Hydro · Controlling a hydro system
Switch off a solar array and the panels sit quietly in the sun. Disconnect a hydro turbine and it accelerates toward runaway within seconds. Shut the valve to stop it and the pressure surge can split the pipe. The energy is still arriving either way, and it has to be given somewhere to go.
The core difference
Every other source in this section can be interrupted safely. A solar array open-circuits harmlessly. A generator is switched off. A hydro system has a mass of water in motion and a rotating machine with stored energy, and both keep going after the decision to stop is made.
That leaves two obvious responses, and each is dangerous in its own direction.
Nothing is absorbing the water's energy any longer, so it all goes into accelerating the runner. The machine speeds up until drag balances the jet, which for a Pelton is near 1.8 times rated speed, reached within seconds of the load disappearing.
A column of water weighing several tons has to stop, and its momentum has to go somewhere. It goes into pressure. Close the valve quickly enough and the surge exceeds what the pipe was built to hold.
Runaway
Load rejection is the term for the electrical load disappearing while the water keeps arriving. A breaker trips, an inverter faults, a wire fails, someone opens a disconnect without thinking. The turbine has no way to know, and it accelerates.
Runaway speed is a property of the machine rather than an accident. Bearings and shafts on a properly built turbine are designed to survive it, because it is treated as the worst-case mechanical load the machine will ever see. Surviving it is not the same as being unaffected by it.
A machine rated at 1,000 rpm reaches somewhere near 1,800. Centrifugal stress rises with the square of speed, so the forces on the runner more than triple.
A generator spun faster produces higher voltage and frequency. Anything still connected downstream sees conditions it was not designed for, which is how a control failure becomes an appliance failure.
There is no window in which a person notices and intervenes. Whatever protects the system has to be automatic and has to be already connected when the fault occurs.
A commercial Pelton installation carries a jet deflector, a hinged plate that swings into the jet in a fraction of a second and sends the water into the casing wall instead of the buckets. The runner slows immediately while the flow-control valve closes gently over tens of seconds, letting the penstock give up its momentum slowly. It is worth understanding that arrangement, because it shows the principle: deal with the electrical problem fast and the hydraulic problem slowly, and never confuse the two.
Water hammer
When flowing water is decelerated, its kinetic energy converts to a pressure rise. The Joukowsky relationship gives the size of it: pressure rise equals the speed of the pressure wave in the pipe, multiplied by the change in water velocity, divided by gravity. Wave speed depends on the pipe material, running near 1,200 meters per second in steel and considerably lower in plastic.
Worked on the running example from this section: 400 feet of 4 inch plastic penstock carrying 150 gallons per minute, water moving at about 1.2 meters per second.
Static pressure at the turbine: 60 feet of head, about 26 psi. Comfortable for any pipe.
Surge from an instantaneous closure: roughly 68 psi added, for a plastic pipe wave speed near 400 meters per second.
Total the pipe would see: about 94 psi, more than three and a half times the static figure.
A pipe selected on static pressure alone, with a comfortable-looking margin over 26 psi, can be destroyed by one fast valve closure. On a steel penstock, where the wave travels three times faster, the surge is three times larger.
The pressure wave travels to the far end of the penstock and reflects back. That round trip takes twice the pipe length divided by the wave speed, and it sets the boundary between a fast closure and a slow one. On our 400 foot plastic line it is roughly six tenths of a second.
Any closure slower than the reflection time produces less than the full Joukowsky surge, and the slower the closure the smaller the rise. On larger systems closure times are chosen at many multiples of the reflection time, measured in tens of seconds, precisely to keep the surge to a modest fraction above static pressure.
Never fit a quarter-turn ball valve as the shutoff on a penstock of any length. It can be closed in half a second by someone who does not know what that means, including a future owner of the property.
Use a valve that closes slowly by design, mark it, and make sure anyone who might touch it understands why. This is one of the few places in household energy where a routine physical action, performed briskly, breaks something expensive.
The resolution
Both failure modes come from trying to interrupt something. The standard answer in household micro-hydro is not to interrupt anything. The water keeps flowing at a constant rate, the turbine keeps turning at a constant speed, and the electrical side absorbs whatever the household is not using.
The device that does this is a diversion controller, sometimes called a shunt or ballast controller. It watches battery voltage. When the batteries are full and the house is quiet, it routes the surplus into a diversion load, which is usually a resistance heating element. When the house draws more, it diverts less.
From the turbine's point of view, nothing changes. It sees a constant electrical load whatever the household does, which is exactly the condition that keeps it from running away.
A solar charge controller protects the battery by disconnecting the array. A hydro diversion controller protects the battery by connecting something else. The turbine cannot be disconnected, so the surplus must be consumed rather than refused.
Diversion loads are typically water or air heating. A system sized for winter produces a surplus for much of the year, and sending it to a hot water tank or a shop heater turns a safety requirement into something useful.
In practice
A resistance element, sized to absorb everything the turbine can produce, permanently wired and permanently available. The requirements on it are unglamorous and specific.
Not the average, and not the surplus on a typical evening. The full output of the machine, indefinitely, because the case it exists for is the one where the household takes nothing at all.
A water heating element depends on there being water around it. An air heater sized for continuous duty works whatever else has failed, which is why it is the common recommendation for the backup rather than the primary path. A conventional water heater thermostat is also not built for direct current at battery voltages, so elements and controls intended for the job matter.
A diversion load on a breaker somebody might turn off for the summer is not a diversion load. It is a decommissioned safety device waiting for a quiet week to become obvious.
Consider the sequence. The batteries reach full. The controller calls for diversion. The diversion load has failed open, or its wiring has, or the controller itself has failed. Now nothing is absorbing the turbine's output, the batteries continue to be charged past full, and the machine has no load to hold its speed. One component failure, three consequences. That is the argument for a second, independent path, and it is an argument the electrical code has taken seriously in a related context.
What the code actually says
It is commonly stated in the micro-hydro world that the National Electrical Code requires a backup diversion controller. That is worth stating carefully, because the underlying requirement is real but it is not written where people assume.
The NEC contains no article specific to hydroelectric systems. There are articles for photovoltaic systems, for wind electric systems, for fuel cells, for energy storage, and for interconnected power production sources. There is no hydro article.
The requirement people are describing is NEC 690.72(B), which sits inside the photovoltaic article. Where a diversion charge controller is the sole means of regulating battery charging, it requires a second independent means of preventing overcharging. It also sets sizing rules: the diversion load's power rating at least 150 percent of the array's, and the conductor ampacity and overcurrent protection for that circuit at least 150 percent of the controller's maximum current rating[1].
The hazard is identical. A battery bank being overcharged by an uncontrolled source does not care what turns the generator. Equipment manufacturers write their hydro guidance against this section, and installers treat it as the governing standard.
Whether a PV article applies to your hydro installation is decided by your local authority having jurisdiction and the code edition your area has adopted. Section numbering has also moved between editions as battery provisions migrated toward the energy storage article. Cite the principle, then ask your inspector.
This page explains why control matters, what the failure modes are, and what the governing principle is. It does not specify controllers, size conductors, or lay out a wiring arrangement for your system, and no page written for a general audience should.
Permanently wired generation feeding a battery bank and a household is licensed electrician territory in most jurisdictions, and it is the part of a micro-hydro project where a mistake is least visible and most consequential. Understand it here so you can have an informed conversation, then have that conversation with someone qualified.
Common mistakes
It is not the part that uses up spare power. It is the part that keeps the machine from running away and the batteries from cooking. It belongs in the budget beside the turbine.
The scenario it exists for is total load loss, where the surplus equals everything the turbine makes. Anything smaller works right up until the moment it is needed.
One controller, one load, one circuit. The code's insistence on a second independent means in the photovoltaic case reflects how these systems actually fail.
A quarter-turn valve on a long pipe is a pressure spike waiting for somebody in a hurry. Fit something that cannot be slammed, and label it.
A hydro system that shuts down in January is a freezing problem within hours. Something has to tell the household, and a light nobody looks at is not that something.
An inverter exporting surplus is a diversion path only while it is working and its destination is available. When it faults, the backup path is what remains, which is the whole reason to have one.
Next
Water measured, configuration chosen, machine matched, pipe sized, control resolved. What remains is whether any of it is lawful on your particular stream, which is the question that should have been asked first and is answered last only because it takes the other seven guides to know what you are asking permission for.
The pipe whose pressure rating this page just made more demanding.
Read the guideThe machine whose runaway speed and control needs start here.
Read the guideThe bank the diversion controller is protecting, and how it is sized.
Read the guideSources
Pressure and surge figures are calculated for the illustrative system used throughout this section and depend strongly on pipe material, wall thickness and length. Calculate your own. Code requirements vary by adopted edition and by jurisdiction, and the application of photovoltaic provisions to a hydroelectric installation is a determination for your local authority having jurisdiction. Permanently wired generation feeding a household requires a licensed electrician in most places.