“Throughout the 20th century, electricity has become ubiquitous and has become a necessity of daily life. It is not difficult to imagine that today’s electricity network that supports our daily electricity needs is extremely complex. People need to deal with a variety of issues, such as maintaining or replacing old systems, connecting old facilities and new green power generation solutions, supporting and coping with fluctuations in energy demand, long-distance transmission of energy, power transmission and distribution and corresponding standards in crowded areas, and Ensure the overall satisfaction of customers.
Throughout the 20th century, electricity has become ubiquitous and has become a necessity of daily life. It is not difficult to imagine that today’s electricity network that supports our daily electricity needs is extremely complex. People need to deal with a variety of issues, such as maintaining or replacing old systems, connecting old facilities and new green power generation solutions, supporting and coping with fluctuations in energy demand, long-distance transmission of energy, power transmission and distribution and corresponding standards in crowded areas, and Ensure the overall satisfaction of customers. In the past few decades, power service interruption has been the focus of attention, and has promoted the research of monitoring, predicting and preventing equipment problems. A physical phenomenon called partial discharge (PD) has been used to detect these problems. This article will briefly introduce the concept and advantages of partial discharge, as well as different capture technologies, focusing on the ultra-high frequency (UHF) system, especially its data acquisition system, and then introduce the data conversion solutions to build this system.
Partial discharge and why it should be detected
Partial discharge is the discharge that occurs in the insulation layer of electrical equipment (cables, switchgear, circuit breakers, etc.). Since this discharge does not completely connect the two conductive terminals, it is called a partial discharge.
Figure 1 Partial discharge
Partial discharge may occur in many parts of the power grid, usually where high Voltage is transmitted and surrounded by some kind of insulating medium (solid, liquid, air). Due to the locality and repetitiveness of partial discharges, the insulation of transformers, power cables and accessories will be damaged over time. Partial discharge is a good indicator to characterize the failure of materials that need to be replaced in the future, and it is very worth monitoring. People can detect faults as early as possible through the interruption of local power grids and carry out preventive replacements, with minimal impact on power users.
Nowadays, the manufacturing process of modern cables is very mature, and defective products are rarely produced. These products are usually detected and discarded before they reach the installation stage. The most important problems caused by partial discharge usually occur at joints and accessories.
As mentioned earlier, monitoring the partial discharge of any type of grid can help to develop a maintenance plan. In addition, by determining the location of the partial discharge, it helps to quickly find and solve the problem. This is particularly useful for underground parts because of the high cost of excavation and other effects such as road closures.
How to detect and locate partial discharge
There are many technologies available to detect partial discharge, and each technology has its own advantages, challenges, and use cases. The main focus of this article is ultra-high frequency (UHF) technology, which requires a high-speed detection system to correctly detect captured short pulses. Table 1 briefly summarizes the different techniques for detecting partial discharges.
Chart 1: Overview of the main partial discharge detection technologies
Note that the techniques listed below are not applicable to all types of equipment. For example, UHF and optical technologies are more suitable for gas insulated (GIS) ultra-high voltage (EHV) transformers. In addition, a variety of techniques can be used to improve the performance of the entire monitoring system.
In principle, UHF partial discharge detectors can monitor the short discharge pulses (usually lasting a few nanoseconds). Because the pulse time is very short, the frequency range of the discharge signal can span from DC to several GHz. There are many advantages to using the UHF part of the signal. This frequency band is less affected by interference, and it is easier to take measures to reduce interference. In addition, the latest UHF sensor and data converter technology can achieve high sensitivity, and the UHF detection system can achieve better positioning accuracy and default pattern recognition. For grid monitoring, this means being able to better find out where the fault occurred and assess its impact.
Partial discharge localization can be achieved through a variety of technologies. Each technique requires multiple sensing channels, and the position is determined by comparing the different parameters of the pulse captured by each channel. Most solutions require at least 4 sensing channels to achieve a partial discharge positioning accuracy of 1 meter or better.
The most eye-catching solution currently is the trilateral measurement technique. The propagation time (time of flight) of the pulse from the partial discharge to the position of the sensing channel is related to the distance between the two. By comparing the relative time of pulse arrival between different sensing channels, the position of the partial discharge can be inferred. Generally, an accuracy of 1 meter or better can be achieved.
Another solution is to consider the signal strength captured by different sensing channels. The signal strength is related to the distance between the partial discharge and the sensing channel. Therefore, by comparing the signal intensity captured by different sensing channels, partial discharge events can be accurately located.
UHF acquisition system is the key to detection performance
The goal of the acquisition system is to accurately capture the analog output of the partial discharge sensor containing partial discharge information. After the signal conditioning link, the analog signal is converted to the digital domain, and then processed to determine whether a partial discharge occurs, and to obtain the location of the partial discharge and any other parameters of interest.
Figure 2: High-level block diagram of the acquisition system
One of the most critical components in the acquisition system is ADC (Analog-to-Digital Converter), which is used to convert the output of the sensor into a digital data stream that the host PC can process. Due to the pulse characteristics of partial discharge, its UHF component can reach a transient time of less than 1 ns. In order to accurately capture the pulse, multiple parameters of the ADC need to be considered. Such as -3dB analog input bandwidth, resolution, sampling speed, number of channels, etc.
-3dB analog input bandwidth: In order to accurately capture the pulse frequency, the bandwidth of the ADC needs to be high enough. If the pulse frequency is higher than the bandwidth of the ADC, part of the pulse information will be filtered out by the system. A rule of thumb is that the bandwidth of the ADC needs to exceed 5 to 10 times the maximum frequency component of the pulse to obtain sufficient accuracy. The following formula can be used to convert pulse transient time to frequency:
Bp is the bandwidth of the pulse, and Tr is the 10-90% rise/fall time of the pulse. This formula is based on the response of the RC low-pass filter and is a simple method of estimating the bandwidth required to capture the pulse. For example, the 10-90% rise time is 1ns and the pulse bandwidth is 350MHz. To recover the pulse accurately, the ADC’s -3dB analog input bandwidth should be between 1.75~3.5GHz.
Please note that different systems have different requirements and therefore have different requirements for higher ADC bandwidth. Generally speaking, the more information we hope to obtain from the device, the higher the accuracy of pulse capture required, and the higher the bandwidth requirements. Conversely, if the goal of the device is only to identify whether a partial discharge occurs, a bandwidth of 2 to 3 times the pulse frequency is sufficient.
Resolution: It can also be understood as vertical (voltage) resolution. It represents the accuracy of the value of each sample. Higher resolution can improve the accuracy of conversion. For example, an ADC with a resolution of 10 bits corresponds to 1024 possible values of full scale. Assuming that the full-scale voltage is 1V, and each step size corresponds to 977μV, for an ideal ADC, the input signal is sampled and converted with a vertical error of +/-488μV. Therefore, it is easy to understand that if the resolution is increased by 2 bits, the accuracy will be increased by 4 times. Although increasing the full-scale voltage in order to capture larger pulses will reduce the voltage resolution, it should be noted that the vertical resolution characterizes the theoretical performance. In practical applications, different types of noise will affect the performance of the ADC. Therefore, when evaluating the vertical resolution, it is best to consider ENOB (effective number of bits) at the same time, because it contains the influence of noise.
Similarly, the requirements of the system determine the requirements of ENOB. Generally speaking, the larger the ENOB, the higher the processing complexity, and the more detailed the information extracted from the partial discharge pulse.
Sampling speed: It can also be understood as the horizontal (time) resolution. It represents the number of ADC samples per second. A higher sampling rate corresponds to a shorter continuous sampling duration and a higher pulse timing accuracy. Theoretically, according to the Shannon-Nyquist theorem, the minimum sampling speed to recover a given pulse is 2*Bp. In our previous example with a pulse width of 350MHz, an ADC with a sampling rate of 700Msps can meet the requirement. As mentioned earlier, the goals of the equipment determine the requirements. If you need to extract more complex information from the pulse, such as the location of the partial discharge, the energy or the energy pattern of the partial discharge, etc., a higher sampling speed is required.
Number of channels: It can be simply understood as the number of available acquisition channels. One of the main advantages of the multi-channel partial discharge system is that when 4 channels are used, the location of the fault can be determined by the trilateral measurement technique. In addition, more channels allow simultaneous measurement, which is very useful for large-scale systems, such as collecting all partial discharge information in a substation control building and/or transmitting this information for remote monitoring.
Another key part of the acquisition system is the front-end processing unit that interfaces with the ADC. In most cases, FPGAs are used to complete this work. The FPGA is connected to the ADC to complete the first stage of processing, and then send the processed data to the host PC. The host PC will perform additional post-processing, storage and translation of the data, and decide how to take action when a partial discharge is detected. The parallel processing capabilities and advanced interface options of FPGAs are particularly suitable for this application.
In addition, FPGAs need to be able to handle the massive amounts of data generated by high-speed ADCs. For example, a four-channel 10-bit ADC operating at a sampling rate of 2Gsps will produce 80Gbps or 10Gbps raw data. FPGA can interface with ADC, recover all data, perform first-level real-time processing (such as digital filtering, nonlinear noise suppression, digital baseline stabilization, etc.), and then select useful data based on complex trigger mechanisms. In some cases, in order to further reduce the amount of data transferred to the host PC, the second-level processing (such as pulse analysis) also needs to be executed in the FPGA. Of course, you can also choose to perform the second-level processing in the host PC.
Figure 3: Overview of processing steps
Teledyne SP Devices develops a high-performance digital acquisition card (digitizer) that integrates ADC and FPGA into a complete hardware solution that supports signal capture and processing. These digitizers can be directly connected to the host PC and provide powerful firmware functions and software solutions.
As shown in Figure 2, these three digitizers provide excellent solutions for UHF partial discharge detection equipment.
As shown in the above table, ADQ8-4X provides a cost-optimized solution with a compact size and a large number of channels. It also supports synchronization between multiple boards and chassis with an accuracy of 200ps, which facilitates the design of multiple complex detection systems in large areas. In addition, an 8-channel 1Gsps sampling rate version (ADQ8-8C) is also available.
ADQ14 provides higher resolution than ADQ8, so it can achieve more accurate pulse measurement. It can be configured as single-channel, dual-channel or four-channel, the latter is more suitable for systems that need to locate or quantify partial discharge effects.
Finally, in order to achieve the ultimate performance, ADQ7DC provides fewer channels, but with a sampling speed of up to 10Gsps, it can be used for high-performance, large-bandwidth equipment.
These three digitizers have different firmware options, including general acquisition and trigger functions, and firmware development tool options. Users can implement their own custom algorithms on the on-board FPGA. In terms of software, the easy-to-use Digitizer Studio GUI can easily configure, collect, Display, analyze and store data. In addition, APIs and design routines can help optimize software to meet the needs of more complex and/or specialized systems.
In addition, both ADQ14 and ADQ7DC can provide 10GbE shape parameters. This is an advantage for harsh environments such as substations, because it provides complete electrical isolation between the digitizer and the host PC. Fiber optics also means that the distance between the PC and the digitizer can be very long, and can be used for large equipment that contains multiple measurement points distributed over a large area.
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