Essential FAQs for Medium Voltage Sensors: Standards, Types, Installation, and More

Medium voltage sensors are essential components in modern electrical systems, enabling precise monitoring and control of voltage levels ranging from several hundred to several thousand volts. These sensors are integral to applications such as power distribution, renewable energy systems, industrial automation, transportation, and smart grid systems. By providing accurate real-time data, they enhance the efficiency, reliability, and safety of electrical networks. This FAQ article aims to address common questions regarding medium voltage sensors, including their types, installation, safety, lifespan, and compatibility, among other key aspects.

Which are the main standards for medium voltage sensors?

The new IEC 61869 standards combine the old standard parts IEC 60044-7 and IEC 60044-8 focused on ”Electronic transformers“. Now split into ”Electronic transformers“ and ”Low-power passive transformers“ in the new IEC 61869 standards.

What are the different types of medium voltage sensors available?

There are various types of medium voltage and current sensors available, including resistive voltage dividers, capacitive voltage sensors, optical voltage sensors, and Rogowski coils. The choice of sensor depends on factors such as voltage range, accuracy requirements, environmental conditions, and application-specific needs. G&W Electric uses Rogowski coil and capacitive divider as technologies (for current and voltage measurement) considered to be more reliable, accurate and proven technologies. Moreover, they have the ability to be linear with the primary current/voltage and cover a wide frequency range measurement which is very important when it comes to power quality measurement (especially in DERs grids).

What are the main differences between Capacitive and Resistive Divider technology?

The main difference between Capacitive and Resistive Divider technology is as follows:

Resistive dividers:

  • Less variation vs. temperature ppm / °C or °F
  • Measure DCV (direct current voltage)
  • Limited frequency bandwidth: strong influence of stray capacitance
  • Very high values of resistors: non linearity per leakage resistance
  • Dimensions related to R.I.L. (Reliability, Integrity, Longevity) of commercial components
  • Burden must be pure resistive and as high as possible

Capacitive dividers:

  • Transformation ratio and phase offset depend on the burden
  • Galvanic isolation from Primary Voltage
  • Very linear and very large frequency bandwidth
  • Primary Capacitance designed & developed by manufacturer
    • Extremely compact solution
    • Characterization of leakage resistance
    • Characterization of ppm/°C or °F
  • Compensated vs. Temperature

How are medium voltage sensors installed and connected?

Medium voltage sensors are typically installed by connecting them to the electrical system’s voltage terminals or using appropriate mounting arrangements. The connection method can vary depending on the specific sensor and system requirements. G&W Electric uses different types of connectors for the secondary signal, including RJ45 for indoor applications (as per IEC 60044-7, IEC 61869-11) and outdoor (depending on customer requirements).

Can medium voltage sensors be used in both indoor and outdoor environments?

Yes, medium voltage sensors are available in both indoor and outdoor versions. Outdoor sensors are designed to withstand environmental conditions such as moisture, temperature variations, and UV exposure.

Are medium voltage sensors safe to use?

Yes, medium voltage sensors are designed with safety in mind. They adhere to safety standards and regulations, and appropriate insulation and protection mechanisms are incorporated to ensure safe operation in high voltage environments. Furthermore, the low energy output (aka LEA) is an additional safety condition compared to traditional CT/VT.

What is the lifespan of medium voltage sensors?

The lifespan of medium voltage sensors can vary depending on factors such as the sensor’s quality, operating conditions, and maintenance practices. Typically, they have a long lifespan of several years under normal operating conditions. G&W Electric has installations in both indoor and outdoor environments that have lasted several years.

Are there any maintenance requirements for medium voltage sensors?

No, medium voltage sensors generally require no maintenance.

Can medium voltage sensors measure both AC and DC voltages?

Yes, there are medium voltage sensors available that can measure both AC and DC voltages. G&W Electric decided to focus on AC measurement to provide a wide portfolio of solutions that cover the main applications.

How do medium voltage sensors communicate data or signals?

Medium voltage sensors can communicate data or signals through various methods, including analog signals (such as voltage or current outputs), digital protocols (such as Modbus or Profibus), or wireless communication technologies. G&W Electric offers a portfolio of active and passive sensors which provide different types of solutions based on customer requests.

Can medium voltage sensors be integrated with control or monitoring systems?

Yes, medium voltage sensors can be integrated with control or monitoring systems, allowing for centralized data collection and analysis. This is the case of power quality, or centralized protection systems in which sensors are more and more used around the globe.

Does the accuracy 0.5% change with frequency?

Generally the accuracy does not vary based on the frequency for active sensors (specifically for power quality measurement where frequency accuracy over a frequency range is required). In case of passive sensors the calibration factors are based on the frequency (50/60Hz). This needs to be set in the protection relay.

Which is the output voltage level?

The output voltage level depends on the sensor. Usually, it is written in the catalog/data sheet. Here are some examples (please note that both the output of the voltage sensor and of the current sensor is a voltage signal):

  1. Example: CVS-24-O: The output is ~ 1V:
    • Voltage sensor 1.2V @ 12 kV (ratio 10,000), ratio can be customized at the order (e.g. 2200, 5000, 6154 …)
    • Current sensor 30mV @ 100A (ratio 3,333), ratio can be customized at the order (e.g. 60, 100, 200, 500 …)
  2. Example: VS-24-BP (VS-24-BP is a voltage sensor only):
    • 1.9V @ 12kV (ratio 6,154)

The sensors will be installed in a 36kV phase to phase voltage. Do I have to consider a 20,78kV phase to neutral voltage level?

No, if you need 36kV phase to phase, you need a 36kV rated voltage sensor. Usually, the rated primary voltage is considered phase to phase.

  • 36kV means 30kV ph-ph (max 36kV), 20kV max ph-neutral
  • 24kV means 20kV ph-ph (max 24kV), 12kV max ph-neutral

Can I have a customer reference list?

Yes, G&W Electric will provide a reference list upon request.

Can I connect more than 1 relay to the same sensor, to cover different functions like protection, power quality and metering (including revenue metering)?

In the case of active version, an RJ45 splitter is enough. In the case of a passive sensor, a merging unit provided with an amplifier may be needed depending on the application.

Can I connect a voltage sensor or current sensor to an existing protection relay using a traditional VT input? i.e. 67V?

Yes, we do have a device designed to allow retrofitting relays with 67V inputs.

Can I connect a voltage sensor or current sensor to an existing protection relay using a traditional CT input? i.e. 1A/5A?

Currently, we do not offer any accessories specifically designed for that application.

Are all Rogowski coils/sensors universal? I see some come with an RJ45 connection on the ends while others are just bare wires.

It really depends on the type of application you are targeting and the type of relay you want to connect with. For outdoor installations bare wires is common. In the case of combined voltage and current, we have specific cables and connectors based on the relay being used. In case of active sensors, the wires connect the sensor to the controller and supply voltage that goes from the controller to the sensor. That’s why the cable is usually specific and related to the application.

In case of an indoor current sensor or voltage sensor passive version, the cable is usually RJ45, with fixed lengths. In this case it is important to know the impedances of the system in order to set correction factors in the relay.

What other items do I need to be aware of when trying to pair a coil/sensor with a relaying device? Different impedances? Maximum wire length? Voltage range?

Please refer to G&W Electric for a relay compatibility list.

What happens in case an existing installation is using oil immersed transformer style device, thus a VA rating is needed; how does this works with sensors?

Electronic sensors do not have VA ratings, it is just a matter of compatibility with the controller.

Are there specific controllers that work with sensors? What kind of “controller” need to be used?

This depends on the application, for example SEL-751 is a feeder protection device with metering (not revenue metering) and power quality capabilities. But there are controllers designed to provide power quality (for example the PQube LV) or metering (for example the SATEC PM175). All of these are compatible with G&W Electric sensors.

Are sensors used to directly replace the instrument transformers for metering purposes?

Sensors can replace the instrument transformer for metering purposes but generally the control (IED) must be replaced. This is because the PT [CT] output is 110V [1A] instead voltage and current sensor typical output is ~ 1V. When fiscal metering is needed, there are no solutions in the market able to fulfill this specific application.

What kind of local certification is the customer usually ask for in case of metering?

Every country is using its own reference standard. For example, Australia adopts IEC standards, so the certifications per IEC standard should be enough. This is valid for NON fiscal metering applications. In case of fiscal metering, there might be specific additional certifications that need to be fulfill.

In terms of general purpose, as a metering device, which tests does G&W Electric perform and which are needed for local regulations?

Usually, IEC or IEEE or any other international standard is valid and enough when it comes to protection or metering functions. G&W Electric has a fully tested portfolio for all the main international standards.

Medium voltage sensors are indispensable for effective management of electrical systems, offering accurate measurements and monitoring capabilities. Their adherence to international standards, minimal maintenance requirements, and versatility for both indoor and outdoor applications make them a reliable choice for various industries. Selecting the appropriate sensor and configuration is vital for optimizing system performance and ensuring safety. For tailored solutions, consulting with manufacturers and reviewing detailed product specifications is recommended.