Introduction
A key element of the propulsion systems for both low-speed urban transportation and high-speed intercity trains is the traction converter responsible for transforming power from an overhead catenary, a third rail, or a diesel engine to drive the electric motors.
牵引变流器由一个逆变器drive the motor and a rectifier, if connected to an AC power source, or a filter if directly connected to a DC link. Connecting the two is the DC link, which requires a constant voltage, regardless of the load, in order to guarantee sustainable performance. Reliable measurement is crucial to regulating the voltage level, making the voltage transducer a key component of the DC link and the traction converter itself.
Figure 1: A traction drive system
A traction converter needs to be able to function in a range of environmental conditions (such as extreme temperatures, dryness, and humidity), putting them under a great deal of stress. The continuing evolution of power electronics, despite significant benefits, places additional constraints on converters and their components. For example, new semiconductors with higher switching frequencies allow for a more compact design and significantly improved efficiency; but they also produce higher magnetic fields and higher common-mode perturbations that can negatively impact voltage transducers.
LEM开发了DVM电压换能器系列，以满足这些不断发展且苛刻的条件（图2）。DVM传感器对外部磁场的免疫力极高，部分放电水平高于最大直流链路电压。
DVM voltage transducers
The DVM series is able to measure nominal voltage from 600 up to 4200 VRMS. To operate, the DVM only needs to be connected to the measuring voltage, without additional resistors inserted on the primary side. It operates with a standard DC power supply range of ±13.5 V to ±26.4 V.
Figure 2: DVM voltage transducers series measures from 600 to 4200 VRMS
在主电压高于零的情况下，DVM最多消耗30 mA（最大内部消耗）加上输出电流（通常以名义值为标称值），当时设置为电流输出。
How does it work?
图3说明了数字式电压表系列作品,starting from the primary voltage input. Here, the voltage would typically be ±4.2 kV. A voltage divider, able to withstand high dv/dt while having low thermal drift, then reduces the supply down to just a few volts before a sigma delta modulator converts that signal from analog to digital as a 16-bit output.
A digital encoder then produces a serial signal enabling data to be transmitted via a single isolated channel, and an amplifier feeds the signal to the primary side transformer required to provide the desired galvanic isolation. The maximum product insulation test voltage is 12 kV, so the transformer needs to withstand this while also guaranteeing the lifetime of the insulation. The DVM has been specifically designed to achieve this, discharging less than 10 pC when a 5 kV voltage is applied between the primary and secondary.
在DVM的次要侧，bitstream被解码并进行了数字过滤。由于主信号方波被变压器扭曲，因此在变压器的次级侧使用了施密特触发器将其恢复为方波。然后将其输入到数字过滤器中，以将数据bitstream解码为标准数字值，该值可用于微控制器的数字到模拟转换器。恢复的输出信号完全隔离到主电压（高电压）上，并且是主电压的确切表示。
通过将编程为微控制器编程的增益修改而无需更改变压器的设计或壳体中电路板的组装，可以轻松地适应不同的范围。微控制器软件会取消偏移，并在将信号从数字转换为模拟输出之前调整增益。微控制器将数据从数字过滤器传输到12位D/A转换器，转移时间约为6μs。然后，使用电流发电机防止短路，将模拟输出电压过滤并转换为电流（±75 mA全尺度）。
微控制器还调节DC/DC转换器，该转换器会产生内部次级调节电源电压。DVM用户通常提供±24 V或±15 V DC电压，而DC/DC转换器允许向Sigma Delta Converter和Digital Encoder提供+/- 5 V和+/- 3.3 V的数字编码器。在此附加电路中，DC/DC转换器在示意图的顶部显示，由微控制器提供。
To the right of the microcontroller is a voltage to current converter for users who prefer current output, typically 50 mA at nominal voltage, in order to comply with electromagnetic compatibility (EMC) regulations. The lower impedance current output is less prone to interferences from external electromagnetic fields. A voltage output version of 10 V at nominal voltage is also available, as well as a 4 to 20 mA output for unipolar measurements.
Figure 3: DVM technology: Working principle of the insulating digital technology
Main characteristics
With a typical accuracy of ±0.5 percent of VPN at ambient temperature, the DVM shows very little temperature drift, resulting in a typical accuracy of only ±1 percent of VPN over its operating temperature range from -40°C to 85°C. Initial offset at 25°C is 50µA max with a maximum possible drift of ±100µA (typical) over the operating temperature range. Linearity is only ±0.1 percent.
DVM换能器的典型响应时间（定义为VPN的90％），在VPN处的电压步骤为48µs（最大60μs）。由于这个快速的响应时间，在-3 dB时以12 kHz的形式验证了大带宽。
Mechanical and standards
Although it offers improved levels of accuracy and temperature stability, the DVM series is 100 percent compatible in terms of functions and performance, thus greatly simplifying retrofits. The DVM series is 100 percent mechanically compatible with earlier products (LV 100 models) for the base mounting footprint, but has slight differences in the outline dimensions, such as the primary and secondary connection locations. The new design (Figure 4) means the DVM is 30 percent shorter, occupies 25 percent less overall volume and is 56 percent lighter than the LV 100.
Figure 4: DVM vs LV 100 voltage outlines
Thanks to the mechanical design and a highly focused internal electronic design on the printed circuit, this reduction in size does not compromise the DVM’s high immunity against the external surrounding perturbations or against the high voltage variations (Figure 5).
With a DVM 4000 transducer, the resulting error in common mode condition with 6 kV/us and 4200 V applied is limited to 0.5 percent of VPN and with a short recovery time of less than 50µs. For comparison, the equivalent conditions result in up to 18 percent inaccuracy and 500µs of recovery time for a LV 100 voltage transducer.
图5：DVM 4000典型的公共模式行为，针对6 kV/us（应用4200 V）的DV/DT：仅恢复时间小于50µs的误差，只有0.5％的VPN作为误差。
As Figure 5 shows, the effect of dynamic common mode is nearly cancelled out by the transducer’s low parasitic capacitance. This is especially important with new technologies like IGBT and MOSFETs SIC that provide a higher dv/dt between primary and secondary. The differential voltage measured by the primary can float, causing perturbations in the secondary, which is generally connected to the ground for safety reasons.
Since filtering this perturbation would reduce the response time, the DVM is designed to reduce the parasitic capacitance between primary and secondary to the lowest amount possible. The previous LV 100 voltage transducers use Hall effect technology in closed loop mode and use a magnetic circuit, making them more sensitive to external magnetic fields than DVM transducers, which do not use magnetic circuits.
The DVM also allows users to easily adjust the input isolator size depending on the input voltage, and adapt the secondary side connection as needed with connectors, shielded cables or terminals (threaded studs, M4, M5, inserts, UNC, etc.).
The DVM series has been designed and tested according to the latest worldwide standards for traction and industry applications, including IEC 61800 for drive applications, IEC 62109 for solar applications, IEC 61010 for safety and EN 50155 (Railways Applications Electronic Equipment Used on Rolling Stock), ensuring overall performance for electrical, environmental and mechanical parameters in railway applications.
特别注意DVM的机械设计，以确保以高压速率的局部排放量低。灭绝局部放电电压越高（> 5kV）越好，因为在正常定义功能期间不会发生放电。部分放电水平定义为10 pc。
As voltage rises, partial disruptive discharge starts between two points, usually at the opposite potentials in any product. Maintaining the discharge levels will reduce the product insulation over the time and then eventually impede the quality of the product until it fails. These discharges happen at a level called the ignition voltage and are defined as disappearing usually when they reach a level of 10 pC when decreasing the applied voltage (extinction voltage). Usually, the extinction voltage is lower than the ignition voltage. To ensure the long life of a product, the goal is to have the extinction voltage at a higher level than the normal working voltage rate. With an extinction voltage of 5 kV, the DVM series achieves this.
Accelerated tests have been performed to estimate failure rate, including temperature cycles as well as complete characterization of the product according to standards. Due to an innovative design, linking the insulation transformer to digital technology, DVM models guarantee insulation and partial discharge levels for high-voltage applications up to 5kV peak.
Filed Under:Industrial automation
