Regardless of electrical expertise, the average wind turbine technician works in close proximity to system voltages of 12 kV and higher all day long.
A good portion of safety is driven by the cost of installing high voltage systems. When a system is designed and built there is great pressure on the designers to keep the cost low. Sometimes components that are selected for the distribution system are suitable for specific functions, but do not necessarily offer the best protection for the people that have to maintain it.
Room for improvement – working with high voltage
Most new turbines today require the turbine’s high voltage step-down transformer to be turned off in order to cut the power to the wind turbine. Turning off the wind turbine’s main disconnect inside the turbine’s control panel does not eliminate the source of voltage entering the tower. Often the small transformer feeding the control circuits is fed from the high voltage lines before they reach the main disconnect. Not knowing this fact could lead to a serious mistake.
Safety precautions for working with high voltage
Any technicians that must turn off power at a transformer to a wind turbine should:
- Have high voltage electrical training;
- Have access to proper protective equipment;
- Have knowledge of, or access to, drawings showing the protective system of the wind farm, starting at the substation, and including overhead lines, drops to the underground, turbine transformers, and the turbine control cabinets;
- Never work on any electrical components without verifying that it is properly locked out;
- Use a hand held voltmeter or tick tracer to verify that the components are not energised.
To turn off all the power entering the turbine usually requires someone to gain access to the high voltage side of the wind turbine’s main distribution transformer. This transformer is usually near the base of the turbine, or it can be placed inside the tower or nacelle. The transformer is usually switched off with a hot stick, at the transformer’s high voltage side.
During this switching a category 3 NFPA safety system should be in use. Some may say this level of safety isn’t needed, but I wonder if the technician who died during a switching event would have lived if he had been protected with such a safety system.
High voltage recloser switches and overhead/underground mixed line systems
The recloser switch is designed to help protect the overhead lines from nuisance faults that may occur from exposure to the 'world'. Items that can cause a trip include things like animals, tree branches, a metallic balloon, or swinging cables that arc.
The recloser is set to trip off during the fault and then re-close the circuit in case the circuit problem has cleared. If the fault has cleared then the system is turned back on. If the circuit is not cleared the breaker will reopen. This may happen more than once depending on the setting of the recloser circuit breaker.
Most wind farms today have a mixed system of overhead lines and underground lines. When there is a mixed system, additional protective devices should be used along with a recloser or main breaker at the substation. It is possible that older wind farms may have been installed with manual disconnects – where the overhead lines drop down to feed underground lines. These individual underground lines serve only a portion of the wind farm.
However, without protective fuses at the pole, their only protection is the recloser back at the substation. If there is an underground fault, the entire fault current at the substation recloser is fed to a single underground line. This can cause severe damage to the entire underground line, which is not sized for the amount of current that the recloser passes. While there is a protective device at the substation, the branch lines are not protected as well as they should be.
With underground cable systems, all faults are considered permanent. Reclosers should not be allowed to reclose on an underground power system fault. However, reclosing is nearly always used on overhead systems. Hence the problem posed by a system with a mix of overhead and underground power lines is no isolated matter.
Turbine step up transformers
Sometimes there have been cases where turbine step up transformers in a wind farm do not have their own on/off switches. For service, the manual switch at the drop from the overhead can be used. However, the transformer itself will see the large fault current from the substation breaker if it does not have its own high side fuses, or fuses at the pole drop.
Further down the line, an older wind farm may have the step up transformer feeding several wind turbines. In that case, there is generally a main breaker on the low side of the transformer. This main breaker serves a buss that contains breakers for the individual turbines. Occasionally a technician may find this setup without the main breaker. This means that the buss is protected only by the transformer high side fuses, or other protective equipment further up the line. These may have higher ratings than the low side buss, so it is not properly protected.
Part of the basic electrical training that workers must learn is how to properly lock out equipment. With larger wind farms, there may be multiple crews working on a site. If a circuit is not properly locked out, other workers may inadvertently re-energise the circuit as they perform their duties.
And finally, it is important for a technician to never work on any electrical component before personally checking with a voltmeter or a tick tracer (non contact voltage sensor) to verify that the circuit is de-energised. Often, electrical accidents are caused by a worker assuming that the equipment is de-energised. In fact the wrong disconnect may have opened and locked out by mistake. So the voltage check is the final line of defence before starting electrical work.
About the authors
Jack Wallace Jr., Wind Turbine Technical Advisor
Frontier Pro Services
With support from Mark Dawson, Wind Turbine Engineer
Frontier Pro Services