1. Benefits of a digital condition monitoring system, with
machinery vibration and generators air gap,
for an efficient plant management:
Cahora Bassa Power Plant, Mozambique
Pascal Maeder Dr. Dominique Vez
Marketing Manager Head of Product Management
Vibro-Meter SA / Meggitt PLC Vibro-Meter SA / Meggitt PLC
Rte. de Moncor 4 Rte. de Moncor 4
1701 Fribourg, Switzerland 1701 Fribourg, Switzerland
Introduction
What are the main reasons for increasing the use of permanent, on-line monitoring systems in hydro-electric
power plants?
• The modernization of power plants makes them evolve from an operation by plant-based personal,
towards remote and automated control.
• The increase of efficiency and power output, that subjects materials to limit stresses, calls for the
use of systems to protect and monitor machines.
• The increasing complexity of machines and of their operation mode, calls for monitoring systems
to “close the loop” of plant control.
1. Project background
The Cahora Bassa dam on the Zambezi river was completed in 1975 and renovated in 2007; its plant comprises 5
Francis turbines, with a total power of 2,1 GW. Within the renovation project, Hydroeléctrica de Cahora Bassa
(Songo, Mozambique) selected Meggitt / Vibro-Meter (Switzerland) to provide machinery vibration, generator
air gap, with a networked Protection and Condition Monitoring System.
Fig.1. The Cahora Bassa lake is Africas second-largest artificial lake

2. 2. Vibration
In the vast field of vibration measurement and analysis, three kinds of measurement are generally applied on
machines in hydro-electric plants:
• Absolute vibration of bearings
• Relative shaft vibration
• Vibration of the structure
2.1 Absolute vibration of bearings
The absolute vibration of bearings is measured in relation to a “fixed” environment (in one or more directions).
The most widely applied standards are VDI 2056 and ISO 2372: The recommended evaluation parameter is
velocity (v), in mm/s, or acceleration (a) in g (or m/sec2).
In Cahora Bassa, each turbo generator has 7 Piezoresistive, low-noise low-frequency SE 120 accelerometers, to
measure the bearings’ absolute vibrations:
• Turbine guide bearing (2: x-y)
• Thrust bearing (3: x-y-z)
• Generator guide bearing (2: x-y)
2.2 Relative shaft vibration
The relative shaft vibration is the shaft’s radial movement, measured in relation to its bearing. In the matter of
hydro-electricity power plants, the standards VDI 2059 and ISO 7919 have gained high credit: The
recommended evaluation parameter is displacement (s) in µm.
In Cahora Bassa, each turbo generator has 6 Eddy Current TQ 402 proximity probes, to measure the shaft’s
relative vibration (in x-y) next to 3 bearings (turbine guide, thrust and generator guide). In addition, a 7th
proximity probe is used as key phasor (shaft’s rpm).
2.3 Vibration of structures
The vibration of structures can be measured in several directions on machine bodies, control and adjustment
devices, bracings, foundations, etc… These measurements are not specifically regulated by standards or
recommendations.
In Cahora Bassa, the generator’s frame has 3 CE 680 Piezoelectric accelerometers, to measure the stator’s
structural vibrations (in x-y-z).

3. 3. Generator air gap
In order to increase generator’s efficiency, the trend is to decrease air gap to a bare minimum, so that poles pass
closer to the stator bars and increase the magnetic coupling. Consequences are that with thermal expansion, or
when the rotor’s eccentricity increases, the poles may rub the stator and cause severe damages to the stator: air
gap measurement is necessary to prevent such damages.
In Cahora Bassa, each generator is equipped with a capacitive air gap measurement system (4x LS 120 sensors).
This on-line system is used when the machine is rotating and withstands the extremely high magnetic fields in
the air gap.
Fig.2. Whole vibration and air gap sensors implementation, with links to the Protection, Condition and
Performance Monitoring System that provides an overall data overview to the plant control centre.
4. Machinery condition monitoring
Machinery condition monitoring is critically important to increase efficiency, plan preventive maintenance and
detect wearing of critical parts.
In Cahora Bassa, five VM600 on-line monitoring systems provide protection and condition monitoring to all
turbo-generator groups. They combine the vibration and air gap parameters with other process parameters
(temperatures, levels, active & reactive power,etc…) to monitor for example the shaft’s and bearings’ condition,
the generators’ condition, its rotors’ shape, etc…

4. 5. Networking
The control centre – where the data server is installed, stands 10 km away from the plant. High speed network
access via optical fibre is established between the control centre and the five VM600 monitoring systems in the
plant, allowing the control centre personnel to monitor the dam’s machinery remotely.
As a conclusion, this application example shows how machinery condition monitoring is fundamental for a
proper plant asset management that brings to the operator benefits such as: safety, higher availability, improved
efficiency and reduced maintenance costs.
References
1. Da Rocha e Silva, Francisco J.C., “Cahora Bassa and its role in ensuring hydro sustainability in Mozambique”,
Hydropower & Dams, Volume Thirteen, Issue 6, 2006.
2. Dr. Fromaigeat, Luc, “Airgap measurements for hydropower generators – application note”, Vibro-Meter SA, 2006
The Authors
Pascal Maeder graduated (1990) in Electrical Engineering from the State University of Applied Sciences (HEIG-VD), in
Canton de Vaud, Switzerland. Worked fifteen years with high-tech companies as engineer, project manager and marketing
manager in the transportation, aerospace and semiconductor industries. Graduated (2005) in Marketing from the Swiss
Marketing and Advertising Institute (SAWI) in Lausanne. In 2006, he joined Vibro-Meter SA (an operating company of
Meggitt PLC and world-wide leader in machinery health and vibration monitoring) in Fribourg, Switzerland, as Marketing
Manager for the Energy and Aerospace Departments.
Dr. Dominique Vez graduated (2000) in Physics from the Swiss Federal Institute of Technology (ETH), where he received
the annual Grant Award. Worked as a development engineer with a high-tech company and was a research assistant in the
Department of Information Technology and Electrical Engineering of ETH until 2004, when he received his PhD Degree
from ETH Zurich. From 2005 to 2008, he worked with Vibro-Meter SA (an operating company of Meggitt PLC and world-
wide leader in machinery health and vibration monitoring) in Fribourg, Switzerland, as Head of Technology Innovation.
Since beginning 2009 he is Head of Product Management in the Energy Department of Vibro-Meter SA.