Reducing Equipment Failure Risks by Redesigning of Products and Processes ^†

Abstract

Low-voltage (LV) network assets, although they do not play a significant role in reliability indices compared to medium-voltage (MV) assets like the transformer and switchgears, are required to be designed in a way that would mitigate the risk of sporadic failures, hence incurring an R&M cost. LV assets like LV cables, distribution panels, molded-case circuit breakers (MCCBs), and miniature circuit breakers (MCBs) generally do not have a planned maintenance (PM) schedule and are procured based on the run-to-failure concept in view of the huge volume. These assets are exposed to the harshest of environmental and operation conditions. Hence, it is imperative that we take the necessary measures during the design stage such that they are able to cater to their stringent duties, which include frequent short circuits, exposure to the environment, and thermal overloads. It is also important to periodically review the product design based on site feedback and product performance to re-calibrate the product and its associated processes. Through this technical paper, several case studies are presented wherein special terminal connectors with shear bolts were designed to mitigate the thermal hotspot issues causing frequent fire and failures—i.e., vertical fuse switch disconnectors (VFSDs) and miniature circuit breaker (MCBs). A case study on condition monitoring through a substation inspection schedule is also presented, through which potential failures were averted in time. The observations and measurements are mapped in an SAP system for trend analysis. With the adoption of effective product and process design, AEML has reduced asset failures.

1. Introduction

Adani Electricity Mumbai Limited (AEML) has a huge asset base of MV switchgears, transformers, and LV panels and pillars for catering to the electricity requirements of its more than 3 million customers. In view of the underground installation, LV cables are excluded from discussion in this paper. Medium-voltage (MV) assets are generally housed in customer substations (CSSs), while low-voltage (LV) panels, pillars, MCCBs, and MCBs connected at 415 V or 230 V are spread out across the distribution area, exposed to an open environment. These LV assets cater to the supply requirements of few customers in a congested pocket, buildings, or parts of buildings, and provide last-mile connectivity to customers over the distribution network. Usually, the customer network starts after the MCB, at the switch disconnector fuse (SDF) terminal. The tropical environment, with an annual rainfall of more than 2400 mm and a maximum temperature reaching 40 °C during the summer, exposes the LV assets to a heavy duty of corrosion and thermal stress. With a yearly rise of approx. 200 MW in the recorded maximum demand over the last few years, the assets are stressed over their performance duty. The short-term loading of assets, although they do not immediately lead to failures, substantially reduces the asset life on account of the accelerated aging of the insulation and electrical contacts. Long-term overloading results in asset failures if appropriate action is not taken in time. The contribution of LV asset failures in customer-hour loss (CHL) is approximately 90%. With the majority of customers supplied over the LV network, it becomes imperative that LV assets are adequately designed, and additional checks and balances are put in place through process re-engineering. This also includes the field installation and monitoring post asset installation.

2. Challenges Faced by Utilities in Design and Maintenance of LV Electrical Assets

LV assets comprise LV panels and pillars with ACBs, VFSDs, MCCBs, SFUs, and MCBs installed at the downstream side of the distribution transformer towards the customer end. These assets are installed in an open environment and are affected by corrosion, external disturbances like pilferage and damage, and thermal stresses due to periodic overloading. The following challenges are faced by utilities in the design, procurement, and maintenance of these assets.

2.1. Limited Vendor Base

LV products like VFSDs are offered by only a few OEMs in India. At AEML, the max LV cable size is standardized to 300 sq mm AL. Since the design of VFSDs offered by Indian OEMs is not suitable and type-tested for a connection with 300 sq mm AL cables, they are imported from European OEMs. Similarly, the SDFs suitable for a connection with AL cables are offered by only four OEMs. The limited vendor base poses challenges in material availability and service support in India.

2.2. Vendor Capabilities in Design and Fabrication

All the LV panel, pillar, and SDF unit designs are customized to be suitable for electricity distribution and a connection with AL cables. They are also designed to be compact to reduce their footprint in cities like Mumbai. Therefore, the vendors who possess a competent engineering setup for the preparation of fabrication, as well as engineering drawings; the latest fabrication facilities with features such as laser and CNC machines; and a well-maintained ten-tank powder-coating plant need to be selected for the manufacturing and supply of LV assets. Properly designed, engineered, and tested products will offer the intended service life after its commissioning in the field. Currently, very few vendors possess these facilities.

2.3. Quality Assurance and Control

Quality assurance and control during manufacturing, installation, testing, and commissioning will be crucial from a product performance perspective. Stringent quality checks including stage inspections and FATs are enforced on vendors during the manufacturing of the products. Similarly, at AEML, field quality controls are carried out during the installation and commissioning of the LV assets by an independent team in order to mitigate the workmanship issues by various agencies, if there are any. The available vendors of fabrication do lack quality control procedures and are dependent on OEMs. Additionally, products like MCCB, MCBs, and panels are rated per the standard 20 °C operating temperature. The performance in actual field conditions, which may reach up to 40 °C ambient, deviates from the given performance duty and results in spurious tripping.

2.4. User Ignorance in Maintenance and Troubleshooting

For LV assets, maintenance is dependent on non-engineering staff, considering the volume of activities. These staff personnel sometimes lack the skills required for troubleshooting. Thermography is being performed for critical cases, although, due to the huge volume, the practice is not cost-feasible or widespread. With proper maintenance planning and the deployment of qualified engineers, this issue is being addressed.

2.5. High Maintenance Cost

Planned or scheduled maintenance is performed one with a specific periodicity, depending on the asset’s network criticality and its impact on reliability, in addition to the cost of maintenance over the life cycle. Due to the large quantum of LV assets, abnormalities, if any, due to the product performance, workmanship, and site compatibility will attract high maintenance costs. Since the LV assets are located close to the customer, all the abnormalities will have customer-hour loss, thus directly affecting the revenue of the company.

3. AEML-D: Challenges Faced and Their Mitigation

AEML-D has a huge asset base of LT equipment. Due to space constraints, all products are designed with a lower footprint and a high safety margin. Through this paper, two case studies are presented wherein, post product implementation, based on a design review, the site issues were addressed. Apart from the design review, the sensitization of the working staff has played a major role in the mitigation of the problem.

3.1. Case Study 1: Improving the Performance and Reliability of Vertical Fuse Switch Disconnectors (VFSDs) by Designing Special Shear Bolt Connectors

In view of space constraints, AEML-D designed and implemented compact LV panels in Mumbai. The compact panels were provided with VFSDs [1,2] (Figure 1) meeting the requirements of IS/IEC 60947 [3] for outgoing LV circuits. These VFSDs are made from thermosetting plastic with a temperature endurance of up to 200 °C. The product was type-tested per IEC 60947-P3 [3] for a temperature rise <−80 °C. The VFSDs were installed in MS powder-coated panels with an IP Class of IP44.

Of late, fire issues have been reported in the VFSD panels and pillar. The fire would start from the bottom of the VFSD and propagate towards the top side (Figure 2). In another case, a fire was reported in the fuse compartment (Figure 2).

The investigation findings are as follows: the LV sector-shaped cables were directly terminated in the V-groove connector of the VFSD. Due to the spreading of the cable strands during cable termination, the contact resistance at termination was increased, resulting in hotspot formation. Over this period, due to the loading and deterioration of the cable insulation, a fire was initiated from the termination area. In Figure 3c, the HRC fuse was not inserted correctly inside the fuse carrier, resulting in hotspot formation.

Hence, design measures need to consider the issues of high contact resistance, and the deterioration of terminal contacts over time due to the oxidation of aluminum conductors and Cu contacts. Thermal run-off failures are also attributed to poor ventilation.

The solution implemented is the design and deployment of the sector-shaped shear-head connector (Figure 4).

A VFSD connector with alloy Al with a matching sector-shape barrel was designed. Due to the alloy Al, the oxidation issues are addressed. The conductivity of the Al alloy is 53%, and, hence, the contact area is proportionately enhanced, which also reduces the ventilation issues. The lug is also tine-plated, and non-oxidizing grease is applied inside the lug barrel. The connectors were tested on 250 load cycles per the IEC 61238 [4] method, and the temperature rise was within 45 °C. The cables were terminated in a round base, which was secured by torque-co-ordinated shear-head bolts. The lugs were insulated with thin-wall polyolefin tubes to ensure creepage. Another advantage of the shear-head connector was its easy installation with a minimal skill set and not requiring special tools like core-shaping and turning tools. The VFSD connectors have been installed for more than 3000 circuits having loading constraints, and the performance was found to be satisfactory. For the last two years, there have been no failures in VFSD panels.

3.2. Case Study 2: Addressing the Spurious Tripping of MCBs by Designing Special Shear Bolt Lug

The metering installations are provided with miniature circuit breakers (MCBs) with Curve C characteristics. These MCBs are designed per IS/IEC 60898 and have been tested for their connection with flexible Cu conductors. These MCBs are designed for a full rating at 30 °C per industry standard. AEML-D uses sector-shaped Al cables for LV service connections. These 25/50 sq mm cables were directly terminated in MCB terminals.

Spurious trippings were reported for MCBs with a rating of 63/100 Amps. These MCBs are used as incomer switches to slum locations with four to eight single-phase meters. The matter was investigated, and the following are the findings:

• The MCBs are designed for 30 °C ambience. During the summer peak, the ambient temperature crosses 38–40 °C. Due to de-rating, the MCBs were tripped on a lower load.
• Al cables were directly terminated in MCB terminals. Al cable strands spread during termination and create a hotspot at the MCB terminal (Figure 5). The MCB tripping mechanism for overloads consists of a bi-metallic strip, and a rise in the ambient and terminal temperature results in tripping on low loads.

The solution implemented is the design of a special MCB connector with Al alloy. The connector head was specially machined for MCB terminals with reduced contact resistance (Figure 6).

Frequent tripping cases were identified through the OMS portal. These sites were inspected, and the wiring was tested through a loop impedance measurement. MCB connectors were installed and monitored over a period of time. The findings are in Table 1 below:

Table 1. Reduction in temperature post MCB connector installation.

Installation	Date	Max Amp	Temp	Date	Max Amp	Temp
4C 25 sq mm	19.12.22	28	39.7	24.02.23	38	30.3
4C 25 sq mm	19.12.22	24.9	37.1	24.02.23	35.6	32.3

Table 1. Reduction in temperature post MCB connector installation.

Installation	Date	Max Amp	Temp	Date	Max Amp	Temp
4C 25 sq mm	19.12.22	28	39.7	24.02.23	38	30.3
4C 25 sq mm	19.12.22	24.9	37.1	24.02.23	35.6	32.3

The condition monitoring of substation assets is as follows: AEML-D Customer Substations (CSSs) house MV switchgears, distribution transformers, and LV panels with ACBs or MCCBs. AEML-D has deployed various diagnostic measures like thermography [5,6] and partial discharge monitoring through state-of-art thermovision cameras and portable partial discharge meters with TEV and acoustic measurement modes. Scheduled maintenance is a costly affair in view of the huge R&M cost and failures between the PM period. With reliability-centered maintenance as the new milestone, scheduled maintenance is being phased out, with a reliance on advanced diagnostic and data analytics.

AEML-D has deployed a yearly substation inspection [7] through which observations and measured values are captured on a mobility platform. The mobile app is integrated with SAP wherein the values are captured as measurement points (Figure 7). An asset-wise trend analysis is performed to predict anomalies through data analytics. The data model has been successfully deployed for MV cable assets with a 98% fault accuracy. Through substation inspection, developing and incipient anomalies are captured before the failures. The condition-monitoring technologies used are thermography, partial discharge monitoring (for MV assets) using TEV (transient earth voltage), and ultrasound.

4. Discussion

AEML-D Engineering Function has established processes [7] to review designs based on user feedback and CAPA post any asset failure. Although the products are designed and procured following IS/IEC standards and stringent QAPs, the performance on site depends on many factors. The controllable factors are addressed through the engineering and administrative control measured. Engineering measures include a re-design of the main equipment, the introduction of accessories to meet the desired performance, and putting up sensors for predictive analysis. Administrative measures include training for staff and engineers, the standardization of QAPs, work instructions, etc. AEML-D has successfully optimized its maintenance strategy based on condition monitoring and working out a plan to deploy reliability-centered maintenance, wherein, based on selective criteria like asset duty, customer class, and ambient conditions, the asset shall be recommended for maintenance.

5. Conclusions

Product failures are indicative of performance gaps in terms of product endurance or practices not aligned with SOP. Electrical utilities can refer to the case studies explained through this paper to mitigate the issues at their end.