Power quality and networks
Embedded generation voltage rise
- where
- voltage rise, line-to-line (kV)
- generator active power export (MW)
- network resistance to the connection point (Ω)
- reactive power absorbed by the generator (Mvar)
- network reactance to the connection point (Ω)
- nominal line-to-line voltage (kV)
- voltage rise (% of U)
Explain this equation
Estimates how much a generator’s export raises the voltage at its connection point. Use it as a first check in a G99 application before a full load-flow study.
Active power P flowing back through the network resistance R pushes the voltage up. Reactive power Q absorbed by the generator flows through the reactance X the other way and pulls it down, which is why running at a leading power factor helps.
This is the in-phase part of the voltage drop across a series impedance, the same expression as a cable volt-drop with the power flow reversed. MW × Ω divided by kV gives kV; dividing again by U gives the per-unit rise.
Sense check: 5 MW through 1.5 Ω at 11 kV gives about 0.68 kV, or 6 %. Absorbing vars at 0.95 pf through 3 Ω of reactance cuts this to about 2 %.
Watch out: on low X/R networks (11 kV overhead, LV) reactive absorption achieves little. Add the rise to the source busbar voltage at minimum load before comparing with statutory limits.
Approximate steady-state voltage rise at a generator’s connection point caused by its export through the network impedance, for G99 connection assessments.
Enter your values to see the result.
Good to know
MW × Ω / kV gives kV. In per unit the same expression is ΔV ≈ P·R − Q·X with all quantities on a common base. Running the generator at a leading power factor (absorbing vars) offsets the rise; this is effective only where X/R is high, which is why it works at 33 kV and 132 kV but much less on 11 kV overhead or LV networks. Compare the result with the statutory limits (ESQCR: ±6 % for HV below 132 kV, +10 %/−6 % at LV) after adding the source busbar voltage and minimum-load conditions.
Common mistake
Using the sign convention backwards. Here Q is positive when the generator absorbs reactive power; with a generator exporting vars enter Q as negative, which increases the rise.
Limits
First-order approximation of the phasor voltage drop, ignoring the quadrature term; accurate to a few percent for rises under about 10 %. Assumes the source voltage is held fixed and no load is connected. A load-flow study is required by the DNO for G99 Type B and above.
Sources
- Xu T., Taylor P.C., Voltage control techniques for electrical distribution networks including distributed generation, IFAC World Congress 2008 (eq. 1, ΔV ≈ (PR + QX)/V)
- ENA Engineering Recommendation G99 Issue 1 Amendment 9 (2022): Requirements for the connection of generation equipment in parallel with public distribution networks
- Grainger J.J., Stevenson W.D., Power System Analysis, McGraw-Hill 1994 – three-phase power and per-unit quantities
For competent high-voltage engineers. Results are calculations from the published methods shown and depend entirely on the data you enter. They are not a substitute for a system study, manufacturer data, the applicable standards and Engineering Recommendations, or your organisation’s safety rules. Never use a result to justify working on or near live HV equipment.