IED FOR USE AS SYNCHROPHASOR MEASUREMENT UNIT (PMUs), FAULT/DISTURBANCE RECORDING SYSTEM OVER

WIDE-AREA COMMUNICATION AND INTEGRATION OF IEC 61850 WITH IEEE C37.118

Some of the major system blackouts can be avoided if fast corrective actions are taken by the system controllers. Such real

time corrective actions sometimes may not be possible, but future occurrence can be avoided by off-real time analysis and

preventing sequence of disturbances which lead to major blackouts.

Real time synchronized data from widely distributed Substations in interconnected systems are required for operation

controllers for evaluation of the state of an Interconnected Power system.

Non-real Time-stamped data is required for non-real time applications like System Planning, Protection upgrade, System

addition/modifications, switching sequences, emergency means etc

Synchrophasor measurement is a mean for real time analysis and Fault/Disturbance recording is mean for non-real time

analysis.

IEEE Standard 1344-1995 on measurement of synchronized phasors of power system currents and voltages has been

revised and published as IEEE Standard C37.118-2005.

Synchrophasor measurement is based on steady state AC waveforms (at constant frequency). When the frequency of the

system is constant, AC waves (Voltage and Current) can be defined as cosine mathematical expression in the form A(t) = Am

, where Am is peak value and ω is angular velocity at frequency (f) and ф is angular starting point for the wave.

As a phasor it can be represented as = Am

Voltage and current phasors can be time stamped to the Coordinated Universal Time reference (UTC time). UTC time

reference is International Atomic Time with addition of leap seconds and is used in various applications worldwide. Based

on IEEE 1588 standard highly accurate time synchronization is possible over communication networks.

Based on UTC time reference any AC waveform peak can be time stamped as ±ѳ (displacement of peak from the UTC

reference). A time stamp is an 8-byte message (4-bytes for Second of Century, 3-bytes for Fraction of second, 1-byte time

quality indicator….a quality indicator indicates about status & quality of source clock as well as pending leap seconds).

The IEEE C37.118 specification describes the standards used for measurement, the method of quantifying measurements, test and certification requirements for verifying accuracy, and the data transmission format and protocol for real-time data communication. Data is usually streamed in this format over UDP/IP or across a serial link.

PMUs located at Substations are used for transmitting Synchrophasors to local, regional and national data concentrators.

Various real time & non-real time functions can be performed at different locations depending on the amount of data

available at the PMUs and capabilities of the communication network.

Due to socio-economic impact of blackouts on top of complexity of larger power systems, most national committees are

imposing the requirement of PMUs at major Power Plants and Substations.

Until the PMU becomes a standard feature of all Protection IEDs, it is required to be located at least at major Substations.

Based on the power flows to which it is possible to determine phasors for its remote substations by compensating the line

drops.

IEEE C37.118 specifies reporting rates and reporting intervals for the Synchrophasors for PMUs.

System Frequency 50Hz 60Hz

10 25 10 12 15 20 30

Reporting rate is evenly divided over a second period. If 60 reports per second is selected, first report is at top of Second is

‘0’, next reporting time is 1/60 second latter (0.0166sec), 0.0333, 0.05, 0.6666, ….and so on as below:

Required communication bandwidth depends on the number of Analog phasors to be transmitted and sampling rate.

AC waveforms normally used for PMU applications are combination of Voltage, Frequency, rate of change of frequency,

Line currents, real power, reactive power, apparent power, power factor.

Synchrophasor measurement must meet the accuracy requirement for its magnitude and phase angle. A term ‘Total Vector

Error (TVE) defines the error (ε) in % as:

For the error to be defined, it is necessary to specify the operating frequency range of Power systems since systems

operate most of the times at a frequency not exactly its nominal. Thus ±5% of nominal 50Hz or 60Hz range may be required

for TVE to be defined in addition to 10% THD and 10% for component of wave outside the ±5% of nominal frequency band.

Component of wave outside the nominal frequency band introduces errors in both magnitude and phase. These outside

nominal frequencies are for example power swings (which are normally low frequency phenomena with oscillations in

range 1-3Hz). For detecting low frequency wave of 3Hz, a sampling rate of greater than twice of that frequency (say 10Hz or

10 samples per second) is required.

Phasor Measurement Units are used as IEDs that collect the Analog information from secondary of CTs and VTs concerned

and transmit that information to various locations in phasor form.

As regards to Phasor Measurement Units, Synchrophasors are basically measurements of ac voltage (& current) and

absolute phase angle, made at a particular point in an electric transmission or distribution system. Absolute phase angle is

phase angle relative to a fixed, universal reference. This reference is based on cosine wave of AC Voltage (& current) at

nominal power system frequency (50 or 60 Hz), with its positive maximum coincident with the on-time occurrence of the

standard one pulse per second (1PPS) signal (see figure below) which is maintained by national laboratories as UTC. These

national laboratories participate in the International Bureau of Time (BIH, also in Paris), ensuring that UTC maintained by

the various laboratories is in agreement. Generally, agreement is held to a fraction of a microsecond.

Figure: Reference cosine wave.

Synchrophasor measurements have been made practical on a large-scale basis by the advent of modern satellite-based

position- and time-determination systems. The best known of these is the Global Positioning System (GPS) operated by the

United States Department of Defense. Others include the Russian GLONASS system and the European Space Agency’s

GALILEO, now in development.

Systems such as these can provide sub-microsecond timing anywhere in the world at low cost. This is important because it

makes possible the ‘universal reference’ described above for absolute phase angle.

Synchrophasor standards are required for AC Voltage & Current and time as well as for absolute phase angle. Such

standards guarantee the necessary accuracy.

The measurement standard for synchronized absolute phase angle should provide for generation and measurement of a 50

or 60 Hz voltage (optionally also a current) synchronized to 1PPS-UTC with the positive maximum of the signal coincident

with the on-time occurrence of the 1PPS signal. The levels of these signals should include low levels, e.g. 1 to 10 volts rms

as well as typical power system voltages, e.g. 120 or 240 volts rms. Currents, if supported, would be in the 1 – 5 amperes

rms range.

The level of uncertainty should be less than 0.025 degree, to allow for transfer of calibration to user equipment at the 0.1

to 0.5 degree level.

Typical Synchrophasor topology:

FIRST LEVEL DATA LINK PMUs TO PDC (Data collection link):

As we see first level of link is between various PMUs to Data Concentrators. Phasor Data Concentrator (PDC) receives

phasor data from different PMUs and sorts it according to time tag. PMUs have serial links (9600 – 57,600bps) &or Ethernet

links (optical/electrical 10MB, 100MB). Data format is specified by the standard. For serial port links if redundancy is

required, multiple serial ports may be required. For more numbers of serial links for different devices, serial multi-drop

configuration may have to be employed. For Ethernet communication link, multicast from PMU or multiple ports may be

required. Ethernet VLAN is useful for communication like point to multipoint data interchange. VLAN identifier 12bit tag

added on to the data packet is read by the Ethernet switch supporting VLAN which switches the data simultaneously to all

devices on the identified VLAN.

NEXT LEVEL DATA LINK PDC TO PDCs AT SAME LEVEL (Data sharing link at same hierarchy level)

(Dashed line in above figure):

This link may contain data from up to 32 PMUs and the link may be of Ethernet, T1 and SONET. PDCs will have to support

application software from multiple vendors. Figure below represents PDC architecture:

DATA LINK PDC TO HIGHER LEVEL:

This link may contain data from up to 32 PMUs and the link may be of Ethernet, T1 and SONET.

Closed-loop controls from SCADA/EMS are typical.

IntelliGrid Architecture addresses some of the application issues at PDCs.

IEC 61850 in relation to Synchrophasors

Most of the Numerical latest versions of utility Protection, Control & Monitoring IEDs (Intelligent Electronic Devices) comply

with IEC61850 standard.

IEC 61850 presently developed for Substation Automation is applicable for both real time and non-real time application. IEDs complying with IEC61850 have measured data used for Substation Automation which are suitable for use as Synchrophasor measurement also. Thus it became necessary for IEDs to be harmonized for meeting IEC61850 with data communication in accordance with IEEE C37.118 standard for Synchrophasor measurement application.

Thus an IED can ultimately have Disturbance Recording, fault recording, event recording and Synchrophasor measurement as its built-in features.

Since IEC 61850 is application within a Substation, in order for it to be adopted for external communication, issues related with external communication becomes a necessity. IEC 61850 standard although presently is for Substation Automation, its use for Wide-area communication is discussed in draft report IEC61850-90-1 for communication between Substations.

Issues such as common time reference, security and other external interference are key issues. Particularly, when it involves control functions. Issues related with time stamp format & time semantic differences between IEC 61850 & IEEE C37.118 are some issues to be resolved. IEEE 1588 standard can address time synchronization aspect. Further, PMU data modeling has to be addressed in IEC 61850, defining any associated logical nodes and attributes. IEC 61850 shall incorporate PMU data exchange and interface requirements in IEDs at logical node level. Working groups within IEC & IEEE are responsible to interact in dealing with and addressing all issues in harmonizing the two standards. North American Electric Reliability Corporation (NERC) extensively works in dealing with issues related with Utilities to tackle blackouts. Working groups thus require interacting with NERC.

Implementation of Disturbance & Fault Recording Functions:

Disturbance and Fault Recording (DFR) functions are handled by Control & Protection IEDs. When a PMU functions as an IED, it can add these functions as its features. In simplest form PMU only sends phasor data to higher level PDCs (which handle external application software). These external software can trigger the DFR functions of PMUs. This is fairly possible by adding additional inputs to the PMUs (Digital and Analog) which are required for DFR application. Wide area cross trigger for may have to be incorporated in the form of Digital channel PMU-PDC communication in addition to other Control Digital channel and Analog phasor channel. Such data archival at Higher levels will lead to more transparency in the operation & evaluation of Grids by various independent operators at Generation, Transmission & Distribution levels.

Advantage of incorporating functions is mainly due to common time reference which is adopted in the PMUs (which otherwise may not be possible due to non-standard approaches of existing DFRs).

At process bus level, presently, it involves sizing the primary system connected devices (like CTs, VTs etc). For IEDs which satisfy the IEC 61850 it becomes fairly simple as CT, VT and other process devices are defined logical Nodes and can be multi-casted to other IEDs (including a PMU which also will become an IED). Implementation of the DFRs in PMUs to be installed in the existing Substations will involve upgrading and modifications to existing systems where devices do not meet IEC 61850 and anticipated progress can be rather slow.

Fault recording functions may be useful in final POSTMORTEM analysis of various events which leads to major Blackouts as such data is not easily and readily available at higher levels.

Disturbance recording is useful in effective system planning and dispatching schedules to minimize the disturbances that trigger inter-operator systems.

Compared with Fault recording function, Disturbance recording function becomes more immediate necessity as:

Most often PMUs & Disturbance recording are required at same exact nodes

Both often deal with same wide area disturbances

Same Analog inputs for Disturbance recording (e.g. Bus Voltages, critical Line currents). Other functions like frequency, rate of change, rms, peak, harmonic, power flows can be derived from Voltage & Current inputs.

Fault recording function implementation is possible due to the requirement of high sampling rates for Synchrophasors of PMUs. However, fault recording requires more numbers of Analog and digital inputs. Thus implementation of fault recording function in PMUs needs case by case approach.

Triggers:

Fault recorder functions involve many triggers from Control and Protection as well as Analog & digital values of Voltage, Current and frequencies of various line and equipment in the Substation. Also trigger is fault based with high sampling rates and duration usually less than 2 seconds….unless there is high-speed Auto-Reclosing with duration reaching few seconds. Some fault recorders are also applied for evaluating high frequency transients also. Sampling rates of few tens to few hundred per cycle based on application (Transients involved with high frequency waves recording will have sampling highest sampling rates).

Disturbance recording functions may involve low sampling rates like few samples per cycle (typically 1 to 2 samples per cycle) but involve total time of up to half an hour or more. Triggers often are based on magnitude of Voltage, current, frequency or derived functions like Power, sequence components (positive, negative or Zero sequence) as well as rate of change of these quantities. Disturbance recording may also find application such as harmonic analysis or THD measurement.

In general, sampling rate and duration depends on the capacity of storage and achieving. In general, the records are over written on basis of first-in first-out basis to avoid recorder getting ‘full’.

With present numerical technology, high storage and GPS clock signaling, all functions of a PMU & DFR may be combined into one device.