Reliability analysis of application of variable frequency drive on condensate pump in nuclear power plant

ABSTRACT

The aim of this paper is to analyze the reliability of Variable Frequency Drives (VFD) when used to control condensate pump motor in a nuclear power plant. IEC 60812 system reliability analysis procedure for Failure Mode and Effects Analysis (FMEA) was used to determine the potential failure modes for VFD and its effects on the operation of the condensate pump motor. In addition, Electrical Transient Analysis Program (ETAP) was used to simulate the performance and evaluate system reliability indices of the VFD for APR1400 auxiliary power system. The objective for adopting these two approaches is to use the FMEA for comprehensive identification and evaluation of unwanted effects of VFD malfunction on the condensate pump operation; and the ETAP was used to quantify the impact of VFD operation on system reliability indices. From the FMEA model, the Insulated Gate Bipolar Transistor (IGBT) and the motor stator winding were identified as the components with high susceptibility to failure. The overall reliability assessment showed that VFD failure effects are generally incipient. Average system availability increased marginally when VFD was used instead of the existing throttle valve for condensate flow control.

KEYWORDS:

• Reliability
• Variable Frequency Drives (VFD)
• failure modes and effects analysis
• nuclear power plant

1. Introduction

Recent problems facing most Nuclear Power Plant (NPP) owner/operating organizations has been economic. Nuclear power plant operational cost has been increasing steadily over the years and this is partly due to implementation of additional safety enhancement and coping capabilities for extended loss of AC power. This became profound after the Fukushima accident [1]. Power plant owners have therefore adopted various strategies to limit the rising operational cost and maximize revenue. One of the key strategies is to reduce the energy consumed by auxiliary power systems and make the energy saved available to the commercial grid for additional revenue [2,3].

In view of these, many energy saving methods have been considered to be used in the auxiliary power system. Variable Frequency Drives (VFD) is one of such methods which showed a remarkable potential for both energy saving and improvement in induction motor control. The basic principle governing the operation of VFD is to match the input power required to a motor’s terminal to the needs of the process, and by matching this parameter, the benefits are magnified using the cube-law. For example, centrifugal pump in general operate based on affinity laws, which implies that the power requirements of the pump increases with the cube of the speed. Thus, there is a potential to save power consumed by the pumps when the speed is regulated to match the needs of the process. Ongoing research showed that when centrifugal pumps are controlled with VFD there is the potential to save energy consumed between 30% – 50% [4].

Despite the popularity of VFDs in other industries, it has not gain significant attention for full deployment in nuclear power plant due to concerns of reliability and economic justification especially when existing systems has to be retrofitted. Notwithstanding that, introducing new technology in NPP are often subjected to rigorous regulatory requirements which are mainly safety and reliability related [3]. Reliability therefore, is a major concern for NPPs and over the years researchers have investigated into ways of improving the reliability of drives and power electronics to enhance their performance [6–8].

The uncertainty and challenge most researches face however, is that, accessing power generation plant performance data to carry out actual analysis and identify areas for system improvement is often very difficult.

The aim of this work is to contribute to ongoing researches by investigating reliability related issues of VFD when installed on a condensate pump motor by adopting the concept of Failure Mode Effects Analysis (FMEA) to determine component level failures and its overall impact on the system. The condensate pump motor was selected for this research because, to achieve full potential of VFD on a combined system, it is important that the controlled motor is not 100% loaded [9]. This provides flexibility for the VFD to modulate the motor power to match the load. The APR1400 nuclear power plant for example comprise of three condensate pump motors capable of delivering 50% of the required condensate flow capacity. Also, these makes it possible for comprehensive reliability analysis when using electrical power systems simulations software.

The paper is organized in the following manner, section 2 describes the FMEA theoretical concept and the methodology for this research. In section 3 & 4 the results for the FMEA model developed and the ETAP system reliability analysis are presented respectively. Section 5 contains a brief conclusion for the findings of this research.

2. FMEA approach to reliability analysis

The FMEA provides a systematic procedure for determining how each component of a device or system can fail, the mechanisms that cause it to fail, and how it can affect the overall performance of the system [10,11]. Therefore, to understand the relation between all component level failures and the overall impact on a combined system, FMEA is a useful tool in carrying out such analysis.

It should be noted that, reliability for VFD is application-dependent, this is because the magnitude and type of failure governing stresses vary with each application [11,12]. Practical understanding of the combine system is crucial in carrying out reliability analysis. By adopting IEC 60,812 procedure for FMEA, this offers useful approach in carrying out practical reliability analysis. In this research, the FMEA analysis was carried out in accordance with the IEC 60,812 procedure [10,13].

2.1 Methodology for VFD reliability analysis

Specific FMEA was performed on VFD installed to control 4600 HP condensate pump motor. For each major component of the VFD, a brief functional description of how the component fits into the design of the VFD was stated. The main failure modes of the component and the failure mechanisms that can contribute to occurrence of that failure mode was also listed. Consequently, the possible effects that can result from each failure mode and its severity was evaluated for each failure mode as determined by the failure effects. The effects were classified as follows:

• Catastrophic: immediate loss of motor function due to VFD malfunctioning

• Degraded: reduced motor performance

• Incipient: early problems that can be repaired easily but would become more severe if left unchecked.

Four main components: converter (rectifier circuit), DC link, Inverter and controller were selected for the FMEA analysis on the VFD. In Figure 1, the harmonic filter and power factor correction were included as optional packages and have not been considered neither for the FMEA model nor the reliability calculation.

Figure 1. VFD general structure arrangement.

2.2 Reliability analysis of combined effect on condensate pump motor

To provide a better understanding of the effects of VFD failure on the condensate pump motor and its effect on the entire condensate system, FMEA was also carried out on major components on the pump motor. Although the condensate pump motor has high number of defects that can occur, only the components which has possible direct impact from VFD malfunctioning were analyzed. Three components: stator winding, bearings, and stator leads & cross ties were analyzed.

The boundary of the study with respect to the reliability of VFD and condensate motor reliability calculation is shown in Figure 2. The assumption is that the VFD and the motor are in series connection and with this configuration, if any one of the system components fails, the entire system fails as show in the block diagram representation in Figure 2. It is important to mention at this point that, VFD bypass mode and redundancy were not considered for the reliability analysis. The aim is to present the worst-case scenario in determining the failure rate.

Figure 2. Boundary of study for reliability calculation.

3. The FMEA model

The FMEA for the VFD main components identified in this study is show in Table 1. In performing subcomponent analysis, harmonics were identified as major concerns because it has a wider potential to induce failure and its likely effect on the condensate pump motor is to cause stator winding to ground faults. The harmonic contents usually come from early frequency converters due to non-linear switching action of VFD inverter electronics. As confirmed in [14] this has the potential to cause failures resulting in heating of the motor winding.

Table 1. Variable Frequency Drive (VFD) Failure Mode Effects Analysis (FMEA).

Item	Component Function	Functional failure	Failure mode	Failure effect	Consequence Evaluation	Corrective Action
Converter	Converts AC input power to DC power	Short or open circuit due increasing voltage and current	Diode failure	Diode fail to open or close	Incipient	Check input power source and replace damage converter
DC link	Holds the DC voltage at constant level as energy source to be used by the inverter.	Capacitor wear due to high temperature, over voltage/current, improper mounting or clamping. Inability to smooth current flow to VFD due to Coil Failure	Transient causing input diode failure	IGBT failure to produce sine wave Inability to reduce harmonic noise,	Catastrophic Degraded	Incorporate AC & DC reactors into drive design to increase reliability
Inverter	Convert constant DC voltage into three phase AC variable voltage to provides Pulse Width Modulation (PWM) output to the condensate pump motor depending upon the control circuit and load conditions.	Output converter failure Failure to control voltage and/or frequency	IGBT not switching Clamp capacitor failure Clamp resistor failure Loss of aux voltage Gate unit supply failure Loss of optical fibre control signal Loose connections	IGBT fault trip IGBT failure – fault trip IGBT failure – fault trip Gate unit supply trip Gate unit supply trip IGBT fault trip IGBT failure – fault trip	Incipient Degraded Incipient Catastrophic Incipient Incipient Incipient	Replace IGBT Replace clamp capacitor Visual checks Consider installation of UPS Random failure Replace Optical fibre Visual checks
Controller	Provides control signal for the operation of IGBT based on sensor outputs	Induced excessive network traffic on shared Integrated Computer System network	PLC failure	VFD control failure resulting in signal scams	Catastrophic	Reset controller and restart VFD. Install network firewall on condensate and VFD

From the analysis, failure effects from gate unit supply is very severe as this would result in uncontrolled voltage and frequency from the inverter and consequently resulting in degraded operations of the pump motor. Similarly, failure of the DC link which is responsible to store constant DC energy for inverter use is catastrophic. This could be due to failure of the capacitor leading to IGBT failure to produce the desired sinewave.

Inverter failure, in particular the IGBT has significant effect on the overall reliability of the VFD and on the motor as well. From literature, it was noted that the rise time of VFD PWM pulse has a major impact on the peak voltage at the motor terminal, and this peak voltage when overshoot results in insulation breakdown and subsequently causing stator coil damage. Therefore, the effect of IGBT mis-operation is high.

As shown in Table 1, it emerged that most of the induce failure effects from VFD malfunction are rather incipient and maintenance and monitoring activities must be designed to focus on identifying the incipient failures before they lead to more severe operating failures. More importantly, out of the eleven failure effects identified for the VFD eight of the failures involve IGBT failures, and only one for controller inoperability.

The FMEA representing possible effect of VFD induced failures on condensate pump motor performance is shown in

Table 2. As mentioned earlier, induction motors have several components that can fail leading to loss of motor functionality. Bearing, insulation faults and loose connections are among the highest causes of failures in induction motors. It is essential to keep in mind that motor bearings and insultations failures generally occur due to underlying factors such as high temperature, wear, motor loading condition, operating environment and overvoltage. Therefore, the overall mode of failure for those components vary greatly.

Table 2. Failure modes effects analysis on combine effects from VFD on condensate pump motor.

Item	Function	Functional failure	Failure mode	Failure effect	Consequence Evaluation	Corrective Action
Stator Winding	Produces a sinusoidally distributed, rotating magnetic field in the stator when 3-phase variable AC voltage and Variable frequency is applied from VFD output to stator winding.	VFD contribution to thermal degradation of coils due to low frequency, voltage imbalance. VFD converter performance issue resulting in uncontrol voltage/frequency supplied to stator winding Insulation breakdown due to electrical transients and surges VFD induced motor insulation breakdown due to interaction between voltage pulses VFD applies to pump motor stator coils.	Winding-to-ground fault Winding-to-ground fault Open winding Open winding	Electrical trip Damage to stator windings requiring motor repair/rewind Same as above Same as above	Catastrophic Catastrophic Degraded Degraded	Good maintenance practices to keep VFD and motor ventilation clean and unrestricted Monitor and trend insulation conditions parameters, ambient temp. winding temp.
Stator leads and Coil cross-ties	Connect motor line terminations to individual stator winding coils	Loosening of leads, coil cross-ties, and fasteners due to vibration, electromagnetic transients from VFD induced harmonics.	Phase to ground fault Loose leads or coil cross-ties	Degradation and damage to insulation and conductors leading to electrical trip	Incipient	Monitor and trend ambient temp. VFD harmonic, cooling and motor amps

Per Table 2, stator winding, and stator lead and cross-ties were identified as the major components with some form of direct impact from VFD malfunction. As shown in the table, winding-to-ground-fault occurs due to issues of low frequency or voltage imbalance attributable to inverter fault to control the supplied voltage and frequency. This event is catastrophic since it leads to electrical trip and subsequent loss of motor functionality. In the same way, a failure mode of open-winding can be attributed to VFD induced motor insulation breakdown as a result of interaction between voltage pulses applied to the pump motor stator coils. This event also degrades the operation of the pump motor.

As a general principle, nuclear power plants operate in specific modes and depending on the operating mode, the requirements for condensate pump motor operation varies. This variation has the tendency to alter the performance of the VFD and consequently affect its interaction with the condensate pump motor. To evaluate the degree of impact, usually FMECA (failure mode effects and criticality analysis) could be performed to categorize and determine the severity of each failure mode and effects. However, in this research, ETAP was used to evaluate and quantify the overall impact on the system reliability indices when VFD is introduced to control the condensate flow system. This results are highlighted in Section 4.

4. System reliability analysis

Generally, determination of system reliability indices can vary greatly depending on published equipment failure rate data obtained from equipment manufacturers. Understanding the assumptions and methodologies used in establishing these failure rates can provide meaningful and realistic system reliability indices. The objective of this section is in twofold; first to present mathematical approach in calculating reliability indices; and second to carry out reliability analysis by ETAP to simulate APR1400 auxiliary power system with and without VFD installed to control the condensate pump motor (see Figure 3). For the simulation analysis, careful consideration was made in selecting the equipment failure rate data for the simulation model. The data used in this research were from IEEE Std493-1997, IAEA-TECDOC-478 and NUREG/CR-6928 nuclear power plants component reliability failure data. Some of these data are shown in Table 3.

Table 3. Some equipment failure rate data.

Availability	Data Source	Failures Rate/Hrs
Main Generator	IAEA	9.47 x 10^–7
Main Transformer	IAEA	1.15 x 10^–6
Power Cable	IEEE	5.27 x 10^–6
13.8kV MV Bus	IAEA	1.5 x 10^–6
15 kV Indoor CB	IAEA	6.51 x 10^–7
Circulating Water Pump	IAEA	1.8 x 10^–5
Condensate Pump	IAEA	3.6 x 10^–6
Level Control Valve	NPRDS	1.0 x 10^–5
Induction Motor	IEEE	1.53 x 10^–5
VFD	Manufacture	7 x 10^–6

Figure 3. Segment of the APR1400 auxiliary power system.

Mathematically, component failure is assumed to be exponentially distributed and assume $T$ is considered as a random variable representing the time to failure of a component, the component reliability at time τ is defined as

(1) $MTTF=\underset{0}{\overset{\mathrm{\infty }}{\int }}R\left(\tau \right)d\tau$(1)

Consequently, the Mean Time Between Failure (MTBT) and Availability can all be calculated using the equations below.

(2) $MTBF=\frac{Totaluptime}{No.ofBreakdowns}$(2)

$Reliability=e^{-\frac{Time}{MTBF}}\left(3\right)$

(4) $MTBF=\frac{1}{FailureRate\left(\lambda \right)}$(4)

$Availabiity=\frac{MTBF}{MTBF+MTTR}\left(5\right)$

Using the above equations, the system downtime with respect to availability can be estimated as shown in Table 4.

Table 4. Downtime of availability.

Availability	Downtime
80%	73 days/year
90%	36.5 days/year
99%	3.65 days/year
99.9%	8.76 hours/year
99.99%	52 minutes/year
99.999%	5 minutes/year
99.9999%	31 seconds/year

4.1 ETAP reliability simulation results

Figure 3 shows a portion of the APR1400 auxiliary power system which was simulated. The comparison results for the system when operated with VFD installed to control the condensate pump and when operated with throttle valve is shown in Table 5. The main reliability indices considered were System Average Interruption Frequency Index (SAIFI); System Average Interruption Duration Index (SAIDI); Customer Average Interruption Duration Index (CAIDI); Average Service Availability Index (ASAI); and Average Service Unavailability Index (ASUI).

Table 5. Summary of system reliability indices.

Reliability Indices	Throttle Valve controlled Condensate Pump Motor	VFD controlled Condensate Pump Motor
ASAI (p.u.)	0.9966	0.9966
ASUI (p.u.)	0.00338	0.00336
CAIDI (hr/customer interruption)	88.927	105.255
SAIDI (hr/customer. Yr)	29.5782	29.4641
SAIFI (f/customer. Yr)	0.3326	0.2799

From the simulation results, it was noted that for load point reliability indices, there was significant difference in the reliability indices for the condensate pump motor when operated with VFD compared to the existing throttle valve control method in the APR1400. The Average interruption rate at the motor terminal was found to be 1.5963 f/yr and 0.2792 f/yr for the throttle valve method and VFD controlled method respectively. As shown in Table 5 below, introducing VFD to control condensate pump motor instead of the existing throttle valves resulted in marginal improvement in the reliability indices. The interruption frequency is lower for VFD control system as compared to throttle valve control. Similarly, the unavailability index and interruption duration for the VFD condensate flow control method is lower than the indices for throttle valve controlled method.

5. Conclusion

In this paper we have developed an FMEA reliability assessment model based on IEC 60,812 for VFD and condensate pump motor. The FMEA model covered the combine effect of VFD malfunction on the entire condensate system operation. In addition, reliability analysis was also carried out using ETAP to determine the overall impact on auxiliary power system reliability when VFD is installed to replace the existing direct online motor starting method which utilizes throttle valve for condensate flow regulation. The assessment results for FMEA showed that the VFD failures are mostly incipient. With regards to the overall system reliability, using the VFD showed marginal improvement in system reliability.

Acknowledgments

This work was supported in part by the KEPCO International Nuclear Graduate School (KINGS), Ulsan South Korea.

Disclosure statement

No potential conflict of interest was reported by the authors.