Paul van Dyk, Pantac Control: Gas turbine controls DC battery bank system failures

14th Australian Gas Turbine Conference

Case Study: Gas Turbine Controls

DC Battery Bank System Failures

Paul van Dyk FS Expert (TÜV R.) ID 215/13

TUV FS Eng training / PanSIL – Safety Management Services / Turbine Control / Compressor Control / HMI Simulators / Eagle Eye Power Solutions

1. INTRODUCTION


Introduction

The objective of a battery backed power system is:

• To ensure that the controls and safety functions are maintainedfor the duration of the primary power supply failure.

• To maintain emergency motor controls and oil supplies following a trip and subsequent coast down.


Introduction

• Batteries are the only element in a Protection System that is a perishable item with a shelf life.

• Batteries require:

– A constant float charge to maintain their freshness (capacity)

– Periodic inspection to minimize age related problems

– Testing to confirm the system can still deliver the required output.

• Diagnostics, Inspection and Service tests are the most common methods to ensure that the battery backed supplies will work when called upon to do so.


2. TYPICAL INSTALLATION


Typical Installation

• A typical gas turbine control system supply consists of a battery charger providing the battery bank float charge and up to 100% of the DC load requirements.

When the AC supply fails or the DC load exceeds the charger rating, the batteries discharge to maintain DC bus voltage.

Battery Charger

and

System DC supply

Primary

AC supply

Battery bank

Emergency DC supply

DC Power

distribution

DC pumps and

controls


3. AUSTRALIAN SYSTEM FAILURES


System Failures

Example 1: Offshore Platform

• Due to the ‘Island’ nature of offshore production platforms, power supply failures can leave you in the dark.

• In the late 1990’s, whilst developing a safety system software specification for Esso Longford, the client engineer was called away for a few days to investigate an offshore platform black out and subsequent production loss.

• The root cause of the incident was a controls back-up supply failing to meet its duty cycle. Operations had failed to ‘cycle the batteries’ (service test) to confirm operation and capacity.


System Failures

Best practice

• Caltex Kurnell refinery currently performs a one hour partial discharge test every 3 months and records battery conditions before, during and after the Service test to confirm operation.

• Partial discharge test data may be compared to the base line data to identify weak and end of life cells.


System Failures

Example 2: ‘A’ black start power station

• Ten years later, whilst commissioning Port Lincoln power station’s GT 3 controls upgrade, there was a story told of a battery bank cell failure during a black start resulting in a coast down without lube oil. Checking the facts with the client found no record of this scenario but it is certainly plausible.

• Consideration to infancy bath tub failures of newly commissioned equipment leading to power outages is particularly relevant to high demand applications such as black start power plants. In this case dual redundant charger / battery banks may be warranted.


System Failures

Example 3: Valley Power Station

• 6 x 60MW Pratt & Whitney twin pack generators experienced battery bank failures in, Dec 2007 – Jan 2008.

• At Valley, the station’s auxiliary AC supply via the battery charger provides DC power for the emergency oil pumps and controls. When the generator is excited and at synchronous speed, the unit switches to the generator’s own AC power to reduce metered electricity consumption.

• During the automatic supply change over (station to generator and vice versa), control system power is maintained by the battery bank.


System Failures


• Due to a single failed battery cell, the DC bus voltage collapsed as soon as the change over switch transferred.

• The control system logic solver (PLC) powered down and rebooted to a ‘Ready to Start’ condition. All pumps and fans were shut down.

• Within 3 and 5 minutes of loss of power, the operator manually re-started the AC pumps but the damage was done.


System Failures


• Normal coast down time of 25 minutes was reduced to 7 minutes with significant white metal bearing damage and rotor shaft wear.

• 2008 - New dual battery banks installed and all cells fitted with continuous individual cell voltage and Ohmic impedance monitoring. Gel type cells

• 2011 – 2013 Governor, safety, sequencer and MCC controls upgraded on all 6 sets


System Failures


System Failures

Bearing

Unit 4

Max °C

Unit 6

Max °C

Trip

°C

Gen A-end bearing oil (drain) 116 108 85

Gen B-end bearing oil (drain) 120 68 85

Exciter bearing - - 85

A Front journal - 165 127

A Rear journal - 103 127

A Thrust journal - 63 127

A Counter-thrust - 73 127

B Front journal 176+ 176 127

B Rear journal 113 119 127

B Thrust journal 47 61 127

B Counter-thrust 63 69 127


System Failures

Example 4: Broken Hill Power Station

• Circa 2011, 2 x 22MW GE Frame V turbo generators experienced miscellaneous control system faults and a number of spurious operations such as fan starting for no apparent reason.

• Investigation found the root cause to be an undersized battery charger leading to low DC supply voltage causing control relays to drop out.

• A portable battery bank and charger was installed to maintain DC control power until the battery system upgrade project completion. The portable equipment was less than 3 years old, but had not been fully tested under discharge for several months.


System Failures


• During the course of scheduled site works, a unit generator breaker inspection inadvertently closed the breaker onto the live bus, exciting the generator and stationary rotor.

• Post incident investigation found the portable battery bank failed under load, with the resultant voltage too low for the protection system to operate.

• Generator protection relays failed to open the breaker, burning out the trip coils and the breaker spring charge motor.


System Failures


• The end result was a significant outage, rotor rewind and extended commissioning time to resolve a number of voltage and turbine control component failures, (GE mark II controls).

• 2012 - Battery banks replaced, NiCad cells & new chargers matched to the Battery banks installed.

• 2013 - New rotor commissioned and returned to service

• 2013 - Controls upgrade tender issued


3. STANDARDS AND RECOMMENDED PRACTICES


Standards

ISO 21789:2009 Gas turbine applications – Safety

• This International Standard covers the safety requirements for gas turbine applications using liquid or gaseous fuels and the safety related control and detection systems and the essential auxiliaries for all types of open cycles (simple, combined, regenerative, reheat, etc.) used in onshore and offshore applications including floating production platforms.

• Section 5.21.9.1, In the case of battery back-up systems, a shutdown shall be initiated before the remaining battery power becomes insufficient for

a safe shut-down of the equipment.


Standards

AS 2676.1:1992 part 1 Vented cellsGuide to the installation, maintenance, testing and replacement of secondary batteries in building

• This standard sets out the recommended practices for the installation, maintenance, testing and replacement of vented secondary batteries permanently installed in or on buildings.

• The US equivalent of this standard is; IEEE 450:2010 Recommended practice of maintenance, testing and replacement of Vented Lead-Acid batteries for stationary applications


Standards

AS 2676.2:1992 part 2 Sealed cells

Guide to the installation, maintenance, testing and replacement of secondary batteries in building

• This standard sets out the recommended practices for the installation, maintenance, testing and replacement of sealed secondary batteries (either valve regulated or gastight) located in or on buildings

• The US equivalent of this standard is; IEEE 1188:2005 Recommended practice of maintenance, testing and replacement of Valve-Regulated Lead-Acid (VRLA) batteries for stationary applications


Standards

North American Electric Reliability Corporation PRC-005-2 – Protection Systems Maintenance

• This US Standard provides recommended maintenance, test schedules and test procedures of batteries systems used for the protection of bulk power systems (generators).

• NERC develops and enforces Reliability Standards, subject to oversight by the Federal Energy Regulatory Commission (USA) and governmental authorities in Canada. Entities under NERC’s jurisdiction are the users, owners and operators of the Bulk-Power System, which serves more than 334 million people.


Recommended Practices

The tables on the following slides merge PRC-005-2

• table 1.4(a) vented lead-acid (VLA)

• table 1.4(b) valve-regulated lead-acid (VRLA)

• table 1.4(c) Nickel-Cadmium (NiCad) batteries



Maintenance Activity

Maximum interval

(months) Notes

VLA VRLA NiCad

Verify station dc supply voltage 4* 4* 4* Automation

possible

Inspect electrolyte level 4* - 4*

Inspect unintentional grounds 4* 4* 4* Automation

possible

Inspect condition of all individual units by

measuring battery cell/unit internal Ohmic

values

- 6 - Automation

possible


4* : The standard drafting team believes that routine monthly inspections are the norm. To align routine station inspections with other important inspections the 3-month interval was chosen for the standard. In lieu of station visits many activities can be accomplished with automated monitoring and alarming.



Maximum interval

(months) Notes

VLA VRLA NiCad

Verify float voltage of battery charger 18 18 18 Automation

possible

Verify battery continuity 18 18 18 Automation

possible

Verify battery terminal connection resistance 18 18 18 Automation

possible

Verify battery inter-cell connection

resistance

18 18 18 Automation

possible




Maximum interval

(months) Notes

VLA VRLA NiCad

Inspect - cell condition of all individual

battery cells where cells are visible

OR

measure battery cell/unit internal Ohmic

values where the cells are not visible

18

18

-

-

-

- Automation

possible

Inspect - physical condition of battery rack 18 18 18

Inspect - cell condition of all individual

battery cells where cells are visible

OR

measure battery cell/unit internal Ohmic

values where the cells are not visible

18

18

-

-

-

- Automation

possible




Maximum interval

(months) Notes

VLA VRLA NiCad

Verify - that the station battery can perform as

manufactured by evaluating cell/unit

measurements indicative of battery performance

(e.g. internal Ohmic values or float current)

against the station battery baseline (6 calendar

months)

OR

Verify - that the station battery can perform as

manufactured by conducting a performance or

modified performance capacity test of the entire

battery bank

18

72

6

36

-

72

Automation

possible

Service test with

condition

monitoring and

comparison again

base line



• PRC-005-2 Table 1.6(f) suggests that most maintenance activities can be automated but no type of battery manufactured today for Protection System applications, is free from age related degradation. There are too many variables in the electro-chemical process to completely isolate all of the performance-changing criteria.

• Inspections, tests, and in-situ Service tests are still recommended even with continuous monitoring.


DIAGNOSTIC COVERAGE

• Device failures may categorised into four (4) categories,

- Safe detected ƛsd

- Safe undetected ƛsu

- Dangerous detected ƛdd

- Dangerous undetected ƛdu

• Continuous monitoring greatly improves diagnostic coverage. Detecting failures, both safe and dangerous, significantly reduces the probability to fail on demand.

• Diagnostic coverage is never 100%, particularly in the case of batteries. A scheduled service test to confirm the battery bank will maintain the controls and oil system throughout the coast down time is still required.


SERVICE TEST

• Service tests confirm the battery bank duty cycle and assists in identifying weak / end of life cells.

• Service test logged data should be compared to the base line acceptance test data to document and verify the system duty cycle.

• Service test versus base line comparisons can identify weak cells that warrant replacement requirements


SERVICE TEST


4. BATTERY BANK MEASUREMENT TECHNOLOGY


Measurement technologies

Digital Hydrometer

– Portable instrument to test battery acid, where acid density is relative to charge.

– Reliable results appear in just 3 seconds - just immerse the sampling tube, pull the trigger, and read the final result.

– Commonly used for battery testing (lead-acid & Ni-cad).



Battery Testers

– Portable instrument to test Internal Ohmic Resistance (R) or Conductance (Siemens), Jar Voltage (V), Ripple Current, Temperature, and Strap Resistance per IEEE and NERC Recommendations.

– The meter injects a small AC current across the cell to determine its measurements. Results appear in just 3 seconds.

– Battery management software may be included to identify bad cells, create reports, save data and ensure the integrity of backup power systems



Continuous Monitoring

– Continuous condition monitoring of each cell (volts, amps, temperature and resistance) can significantly reduce operator manual test requirements.

– When wired to the site’s monitoring system, battery faults can be alarmed for action.

20 x 6 V battery string with individual

cell condition monitoring




Continuous on-line monitoring system

5. CLOSING COMMENTS & RECOMMENDATIONS


Closing comments & recommendations

• Battery back-up supply failures can have serious and costly consequences. Although routine manual inspection and testing is common practice, most functions are reasonably easy to automate.

• Continuous on-line monitoring of each cell in the battery string dramatically improves diagnostic coverage.

• On-line monitoring reduces manual testing requirements.

• Modified performance tests (partial discharge or service tests) are still required to confirm that the installation is able to perform the system’s duty cycle. A suitable frequency would be at least twice as often as the system demand rate, i.e. if the primary power supply is likely to fail every 24 months, a service test should be scheduled every 6 to 12 months.


Paul van Dyk, Pantac Control: Gas turbine controls DC battery bank system failures

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