--- title: "AC voltage controller in ELC for micro-hydro" subject: "Power Electronics" chapter: 6 key: "semester_7/power_electronics:acvc-elc-micro-hydro" source: https://www.bctengineer.com/q/semester_7/power_electronics/acvc-elc-micro-hydro site: BCT Engineering --- # AC voltage controller in ELC for micro-hydro Power Electronics · Chapter 6: AC voltage controller - Asked in: 2080 Bhadra, 8 marks; 2075 Asoj, 8 marks; 2074 Asoj, 8 marks; 2073 Shrawan, 8 marks; 2070 Asar, 8 marks; 2070 Chaitra, 8 marks (asked 6 times) - Video: [Electronic dump load controller (ELC) for a hydro electric turbine - demo](https://www.youtube.com/watch?v=ehfiMbg0JCI) — E & S Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_electronics/acvc-elc-micro-hydro - Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-6 ## Question Explain the operation of ac voltage controller and its application in electronic load controller (ELC) for micro hydro power plant. ## Short answer An AC voltage controller uses phase control to vary power to a resistive ballast load. In micro-hydro, an electronic load controller keeps total generator power constant by adjusting firing angle: decreasing α diverts surplus power to the ballast when consumer load drops, stabilizing frequency and voltage. ## Answer An AC voltage controller is a thyristor (or TRIAC) circuit that varies the rms value of AC voltage applied to a load at the same frequency, by phase control or integral cycle control. In micro hydro (MHP), it is the heart of the **electronic load controller (ELC)**, which keeps the generator load constant. #### Operation of the AC voltage controller ```text T1 --->|--- +---| |----+ | ---|<--- | | | T2 | ( ~ ) vs R vo | | +-------------------+ ``` - T1 is fired at $\alpha$ in the positive half cycle and conducts until $\pi$; T2 is fired at $\pi + \alpha$ and conducts until $2\pi$. - Before firing, the output is zero; after firing, $v_o = v_s$. - With a resistive load: $$ V_o = V_s\sqrt{\frac{1}{\pi}\left(\pi - \alpha + \frac{\sin 2\alpha}{2}\right)}, \qquad P = \frac{V_o^2}{R} $$ So by changing $\alpha$ from 0 to $\pi$, the power taken by the load changes smoothly from full to zero. #### Why an ELC is needed in micro hydro - Small MHP plants (up to about 100 kW in Nepal) usually have no mechanical governor, because it is costly and slow. - The turbine gets a fixed water flow, so it always produces nearly constant power. - When consumer load drops, the extra power would speed up the generator, raising frequency and voltage and damaging appliances. #### ELC principle The generator always supplies a constant total power: $$ P_{gen} = P_{consumer} + P_{ballast} = \text{constant} $$ The ELC sends the surplus power to a **ballast (dump) load**, usually resistive water or air heaters, through an AC voltage controller. ```text Turbine-Gen ---+------> Consumer load | | | +--[T1/T2 ACVC]--> Ballast R | ^ +--> f / V sense --+ controller (alpha) ``` #### Working 1. The controller measures the generator frequency (or voltage). 2. If consumer load falls, frequency starts to rise. The controller **reduces $\alpha$**, so more power goes to the ballast. 3. If consumer load rises, frequency starts to fall. The controller **increases $\alpha$**, so less power goes to the ballast. 4. Thus the total electrical load and hence speed, frequency and voltage stay nearly constant. #### Features - Cheap, simple, no moving parts, fast response. - Ballast heat can be used for water or space heating. - Phase control causes harmonics and poor power factor at mid firing angles; for this reason some ELCs use several binary-weighted ballast steps or integral cycle control. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does an AC voltage controller change the power delivered to a resistive load? A. By changing the supply frequency B. By varying the firing angle, which changes the rms voltage at the same frequency C. By rectifying the supply to DC and filtering it D. By changing the number of turns of the transformer Answer: B. Phase control of anti-parallel thyristors changes the rms output voltage while the frequency stays equal to the supply frequency. (Key point: Role of AC voltage controller) 2. Why do small micro hydro plants need an electronic load controller? A. Turbine water flow is fixed and there is no governor, so a drop in consumer load would raise speed, frequency and voltage B. The generator produces DC that must be converted C. The consumer load is always constant, so the ELC stores excess energy D. The ELC increases the water flow when load rises Answer: A. With constant water input and no mechanical governor, surplus power would overspeed the generator; the ELC diverts it to a ballast load. (Key point: Why an ELC is needed) 3. What condition does the ELC maintain? A. Ballast power equal to consumer power B. Consumer power constant at all times C. P_consumer + P_ballast = constant generator power D. Zero power in the ballast load Answer: C. The generator always supplies constant total power, so whatever consumers do not use is sent to the ballast (dump) load. (Key point: ELC principle) 4. When consumer load falls and frequency starts to rise, what does the ELC do? A. Increases the firing angle so less power goes to the ballast B. Switches off the ballast load C. Reduces the firing angle so more power goes to the ballast D. Reduces the turbine water flow Answer: C. A smaller alpha gives a larger rms voltage on the ballast, absorbing the surplus power and holding frequency steady. 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