condition based maintenance program for ground
TRANSCRIPT
CONDITION BASED MAINTENANCE PROGRAM FOR GROUND MOUNTED 11 kV FIXED Y-Y UNGROUNDED POWER CAPACITORS
IN MALAYSIA
M.F. Faisal Tenaga Nasional Berhad, Malaysia,
ABSTRACT
A capacitor bank represents an effective and low cost alternative for maintaining a desired voltage profile and improving the power flow in transmission and distribution power systems. A capacitor bank is also an effective equipment to improve low power factor in the power systems. In Malaysia, customers installed capacitor banks for the objective of power factor correction. If their monthly power factors are less than 0.85 (0.9), penalty surcharges will be imposed to their monthly electricity bills. To ensure optimum results are obtained from the capacitors, a comprehensive maintenance program must be implemented. The existing maintenance programs for medium voltage (MV) capacitors are time based maintenance programs. This paper shares a new condition-based maintenance program implemented by TNB Malaysia for ground mounted 11 kV Y-Y ungrounded capacitor banks. KEYWORDS: Capacitors, Condition Based Maintenance, Reactive Power, Time Based Maintenance. 1.0 INTRODUCTION Within an AC electric power system, there are two components of current needed to make possible the transfer of energy. One is the power component, or working portion of the current, sometimes referred to as active power. This is the component that is converted by the equipment into work, usually in the form of heat, light, or torque in rotating machines. The unit of measurement of active power or real power is the watt (W). The second is the reactive component, or nonworking portion of the current. This reactive component is responsible for the magnetic flux surrounding the conductors and magnetizing the iron in transformers and rotating machines. The unit of measurement is Volt-Amps reactive (VAr). Without the reactive component or magnetizing current, it is impossible for the power component (W) to be transmitted through the transmission and distribution systems. The magnetizing current allows energy to flow through the core of a transformer and across the air gap of an induction motor. The magnetizing current, establishes the magnetic field so the motor will spin, its current is constant and does not vary with load, it uses no energy and its current sine wave leads the voltage's sine wave by 90 degrees. Figure 1 shows a typical voltage & current profiles for a very lightly loaded motor. The motor has a power factor of less than 0.3. There is a lot of magnetizing current compared to the load current.
Figure 1 Voltage & current profiles for a motor with power factor of less than 0.3. All inductive loads require magnetizing currents to produce magnetic fields, which produce the desired works. The total or apparent power (VA) required by an inductive device is the addition of real power (W) and reactive
power (VAr). The equation for apparent power is shown in Eq(1).
22 VArWVA += (1) More reactive power requirement by inductive loads will increase apparent power and cause the power factor (PF) to decrease. The equation of power factor is shown in Eq(2).
VAWPF = (2)
Low power factor will require a higher current draw, which leads to larger cables in the electrical distribution system. Higher currents lead to higher copper losses (I2R) in cables and transformers. Higher current flow in the power distribution system will also cause voltage drops. Low voltage can cause overheating and premature failure of motors and other inductive devices. 1.1 Reactive power compensation The voltage and current shift by inductive loads can be offset using compensation devices. Reactive compensation commonly comes from three types of devices:- 1) Power capacitors are the largest source of compensating reactive power and are commonly used throughout the power system. 2) Synchronous condenser is a type of rotating machine -like a generator - but it does not produce real power, only reactive power. 3) Conventional generators, in addition to supplying real power, are an important source of reactive power. Overall, power capacitors are the most common devices used for reactive power compensation. Strategically placed capacitors in the power system can reduce the magnitude of reactive power of the power system, between the loads and the metering point. Thus, capacitors will reduce total current flow in the power systems. Examples of capacitor installations in Malaysia are shown in Figure 2.
(a) Interior of a capacitor bank
(b) Exterior of a capacitor bank
Figure 2 Example of a ground mounted Y-Y capacitor bank.
To ensure optimum results are obtained from these capacitors, a comprehensive maintenance program must be implemented. Currently, all the existing maintenance programs for medium voltage (MV) capacitors are time based maintenance programs. Annual time based maintenance is a schedule inspection, maintenance and replacement at fixed interval. The activities are performed at pre-determined intervals without regard to equipment condition or degree of use. The maintenance schedule is based on MTBF (Mean time before failure) or manufacturer recommended schedule. This maintenance approach can be costly and ineffective when it is the sole type of maintenance program practiced. In this paper a new condition based maintenance (CBM) program implemented by TNB Malaysia for ground mounted 11kV Y-Y ungrounded capacitor banks is presented.
2.0 DESIGNS OF GROUND MOUNTED 11 kV Y-Y CAPACITOR BANKS Currently, there are two designs for ground mounted 11 kV Y-Y capacitor banks. The first design is a fixed step capacitor banks and the second design is a switch step capacitor bank. Both designs are for installed indoor & outdoor installations. The single line diagrams for both capacitor designs are shown in Figure 3.
Fixed capacitor bank
Switched capacitor bank
Figure 3 Single line diagrams for fixed and switched Y-Y capacitors. 3.0 TIME BASED MAINTENANCE PROGRAM During normal operating conditions, capacitor bank needs no maintenance, but it should be inspected regularly as to ensure trouble-free operation. According to IEEE Std.18 [1], regular inspection of the capacitor installation on an established schedule should include a check of ventilation, fuses, temperature, and voltage conditions. In contaminated areas, capacitor bushings and insulating surfaces should be cleaned periodically, the interval between servicing depending on the severity of conditions to which the insulators are exposed. Capacitors exposed to weathering may require repainting periodically to prevent excessive corrosion and to maintain a good radiating surface. Time based maintenance program for capacitor comprises of annual inspection and annual maintenance programs. 3.1 Annual inspection program 3.1.1 Measurement of unbalance current in the Y-Y capacitor bank Unbalance current in the capacitor bank neutral conductor can cause the unbalance relay to operate. For VCB with current transformer (CT) with secondary ratio of 1, detail assessment must be done to ensure the unbalance reactive currents from the capacitor will not trigger the earth fault (EF) relay to operate. For VCB that uses CT with secondary ratio of 5, the EF relay will not be too sensitive. The common setting for the unbalance relay with unbalance CT of 10/5 is 60% (stage 1) and 90% (stage 2). The value of the unbalance current must be recorded during the annual inspection. If the unbalance current readings differ more than 20 % from unbalance CT ratio, there may be breakdown in one or more of the internal capacitor elements. The change in current due to breakdown in one element depends on the total number of elements in the capacitor unit and on the connection arrangement of the capacitor bank. A capacitance measurement of all capacitor units in the bank must be performed and faulty units replaced.
CT
REACTOR
REACTOR
REACTOR
BUSBAR 11 KV
2 MVar 3 MVar
3.1.2 Regular inspection An annual inspection is sufficient unless the capacitor bank is exposed to abnormal conditions of any kind, such as excessive contamination. At each inspection, proceed as follows- • Perform a visual inspection of pollution, damage to the finish, leaking capacitor units etc. The insulators
and bushings should be wiped clean if necessary. Under heavy pollution condition, cleaning should be carried out more frequently.
3.2 Annual maintenance program It is important to note that, the time based maintenance is performed based on a calendar schedule. This means that time is the maintenance trigger for this type of maintenance. Time-based maintenance is a planned maintenance (PM). Please follow all safety procedure to isolate and disconnect the capacitors. When working with capacitor banks, the following safety regulations, in particular, must be observed [2]. • Do not touch a capacitor bank until it has been completely discharged, short circuited and grounded. Short
circuit the capacitor units individually also, as if there is an internal fault, such as a damaged discharge resistor, there may still be a voltage even though the capacitor bank has been discharged.
• Normally capacitors have an internal discharge resistor, which reduces the voltage of the unit to: 50 V
within 5 minutes – or 75 V within 10 minutes. • Avoid skin contact with the impregnation fluid in the event of leakage, and avoid breathing in fumes or
gases from the impregnation fluid. In the event of skin contact, wash with soap and water. If fluid gets into your eyes, rinse with lukewarm water.
After the capacitor bank has been isolated, these inspection & maintenance programs are performed. 3.2.1 Enclosure exterior • Ensure enclosure is receiving adequate ventilation. Check to see if filters are clean and airflow is not
restricted. Replace filters and remove obstacles as required. • Ensure enclosure, enclosure doors, louvers, and rodent guards are adequate to prevent entry of liquids,
insects, and rodents. Clean and correct as required. • Inspect operation and adjustment of key interlock systems to determine if security features are working
properly. Correct deficiencies. • Examine enclosure for corrosion and paint adherence. Repaint scratched or marred exterior surfaces to
closely match original finish, as required. • Examine warning signs and placards. If not legible, replace. • Clean all enclosure windows. 3.2.2 Enclosure interior • Remove accumulated dust and dirt. • Remove insects, nests, or animal material. Inspect for condensation. Clean and inspect strip heaters and
check thermostats. • Check proper operation of ventilation equipment (fans) and thermostats. Correct as required. • Examine enclosure for corrosion and paint adherence. Repaint scratched or marred exterior surfaces to
closely match original finish, as required. • Examine warning signs and placards. If not legible, replace. • Clean all enclosure windows.
3.2.3 Bus bar & wiring • Inspect for loose bus bar connections and discoloration. Tighten as required. • Inspect for proper phase to phase and phase to ground clearance. • Remove excess surface oxides from aluminum connectors. • Inspect control wire connections, tighten as required. • Inspect wire insulation for cuts, breakdown, or burns. Replace as required. 3.2.4 Capacitors • Check for physical damage, leaks, bulges, or discoloration. Replace as required. • Clean capacitor case, insulation bushings, and any connectors that are dirty or corroded. • Check each capacitor for capacitive reactance using a capacitance bridge. • Confirm kVAr, voltage, and BIL rating for each capacitor. Verify with specifications. • Verify internal discharge resistors are working properly. Replace cells as required. 3.2.5 Insulators • Check for cracks, chips, and signs of arc tracking. Replace as required. • Clean insulators and barriers. • Check all mounting hardware, tighten as required. 3.2.6 Fuses • Check all capacitor fuses, control fuses, and PT fuses for blown fuses. Replace as required. • Check all mounting hardware, tighten as required. • Confirm proper fuse rating. Verify with specification. • Check functioning of neutral unbalance sensor. • Clean contact area of fuses and fuse holders. 3.2.7 Controls • Perform several manual switching operations of each capacitor stage to ensure equipment is in proper
working order. • Check all indicator lights to ensure proper functioning. Replace lights as required. • Check controller for proper functionality to specification. • Check for physical damage and proper operation of ammeters and voltmeters. • Check that blown fuse detection system is working properly. Correct as required. • Check that all alarms and indicators are per specification. 3.2.8 Air Disconnect Switch • Check torque on all bolts. Tighten as required. • Apply contact grease as required. • Check chains for proper tension. • Check for proper operation • Check that all contact are free of pitting and corrosion. • Check that all moving parts are free of binding and lubricate moveable parts. 3.2.9 Vacuum Switches • Check for proper rating. • Check for physical damage and leaks. Replace or fill as needed. • Check torque on all bolts. Tighten as required. • Check for continuity when closed and discontinuity when opened. • Physically check contacts on switches, clean as required. • Perform vacuum testing on vacuum bottles.
3.2.10 Harmonic filter reactors or inrush reactors • Confirm nameplate rating. • Check torque on all bolts. Tighten as required. • Check for physical damage. • Measure inductance of each reactor phase by applying 35 volts (filter reactor) or 63 mVolt (inrush reactor)
across each reactor and measuring corresponding current. Calculate the corresponding mH. • Calculate tuning point of each phase by using capacitive and inductive reactance. 3.2.11 Surge arresters • Check for physical damage. Replace as required. • Check torque on all bolts. Tighten as required. • Check for proper rating of surge arrester. 3.2.12 Insulation Test • Check phase to ground insulation level. Note: If possible, cable to the capacitor bank should be connected
during this test. Please ensure the switch /VCB to the capacitor is in open position. After all the correction measures have been implemented, the capacitor bank can be re commissioned. All the results of the inspection & maintenance programs must be documented. The engineers in charge must assess the results and all corrective measures done on the capacitor assets. 3.3 Corrective maintenance If the unbalance protection has tripped the VCB to the capacitor bank i.e. the unbalance current is more than 90% of the CT ratio, all capacitor bank units should be capacitance measured and faulty units replaced. If the overcurrent or earth fault relay has operated, it is possible there’s a permanent fault in the cable or inside the capacitor bank cubicle. 4.0 CONDITION BASED MAINTENANCE PROGRAM In this paper, a condition based maintenance (CBM) program implemented by TNB Malaysia for 11 kV capacitor banks is presented. The main goal is to identify a cost effective maintenance program through effective asset management. An effective CBM program for power capacitor is crucial as capacitors are the main component in managing reactive power compensation for the distribution network. Unlike in planned scheduled maintenance (PM), where maintenance is performed based upon predefined scheduled intervals, CBM is performed only when it is triggered by certain asset conditions. Compared with PM, CBM increases the time between maintenance tasks, because maintenance is done on an as-needed basis. Condition Based Maintenance (CBM) is a maintenance strategy that uses the actual condition of the asset to decide what maintenance needs to be done. CBM dictates that maintenance should only be performed when certain indicators show signs of decreasing performance or upcoming failure. Checking a capacitor for these indicators may include non-invasive measurements, visual inspection, performance data and scheduled tests. Condition data can be gathered at certain intervals, or continuously (as is done when equipment has internal sensors). CBM can be applied to mission critical and non-mission critical assets. 5.0 CBM PROGRAM FOR 11 kV Y-Y CAPACITORS The CBM strategies for power capacitors are shown in Figure 4.
Figure 4. CBM program for 11 kV capacitor banks The description of the CBM program is explained in Table 1. Table 1 Description of CBM program
Program Description On line
assessment Online condition assessment as routine maintenance for all in service power capacitor banks (Tier 1)
Off line assessment
Offline condition assessment performed when Tier 1 tests indicate abnormal conditions (Tier 2). Under normal condition, this assessment is performance as time based maintenance or preventive maintenance.
Corrective maintenance
It is intended to confirm the need for more extensive maintenance, rehabilitation or equipment replacement after the capacitor bank has tripped. Corrective maintenance is further categorized as planned and unplanned maintenance.
5.1 Tier 1 Tier 1 online assessment is performed to assess for presence of faults and evaluate the operating performance of all the capacitors. To obtain trouble free operation, it is recommended an online condition assessment to be performed for all in service power capacitor banks. Descriptions of all programs under Tier 1 are explained as follows. Table 2 Detail CBM programs for Y-Y ungrounded fixed 11 kV capacitor bank
Group Diagnostic procedure Frequency Application
Tier 1 (on line condition assessment)
Visual inspection 3 month
All these procedures are to be performed on all capacitor
banks in the power systems.
Record these values:- • Transformer current • Capacitive current • Voltage • Power factor • Transformer tap position • Unbalance current
3 month
Harmonic measurement (THDV) 12 month
Infrared thermography 12 month
Ultrasound & TEV scanning 12 month
Tier 2 (off line condition assessment)
Capacitance measurement 24 month
All these procedures are to be performed on all capacitor
banks in the power systems. Test on reactor 24 month
Test on insulation 24 month
Figure 5. CBM Flow chart for 11 kV capacitor banks
5.1.1 Visual inspection on physical conditions. During normal operating conditions, capacitor bank needs no maintenance, but it should be checked regularly as described below to ensure trouble-free operation. At each inspection, proceed as follows-
• Perform a visual inspection of pollution, damage to the finish, leaking capacitor units etc. • Under heavy pollution condition, cleaning should be carried out more frequently.
Document all the findings. Go to Tier 2 if the insulators and bushings should be wiped clean. 5.1.2 Record the value of the unbalance current in the Y-Y capacitor bank The value of the unbalance current should be evaluated from the unbalance relay. Description of unbalance current protection scheme is as follows:- The unbalance CT monitors the neutral current inside the capacitor bank. A healthy capacitor bank that’s operational will have value of the neutral current equal to 0 Amp. When one of the capacitor can deteriorate or fails, the value of the neutral current will be more than 0. If the neutral current exceeds the prescribed limits > 60 % of the rating of the unbalance CT, then the voltage across the capacitors will exceed 110 % of the rated voltage. Voltage more than 110% can damage capacitor units. Therefore, it is recommended to go to Tier 2, and performed a full capacitance inspection on all the capacitor units. Document all the findings. Faulty capacitors must be identified and replaced. 5.1.3 Record the power factor at transformer and reactive current from the capacitor bank Record the value of the power factor and reactive current. The power factor can be recorded using a power factor meter or a power quality meter. The value of the power factor must be measured at the incoming VCB i.e. power transformer. The reactive current must be measured at the VCB for the capacitor bank. Table 3 Examples for power factor and reactive current measurement
Step MVAr PF Rated Amp Measured Amp % Diff Amp
1 5 0.8 262.44 260.00
0.76
2 5 0.9 262.00 250.00
4.6 If the power factor is less than 0.9, please perform a load profiling at the substation to verify the actual reactive power demand at the substation. If the difference between the rated Amp and measured Amp is > 8%, please go to Tier 2 and perform a full capacitance measurement. 5.1.4 Record the value of the line voltage at the substation Voltage more than 110% of the capacitor voltage rating can damage capacitor units. Please record the voltage value and implement immediate measures if the value of the voltage is more than 110 %. It is possible either the automatic voltage regulator (AVR) or transformer tap changer has malfunction. The immediate measure will be to contact the respective protection engineer and reduce the transformer tap positions. The faulty AVR or transformer tap changer must be repaired ASAP. 5.1.5 Thermal imaging Thermography is an important factor in determining the operating condition of the capacitor banks. Performing periodic thermography tests can provide an early indication of a potential problem. These tests can identify bad electrical contacts, hot spots, partial discharge or overheating of capacitor units. Table 4 Assessment of the results of thermography
Category Results Action 1 1 to 4 OC Normal. No action is required 2 5 to 10 OC Monitor. Inspect and repair within 6 months 3 11 to 14 OC Inspect and repair within 3 months 4 ≥ 15 OC Inspect and repair immediately.
If the result of the thermography is Level 2 or 3 please go to Tier 2 and planned the maintenance work.
If the result of the thermography is Level 4 please go to Tier 2 and implement the maintenance work. 5.1.6 Internal Partial Discharge/ Transient Earth Voltage (TEV) Partial discharge creates current spikes in the conductor and hence also in the earthed metal surrounding the conductor. TEVs are a very convenient phenomenon for measuring and detecting internal partial discharge as they can be detected without making an electrical connection or opening any panel. Each discrete partial discharge is the result of an electrical breakdown of void within the insulation. These discharges degrade insulation and eventually result in insulation failure Table 5 Result and action for internal partial discharge test
Results Action ≤ 20 dB Normal. Monitoring frequency can be maintained
20 dB < dB, dB ≤30 dB Monitoring frequency should be revised to 3 months >30 dB Make arrangement for Tier 2 tests – PD Locator
From table 4, if the TEV reading is more than 29dB, please implement the actions in Table 5. Table 6 Guide to interpretation of TEV readings
TEV reading Conclusion Background reading
< 20 dB Suspect internal PD. Further investigation require: 1. PD Locator 2. Sequential switching 3. Visual inspection
Background reading > 20 dB
Further investigation is required to detect source of TEV reading either from internal PD or surrounding by using PQ monitor for a period of 1 week.
5.1.7 Recording of the harmonic voltages Harmonic measurement must be taken at the respective points at the VCB. If the total harmonic distortion voltage (THDV) > 4 % (11 kV), please go to Tier 2. If the harmonic spectrum also shows the existence of the 5th & 11th harmonics, please go to Tier 2. 5.2 Tier 2 Tier 2 diagnostic techniques are applied based on the results of Tier 1 diagnosis which could not definitely classify the capacitor bank as normal. Table 7 Actions to be taken based on results from Tier 1
Result from Tier 1 Action plans Unbalance current > 60% from the rating of the unbalance CT.
• Shutdown the capacitor • Perform a full capacitance measurement on all capacitor units • Capacitor units that have % difference more than 10 % should be replaced
Measured reactive current > 8% of rated capacitive current
• Shutdown the capacitor • Perform a full capacitance measurement on all capacitor units • Capacitor units that have % difference more than 10 % should be replaced • Inspect on HT Fuses. For damaged fuses, please replace with new ones • Please perform reactance test on detuned reactor/reactors. • For reactors with ≥10% difference, please change the reactors.
TEV > 30 dB • Shutdown the capacitor • Perform a PD locating • Check cable for crossing condition • Repair the faulty cable
Thermal difference ≥ 15 OC
• Shutdown the capacitor • Check for hot spots • Repair the problem area
If the THDV > 4 % (11kV)
Or harmonic spectrum shows the existence of harmonics from 5th or 7th…
• Shutdown the capacitor • Perform a full capacitance measurement on all capacitor units • Capacitor units that have % difference more than 10 % should be replaced • Perform a harmonic penetration study to evaluate the possibility of
harmonic resonance and finding solutions.
5.2.1 Requirement for overall maintenance For effective maintenance, all the action plans in Table 6 must be performed together with the maintenance program described in section 3.2. 5.3 Corrective maintenance Corrective maintenance program is implemented when the VCB, that connects the capacitor bank to the power system, trips. The cause of the tripping must be identified and corrected. The common cause of the VCB tripping may be due to: - a. Unbalance relay operated.
The cause is due to the existence of unbalance current more than 90% of the CT ratio. If the unbalance protection has tripped the VCB, all capacitor bank units should be capacitance measured and faulty units replaced.
b. Overcurrent relay operated.
The cause could be due to phase to phase shorting caused by conducting objects or conductors shorted phase to phase inside the capacitor banks.
All the faulty components inside the capacitor banks must be repaired or replaced. Cable failure i.e. termination failure, could also cause the overcurrent relay to operate. Test the cable to verify the cause of fault. All the causes of the faults must be identified and rectified.
c. Earth fault relay operated.
Severe imbalance in the phase currents can cause neutral to be present in the earth fault relay scheme. If the value of the imbalance current is more than the settings of the EF relay, then the relay will operate. Single phase fault can also trigger and initiate the EF relay. The cause of the EF relay operation must be identified and rectified.
6.0 CONCLUSION The existing maintenance program for all MV power capacitors are time based maintenance program. In order to meet availability targets and minimize costs, new appropriate maintenance plans need to be established for power capacitors. Unlike in planned scheduled maintenance (PM), where maintenance is performed based upon predefined scheduled intervals, CBM is performed only when it is triggered by asset conditions. Compared with PM, this increases the time between maintenance tasks, because maintenance is done on an as-needed basis. In this paper, a CBM program for 11 kV power capacitors is presented. The program comprises of 3 phases of CBM and summarized as follows:-
Phase of CBM Description Surveillance Monitoring the capacitor condition to detect developing problems Diagnosis Isolating the root cause of the issue and developing a corrective
plan based on priority, capacitor condition and its remaining life Remedy Performing the corrective action
The advantages of this CBM program for power capacitors are:- • CBM is performed while the capacitor is working, this lowers disruptions to normal operations • Reduces the cost of capacitor failures • Improves capacitor reliability • Minimizes unscheduled downtime due to catastrophic failure • Minimizes time spent on maintenance • Minimizes overtime costs by scheduling the activities • Minimizes requirement for emergency spare parts • Optimized maintenance intervals (more optimal than manufacturer recommendations) In summary, this CBM program can effectively maintain and extend the life cycle of MV power capacitors. 7.0 REFERENCES [1] IEEE Std.18: IEEE standard for shunt power capacitors. [2] IEC 60871 Shunt capacitors for a.c. power system having a rated voltage above 660V. Author
Dr.Mohamed Fuad bin Faisal is the Specialist (Power Quality & Energy Efficiency) in the Asset Management Department, Distribution Division TNB Malaysia. He graduated from CWRU, Cleveland, Ohio in 1990 with a BSc in Electrical Engineering. He obtained his MSc in Electrical Engineering from UiTM, Shah Alam in 2003 and his Phd in Electrical Engineering from UKM in 2011. He is a senior member of IEEE.