Shaft Grounding Systems

Current Situation

  • All rotating machines because of inherent electrical, mechanical & operational characteristics induce an electrical potential (voltage) on their shafts. If this voltage is not managed or if the voltage mitigation systems (shaft grounding brushes) are ineffective, the voltage seeks an alternate path. That path is the metal component, typically a bearing or seal or gear train which have least clearances.
  • When the voltage seeks path to the ground through components separated by a dielectric medium, arcing happens. Arcing erodes metal surfaces and disturbs the tight clearances that these components depend on for proper operation.
  • The problems caused by electrical discharge are of concern to plant engineers because their presence is not easily detected. The confusion is compounded by the fact that while the source of the damage is electrical, the damage manifests itself as mechanical damage.
  • Since there is not enough awareness on the subject, damage is most often mis-diagnosed, and the components are routinely replaced prematurely. This causes unscheduled shutdowns or prolongs routine outages.

Source of Shaft Voltages

1. Electrostatic
  1. Due to brushing effect of the wet steam with the blades of the turbine in the low-pressure stage of the turbine.
2. Electromagnetic
  1. Residual magnetism is left in a machine after conducting MPI on the machine.
  2. Making use of improper welding methods can also magnetize a machine. Welding in the proximity of big machines and not having the earth probe close to the welding electrode can cause this.
3. Externally Applied Voltages
  1. Leakage current from active winding protection
  2. Thyristor controlled excitation systems that contain relatively high frequency in their output
  3. Rotor winding breakdown caused by inter-turn and rotor earth faults resulting in current asymmetries
  4. Residual magnetism within the machine caused by magnetic particle inspection, welding & the flow of shaft currents

Identification & Types of Damage

  • Shaft current damage can be identified by close examination of the damaged component.
  • Some indications are clearly visible.
  • Often a 30x magnification is required.
Types of Electrical Damage Caused by Shaft Currents

Damages observed can be classified as:

    Frosting
  • Not visible unless magnified to 30X
  • Caused by very low current ( < 0.5 amp)
  • Long term damage (3 to 5 years)
  • Reduces thrusting capabilities leading to eventual wipe
  • Contributes to many thrust bearing wipes
  • Often misdiagnosed as thermal or abrasive damage

    Pitting
  • More severe than frosting
  • Caused by higher currents (~ 5 amps)
  • Short term damage (~ 6 months)
  • Random and Pits are > 1 mm in diameter
  • Found often on rollers, balls, thrust pads, gear teeth and journal bearings
    Spark Tracks
  • Discharge is sustained by weak oil
  • Current path is random, sometimes straight.
  • Often regarded as scratches caused
    • During handling or
    • Due to debris in oil
  • High energy - proof of residual magnetism within the machine
  • Cause for concern – further severe damage can be expected.
    Welding
  • Damage is very severe
  • Very high current discharge - sometimes over 1000 amps.
  • Machine fails within minutes of the discharge
  • Tremendous amount of potential energy.
  • Energy can be dormant for many years.
  • Energy often released during a rub or machine upset.
    Melting
  • Damage is very severe.
  • Very high current discharge over a short period of time.
  • Machine fails within days or weeks.
  • Machine could self-excite and lead to self-destruction.
  • Damage can be prevented if detected by monitors.

Solution

Multiple studies by the world’s largest power utilities have confirmed that block brushes (carbon-graphite, silver-graphite, etc.), copper braids brushes, etc. are ineffective in grounding shaft currents on high speed shafts for several reasons, including:

  1. Loss of contact due to build up a film of graphite dust over the brush surface
  2. High electrical contact resistance
  3. Brush surface becomes impregnated by oil, etc.
  4. Performance deterioration unless cleaned at regular intervals

Over the years, OEMs, including BHEL, Siemens, GE, Alstom, etc. and utilities have OEM installed & retrofitted specialized gold-silver alloy brushes on turbo-generators. These brushes have a contact resistance of less than 6 mili-Ohms, and therefore provide the least path of resistance. This ensure that the machine components are protected, thereby preventing unscheduled shutdowns owing to premature component wear.

Monitoring and Fault Conformation

  • The brushes can be connected to an online monitor that provides continuous monitoring of shaft voltage & current data. The online monitoring system serves as a fault confirmation tool and is an invaluable tool for plant engineers to determine the health of machines and confirm developing machine faults at an early stage.
  • Protection of turbo generators usually involves the installation of 2 Shaft voltage sensing brushes & current grounding brushes and the installation of 1 Online Monitoring System.
  • Problems detected are determined by reducing shaft current and shaft voltage signals into minimums, maximums, frequency, ratios and rate of changes. These may indicate several possible causes requiring positive identification and confirmation from other factors to pinpoint a specific fault.
  • By narrowing possible problems, effort can then be focused on effective troubleshooting, leading to orderly planning and/or execution of maintenance or shutdowns, as appropriate.
  • When used in conjunction with other monitors, abnormal trends of shaft voltages & current signals have been used in the past to confirm the following faults:
    • Confirmation of Shaft Rub
    • Wet Steam in Turbine
    • Loss of Bearing or Seal Insulation
    • Developing Stator winding faults
    • Stator Lamination Shorting & Melting
    • Stator Lamination shorting and melting
    • Shift in air gap
    • Rotor winding faulting
    • The need for a shaft voltage harmonic filter.
    • Trending of each brush grounding current, with maximum current alarm on each brush. Each brush will alarm on low brush grounding current indicating brush or grounding maintenance required.

Altair specializes in the detection, confirmation & resolution of faultscaused by Shaft currents. We provide the following services:

  • Online shaft voltage audits to confirm presence of shaft voltages.
  • Supply of Brushes
  • Supply of Online monitoring systems

Altair Industrial has a track record of over 12 years in trouble shooting & solving problems at leading power plants (Reliance, Adani, NHPC, etc.), petrochemical (IOCL, BPCL, RIL) and fertilizer complexes (GNFC, IFFCO, Tata Chemicals, etc.) on a proprietary basis.

Clients

  • Reliance Industries Ltd. – Hazira Complex
  • Reliance Industries Ltd. – Patalganga Complex
  • Reliance Industries Ltd. – Dahej Compex
  • Reliance Industries Ltd. – Baroda Complex
  • Reliance Industries Ltd. – Nagothane Complex
  • IOCL – Panipat Refinery
  • IOCL – Baroda Refinery
  • National Fertilizers – Guna
  • National Fertilizers – Panipat
  • Adani Power (Mundra, Udupi, Kawai)
  • BCPL (Kochi Refinery)
  • BPCL (Mumbai Refinery)
  • Haldia Petrochemicals Ltd
  • IFFCO – Aonla Unit
  • IFFCO – Kalol Unit
  • IFFCO – Phulpur Unit
  • GNFC Ltd.
  • Kribho Shyam Fertilizers Ltd.
  • Indo Gulf Fertilizers
  • Chambal Fertilizers
  • NHPC Salal Power Station
  • Siemens
  • JSPL
  • TNEB
  • GSEG
  • BHEL
  • Triveni Power
  • Essar Steel
  • Lanco Power
  • Aban Power
  • Tata Chemicals
  • Tata Power
  • IPCL
  • JITPCL
  • Reliance Energy
  • HPCL (Mumbai Refinery)
  • GSFC Ltd.
  • Zuari Fertilizer