In the context of the expanding adoption of battery energy storage systems (BESS) across various energy sectors, the demand for specialized cables that can endure extreme operational conditions has become increasingly critical. As a researcher involved in this field, I have focused on designing and developing cables capable of withstanding temperatures up to 125℃, which are essential for ensuring the reliability and safety of BESS installations. These systems require cables that not only handle high voltages, such as DC 1500 V, but also exhibit resistance to temperature fluctuations, chemical exposure, battery acids, and flames. Currently, there is no unified national or international standard for cables used in BESS, leading to reliance on certification guidelines like CQC 1143—2019 and TÜV 2 PfG 2693/06.19. However, these standards often overlook rubber-based cables, prompting my investigation into optimizing materials and structures for enhanced performance. This article details my comprehensive approach, including material selection, structural design, manufacturing processes, and validation testing, all aimed at advancing cable technology for battery energy storage system applications.
The primary objective of my work was to design a cable that meets the stringent requirements of battery energy storage systems, with a focus on 125℃ operational capability. I began by defining the key technical parameters, as outlined in Table 1, which cover electrical, mechanical, and environmental aspects. These parameters ensure that the cable can perform reliably under the harsh conditions typical of BESS environments, such as high temperatures and exposure to corrosive substances.
| Parameter | Requirement |
|---|---|
| Temperature Range | −40 to 125℃ |
| DC Resistance at 20℃ (Ω/km) | According to GB/T 3956—2008 |
| Insulation Volume Resistance at 20℃ (Ω·m) | ≥1.0 × 1011 |
| Voltage Test (3.5 kV, 5 min) | No breakdown |
| Tear Resistance (N) | ≥6 |
| Flexibility (Rebound Angle, °) | ≤40 |
| Temperature Rise (℃) | ≤30 |
| Chemical Resistance to Electrolyte | No significant change |
For the conductor design, I selected bare or tinned annealed copper conductors conforming to GB/T 3956—2008, specifically the 5th or 6th class, to ensure optimal flexibility and current-carrying capacity. For instance, in a 35 mm² configuration, I employed a multi-strand structure with a 1+6 lay pattern and small pitch twisting, which minimizes the conductor diameter and enhances bending performance. This design is crucial for battery energy storage system installations where space constraints and movement are common. The DC resistance of the conductor at 20℃ can be calculated using the formula derived from standard methods:
$$ R_{20} = R_t \times \frac{254.5}{234.5 + t} \times \frac{1000}{L} $$
where \( R_{20} \) is the resistance at 20℃ in Ω/km, \( R_t \) is the measured resistance at temperature \( t \) in Ω, \( t \) is the measurement temperature in ℃, and \( L \) is the sample length in meters. This equation ensures accurate assessment of conductor performance under varying conditions, which is vital for maintaining efficiency in battery energy storage system applications.
In terms of material selection, I evaluated several options to achieve the desired balance of electrical insulation, mechanical strength, aging resistance, chemical durability, tear resistance, and low temperature rise. I prepared four distinct cable samples for comparative analysis: Sample 1 with PVC insulation, Sample 2 with silicone rubber insulation, Sample 3 with halogen-free rubber insulation, and Sample 4 with both halogen-free rubber insulation and sheath. These materials were chosen based on their properties; for example, PVC offers cost-effectiveness but may contain halogens, silicone provides excellent temperature resistance but has mechanical limitations, and halogen-free rubbers deliver superior environmental safety and performance. The insulation volume resistivity was assessed using the following formula to ensure compliance with requirements:
$$ \rho = \frac{2 \pi L R}{\ln(D/d)} $$
where \( \rho \) is the volume resistivity in Ω·cm, \( L \) is the sample length in cm, \( R \) is the measured insulation resistance in Ω, \( D \) is the average outer diameter of the insulation in mm, and \( d \) is the average inner diameter in mm. This parameter is critical for preventing electrical failures in battery energy storage system cables, especially under high-temperature operations.
The structural design of the cables was tailored to each material type. For Sample 1 (125℃ PVC insulated cable), I adopted a single-layer insulation structure similar to RV-type cables in GB/T 5023.5—2008, using a 6th class copper conductor. Sample 2 (125℃ silicone insulated cable) also featured a single-layer design, referencing the ESP15S model from 2 PfG 2693/06.19, to leverage silicone’s high-temperature tolerance. Sample 3 (125℃ halogen-free rubber insulated cable) included a non-hygroscopic wrap around the conductor and a single insulation layer, modeled after TYPE 4 series in BS 6195. Sample 4 (125℃ halogen-free rubber insulated and sheathed cable) employed a double-layer structure with both insulation and sheath made of halogen-free rubber, inspired by the NSGAFOU series in DIN VDE 0250-602:1985. This layered approach enhances durability and safety, which are essential for the dynamic environments of battery energy storage systems. The mechanical properties of the insulation and sheath materials, both before and after aging, were rigorously tested, as summarized in Table 2, to ensure long-term reliability in BESS applications.
| Stage | Property | Insulation Requirement | Sheath Requirement | Sample 1 (PVC) | Sample 2 (Silicone) | Sample 3 (Halogen-Free Insulation) | Sample 4 (Halogen-Free Insulation and Sheath) |
|---|---|---|---|---|---|---|---|
| Before Aging | Tensile Strength (MPa) | ≥8 | ≥8 | 10.6 | 4.6 | 8.8 | 10.3 |
| Elongation at Break (%) | ≥150 | ≥150 | 180 | 265 | 360 | 285 | |
| After Aging | Aging Temperature (℃) | 158±2 | 158±2 | 158 | 158 | 158 | 158 |
| Aging Time (h) | 168 | 240 | 158 | 168 | 168 | 240 | |
| Tensile Strength Change (%) | ≤±25 | ≤±25 | -19.8 | 15.2 | -6.8 | 3.8 | |
| Elongation Change (%) | ≤±25 | ≤±25 | -25 | -4.9 | -3.9 | -5.9 |
Moving to the manufacturing process, I oversaw the production of conductors and cable samples with meticulous attention to parameters. For the 35 mm² bare copper conductor, I used a DBT-type 16-head fine wire drawing machine to achieve a single wire diameter of 0.191 mm, controlling elongation between 25% and 30%. The stranding involved a 168-wire/0.191 mm structure with an S-lay direction and pitch of 80–90 mm, followed by a 1+6 bunching on a CLY6+12 strander with an S-lay and pitch of 150–160 mm. This process ensured high flexibility, which is crucial for installation in compact battery energy storage system setups. For the cable samples, extrusion parameters were optimized based on material properties, as detailed in Table 3. For instance, Sample 1 required PVC pre-drying at 80℃ for 2 hours and extrusion at 35 m/min, while Samples 2–4 involved rubber extrusion with specific temperature profiles and steam curing to achieve cross-linking. These steps highlight the importance of tailored manufacturing for maintaining cable integrity in BESS environments.
| Sample | Material | Extrusion Machine | Key Parameters | Notes |
|---|---|---|---|---|
| Sample 1 | PVC | SP90/45 Extruder | Barrel Zones: 115–145℃; Head: 135–145℃ | Pre-dried at 80℃ for 2 h; cooled in 15 m water trough |
| Sample 2 | Silicone Rubber | XJW-100 Rubber Extruder | Barrel Zones: 20–40℃; Head: 35–40℃ | Low-temperature extrusion; steam pressure 1.5 MPa; 13 m vapor balance |
| Sample 3 | Halogen-Free Rubber | XJW-100 Rubber Extruder | Barrel Zones: 50–90℃; Head: 80–90℃ | High-temperature extrusion; 40 m/min rate; steam curing |
| Sample 4 | Halogen-Free Rubber (Insulation and Sheath) | XJW-100 and XJW-90 Extruders | Barrel Zones: 50–90℃; Head: 80–90℃ | Co-extrusion at 30 m/min; steam pressure 1.5 MPa; 13 m vapor balance |
To validate the designs, I conducted a series of tests focusing on electrical, mechanical, and thermal properties. The DC resistance was measured using an LCR digital bridge, with samples conditioned at 5–35℃ for over 12 hours to stabilize temperature. Insulation volume resistance was assessed with a ZC-90 high-insulation resistance tester, applying 2500 V AC for breakdown testing followed by 500 V DC for resistance measurement. Voltage withstand tests were performed according to GB/T 3048.8—1994, ensuring no breakdown under 3.5 kV for 5 minutes. Mechanical properties, including tensile strength and elongation, were evaluated before and after aging in an oven at 158±2℃ for specified durations, using universal testing machines and aging chambers based on GB/T 2951.11—2008 and GB/T 2951.12—2008. Flexibility was tested by fixing one end of the cable, applying a load to bend it 90° around a mandrel, and measuring the rebound angle after load removal, as per CQC 1143—2019. Tear resistance was determined by preparing notched samples and pulling them apart at 200–300 mm/min, following VW 60306-1:2018. Temperature rise tests involved passing 120 A DC current at 1500 V through a 5 m cable coiled in five turns in a 20℃ environment, measuring the maximum temperature difference over 2 hours, referencing TICW 15—2012. Chemical resistance was evaluated by applying lithium iron phosphate electrolyte to the cable surface and incubating at 45℃ for 72 hours, observing any changes. The results of these tests are compiled in Table 4, demonstrating how each sample performs relative to the requirements for battery energy storage system cables.
| Test Parameter | Requirement | Sample 1 (PVC) | Sample 2 (Silicone) | Sample 3 (Halogen-Free Insulation) | Sample 4 (Halogen-Free Insulation and Sheath) |
|---|---|---|---|---|---|
| DC Resistance at 20℃ (Ω/km) | ≤0.554 | 0.543 | 0.543 | 0.543 | 0.543 |
| Insulation Volume Resistance at 20℃ (Ω·m) | ≥1.0 × 1011 | 2.35 × 1010 | 1.36 × 1014 | 3.55 × 1013 | 3.68 × 1013 |
| Voltage Test (3.5 kV, 5 min) | No breakdown | Pass | Pass | Pass | Pass |
| Tear Resistance (N) | ≥6 | 7.2 | 3.6 | 7.5 | 8.3 |
| Flexibility (Rebound Angle, °) | ≤40 | 36 | 15 | 22 | 23 |
| Temperature Rise (℃) | ≤30 | 34 | 25 | 29 | 29 |
| Chemical Resistance to Electrolyte | No significant change | Slight stickiness | Bubbling and exposure | No change | No change |
| Halogen-Free Test: pH | ≥4.3 | — | 6.7 | 8.4 | 8.4 |
| Halogen-Free Test: Conductivity (μS/mm) | ≤10 | — | 1.5 | 3.4 | 4.3 |
| Halogen-Free Test: Halogen Acid Gas (%) | ≤0.5 | — | 0.32 | 0.35 | 0.3 |
| Halogen-Free Test: Light Transmittance (%) | ≥60 | — | 65 | 68 | 81 |
| Flame Retardance | Class C | Pass | Pass | Pass | Pass |
Analyzing the test outcomes, I found that Sample 4, with its halogen-free rubber insulation and sheath, delivered the best overall performance, excelling in tear resistance, temperature rise, and chemical durability. Sample 3 also showed promising results, particularly in electrical properties. This underscores the advantage of halogen-free materials in battery energy storage system applications, where safety and environmental factors are paramount. The temperature rise, for instance, is a critical parameter because excessive heating can degrade cable performance and pose risks in BESS. The formula for temperature rise assessment involves measuring the difference between the maximum cable temperature and ambient conditions under load, which I optimized through material and structural choices. Additionally, the halogen-free tests confirmed that Samples 2–4 meet low emission standards, reducing toxic fume risks in fire scenarios—a key consideration for battery energy storage system safety.
In conclusion, my research demonstrates that halogen-free rubber cables, especially those with dual-layer insulation and sheath, offer a robust solution for the demanding conditions of battery energy storage systems. By balancing electrical insulation, mechanical strength, thermal stability, and chemical resistance, these cables enhance the reliability and longevity of BESS installations. As the industry moves toward standardization, my findings contribute to establishing guidelines for cable design, promoting safer and more efficient energy storage solutions. Future work will focus on further optimizing materials and expanding testing to real-world BESS environments, ensuring continuous improvement in cable technology for global energy needs.