AC voltage controller in ELC for micro-hydro

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

        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.

 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.