Keeping Harmonics in Check with Electronic Voltage Sensors

From wind turbines to railway traction systems, the protection of critical motors against abnormal situations has always been vital. With increasing amounts of energy supplied by renewable sources such as wind power, it is also vital to ensure that motors receive a high quality power supply.

 

One of the key considerations is to ensure the reliability and safety of the system when converting the primary voltage from a DC to an AC system, and to do so in all environmental conditions and temperatures.

The increasing demand for green energy is one of the drivers of the adoption of wind power. Wind turbines commonly use a variable speed controller that works with the power electric converter to connect the generated power to the grid. However, a common problem of power electronics is the generation and emission of harmonic currents, which dramatically reduce the quality of the injected current.

The table below summarizes the requirements of IEEE 519, showing the allowable harmonics limit for both voltage and current at various bus voltages.

 

Figure 1: Allowable harmonics limits for current and voltages at different bus voltages

 

Various types of filters are used to reduce the harmonic distortions to acceptable levels. These filters can compensate for the harmonics of non-linear loads and are usually installed close to the point of distortion. One of the more common techniques is to use an Active Power Filter, which repairs the distorted waveform by injecting an antiharmonic waveform. A typical circuit schematic is shown below.

An anti-harmonic signal waveform is then generated using the p-q theory as a reference signal for APF switching. Clarke transformation is used to transform the three-phase coordinates into equivalent twophase α-β coordinates for voltage and current.

In a high current/high voltage system, the demands on the sensor are relatively high, with a requirement to bring the measured voltage to the controller in a safe and insulated manner.

The DVC 1000 family of electronic based voltage sensors from LEM offers this high level of insulation, up to 4.2kV. Its modular design also allows it to be installed as close as possible to the load, ensuring it can fit into restricted spaces.

In railway applications, the auxiliary converter which brings power to loads such as fans, blowers, lighting or battery chargers needs to perform stringent monitoring of the harmonics. In particular, it is very important not to re-inject any signal back to the tracks at a 50Hz frequency, which could result in disturbances to track equipment and a consequent risk to safety. This is why the converters are often coupled with a voltage sensor which monitors the signal and plays an important role within the safety loop.

In railway applications, the auxiliary converter which brings power to loads such as fans, blowers, lighting or battery chargers needs to perform stringent monitoring of the harmonics. In particular, it is very important not to re-inject any signal back to the tracks at a 50Hz frequency, which could result in disturbances to track equipment and a consequent risk to safety. This is why the converters are often coupled with a voltage sensor which monitors the signal and plays an important role within the safety loop.

 

Figure 2: An Active Power Filter can be used to repair distorted waveforms

 

As previously mentioned, protecting the motor is a permanent requirement. In particular, most electric drives incorporate an inverter circuit which rectifies AC into a high-voltage DC signal. Called a DC-link, this serves as a power supply for circuitry that generates drive signals to power the motor. The DC-link voltage must be continuously controlled. Under certain operating conditions, a motor can act as a generator and deliver a high voltage back into the DC-link through the inverter’s power device and/or the recovery diodes. This high voltage adds to the DC-link voltage, and the IGBTs (insulated-gate-bipolar transistors) driving the motor can be stressed by a high (and potentially damaging) surge voltage. An isolated voltage sensor is then necessary to monitor voltage back to the control application, which will shut down the whole application safely in the event of an overvoltage.

As well as overvoltage conditions, under voltages can be just as dangerous. If we consider a 600V rated drive, the overvoltage should be around 1000V while undervoltage would be around 400V. The same sensor should be capable of being used between these two limits.

There are several conditions that could lead to a voltage drop, but the most common one is the loss of one phase. Having said that, the volt age sensor can be located at the AC input side of the rectifier, or most commonly, directly at the DC-link. Installing a voltage sensor on both sides will bring more information and will be safer for the system.

Motor drives are not the only application that require galvanic isolated voltage sensors. Other applications such as solar inverters and UPS also require these functions to ensure the protection and safety of the application. For such cases, a voltage sensor must accurately measure the DC-link voltage and provide isolation between the highvoltage side and the low-voltage controller side.

The question is, when do we need isolated voltage sensing and when should we choose a non-isolated sensor or simple resistance divider?

 

There are two main reasons to use such a sensor:

First case: When the microcontroller, which hosts the AD converter, is not referenced to the same point (so DC-) as the voltage measurement (the resistive divider). Care is needed to delineate between different grounding in the system. By isolating both the signal to measure and the microcontroller, we can avoid damage to the system from events such as inductive spikes or lightning strikes.

Second case: The second question is, what safety regulations are in place that require an actual isolation barrier rather than simply relying on the attenuation network? Often, for safety certifications, resistive attenuation is not enough to ensure safe grounding of the system. The voltage sensing then needs to go through an isolated channel in order to ensure an adequate safety barrier.

When the whole system is being designed around different equipment in place, adding the safety requirements, without defining specific equipment, can be more cost-effective by using the existing devices (such as a controller or a PLC) to do the monitoring. In this case, the DVC 1000 offers the necessary easy-to-use voltage sensing with the appropriate isolation to the control application. This direct sensing will be even easier using the standard interface (+/-10V or 4-20mA) of the DVC 1000, which is compatible with the standard analogue inputs of the controller. The versatility of the DVC 1000 to measure AC and DC will allow a wide range of applications.

 

Figure 3: Isolating with a DVC1000 allows existing control elements to monitor power applications

 

For higher integration, the DVC 1000 is also available in a PCB mounted version, for further space saving. Likewise, the panel mounted version, the DVC 1000-P, is self-sufficient and does not require additional components.

With installations becoming ever smaller, LEM developed a new technology for these voltage measurements. Based on the isolating amplifier technology, these developments gave rise to the DVC series.

To complete its range of digital voltage sensors offering smaller dimensions, LEM has developed a new voltage sensor for measuring voltages up to 1000V RMS nominal and 1500V peak.

To measure voltage (VP), the DVC series uses only well-known electronic components, the major one being an isolating amplifier. The voltage to measure (VP) is directly applied on the sensor primary connections through an internal resistor network and some components allowing the signal to feed an isolation amplifier.

This allows an isolated signal to be recovered and then conditioned in order to supply a voltage or a current at the sensor output connections, which is an exact representation of the primary voltage.

 

Figure 4: An isolating amplifier provides an exact representation of the input voltage at the output terminals

 

An internal isolated DC-DC converter is used to supply the electronics on the primary side.

 

Isolating Amplifier technology features

• Any kind of signal - DC, AC, pulsed, complex - can be measured.
• Galvanic isolation between primary (high power) and secondary circuits (electronic circuit).
• Fast dynamic response for a wide frequency bandwidth.
• Small volume needed.

This new family, known as DVC, consists of two main products, one for PCB mounting (DVC 1000-P) and one for panel mounting (DVC 1000), where a DIN rail mounting adaptor is also proposed as an option.

 

Figure 5: DVC 1000x series mounting options

 

The DVC 1000-P (soldered to a PCB) is powered with + 5V power supply and converts a bipolar input voltage into an output voltage centered on a 2.5V voltage reference. This reference is accessible by the user, who can use their own reference as an alternative.

The DVC 1000 (panel mounted) is powered with +/- 15 to 24V power, and converts a bipolar input voltage of 1000VRMS (+/- 1500V peak) into a bipolar instantaneous current output of +/-30 mA peak (DVC 1000 model) or a voltage output of +/- 10V peak (DVC 1000-B model) or a 4-20 mA unipolar instantaneous current output at 0…+1000V DC (DVC 1000-UI model) (unipolar DC Voltage measurement only).

 

Figure 6: DVC 1000-P for PCB mounting - DVC 1000 models for panel mounting

 

Figure 7: DIN rail mounting adaptor proposed as an option with DVC 1000 models

Figure 8: Performance provided by the DVC series

 

This technology brings a significant space saving. For comparison, a standard voltage sensor using insulating digital technology occupies around 304 cm3 of volume, whereas the DVC 1000-P needs only 37.4 cm3, representing an 87 % space saving.

The unit also weighs only 22g, 67% less compared to a LV 25-1000 model based on Hall effect closed loop technology. For instance, the panel mounted DVC 1000 versions measure only 29 x 51 x 89 mm and can be mounted on panels for a total occupied volume of only 131.6 cm3 with a weight of only 57g, making it unique on the market.

The DVC series complies with a range of internationally-recognized safety standards in addition to its adherence to IRIS specifications and uses materials that conform with all relevant fire and smoke requirements (EN 45545), which are mandatory in railway applications.

 

About the Author

Damien Leterrier graduated Electronics Engineering for Embedded System and Automation at ESIEE Paris. He worked as a Global Product Manager at LEM that is a focused manufacturer and global market leader in producing high-quality transducers for measuring electrical parameters.