Electric generators are essentially motors in reverse: rather than drawing power from the line and using it to do work, they are driven by an external force, such as wind, which is used to produce electric power. Unfortunately, wind and other generators are, also like motors, vulnerable to electrical damage caused by inverters. This causes premature failure and the huge expense of unplanned downtime.
Inverters and Stray Current
The most common types of WTGs are doubly-fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs). Both use inverters to regulate their power output, and DFIGs additionally use an inverter for excitation. Unlike PMSGs, DFIGs also use gearboxes.
The problem with inverters is that their high switching frequencies and unbalanced phase voltages produce stray voltage and current in their systems. In WTGs, this can lead to electrical damage to the generator's bearings and gearboxes. This damage accumulates and can cause premature system failure. The image shows "tiger striping" on a gearbox, which was caused by electrical damage.
In addition to inverter-induced electrical damage, wind turbines are prone to large static charges that can damage bearings and gearboxes. This problem is most severe in doubly-fed systems, but it also occurs in PMSGs.
Protection from Electrical Damage
Many WTGs include one or more insulated or hybrid bearings, which are resistant or immune (respectively) to electrical damage. They are particularly effective at blocking high-frequency circulating currents. Two insulated bearings are not a complete solution for DFIGs because there will still be stray voltage on the shaft, which, with the bearings blocked, will tend to discharge through the gearbox instead.
Most WTGs also include a carbon brush to ground the shaft. These brushes can carry high-amperage DC and low-frequency current, but they have higher impedance at high frequencies. Since the stray current produced by inverters is largely high-frequency, they are less than a perfect match.
The AEGIS® PRO Series Shaft Grounding Ring was developed specifically to address high-frequency stray current in large motors and generators. AEGIS PRO Rings ground the shaft by surrounding it with millions of conductive microfibers that bleed off voltage before it builds up high enough to discharge by arcing through a bearing or gearbox.
An installed AEGIS PRO is shown in the second photo - the AEGIS ring is the outermost metallic circle. The conductive microfibers (black) are visible around the inner diameter of the ring.
The best practice for electrical bearing protection is to combine these three approaches: One hybrid bearing to interrupt circulating current, a carbon brush or brushes to carry DC and low-frequency current, and an AEGIS PRO Ring to dissipate inverter-sourced high-frequency voltage.
With this combination, generations and gearboxes are essentially totally protected against electrical damage to rotating equipment. This electrical protection is, of course, only one component of reliability, but it is one less thing for wind farm managers to worry about.
AEGIS Rings also come with a 2-year extended warranty against bearing fluting damage. No other form of protection against VFD-caused bearing damage offers a warranty like this.
To learn more about AEGIS shaft grounding and best practices for electrical bearing protection - for motors and generators - sign up for a training. We offer monthly live training webinars, and we can also visit your facility to review your exact application.
