, an inverter takes the DC power on its input side and converts it to AC power by transistors driven by a PWM-signal (Pulse-Width Modulation) which creates the AC waveform needed. Filtering components, like inductors and capacitors, help convert that PWM-driven transistor output into a smooth, sinusoidal waveform (see image below).
PWM-Generated AC Waveform
Inverters are most efficient with a DC voltage level that is closest to the peak voltage of the AC waveform. When we say voltages like 120 Vac, 240 Vac, 480 Vac or 600 Vac, these all represent the RMS voltage, or Root Mean Squared Voltage which is the average voltage of the waveform.
The peak voltage for 600 Vac, for example, is actually 848.5 volts (Peak Voltage=V_RMS*√2 ). This means that the minimum DC voltage required to make 600 Vac is 848.5 volts. However, there are a few aspects that will affect what DC voltage is actually needed:
- Transistor and Diode Voltage Drop — The power electronics (Insulated Gate Bipolar Transistors (IGBTs), Silicon Carbide (SiC) transistors, etc.) will introduce a voltage drop that needs to be accounted for by the DC voltage source. The voltage drop will change based on the current flowing through the transistor, so the higher the current, the higher the voltage drop.
- Natural Grid Voltage Fluctuations — Grid voltage will fluctuate depending on how close your project’s interconnection point is to a substation or generation source. In general, the closer to a substation, the higher the grid voltage will be. If the grid is 5% high, then the DC voltage needed to make the waveform will also need to be 5% higher.
- High Voltage Ride Through (HVRT) — During certain grid events, like generator faults, large load dropouts, etc., the grid voltage may spike to higher levels, sometimes as high as 1.2x the nominal voltage and IEEE 1547 stipulates that the inverters must stay connected during these events, depending on how long the event lasts for. If the grid is 20% high during an event, the DC voltage required will also increase by 20%.
- Overexcited, or Capacitive, Reactive Power Support — Inverters and other power conversion devices connected to an AC grid, may be called upon to provide overexcited reactive power as a means to help stabilize the grid voltage. This is usually done automatically by setting a power factor range and setpoints in a volt-var curve, where the inverter will automatically adjust its reactive power based on the voltage measured. Overexcited reactive power will require higher DC voltage bus voltage than during normal conditions.
Given all these conditions, which could be taking place simultaneously, you can see why having a reliable DC bus is critical to system availability.
Efficiency and PV Module Characteristics
Having sufficient DC bus voltage is just one aspect of how the grid voltage can affect inverters. Efficiency is also impacted by the DC voltage. As mentioned previously, PWM is used to create the AC waveform necessary to inject power into the grid/loads. As the voltage increases to the upper limits of the inverter, 1500 Vdc for example, the device will have to manipulate its duty cycle and frequency to maintain the AC waveform.
As a result of these changes, the power electronics will see higher switching losses, more heat will be generated, and it will need to be properly dissipated, or the output power will need to be reduced. Due to this point, inverters are most efficient when operating closer to the peak AC voltage point while also taking into consideration DC voltage headroom needed to accommodate any grid fluctuations or events, as noted previously.
This is all fine and dandy but how do power electronics, whether converters or inverters, regulate DC voltage?
Inverters and converters regulate DC voltage by changing the current draw from the solar array connected to the inverter/converter. While a PV module does provide a DC power output, the voltage and current have a unique profile that can be shown as an I-V curve, where current is the Y-axis and voltage is the X-axis. Every module has a unique I-V curve however they generally look like the example below.
Example I-V Curve of a Photovoltaic Module
As you can see in the example graph, the voltage is dependent on the output current of the module. As the current increases, the voltage will decrease. If the current hits its maximum value, better known as its short circuit current, the voltage will collapse and will cease providing power (only current with essentially no voltage). All modules have a point in their I-V curve which is known as the MPP or Maximum Power Point which is the optimal value for voltage and current to provide the most power that the module can produce. Remember: Power=Voltage*Current.
Since the voltage and current values change with irradiance and temperature of the solar module, so too will the MPP values. Inverters and converters employ an algorithm known as MPPT, or Maximum Power Point Tracking, to always know where the MPP voltage and current setpoints are, which are constantly changing based on the local, ambient environmental conditions (i.e. irradiance, temperature, etc.), and so ensuring that the system produces as much energy as possible.
It's important to remember that the MPP voltage and the optimal inverter voltage are normally different values so the inverter or converter system must balance its switching losses associated at higher DC voltages with the MPP voltage of the inverter. In the case of any grid anomaly or curtailment of the AC power, the voltage will shift away from the MPP voltage setpoint and move higher as a means of meeting the grid condition or limiting the power output.
Another important thing to note is that every PV module is slightly different: their MPP voltage and current are different, the modules lose efficiency differently over time and any imperfection or soiling on the glass will affect the MPP values.
Optimizing the DC collection system is in our DNA, and this is where SolarEdge provides technology-leading products in the inverter space, with the TerraMax inverters and H-Series Power Optimizers. The power optimizers are DC-to-DC converters which are handling the MPPT for the two modules connected in series to its input. The outputs are regulated DC voltage.
The outputs of the optimizers are then connected in series with each other in order to get a total nominal voltage of 1250 Vdc. This allows for maximum energy production regardless of module type, degradation or soiling. The 1250 Vdc regulated voltage from the power optimizers in series is the sweet spot for the TerraMax inverters in terms of efficiency and headroom to account for fluctuations in the grid.
What are the Technical Advantages of Buck-Only Optimizers?
Let’s now talk a bit more about the optimizers and how they impact system design as well as savings on the balance of systems, but first, let’s go through the basics of DC-to-DC converters. There are generally three types of DC-to-DC converters for power optimizer architecture: boost-only, buck-only, and buck-boost.
Buck, Boost, and Buck-Boost Converters: A Clear Comparison
|
Converter Type |
Voltage Behavior |
Key Use Case |
Advantages |
| Boost-Only | Output voltage is always higher than input voltage | Useful in repowering older 600 Vdc arrays for use with modern inverters | Helps reuse legacy systems when inverter replacement options are limited |
| Buck-Only | Output voltage is always lower than input voltage | Ideal for high-efficiency 1500 Vdc utility-scale systems | Simpler design, fewer components, typically >99% efficiency, high reliability |
| Buck-Boost | Output voltage can be higher or lower than input voltage | Flexible option for rooftops with varying string lengths, 600–1000 Vdc limits | Suitable for projects with complex roof geometries and layout constraints |
Buck-only power optimizers are best suited for large-scale PV plants designed with a maximum voltage of 1500 Vdc. Large-scale PV plants can have long strings and are typically less space constrained than rooftops, so PV plant designers have more freedom with the number of modules per string and number of strings altogether. Having longer strings equates to higher total system voltage which is a perfect operating space for Buck-only optimizers, and thus the most efficient way of operating the optimized system.
How do DC-to-DC converters create Fixed String Voltage?
- DC-to-DC Conversion: Each H-Series Power Optimizer contains high-current transistors, inductors, and capacitors that convert one DC voltage level to another. This is similar in principle to how inverters work.
- MPPT at the Module Level: The Power Optimizer actively tracks the Maximum Power Point (MPP) of the connected modules to extract maximum available power, even under mismatch or partial shading conditions.
- Voltage Regulation: While performing MPPT, the Power Optimizer also regulates its output voltage. Each optimizer adjusts dynamically so that the combined output voltage of all optimizers in a string equals the system target—typically 1250 Vdc.
- Dynamic Compensation: If one Power Optimizer experiences reduced power (due to shading or degradation), neighboring optimizers increase their voltage contribution to maintain a consistent string voltage.
- Anomaly Handling: In case of a module issue—such as a bypass diode failure—the connected optimizer enters bypass mode, allowing current to flow. The remaining Power Optimizers continue to compensate for voltage stability.
- Optimized for TerraMax: This consistent 1250 Vdc is ideal for TerraMax inverters, enabling them to operate at peak efficiency across varying conditions.
- Buck-Only Advantage: As buck-only converters, the H1300 and H1500 models are optimized for reducing input voltage to a fixed output efficiently. Fewer components mean higher reliability and lower cost, all while supporting flexible string design in 1500 Vdc large scale and utility-scale systems. If you’re interested in learning a bit more about how the Power Optimizers work the system, check out our YouTube video on the TerraMax Concept of Operation.
Now that we’ve gone through how inverters and Power Optimizers work, let’s discuss how these products impact the design and operation of the solar power plant.
Maximizing Design Flexibility and Balance-of-System (BoS) Savings with Fixed String Voltage
SolarEdge’s TerraMax Inverters and Power Optimizers are designed to be installed in a centralized inverter configuration, meaning that all the inverters should be centrally located to one another with the Medium Voltage Step-Up Transformer located nearby. This provides significant savings on wiring since AC wiring from the inverters to the transformer is now very short. The DC home run cables are now much longer. However, this provides two main benefits:
- The DC home run cables can be a much smaller sized wire since the DC voltage is operating at 1250 Vdc rather than the AC operating at 690 Vac.
- DC home runs only require 2 current carrying conductors, one for positive pole and one for negative pole plus a ground conductor, whereas AC requires three current carrying conductors plus a ground conductor.
What is String Stretching with Fixed String Voltage for TerraMax Inverters?
Having the ability to have very long strings is one of the main benefits of power optimizers for utility-scale plants. Typical string or central inverters will typically have a maximum string length of 25 to 27 modules. Maximum string length is defined by the VOC (Open Circuit Voltage) of the PV module used and the coldest temperature recorded in the region where the PV plant is located. For utility-scale plants, the maximum string voltage must not exceed 1500 Vdc so the designer must properly calculate what the VOC will be for every plant they design. Using power optimizers drastically reduces and simplifies these calculations by eliminating the problems that arise from max VOC requirements:
- Cross-row stringing or unused racking/tracker tables – String lengths of 27 typically don’t fit nicely into conventional racking or tracker tables so a decision must be made: either have unused portions of racking and not use the land fully or do cross row stringing which can be expensive due to trenching.
- Unable to do cookie cutter designs – As mentioned previously, the VOC is a unique calculation done for the module and the location of the power plant, so even plants located relatively close to one another may have very different absolute minimum temperatures which will change the stringing design.
SolarEdge’s Power Optimizer fix this issue by allowing designers to change string lengths to fit the racking needed for each site, without needing to do cross row stringing, or they allow you to standardize on a particular string length regardless of module type or changes to minimum temperatures.
Another benefit of the stringing stretching is reducing the number of combiner boxes needed for the same site. By having more modules in a string, you need less strings overall which means you need fewer fuses and disconnects, while also reducing the number of home run connections.
One of the first projects to utilize TerraMax inverters with our H1300 Power Optimizers, located in Yreka, California, has string lengths of 50 modules which in turn, lead to significant savings in BoS costs for the site as well as higher energy yields than a traditional string inverter or central inverter design.
Trunk Bus Systems
Trunk Bus systems for utility-scale PV projects have been around for many years now but when used in conjunction with SolarEdge’s Power Optimizer, the savings and ease of installation dramatically increases. Trunk bus systems are designed for easy terminations in the field and essentially eliminating the need for combiner boxes out in the array by having inline fuses, thus reducing labor and CAPEX costs. When you factor in SolarEdge’s ability to do string stretching, this equates to even more savings by reducing the total number of terminations required. We recently published a white paper on this topic, check it out here.
Mixing Module Types
Flexibility is the name of the game most of the time with designers. The more flexibility in a product, the easier it is to design with. While mixing module types in a project may seem like a strange concept, it is actually rooted in financial models where a developer may have safe harbored modules that would qualify as a major material purchase so they can claim the ITC. These safe harbored modules typically have a higher $/W price tag and only a small portion of the total modules actually requires them so mix module types makes financial sense in most cases.
Mixing module types without power optimizers though will result in reduced efficiency of the whole system and therefore will not perform as modeled. Even though two modules may have the same standard power output, their electrical characteristics are most likely very different to each other. Using SolarEdge’s Power Optimizers mitigates this effect by doing module level MPPT, rather than string or system level MPPT, thus ensuring maximum energy harvest from the system.
In a Nutshell: Why Fixed String Voltage is a Game-Changer
- Optimized Inverter Efficiency: TerraMax Inverters operate most efficiently within a narrow voltage band. Fixed string voltage ensures they stay there.
- Higher Energy Yield: Consistent voltage improves energy production by avoiding efficiency losses due to voltage fluctuation.
- Longer Strings: Enables up to 80 modules per string, reducing BoS costs.
- Greater Design Flexibility: Works across varying tilts, orientations, and string lengths.
- Improved O&M: Module-level monitoring enables faster fault detection and troubleshooting, perfect for O&M teams to know before they go to the site.
Other Key Benefits
SolarEdge’s Power Optimizer also brings to the table:
- SafeDC™ for Enhanced Safety: When the inverter is off or the grid is disconnected, optimizers reduce output to just 1 volt—keeping voltage levels safe for installers, O&M technicians, and first responders.
- Sun-Powered Pre-Commissioning: Inverters can be pre-commissioned using the optimizers’ safety voltage, enabling testing and setup even before AC power is available—saving time and simplifying final commissioning.
- Nighttime PID Mitigation: Automatically reduces the impact of Potential Induced Degradation (PID) during overnight hours on ungrounded PV systems, extending module performance and life.
- Reactive Power Control: Full reactive power capabilities from 0 to 0 leading or lagging help support grid stability and create opportunities for additional revenue through ancillary services.
- Arc Fault Detection: Identifies and mitigates electrical arcs to prevent fire hazards.
The Winning Combination for Large-Scale Solar Installations
SolarEdge’s TerraMax inverter and H-Series Power Optimizer architecture represent a significant advancement in large-scale PV system design. By combining fixed string voltage, 1500 Vdc support, and module-level MPPT, this solution simplifies system design, boosts performance, and causes BoS cost reduction. With features like string stretching, module-level monitoring, and compatibility with mixed module types, SolarEdge empowers designers with the flexibility and reliability needed to optimize performance and ROI in modern solar installations.
Ready to elevate your large-scale solar installations with optimized performance and significant cost savings?
Contact SolarEdge today to learn more about TerraMax Inverters and H-Series Optimizers, or explore our large-scale solutions to discover how fixed string voltage can revolutionize your next 1500 Vdc project.
FAQs
Why is fixed string voltage important for 1500 Vdc large-scale solar projects?
It keeps inverters operating within a narrow “sweet spot” voltage band, improving energy harvest, reducing switching losses, enhancing inverter longevity, and enabling simpler, more cost-effective system designs.
How do SolarEdge TerraMax inverters and H-Series Power Optimizers work together?
TerraMax inverters convert DC to AC power at a fixed string voltage, while H-Series optimizers regulate module-level DC voltage and perform MPPT, ensuring stable DC bus voltage and maximizing energy yield across varying conditions.
Why is maintaining a stable DC input voltage crucial for high-power inverters like TerraMax?
Stable DC voltage reduces inverter switching losses and heat generation, ensures reliable operation during grid fluctuations, and keeps the inverter operating efficiently within its optimal voltage range.
What grid conditions affecting DC voltage do PV systems need to be built to withstand?
For efficient and safe production, PV systems must be built with an inverter and converter that can keep the system working during normal grid operation events such as grid voltage fluctuations and reactive power support.
What are H-Series Power Optimizers and how are they designed for 1500 Vdc systems?
H-Series optimizers (including the H1500 model) are buck-only DC-to-DC converters designed for 1500 Vdc solar systems. They support two modules in series per input, enable high efficiency (~99.5%), and accommodate higher wattage and bifacial modules.
What advantages do buck-only optimizers provide over other topologies?
Buck-only converters offer simpler design, higher peak efficiency, fewer components (reducing cost and potential losses), and improved reliability compared to other architectures.
How does the 2-in-series input configuration of H-Series optimizers benefit system design?
Connecting two PV modules in series per optimizer input raises input voltage, helping maintain fixed string voltage with buck-only converters, supporting longer strings, and enabling higher power density in the system layout.
What is string stretching and how does fixed string voltage enable it?
String stretching allows significantly longer strings (up to 80 modules in some cases) by regulating voltage at the optimizer level. This reduces the number of combiner boxes, fuses, disconnects, and field terminations, cutting balance-of-system (BoS) costs.
How do longer strings impact balance-of-system costs and installation complexity?
Longer strings reduce the quantity of wiring, combiner boxes, and field terminations needed. This means less labor, materials (like #10 PV wire), trenching, and simpler tracker designs, leading to substantial BoS savings and easier site layouts.
Can fixed string voltage accommodate complex site layouts and mixed module types?
Yes. It provides flexibility to manage varying string lengths, orientations, tilts, and supports mixing module types for strategies like safe harbor without sacrificing energy yield.
How does module-level MPPT in H-Series optimizers mitigate power losses?
Module-level MPPT optimizes each module’s output individually, reducing losses from mismatch, degradation, shading, or soiling, thus improving overall system performance and energy production.
What safety and monitoring features do TerraMax inverters and H-Series optimizers include?
Features include SafeDC™ (reducing voltage to ~1V when off and detailed module-level monitoring for advanced O&M, enabling fault detection, remote diagnostics, and reducing truck rolls.
How do fixed string voltage systems improve long-term energy yield and system longevity?
By keeping inverter operation consistent and optimal, mitigating mismatch and environmental impacts at the module level, fixed string voltage enhances energy production reliability and reduces stress on system components.
How does the SolarEdge monitoring platform support solar operations?
It provides granular, real-time insights at the module and string level, allowing O&M teams to proactively identify issues, perform remote troubleshooting, and minimize costly on-site interventions.
Why is the combination of fixed string voltage, H1500 optimizers, and TerraMax inverters the winning solution for 1500 Vdc large-scale solar?
This combination delivers design flexibility, maximized inverter efficiency, enhanced energy harvest, simplified O&M, and significant balance-of-system cost reductions—resulting in better ROI for large-scale solar projects.
