Phasor Measurement Unit (PMU)

Phasor Measurement Unit (PMU)

Phasor Measurement Units (PMUs) and its measured synchrophasors make a valuable contribution to the dynamic monitoring of transient processes in energy supply systems.

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Features & functions

Phasor Measurement Unit (PMU)

Features

Phasor Measurement Units (PMUs) measure current and voltage by amplitude and phase at selected stations of the transmission system. The high-precision time synchronization (via GPS) allows comparing measured values (synchrophasors) from different substations far apart and drawing conclusions as to the system state and dynamic events such as power swing conditions.

 

All SIPROTEC 5 devices can be used as a Phasor Measurement Unit. When selecting the option “Phasor Measurement Unit”, the devices determine current and voltage phasors, provide them with highly accurate time stamps and transmit them for analysis together with other measured values (frequency, speed of frequency change) using the IEEE C37.118 communication protocol, which are typically sent to the control center.

 

SIGUARD PDP (Phasor Data Processor) is a system for Wide Area Monitoring and Grid Monitoring that, as its measured variables, uses synchrophasors from phasor measurement units serving as sensors. It helps with quick recognition of the current network situation and indicates both power swings and transient phenomena, transparently as well as instantly. Replication of the respective network topology is not required. The tool supports control center personnel in assessing critical grid situations and contributes to the taking of suitable action. As all measured results are stored, power system disturbances can be promptly analyzed.

Phasor Measurement (PMU) with SIPROTEC 5

PMU - Overview of Function
A Phasor Measurement Unit (PMU) measures the phasor values of current and voltage. These values get a high precision time stamp and together with the values of power frequency, power frequency change rate and optional binary data that are also time stamped are transmitted to a central analysis station. The standardized transmission protocol IEEEC 37.118 is used to do this.

Structure of the Function Group

The PMU function group is activated by selecting the protocol IEEE C37.118PMU on an Ethernet module (electrical or optical). The PMU obtains the measured values from the measuring points and the precise time from time synchronization. The time-stamped synchrophasors for current and voltage are formed from this and transferred together with additional values via the communication module to a server (PDC, Phasor Data Concentrator).

PMU - Function Description 
The synchrophasors, sent from the PMU in a continuous data stream to a PDC, are provided with time stamps and thus are comparable with the measured values of other PMUs. The power frequency, the power frequency change rate and optional binary information are also transmitted as time-stamped measured values. Therefore, you receive an overview of the transient processes in a distributed energy transfer system, for example network fluctuations and compensating processes. 

The following table shows the differences between the PMU measured values and the remaining measured values of the device.

Table: Comparison of Synchrophasors and Conventional Measured Values
Synchrophasors of the PMU
Measured Values from the Measuring Points
Continuous updating (measured value of current) with, for example, 10 values per second (reporting rate)
Slow updating (typically every 5 seconds)
Every measured value has a time stamp
No time stamp for the measured values
Phasor values of current and voltage (amplitude and phase angle)
RMS values without phase angle

The following figure shows the structure of such a Wide Area Monitoring System. The data delivered from the PMUs on the PDC are transmitted via the Inter-Control Center Communications Protocol (ICCP) according to DIN EN 60870-6 to the network control Center.

 

The phasor-measurement units each have 2 communication interfaces: an Ethernet module for synchrophasor communication via IEEE C37.118 and another module for communication with the substation automation technology, for example, through the IEC 61850 protocol.

The central evaluation system, e.g. SIGUARD PDP Phasor Data Processor, receives the data, files, archives them and graphically displays them on a User Interface. In this system, a self-checking function may also be performed, for example, on undamped power swings. The further distribution of information to other PDCs or to a control station is done here. 

To maintain the required maximum errors (TVE) required in the standard IEEE C37.118, the time tolerance relative to the UTC time reference (UTC = Universal Time Coordinated) may be maximum 10 μs. Therefore, the device must be directly synchronized with a GPS precise time signal for the correct function of the PMU.

Phasors 

A phasor u(t) = Ue jωt can be displayed as a phasor that moves counterclockwise in the complex plane at the angular frequency ω. Therefore, the voltage function u(t) = Re{u(t)} is obtained as a projection of the phasor u(t) on the real axis.

Reference Point for Determining the Angle

The phase angle of a measuring signal X m is determined relative to a cosine function having a rated frequency, which is synchronized with the UTC time reference.

 

The number of phasors that are transmitted per second is configurable. The transmission rate is defined according to IEEE C37.118 as the reporting rate. The reporting rate defines the number of phasors that are transmitted per second. Extremely precise time synchronization is essential so that phasor measurement can be carried out to enable phasors from different sites to be compared.

Reporting Rate
You use the configurable reporting rate to specify the number of telegrams that are compiled and sent to the PDC per second. It is adjustable, depending on the rated frequency, and applies to all currents and voltages of the relevant PMU function group. If several PMU function groups are created on the device, these can work with different reporting rates.

Total Vector Error (TVE)

The TVE describes the error between the actual and the measured values of the input signal. The Synchrophasor Standard IEEE 37.118 defines upper limiting values of 120 % VN and 200 % IN, among other things. Up to these limiting values, the TVE of 1 % must not be exceeded for stationary signals. The standard defines the 2 performance classes, Class P and Class M, as ranges within which influencing variables are acceptable and the TVE of ≤1 % must be maintained.

The SIPROTEC 5 devices support Class P and Class M, which specify the following influencing variables for a TVE of 1 %:

  • Signal frequency (relative to f N)

  • Signal magnitude (relative to 100 % nominal magnitude) for voltage and current

  • Phase angle (relative to 0°)

  • Harmonic distortions (relative to < 0.2 % (THD) up to the 50th harmonic

The following figure graphically represents the total vector error. As well as the amplitude, the TVE also enters into the angle error.

Variables that influence the TVE are:

  • Amplitude errors

  • Phase errors

  • Synchronization accuracy (deviation from the UTC)

Synchronization accuracy is affected by a GPS timer and the precise correction of the time delay within the GPS receiver module, as well as by the optimal setup of the GPS antenna.

 

Transmitted Data
The following data is transmitted from the PMU to the PDC:

  • Current and voltage phasors

  • Frequency

  • Frequency rate of change

  • Binary information

The current and voltage channels transferred from a PMU function group are selected through the Function group connections editor in DIGSI 5. The frequency and frequency change rate are determined once per PMU. In this case, only the current and voltage channels selected via the Function group connections editor for this PMU are taken into consideration. You may route the binary information in the information routing matrix of DIGSI 5. 

The channel used for specifying the frequency is selected dynamically during the device runtime. Each channel is checked for the presence of a signal in the following order:

  1. 3-phase voltage measuring points

  2. 1-phase voltage measuring points

  3. 3-phase current measuring points

  4. 1-phase current measuring points

The 1st measuring point found with a valid signal is used for specifying the frequency in the PMU function group. 

In the case of 3-phase measuring points, the positive-sequence system can also be transmitted instead of 3 individual sychrophasors. This setting can be made via the Parameter. 

PMU Communication (IEEE C37.118)
PMU communication according to standard IEEE C37.118 is a client-server communication format in which the PDC (Phasor Data Concentrator) functions as both the client and the server. 

Once the PDC has been successfully connected to the device PMU and the PMU configuration data has been queried, the PDC initiates transmission of the synchrophasor data by sending a close command to the PMU. In addition to the phasors, the channel names of the routed measuring points and the binary information are transferred. Per default, these are automatically generated by the respective PMU function groups. Where applicable, you can also edit the channel names of the measuring point and assign a designation of its own for this purpose.

SIPROTEC 5

The PMU functionality is fully supported by the SIPROTEC 5 system and optionally activatable . Find the right devices for your application.