This document discusses the risk of steam turbine damage from reverse power or anti-motoring. It provides an example where a steam turbine experienced mechanical damage after a generator it was connected to drove the turbine like a synchronous motor for 20 minutes following a power disconnection from the network. Key contributing factors were a failure of the anti-motoring protection relay and lack of dedicated electrical technicians during commissioning. It recommends annual testing of anti-motoring protection systems and training operators on identifying generator motoring conditions and shutdown procedures.

1. This document is © AXA Matrix RC – March 2015Version
For further information or specific questions contact: techcare@axa-cs.com
Steam turbine anti-motoring (reverse power)
Rotor damage risk
Motoring can happen when a turbine’s steam supply is lost whilst the generator is still coupled to a
network (or operated in parallel with another generator). The generator will then become a synchronous
motor and will actually drive the steam turbine with the potential for significant mechanical damage.
A reverse power protection device should be inserted within the generator protection scheme, in
compliance with Institute of Electrical and Electronics Engineers standard IEEE C37.102-2006 – Guide
for AC Generator Protection (Section 4.5.5).
EXPERIENCE FEEDBACKTechC@RE machinery breakdown
GENERATORTURBINE ELECTRICAL CURRENT
GENERATORDAMAGE TO TURBINE ELECTRICAL CURRENT
Steam turbine are used to drive synchronous alternators for electricity production.
Mechanical
damage to turbine
Mechanical repair
of steam turbine
15 days
Start of
acceptance test
5 days
Black out
on public network
2 hours
Turbine running
as a synchronous
motor
20 minutes Breakdown duration: 1 month
Return of power

2. This document is © AXA Matrix RC – March 2015Version
For further information or specific questions contact: techcare@axa-cs.com
Steam turbine anti-motoring
EXPERIENCE
FEEDBACK
Facts
1. A large paper mill has a steam turbo-generator connected to the public network
or can be operated in island mode.
2. In order to increase its power output, the steam turbo-generator was refurbished/
upgraded in 2012.
3. The upgrade focused on mechanical components, as the electrical components
were overhauled/ upgraded some years earlier by another contractor.
4. During re-commissioning; the sequence of power disconnection and power
reconnection to the public network induced motoring of the steam turbine.
Aggravating factors
Operator waited more than 15 minutes before calling an electrical technician
to open the main generator breaker.
Lack of a dedicated electrical technician during the commissioning/test
procedures.
Suggested recommendations
Annual testing of reverse power (anti-motoring) protection.
It should provide backup for sequential tripping schemes.
Control the presence of safety chains, test safety valves, surge
valves on steam supply, pressure/ level devices on lubrication
system, bearing temperature, over speed protection, vibration
monitoring, earth fault, loss of field, over voltage/current
instrumentation, including anti-motoring. Annual testing and
calibration is recommended.
Operators should be trained on identifying generator motoring
conditions and on subsequent shutdown/isolation procedures.
Main causes
Total loss amount: several millions euros (shaft, bearings, blades, packing seals)
Failure of protection
relay on anti-motoring
chain (partially wired)
Lack of bearings lubrication causing
significant wear/vibration to rotor
shaft and thermal stresses
Generator running as
synchronous motor
Client agreement
on testing without
environment analysis
Lack of formalized procedures
for testing protection systems on
re-start up/commissioning.
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