AC coupling in existing systems: Does a second storage system really work alongside KOSTAL?
AC Coupling in existing systems: Does a second storage system really work alongside KOSTAL?
21 kW PV on the roof, DC battery on the hybrid inverter – and now the question arises: Can I simply add another AC-coupled storage system to continue using solar power in a blackout?
This is exactly the situation we are currently encountering frequently in the German market.
This article technically explains the limitations – and which solutions really work.
Initial Situation (Typical Practical Case)
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Existing PV system approx. 6 kW
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Expansion by another 15 kW
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New inverter: KOSTAL Plenticore G2 10.0 Plus (3-phase)
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DC-connected battery (~5 kWh)
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Separate emergency power circuit with switch (galvanically isolated)
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Desired: 5 kW backup power, if necessary 3 kW single-phase
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Key question:
Why "providing 50 Hz" alone is not enough
Many users think:
If I provide a stable 50 Hz signal during a blackout, my PV inverter will recognize it as the grid – and continue to operate.
Technically, this is too simplistic.
A grid-connected inverter not only requires:
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Frequency (50 Hz)
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Voltage (e.g., 230/400 V)
But also a grid-like reference system with:
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defined impedance
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stable short-circuit power
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clean phase synchronization (for 3-phase systems)
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standard-compliant protection logic
Especially for three-phase systems (as in the example with KOSTAL G2), a single-phase "island generator" is not sufficient.
The Core Problem: Two Energy Management Systems at One Grid Connection Point
Here lies the crucial point – and it is often underestimated.
A hybrid inverter like the KOSTAL Plenticore G2 with a DC battery operates in a closed control loop:
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Own energy management
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Own power control
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Own measurement (Smart Meter / CT)
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Own strategy for charging, discharging, zero-export
If an AC-coupled storage system with its own CT control is additionally installed at the same grid connection point, two parallel control loops are created.
In practice, this leads to:
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Power oscillation
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Incorrect feed-in limitation
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Mutual "ramping up and down"
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Unclear priority in grid or island operation
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Error messages or protective shutdowns
This is simplified as a "measurement and control conflict".
Therefore:
Two AC-coupled systems with their own grid power control should not be operated uncoordinated at the same grid connection point.
21 kW PV in island mode – what does that really mean?
In our practical case, we are talking about ~21 kW of PV power in the long term.
However, the desired backup power requirement is only 5 kW.
This means:
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PV power must be massively regulated in island mode
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Surpluses must be immediately stored or curtailed
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The grid-forming device must take over frequency and voltage control
A small "emergency power device" without true grid-forming capabilities can become unstable here.
What solutions are technically sound?
Solution 1: Implement backup power within the hybrid system (recommended option)
If a hybrid inverter with a DC battery is already installed, the backup power function should be resolved within this system.
Advantages:
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Only one energy management system
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No competing CT signals
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Clean phase control
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Clearly defined emergency power circuit
This solution is technically consistent and long-term stable.
Solution 2: Separate emergency power system – but as an independent supply area
If the goal is simply:
"In a blackout, the refrigerator, router, and lights should continue to work."
Then a separate AC system can be useful – but as an independent circuit, not as a parallel system at the main connection point.
This means:
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Separate emergency power socket circuit
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No common power control with the hybrid inverter
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Clear electrical separation
Important: In this scenario, the existing PV inverters usually do not continue to operate during a blackout, as they do not see a standard-compliant reference grid.
Solution 3: Separate AC storage subsystem with its own distribution (cluster approach)
A third – technically more sophisticated – option is to set up an independent AC storage subsystem.
The existing hybrid system (e.g., KOSTAL + DC battery) remains untouched.
Additionally, a Sunpura S2400 system is integrated via a separate distributor with its own measurement and control circuit.
It is important that:
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No common CT control at the same grid connection point
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Clear physical separation of power ranges
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Clear responsibility in island operation (only one system grid-forming)
This concept allows for scalable storage expansion (e.g., multiple S2400 in a cluster), but requires a clean system architecture. Without defined separation, control conflicts can arise between the energy management systems.
This solution is primarily suitable for projects where a structured subsystem architecture is planned – not as a "simple retrofit" into existing hybrid systems.
Common Misconceptions in AC Coupling Context
| Assumption | Reality |
| "50 Hz is sufficient" | Grid impedance, phase control and protection logic are just as crucial |
| "More storage = more security" | Incorrect control architecture can cause instability |
| "You can combine AC systems arbitrarily" | Measurement and control conflicts are common |
| "PV continues to run automatically in a blackout" | Only with grid-forming architecture |
Conclusion: AC coupling is not a modular principle
Especially in the German market with:
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3-phase systems
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Hybrid inverters
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DC batteries
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Smart meter control
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Zero-export requirements
a clean system architecture is crucial.
AC coupling is a powerful concept – but only if the control strategy is unambiguous.
Our Recommendation for Existing Systems
If you:
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Already operate a hybrid inverter with a DC battery
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Have planned an emergency power circuit
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Want to continue using PV in a blackout
Then the backup power solution should be implemented internally within the system – not by a second independent control system at the same grid connection point.
If, on the other hand, only a basic emergency supply is desired, a separate AC system can be a pragmatic solution – clearly separated from the existing energy management.
Discussion desired
Are you currently planning:
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A PV expansion?
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A second storage system?
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Island or backup power in an existing system?
Tell us the key data of your system.
We will technically analyze your system architecture – before control conflicts arise later.