By Cathal Sheehan, Senior Technical Magnetics Marketing Manager at Bourns and Suker Zeng, Signal Magnetics Product Manager at Bourns.
The demand for renewable energy continues to grow and has led to the development of flexible energy systems to allow renewable sources to be reliably and safely connected to the energy grid. These next-generation systems need the support of equally advanced magnetic component technologies that provide power protection, switching and conversion.
One of the potential solutions seen to help support system flexibility is in the use of energy storage systems, which have the unique capability to quickly absorb, hold and then reinject electricity. Over the last few years, DC voltages in energy storage systems have continued to increase leveraging higher lithium-ion battery voltages where today, 250 VDC, 600 VDC, 1000 VDC and now even 1500 VDC are employed.
One of the key drivers of these higher-voltage systems is the availability of advanced solar inverters and power converters. Considering that most utility-scale battery energy storage systems are now being deployed alongside utility scale solar installations, it makes sense that the battery systems match the input DC voltages of the inverters and converters. Most utility-scale solar inverters and converters now use 1500 VDC input from solar panels.
Magnetic Components Rated to 1500 VDC
There are several ways energy sources can be connected to the grid such as with breakers, disconnect switches, insulation monitors, fuses and ground fault devices. Because these systems have evolved to support higher DC voltages, designers are increasingly challenged to find components rated at higher voltages and ones that also provide embedded protection features.
Meeting the needs of these new systems, Bourns has developed insulated magnetics products rated at 1500 VDC. For example, Bourns’ new Model SM91527L single channel signal transformer with integrated common mode choke is designed to meet the demanding isolation and EMI filter requirements on isoSPI™ communication buses in a Battery Energy Storage System (BESS). Bourns® Model SM91527L features a creepage of 15 mm between primary and secondary so it is an ideal solution for use in environments requiring reinforced isolation with working voltages of 1500 V. In addition, Bourns’ new signal transformer has UL approval, which gives designers higher safety confidence.
The Bourns 1500 V signal transformer isolates and protects the battery management system (BMS) board isoSPI™ interface (shown in Figure 1). Of importance in this application is that the BMS interface operates in a nonhazardous environment separate from the battery pack voltage, and at the same time, provides the necessary matching and common mode noise removal. This design is so that the communications between the monitoring ICs and the central management board are also secure and uninterrupted. In addition, the features of the Model SM91527L transformer allow it to still meet isoSPI™ driver recommendations in terms of insertion loss (Max. 1.2 dB at 4 MHz) and EMI (-50 dB 1-100 MHz CMRR).
Figure 1: Block Diagram Showing the Connection of the IsoSPI™ Bus between Primary and Secondary
Transformer Testing Requirements
Transformers with reinforced insulation up to 1500 V are required to meet test specifications as defined by IEC 60664. The standard was developed to prove the insulation supplied will survive sudden surges, which are possible from the electricity grid or from the battery pack itself. A key consideration for designers is that the application determines the level of surge testing that is required during qualification. That’s because consumer equipment has a particular overvoltage category requirement while industrial equipment may need to be rated to a different category.
Surge tests are used to primarily check for adequate clearance, so that if an event such as a lightning strike were to occur, the gap between the windings on the transformer would not break down and ionize causing a crossover. To be rated at 1500 V for an overvoltage category 2 up to a 5000-meter altitude, a reinforced transformer must be able to withstand an impulse test of 12 kV peak. The Bourns® Model SM91527L can meet the above requirements.
A separate series of mandatory tests are necessary to check the quality of the insulation material itself. Reinforced insulation is tested by applying a DC or AC voltage between the primary and secondary side and measuring the amount of leakage current between the two sides. This is also known as a dielectric test as the permittivity of the insulation material will accumulate charge, which translates into a leakage current. Another name for this test is Hi-POT.
Another required procedure for systems with working voltages higher than 750 VDC is to put the transformer through an ageing process using high temperature and humidity. The ageing test consists of ramping up the DC voltage from zero to a value dependent on the working voltage while at the same time measuring the amount of charge in coulombs gathered in the insulation. This test is also known as partial discharge.
Additional Transformer Requirements
Transformers with reinforced insulation (color red) are now also required at the point of entry into networks, especially if the battery management board could be exposed to enable testing and diagnostics by a technician. One exception is for racks that hold battery modules that are shielded by plastic walls and use transformers for communications in a daisy chain configuration. This part of the application does not require reinforced insulation. For communications in a daisy chain configuration, an ideal solution is Bourns® Model SM91501ALO signal transformer with functional insulation for 1500 V, and this product is also UL listed. This device has two channels instead of one and is well-suited for placement in the middle of a daisy chain. This Bourns model will still require Hi-POT and impulse tests, but the procedure is less severe (4.3 kV DC Hi-POT and 8 kV Impulse).
In a 1500 V BESS, Figure 2 highlights where a reinforced single channel transformer and a functional two-channel transformer would be located. The BESS is divided into strings of 50 V battery modules where each is monitored and each has a two-channel functional insulation transformer in the isoSPI™ bus. This configuration enables the string to be protected with the reinforced single-channel magnetics component such as the Bourns® Model SM91501ALO transformer.
Figure 2: Reinforced Transformer and Signal Transformer Placements in a High Voltage Battery System
Achieving Higher Battery Storage Efficiency
This article has presented an established reinforced insulation solution for isoSPI™ buses in 1500 V renewable energy systems. Using the example of Bourns® Model SM91527L was shown to help meet higher battery energy storage efficiency needs. This UL listed single-channel signal transformer with common mode choke is designed for entry points into strings of battery modules. The signal performance of the Model SM91527L has been tested to meet recommendations by IC manufacturers for operation with their drivers, and is shown to meet energy storage system specifications in terms of surge voltage and other tests for insulation.
In addition, the Bourns® Model SM91501ALO has UL recognition and is a two-channel transformer rated to 1500 VDC (functional insulation), which is suitable for battery module enclosures inaccessible to human contact.
To meet the requirements of the rapidly expanding renewable energy system market, Bourns is committed to continued innovation in the development of magnetics components for 1500 V and beyond to help keep these applications safe and maximize efficiencies. Bourns’ high-voltage BMS signal transformers have been designed to work with selected ICs as indicated in the datasheets for Model
SM91501ALO and Model SM91527L. Bourns® Model SM91501ALO is developed for use with Analog Device’s Model LTC6804/681X, NXP’s Model MC33771/33772 and Texas Instruments’ Model BQ79616. Bourns® Model SM91527L is developed for use with NXP’s Model MC33771C series and Analog Device’s Model LTC6804/681x series.
*“isoSPI” is a trademark of Linear Technology Corp.