Based on the provided data for TenneT Holding B.V., here is an assessment of the advisable level of hybrid bond issuance for the next 18 months.

### Summary of Key Facts

- **Entity:** TenneT Holding B.V., a Transmission System Operator (TSO) owned by the Dutch State.
- **Current Hybrid Capital:** €2,125 million outstanding as of January 1, 2023 (and January 1, 2022), first issued in 2010.
- **Total Adjusted Capital (approx.):** Total Equity (€7,713M) + Hybrid Capital (€2,125M) + Long-term Borrowings (€19,006M) + Short-term Borrowings (€709M) = ~€29.5 billion. The current hybrid capital is roughly 7.2% of this total, and S&P's cap is 15% of total adjusted capital.
- **Financial Performance:** The company reported a net loss of €879M in 2022 (€340M loss in 2021). FFO/Net Debt was -0.01x, and Net Debt/EBITDA was 83.85x, indicating extreme leverage under S&P's methodology.
- **Capex and Cash Flow:** Massive investment program underway. Capex (Purchase of PP&E) was €4.4 billion in 2022, up from €3.7 billion in 2021. Cash flow from operations excluding EEG working capital was a modest €596M. Free cash flow after capex is deeply negative, met by massive financing inflows (€6.6 billion). Proceeds from borrowings were €7.3 billion.
- **Market Context:** Interest rates (swap curves) rose dramatically in 2022. The cost of subordinated (hybrid) debt for non-financial IG corporates averaged around 2.295% in 2022, versus 1.298% in 2021.
- **Moody's View:** Adjusted leverage trend is "Deteriorating."

### Analysis of Hybrid Issuance Advisability

**1. Current Starting Point and Historical Precedent**
The entity already has €2.125 billion in hybrid capital, which it has maintained since 2021 (and first issued in 2010). This indicates a clear and established precedent for using this instrument as part of its capital structure. The current level is between 7.5% and 11.25% of the rough adjusted capital calculation. The fact that they are a seasoned issuer is a strong signal that further issuance is a readily available tool.

**2. Financial and Business Risk Profile**
- **Business Risk:** As a state-owned TSO with a strong regulatory advantage in a developed European country (the Netherlands), the business risk profile is strong. Revenue is highly predictable and protected (energy transmission charges).
- **Financial Risk:** This is where the stress lies. The company is in the middle of an extraordinary, multi-year capex program to facilitate the energy transition. This has led to severely stretched credit metrics: negative FFO, enormous net debt to EBITDA, and substantial negative free operating cash flow.
- **Rationale for Hybrids:** Given the "very high" and ongoing capex needs (€4.4B+ pa), the company has "significant leverage pressure" and faces "material downgrade risk without hybrid support." The equity credit from hybrids is crucial for management and its owner (the Dutch State) to fund the capex program while trying to defend the credit rating. The €1.23 billion equity injection from the parent in 2022 shows the state's support but also underscores the funding deficit.

**3. Matching Conditions to Guidance Tiers**
- **15% Tier:** Conditions like "transformational capex program," "material downgrade risk without hybrid support," and "hybrid issuance substantially required to stabilize adjusted leverage trajectory and preserve rating" perfectly describe TenneT's situation. The company's financial metrics are under extreme duress from its capex program, and hybrid equity credit is a critical tool to manage this.
- **11.25% Tier:** "High capex intensity," "significant leverage pressure," and "Investment grade preservation strongly dependent on capital structure optimization" also strongly describe TenneT. The annual €3 billion cap is a limiting factor, but the advisory scope is for the next 18 months.
- **Cost of Debt:** The cost of hybrid capital (Sub-Sen Delta for iBoxx EUR Non-Financial IG) averaged 2.3% in 2022, while the iShares Core Euro Corp Bond yield was 1.1%. The 1.2% delta is material. However, the company's leverage is so high that the cost of *not* issuing hybrids (a potential downgrade, loss of market access, or forced equity injection) far outweighs the incremental coupon cost, which is likely "in line with" or only marginally higher than the current blended cost of new senior debt given the rating pressure.

**4. Advisable Level**
Given the extreme and sustained capex program, the deteriorating credit metrics (FFO/Debt at -0.01x), and the established history of hybrid use, the company should be advised to maximize this tool within the rating agency cap and its own ceiling. A 15% issuance level is the most appropriate instrument to preserve the rating trajectory. The established history of issuing these instruments makes the maximum level a realistic and advisable target for the next 18 months.

<answer>15%</answer>