ONLINE MONITORING DEVICES

Constructional Features Most hydro generators in service today are of the vertical type wit the turbine or

waterwheel at the bottom of the assembly and the generator on top. There are

exceptions to this statement with bulb units being the most common. A bulb unit

places the generator in a submerged chamber, and the orientation of bulb units is

usually horizontal .

Unit ratings can vary from the kW range to over 800MW. Speeds can vary from 60

to over 600 RPM and are determined by both the waterwheel type and the

hydraulic head, generally the higher the head, the higher the waterwheel speed.

The highest head and therefore highest speed water wheels are of the Pelton or

impulse type. Intermediate head machines usually employ Francis or reaction

turbines while the low head machines are typically Kaplan or propeller type

turbines.

Conventional Hydro Generator. Vertical hydro generators are generally manufactured in one of two types,

conventional or umbrella type. Figure above is a conventional hydro generator

with its distinguishing feature being a combined guide and thrust beating above

the generator that is supported by a massive thrust girder. Conventional hydro

generators are usually three bearing machines as show in figure above .

The umbrella type hydro generator is most commonly applied on new machines

he umbrella hydro generator, which is depicted on the cover of this best practices

document is usually a two bearing machine with a turbine guide bearing and a

combined guide the thrust bearing mounted below the generator rotor. The

umbrella design is more economical to manufacture and easier to maintain

because of the lack of the thrust girders above the generator.

Hydro Generator Cross Section. Hydro Generator Rotor Large hydro generator rotors are manufactured at the plant site because it is

impossible to ship an assembly that can be over 50 feet in diameter and can

weigh one thousand tons. From the inside out, the hydro generator rotor consists

of hallow rolled steel shaft sections that are bolted to the top and bottom of a

frame member called a spider. The rotor spider usually consists of eight to 16

spider arms. The spider is a welded steel assembly at least partially built offer site

and shipped to the site. On large machines, the spider is built in sections

assembled on site.

Installed around the outside of the spider is a laminated assembly called the rotor

rim. The rim is built up into a steel cylinder at the plant site from steel plates

approximately 0.2 inch 95mm) thick and installed onto the spider by one of

several means. There are two basic type of rim-spider interfaces, shrink fit and

floating rim. With the shrink fit rim, the rim is heated and then shrunk on to the

rotor spider. The amount of shrink fit is designed to be sufficient to maintain a

tight fit at greater than full over speed conditions (which can reach over 200% of

normal operating speed). The floating rim design usually utilizes some type of

driven key to secure the rim to the spider circumferentially while allowing for radial

expansion with speed and temperature. The shrink fit rim is the most commonly

used method on new hydro generators. The rotors salient poles are keyed to the

outside diameter of the rotor rim.

Rotor Rim Joint

With all those bolted, welded, shrunk, and keyed joints, a properly designed,

manufactured, assembled, and operated hydro generator rotor can remain in

service for well over 50 years. Problems that lead to an out-of round condition or

off center operation can lead to a major failure. Because of the immense weight of

hydro generator rotor assemblies and their slow operating speeds, out of round or

off center rotor rim condition do not always show up as a measurable vibration.

Hydro Generator Stator Hydro generator stators can either be manufactured in sections in the factory or

they can be manufactured in one piece at the plant. Both methods of

manufacture have advantages and disadvantages. Factory manufacturing usually

entails clean-room condition that can be difficult to obtain in the field, and it may

be difficult to duplicate the quality controls inherent in the factory under field

conditions. Manufacturing in the field requires significant floor space for the stator

with an outside diameter that could exceed 70 feet (20 meters) .

Field manufacturing involves a continuous stack or pile of the stator core

laminations with the entire circle of laminations interwoven and a continuous stator

frame. Factory manufacturing allows for the laminations to be interwoven only in

each section (upto six sections per stator core). And the core/frame sections must

be connected in the field. The joint between core sections must be carefully

designed and assembled to assure mechanical, thermal, and magnetic integrity.

Mechanical integrity of the factory manufactured and field assembled stator is a

concern, as a stator assembled from per-manufactured sections simply cannot

have the hoop strength of a single piece field manufactured stator.

Both methods of manufacturing are in use today. The ultimate decision on which

type of manufacturing is utilized usually depends on schedules, available floor

space, and certainly pricing.

Figures below are pictures of a hydro generator stator during construction.

PLACEMENT OF th STATOR SEGMENT JOINING OF STATOR SEGMENTS

CORE RELAMINATION AT JOINTS

HYDRAULIC PRESSING OF STATOR CORE

ASSEMBLED STATOR BEING LOWERED STATOR CONCRETE BARREL ON SOLE PLATES

INTO GENERATOR BARREL

Once the stator core is staked, stator coils will be installed in the vertical slots in

the stator core. The completed stator is set on a sole plant, which for large

machines provides a slip plane to allow for radial movement of the stator due to

thermal expansion. Large stators are known to expand over 0.5 inches (1.25cm)

on the diameter from ambient conditions to full temperature operation. Great care

is taken to assure that the expansion and contraction of the stator is uniform over

the entire stator to assure the stator remains round and concentric with the rotor.

After going through constructional details of Hydro Generator, which is mainly

required to be monitored online, we will discuss reasons for online monitoring and

various devices used for online monitoring.

Why Condition Monitoring on Hydro Units? Historically, Hydro units have been reliable and robust, and have been used for

base load with only limited outages for scheduled maintenance. Utility

deregulation, environmental and water concerns have resulted in a change of

duty from base loading to peaking only. This change has resulted in multiple

starts and stops each day for units which were never designed to do so, causing

premature ageing, un-foreseen mechanical and electrical stress, resulting in

degraded reliability and performance. Financial pressures on operators have also

resulted in reduced routine maintenance. This is particularly the case with

pumped storage units.

Why do Thrust Monitoring? Large Hydro units can weigh over 1000 tons, with the entire weight carried by the

thrust bearing. It is therefore critical that the oil film is present between the bearing

shoes and the rotor before the valves are opened and the high pressure water is

allowed to enter the turbine.

Monitoring of the thrust position is able to provide a permissive to operate,

indicating the presence of the oil film by measuring the lift of the rotor.

Why do Vibration Monitoring? Measuring vibration on Hydro Rotors provides the same benefits as in monitoring

conventional units. Parameters such as rough zone operation, cavitation,

unbalance, bearing problems and wicket gate problems can be diagnosed from

vibration signatures during day-to-day operation. Developing Faults can be

diagnosed and repairs scheduled long before they become critical or catastrophic

failure occurs.

Why do Air Gap monitoring?

Monitoring Air Gap has been proven itself to be an efficient tool to detect and

diagnose structural problems related to :

loose rotor rim weak rotor structure uneven thermal and magnetic forces causing expansion of the

rotor and stator structural change of stator foundation (concrete

deformation).

Monitoring generator problems enables plant owners avoid costly forced outages

for breakdowns or rotor-stator repairs, caused by

Overheating rotor-to-stator rubs structural issues

Air Gap monitoring can help to extend safe generator usage up to the next major

refurbishment. Uneven air gap also diminishes efficiency performance.

What is Air Gap?

The air gap is the distance measured between stator and rotor of a generator.

Large hydroelectric turbines are subjected to constantly varying centrifugal,

thermal and magnetic forces that are capable of distorting the stator and rotor of

the turbine, thereby causing this air gap to vary. Monitoring of air-gap is essential

part of online condition monitoring as it can provide early warning of impending

problems and facilitated timely maintenance procedures.

Air Gap Monitoring

Air gap is a measure of the distance between rotor and stator in the hydro

generator. Monitoring of air gap is critically important because unlike high speed

generators. Both the stator and the rotor on large hydro machines can be quite

flexible the bore diameter on the largest hydro machine can be over 50 feet(>15

meters) and the height of the stator can be over 12 fee (>3 Meters). Nominal air

gaps on large hydro machines vary from approximately 0.3 inches to 1.6 inches

(0.7 to 4cm.) with operating speeds ranging from 60 to over 600 RPM. Peak

operating efficiency is achieved when both rotor and stator are rotor and

concentric. Air gap monitoring provides the operator with early warning of

impending problems with shape and concentricity.

Out of round or off center conditions of the rotor and/or the stator can cause

problems. Any decrease in nominal air gap is a concern because of the magnetic

attraction between rotor and stator. Under normal conditions, the magnetic

attraction between rotor and stator increases approximately with the inverse

square of the air gap. So any anomaly that leads to a decreased air gap tends to

worsen with time as the increased magnetic attraction further decreases the gap.

The increased magnetic field at the location of a small air gap also increases the

magnetic heating in the stator, thus increasing the thermal aging of the stator

winding and core.

In service air gap measurements are made with Stator Mounted air Gap (SMAG)

probes. Measurements are made from the inner surface of the stator core to the

rotor poles. These measurements provide a direct measure of rotor shape. Stator

shape is calculated utilizing multiple SMAG probes. The table below lists standard

tolerances for air gap, rotor and stator roundness and rotor and stator

concentricity as defined in the Guide for erection Tolerances and Shaft alignment

published by CEA (Canadian Electrical Association). These numbers are provided

for reference purposes.

Parameter Definition Erection Deviation1

Acceptable Deviation

Critical Deviation

Air Gap Maximum difference between air gap measured at any point on a single plane and nominal air gap

13% 20% 30%

Stator Roundness

Difference between maximum inside radius and minimum inside radius measured from the rotor rotation axis.

7% 20% 12%

Stator Concentricity

Difference between rotor rotation axis and the best stator centre measure on the same plane and calculated from the for reference pole

5% 7.5% 10%

Rotor Roundness

Difference between maximum outside radius and minimum outside radius on the same plane.

6% 8% 10%

Rotor concentricity

Difference between the rotor rotation axis and the best rotor centre calculated from the outside radius of each pole on he same plane

1.2% 2.55 4%

1Deviations expressed in percentage of nominal air gap.

A minimum of four SMAG probes per plane are recommended for stator bore

diameters upto 25 feet (7.5 meters) with a minimum of eight SMAG probes

recommended for larger machines. More SMAG probes will provide a better

approximation of stator shape. Single plane measurements should always be

made at the top of the stator, as the top of the machine is more flexible because

the bottom rests on sole plates. Measuring air gap in both top and bottom planes

will provide more protection for the user.

Location of sensors for Air gap Air gap Sensors

Bearing Vibrations Hydroelectric turbines are subject to unique forces and operating conditions,

typically operate at low operating speeds (60-600 rpm), and generally incorporate

vertical shaft arrangements. For these reasons, they exhibit unique vibration

characteristics and require specialised filtering for monitoring the various relevant

machinery condition parameters.

TGB Vibration Sensors

Some of vibration behaviours typical of hydroelectric turbines are caused by

rough load conditions (NOT 1X), shear pin failure (nX), faults with or debris in

wicket gates (nX), stator faults, and various sources of unbalance(1X).The system

provides alarm indications for these and other vibration conditions with low

frequency sensitivity necessary to effectively monitor hydroelectric turbines.

Inherent with these low speed turbines are longer response times and larger

vibration levels; therefore, the design of the system is such that it covers this

unique behavior of hydro turbines. Bearing vibration can indicate problems related

to fluid filmed bearings, including overload, misalignment, rough load zone (NOT

1X), shear pin failure (nX), faults with or debris in wicket gates (nX), stator faults,

and various sources of unbalance (1X). Bearings can be drilled and tapped to

accept probe mounting hardware and cable tie-downs during the manufacturing

process. For retrofit installations, the bearings can always be removed and drilled

to accept the probes mounting hardware and cable tie downs.

Stator Core and Frame Vibration Monitoring Vibration between the stator core and frame and between the stator frame and

sole plate is monitored as a predictive measure for various problem scenarios.

Loose stator core laminations will cause a vibration twice the electrical frequency

either 100 or 120 Hz. A seismic transducer, such as the Velomitor CT, is the

recommended probe for this measurement.

Uneven air gap forces will cause stator core and frame vibrations twice the

operating speed of the generator. Since the vibration caused by uneven air gap

forces can be as low as one or 2Hz., the 3300XL is the recommended probe for

measurement of this low frequency vibration.

The table lists standard tolerances for stator core and frame vibrations as defined

in the Guide for erection tolerances and Shaft alignment published by CEA

(Canadian Electrical Association). These numbers are provided for reference

purposes.

Parameter Definition Erection Deviation1

Acceptable Deviation

Critical Deviation

Stator Core vibration

Measured between the core and frame

1.0 1.4 2.0

Frame vibration

Measured between the frame and the sole plate

1.0 2.5 5.0

1Vibration expressed in mils peak to peak.

Thrust Bearing Oil Film Thickness: Unlike horizontal, high speed generators, hydroelectric generators have a great

amount of freedom in the axial direction. During full load reject, some rotors move

over an inch (2.54cm) in the axial direction. For this reason axial position

measurement (rotor position relative to the machine case) are not recommended.

Most large hydroelectric generators are vertical machines. The thrust pads have

to carry the static weight of the machine and any load generated by the water flow

through turbine. An absence or reduction in oil film thickness at the thrust pads

results in rapid breakdown of the bearing Babbitt and can result in rotor/bearing

damage if not caught. Thrust pad oil film thickness can indicate problems related

to fluid-filmed bearings, including overload, bearing fatigue or insufficient

lubrication.

Thrust pads can be drilled and tapped to accept probe mounting hardware and

cable tie-downs during the manufacturing process. For retrofit installations, the

pads can always be removed and drilled to accept the probes mounting hardware

and cable tie downs. For most cases, two pads, separated by approx.. 90 degree,

are instrumented. If the machine is large (more than 100MW) it may be advisable

to place two probes at opposite corners of the each thrust pad because pads on

large machines can saddle under load.

Generator Temperature Monitoring Generator temperatures are commonly measured with RTDs and thermocouples.

Virtually all generators have RTDs installed between coils in the stator slots.

Unless specified otherwise by the user, 12 slot RTDs are installed in most

generators. Note that hydro generator can have over 500 slots, so monitoring

coverage of stator temperature is minimal.

Cooling air is generally measured with RTDs or thermocouples monitoring cool

inlet air and hot outlet air. Units with air to water coolers will measured hot and

cold (inlet and outlet) air on each cooler alongwith cooling water inlet and outlet

temperatures.

Some very large hydro generators are equipped with water inner-cooled windings

where deionized water passes through each stator coil. In these cases, each

stator coil will be equipped with a water inlet and/or outlet thermocouple.

Generator air designed to operate at full load within a specified temperature rise

over ambient temperature. A 40C ambient is generally specified. The rated

temperature rise will be stated such that at full load, stator temperature measured

by the embedded RTDs will not exceed the maximum allowable hot spot

temperature for the stator winding insulations system. The most common

insulation classes in modern machines and their maximum temperature limits are

listed in the table below:-

Insulation Class Maximum allowable temperature

B 130C

F 155C

H 180C

Most operators will limit operation to some level under the maximum allowable

temperature as with only 12 RTDs covering over 500 coils, you can be virtually

assured that the actual hot spot is not measured. A conservative method of

specifying a new machine that is commonly used is to require class F insulation,

but specify a class B temperature rise of 90C over a 40C ambient. In this way, a

machine with Class F insulation will operate at a maximum temperature of 130C

at full load.

Partial discharge Analysis Partial discharge (PD) is a radio frequency noise measured in the MHz range

that is generated by low level arcing and sparking in generator stator insulation.

There are three basic types of PD of concern in medium voltage generators ( 5-

20kV range)

Internal void discharges occur where the insulation on high voltage coils has

internal voids across which a charge builds up. Generator coil insulation is made

up of many overlapped layers of insulating tape all glued together with a resin,

polyester or epoxy on modern machines. If done correctly, the system should be

void free. Where voids exist, a sufficient charge will build up and arc across the

void and create some measurable radio frequency noise. Void discharges occur

only at the high voltage end of the winding, usually on coils operating at 4,000

Volts to ground or higher. In air cooled generators (Virtually all hydro generators

are air-cooled) and motors, internal void PD can be quite damaging as the arc

produces ozone which erodes the surrounding insulation. This is the reason PD is

not a big concern in hydrogen cooled generators like large high speed units, no

oxygen, so no ozone.

Slot discharge occurs where the stator coil is loose and vibrating in the stator slot.

Stator coils have a semi conducting coating on the outer surface to ground the slot

portion. If the coil vibrates, making and breaking the connection between the semi

conductor surface and the grounded core will create arcing. Slot discharge is not a

function of coil voltage, and can occur anywhere in the winding from high voltage

to neutral side. Like the void discharges, slot discharges are damaging in air

cooled winding because the ozone produced attacks the outer surface of the coil,

first destroying the semi conductor costing, and then eating away at the coil

insulation .

End winding pollution discharge occurs when sufficient conductive dirt build up

occurs on the stator end windings, usually bridging between coils with significant

voltage differences. In most cases, end winding discharges are not damaging, just

an indication that the winding needs cleaning.

The hydro Trac instrument takes PD measurement automatically and stores the

data for analysis and tending. Presence of each of the three types of PD

described above can be determined by analysis of the data, particularly where on

the voltage waveform the PD is detected. Trending of the PD data over time is

used to predict long term insulation degradation. Unlike temperature or vibration

measurements, there is no established good or bad level of PD. New machines

by the same manufacturer can have significantly different PD levels and still be

acceptable for operation. Increasing levels of PD with time is the primary indicator

of degrading insulation.

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References:- 1. Bentley Navada Paper on online monitoring 2. R, M & U of Bhakra Right Power House, Er. Surinder Singh