80 MW Wind Farm on a 60 km 115 kV Radial
A distribution utility has received an interconnection application from an 80 MW wind farm (Type 4 turbines, 0.95 pf leading or lagging at the point of interconnection) to connect to its existing 115 kV radial line. The line is 60 km of single-circuit 336.4 kcmil ACSR with per-phase impedance of 0.08 + j0.40 ohm/km and a thermal rating of 100 MVA. Existing peak load on the line is 55 MW at 0.92 pf. The point of common coupling is an existing 115/34.5 kV substation with on-load tap changers at both ends. Three-phase fault level at the 115 kV bus is 1,800 MVA today, falling to 950 MVA when the adjacent 230 kV transformer is out of service. The grid code requires the voltage at the point of interconnection to stay within plus or minus 5 percent of nominal in steady state and requires generators to ride through faults. The developer's consultant has submitted only a thermal rating calculation and asks for approval.
1. Carry out a first-order steady-state voltage-rise screen for 80 MW of export at unity power factor and state what the result implies for the interconnection.
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Screen. For a radial line fed from a stiff source, the per-unit voltage change at the receiving end is approximately
ΔV/V ≈ (P·X − Q·R) / V²
with P in MW, Q in MVAr, X and R in ohms and V in kV.
X = 0.40 × 60 = 24 Ω, R = 0.08 × 60 = 4.8 Ω.
- Unity power factor (Q = 0): ΔV = (80 × 24) / 115² = 1,920 / 13,225 = 0.145, that is +14.5 %.
- At 0.95 pf absorbing (Q = −26.3 MVAr): (1,920 − 26.3 × 4.8) / 13,225 = 1,794 / 13,225 = 0.136, that is +13.6 %.
- Export that would hold 5 % at unity power factor: P = 0.05 × 13,225 / 24 ≈ 27.6 MW.
Interpretation. The thermal rating of 100 MVA is not the binding constraint; voltage is. Because X is five times R, the rise is driven almost entirely by active power, so absorbing reactive power is a very inefficient remedy: the full 0.95 pf capability of the farm removes barely one percentage point. The OLTC at the 115/34.5 kV transformer cannot fix this either, since it changes the 34.5 kV side and not the 115 kV line voltage. On this screen the 115 kV radial can absorb roughly one third of the requested export. The realistic options are a 230 kV connection, series compensation of the line, or firm curtailment to about 28 MW, and the final answer must come from a full load flow over all dispatch scenarios, not from this screen alone.
2. The wind farm's protection engineer proposes to rely on the existing mho distance relays at both line ends, unchanged. Identify the technical problems this creates and recommend a protection scheme.
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Problems with unchanged distance protection.
- Low, controlled fault current. A Type 4 wind farm contributes roughly 1.0–1.2 pu of its own rating during a fault, not the multiples of load current a synchronous source produces. Distance elements at the grid end measure an apparent impedance inflated by the wind farm infeed, so reach settings no longer correspond to line length and coordination with downstream protection fails.
- Polarising quantities. Mho and quadrilateral elements rely on healthy polarising voltage and on negative-sequence or zero-sequence quantities for directional integrity. If the wind farm transformer is delta-connected on the line side, there is no zero-sequence source at that end and ground distance elements see almost nothing; polarisation collapses during close-in faults with depressed voltage.
- Converter control behaviour. Current is injected on a controlled ramp and may be balanced even for unbalanced faults, so phase selection and negative-sequence directional elements can misoperate or time out.
- Weak-infeed conditions. With the 230 kV transformer out, the fault level drops to 950 MVA; the grid-end relay settings were computed for 1,800 MVA.
Recommendation. Use line current differential (87L) over a dedicated fibre channel as the primary protection for the 115 kV line, because it is immune to source strength and infeed. Retain permissive overreaching transfer trip or directional comparison as backup, and re-set the grid-end distance elements using fault studies that model the converter's current-limiting behaviour. Verify that the farm's own protection does not trip for external faults and that it meets the ride-through requirement, and confirm the communication channel availability, since 87L depends on it entirely.
3. The developer asks you to confirm in writing that the line thermal rating is adequate and to approve the interconnection on that basis. How do you respond, and which studies do you require?
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Response. I would decline to write such a confirmation. A statement that the thermal rating exceeds 80 MVA is true but misleading: it answers a question nobody asked and implies an engineering conclusion I have not reached. Approving on that basis would be a failure of competence and of professional responsibility, because my signature would be read as an endorsement of the interconnection as a whole.
What I would put in writing instead: thermal adequacy is confirmed; steady-state voltage adequacy is not, and the first-order screen shows a rise of about 14.5 %, well outside the plus or minus 5 % requirement. Interconnection approval is withheld pending system impact studies.
Studies required:
- Load flow across all credible dispatch scenarios, including maximum export at minimum load (the worst voltage-rise case) and maximum import with the farm off.
- Short-circuit study for both 1,800 MVA and 950 MVA source conditions, to support protection re-setting.
- Transient and dynamic stability, including fault ride-through, post-fault recovery, and the response to the loss of the 230 kV transformer.
- Reactive capability and voltage control study, including whether a STATCOM, SVC or line reactor is needed and where.
- Harmonic and subsynchronous interaction study if series compensation is proposed with Type 4 turbines.
- Frequency response, because the farm displaces synchronous inertia on a small system.
- Protection coordination study covering the changed fault-current profile.
Only when these are complete and the voltage performance is demonstrated can a written approval be defensible.