String PCS vs. Centralized PCS

In the ever-evolving field of electrical energy storage, the architecture of the Power Conversion System (PCS) is a critical factor that can make or break the operational reliability of a project. As the industry shifts towards larger installations—think 5 MWh, 50 MWh, or even 100 MWh—the consequences of a single point of failure become more pronounced. For engineers, asset managers, and utility planners, the choice between a centralized PCS and a modular String PCS is more than just a technical preference; it is a decision that directly impacts system uptime, maintenance strategies, and ultimately, the bottom line.

Let's set the scene. Imagine a 5 MWh battery energy storage system humming along at a renewable-rich substation. The PCS sits at the heart of the installation, quietly converting DC from the battery racks into grid-ready AC. Now, picture the PCS suddenly failing. In a centralized setup, the entire 5 MWh goes offline—no warning, no half-measures, just a complete blackout until the issue is resolved. Scale that scenario up to a 100 MWh project and the stakes become even higher. Outages can mean contractual penalties, lost revenue, and a tarnished reputation for reliability.

But what if there was a way to limit the fallout from such failures? Enter String PCS—a modular approach that slices the PCS function into smaller, independent units. Instead of one monolithic converter, you have a dozen or more, each responsible for a fraction of the total capacity. If one fails, only its share is lost, and the rest of the system keeps delivering power. It's a simple shift in architecture, but the impact on availability and resilience is profound.

What Do We Mean by PCS?

For those new to the terminology, the Power Conversion System is the bridge between the DC world of batteries and the AC world of the grid. It manages charging and discharging, ensures compliance with grid codes, and often incorporates protection and control functions. In large-scale systems, the PCS is not just a piece of hardware—it is the nerve centre that keeps everything running smoothly.

Centralized PCS: The Traditional Approach

Historically, most energy storage systems have relied on a centralized PCS. This means a single, high-capacity inverter or converter handles the entire load. The advantages are clear: fewer components, simpler wiring, and sometimes lower upfront costs. Integration with the grid is straightforward, and control logic can be less complex.

However, the centralized approach comes with a glaring vulnerability: a single point of failure. If the PCS needs maintenance, or if it suffers a fault, the entire system is down. In a 5 MWh installation, that's 5 MWh offline. In a 100 MWh project, it's a complete outage—potentially for hours or even days, depending on the nature of the problem and the availability of spare parts or technical support.

From an operational perspective, this risk is hard to ignore. Downtime not only affects revenue but can also breach service level agreements and erode trust with stakeholders. As systems scale up, the probability and impact of such failures become more pronounced.

String PCS: The Modular Alternative

String PCS turns the traditional model on its head. Instead of one large PCS, the system is divided into multiple smaller units—often called "strings"—each with its own converter and control. In our 5 MWh example, you might have twelve String PCS units, each managing about 0.417 MWh. If one unit fails, only 1/12^th of the capacity is lost. The rest of the system continues to operate, maintaining service and minimizing disruption.

This modular approach offers several key benefits:

Fault Tolerance
The most obvious advantage is resilience. A single failure does not bring down the entire system. In a 100 MWh project, losing one string (about 8.33 MWh if divided into twelve) is far less catastrophic than a total outage. In practice, String PCS can improve system availability by up to 4.8% compared to centralized designs—a figure that translates directly into higher revenue and fewer headaches.
Maintenance Flexibility
With String PCS, maintenance can be performed on individual units without shutting down the whole system. This means planned servicing can be scheduled during periods of low demand, and emergency repairs have a much smaller impact.
Scalability
Adding capacity is as simple as adding more strings. There is no need to overhaul the entire PCS or reconfigure the grid connection. This makes String PCS ideal for projects that anticipate future expansion.
Redundancy and Upgrades
If technology improves or new features are needed, individual strings can be upgraded or replaced without affecting the rest of the system. This future-proofs the installation and allows for gradual improvements over time.

Quantifying the Impact

Let's put some numbers to these concepts. In a centralized PCS setup, a failure results in 100% loss of capacity until repairs are made. In a String PCS system with twelve strings, a single failure results in only 8.33% loss (for a 100 MWh system, that's 8.33 MWh offline). The rest of the system remains operational.

Availability can be calculated as:

\displaystyle \text{Availability} = 1 – \frac{\text{Downtime}}{\text{Total Time}}

Suppose the average repair time for a PCS failure is 12 hours, and such a failure occurs once per year. In a centralized system, that's 12 hours of total outage per year:

\displaystyle \text{Availability}_{\text{centralized}} = 1 – \frac{12}{8760} = 99.86\%

In a String PCS system, the same failure only affects 1/12^th of the capacity:

\displaystyle \text{Availability}_{\text{string}} = 1 – \frac{12}{8760} \times \frac{1}{12} = 99.99\%

That 0.13% difference may seem small, but for a 100 MWh project, it equates to 130 MWh of additional delivered energy per year. At typical market rates, this is a substantial financial benefit—not to mention the improved reliability for grid operators and customers.

Operational Considerations

Of course, String PCS is not a silver bullet. The modular approach introduces more components, which means more potential points of failure. However, because each failure is isolated, the overall system is more robust. Control logic can be more complex, as the system must coordinate multiple converters and ensure balanced operation. Integration with the grid may require additional communication and synchronization.

There can also be a slight increase in initial investment, as more units and associated infrastructure are needed. However, this is often offset by the reduced risk of catastrophic failure and the ability to perform maintenance without full system shutdowns.

Case Study: A Tale of Two Outages

Let's consider two hypothetical 100  MWh projects—one with a centralized PCS, the other with String PCS architecture. Both systems have a rated output of 50 MW (c-factor 0.5), and both experience a PCS failure requiring 12 hours of repair.

Centralized PCS:
The entire 50 MW output is offline for 12 hours. The lost energy is:
\displaystyle 50\ \text{MW} \times 12\ \text{h} = 600\ \text{MWh}
String PCS (12 strings):
Each string represents about 8.33 MWh of storage and roughly 4.17 MW of output. If one string fails, only 4.17 MW is unavailable. Over 12 hours, the lost energy is:
\displaystyle 4.17\ \text{MW} \times 12\ \text{h} \approx 50\ \text{MWh}

The difference is clear. With centralized PCS, a single outage results in 600  MWh of lost energy. With String PCS, the same event leads to only about 50 MWh lost—less than one-twelfth of the total impact. This modular approach dramatically limits the operational and financial consequences of equipment failures.

Takeaway

For large-scale energy storage projects, the choice between centralized and String PCS is more than a technical detail—it is a strategic decision that affects reliability, revenue, and long-term viability. String PCS offers tangible advantages in fault tolerance, maintenance flexibility, and scalability. By minimizing the impact of individual failures, it ensures that the system keeps delivering value, even when things go wrong. For operators and asset owners, this translates to higher availability, lower risk, and a future-proof investment.

Have Questions or Need Guidance?

If you're considering a new energy storage project or looking to upgrade your existing system, our experts at CLOU are here to help. Whether you want to discuss the best PCS architecture for your needs or need tailored technical advice, just reach out. We're always happy to support you in making your project reliable, efficient, and future-ready.

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