Phasor Measurement Unit (PMU)

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Smart grid is an advanced version of the electrical power system that uses modern technologies to provide a reliable, efficient, and sustainable electricity delivery system. Phasor Measurement Units (PMUs) are the key components in this transformation of the grid that provide real-time monitoring and remote-control facilities.

Read this chapter to get a clear understanding of Phasor Measurement Unit (PMU) and how they work.

What is Phasor Measurement Unit?

A Phasor Measurement Unit (PMU) is a device used in smart grid systems to collect and differentiate the power system signals from the voltage and current sensors and convert them into phasors. In other words, a PMU is an advanced monitoring device that can measure and describe each and every peak and down of the power system. It uses GPS signals to synchronize the measurements of the phasors and provides accurate data in real-time. That is why PMUs are also referred to as Synchrophasors.

As per Institute of Electrical and Electronics Engineers (IEEE), a phasor measurement unit is a device that can estimate synchronized phasor, frequency, and rate of change of frequency from voltage and current signals in the power system.

In a smart grid system, the phasor measurement units are the fundamental building blocks of the wide-area monitoring system (WAMS).

In smart grid, a PMU is similar to a heart rate monitor screen in an ICU that provides time-based heart beats of a patient. As we know, a SCADA system is capable in providing x-ray of the power grid, but the PMU is more advanced than SCADA and can provide information about stress on the system, disturbances in the system, etc. In simple words, a PMU provides a wide area snapshot of the power system.

Understanding Phasors and Synchrophasors

Phasors and Synchrophasors are two key terms associated with the phasor measurement units, and are described below −

• Phasor − A phasor is a mathematical representation of any waveform. In electrical, phasors are used to determine any kind of discrepancies or power quality problems from the waveform representation.
• Synchrophasor − The phasor measurement that takes place at the same time is called synchrophasor.

Components of Phasor Measurement Unit

The block diagram representation of a phasor measurement unit is depicted in the following figure −

A phasor measurement unit consists of various components that work together and provide accurate and time-synchronized measurement of phasors in power system. All these components are explained in detailed below −

1. Analog Input Unit

The analog input unit of the PMU acts as an interface between the internal circuit of the PMU and the electric grid. This component of PMU collects current and voltage signals from the secondary windings of current and voltage transformers and performs signal conditioning to convert these signals into required format. This process includes amplification, filtering, etc.

2. Anti-Aliasing Filter

Anti-aliasing filter is another major component of a PMU that prevents aliasing effect and signal distortion during digitalization of signals. It band-limits the analog signals before their digitalization to avoid aliasing.

The anti-aliasing filter is nothing but a low-pass filter that removes high-frequency components above half of the sampling rate to prevent aliasing. It also ensures that all the analog signals should have the same phase shift and attenuation.

3. Analog-to-Digital Converter (ADC)

ADC (Analog-to-Digital Converter) is provided in a PMU to perform digitalization of the input signals i.e., it converts analog signals from anti-aliasing filter into digital signals. In general, a 16-bit or more ADC is used for this purpose so it can capture fine details of the signals.

4. GPS Receiver

GPS receiver is provided in a phasor measurement unit for precise time synchronization of the measurements. It receives a time signal from GPS satellites that provides a time accuracy of better than one microsecond. The output signal produced by the GPS receiver acts as a reference clock for synchronization of measurements across different PMUs.

5. Phase Locked Oscillator

This component of the phasor measurement unit is responsible for ensuring that the digitalization of the signals is synchronized with the reference clock provided by the GPS receiver.

This component of PMUs is important for providing a stable and accurate time reference for all other components. It also produces a stable clock for synchronization of the sampling process in the analog-to-digital conversion.

6. Phasor Microprocessor

The phasor microprocessor is a digital device that processes the digital data to calculate the magnitude, phase angle, frequency, rate of change of frequency, etc. of the electrical signals.

For this purpose, the phasor microprocessor makes the use of algorithms and signal processing techniques like DFT (Discrete Fourier Transform).

7. Modem

It is a communication device that establishes a fast communication between the PMU and the phasor data concentrators. It transmits the output of the phasor microprocessor to the data concentrators for further processing and analysis.

Working of Phasor Measurement Unit

By referring the above block diagram representation of the phasor measurement unit. We can understand the working of it by breaking it down into the following steps −

• Signal Inputting − First of all, the PMU captures input voltage and current signals from the electric grid by using voltage and current transformers respectively.
• Filtering − In the next step, it performs anti-aliasing filtering on the signals to remove high-frequency components from the signals.
• Digitalization − Next, it converts the filtered analog signals into digital signals by using a high-resolution analog-to-digital converter.
• Synchronization − In this step, the digitalization process of the signals is synchronized with the reference signal received from the GPS receiver.
• Data Communication − At the final stage, the processed data by the PMU is transmitted to data concentrators.

These points briefly explain the operation of a typical phasor measurement unit.

Advantages of Phasor Measurement Unit

The use of phasor measurement unit in a smart grid system offers the following benefits −

• Phasor measurement units enable the real-time monitoring and remote control in the electric grid. Thus, they implement advanced protection schemes in the system to provide a stable and resilient electric grid.
• Phasor measurement units provide detailed reports on the grid conditions that enable operators to understand the behavior of the system more effectively.
• PMUs provide continuous monitoring of grid parameters like voltage, current, frequency, phase angle, and many more. This helps grid operators to implement predictive protection schemes and reduces the risks of blackouts.
• Most phasor measurement units support an adjustable sampling rate that ensures high-resolution capturing of data which is very important for dynamic analysis of the grid.
• By providing precise measurement and real-time data on grid performance, PMUs allow to improve the grid stability through better control and accurate decision making.
• Phasor measurement units also improve the situational awareness by providing real-time and time-synchronized data. This helps grid operators to rapidly detect and respond to faults.
• As we know, PMUs are the important parts of the wide-area monitoring system (WAMS). Hence, they provide improved visualization of the grid and help in better coordination over a wide geographical area.

Applications of Phasor Measurement Unit

In smart grid systems, the phasor measurement units are used for the following purposes −

• Monitoring the grid over a wide geographical area
• Accurately estimate the grid state
• Rapidly identify and isolate the faults to minimize damages in the grid
• Real-time monitoring of the grid conditions
• Precisely measure the grid parameters and improve the protection
• Better management of power flow across the grid, etc.

Challenges with Implementation of PMUs

The implementation of phasor measurement units in the smart grid faces the following major challenges −

• Identification of optimal location for placement of the phasor measurement unit in the grid.
• Compatibility issues in the integration of PMUs with the existing SCADA system and electric grid.
• The working of PMUs is dependent of communication that may have issues due to delay in data transmission.
• Monitoring low-frequency oscillations is challenging for PMUs.
• Distorted waveforms reduce the accuracy of predictions by PMUs.
• PMUs require high computation for data processing and analysis.
• Development of robust tools is required for in-depth post-facto analysis, etc.

Conclusion

Phasor Measurement Units (PMU) are an integral part of the wide-area monitoring system in smart grids. They are used in smart grid systems for post disturbance analysis, grid stability monitoring, thermal overload monitoring, remote power system restoration, state estimation and real-time monitoring and control of the grid.

The use of PMUs in a smart grid results in improved stability, efficiency, and reliability. Therefore, the phasor measurement units or PMUs are crucial component of a smart grid and modern power system.