FIELD OF THE INVENTION
The present invention generally relates to the electric power systems, and more
particularly to a method and apparatus for IEC61850 communication protocol of
a Merging Unit.
BACKGROUND OF THE INVENTION
Substations in power networks include primary equipment such as transformers,
switchgear, bus bars; and secondary devices such as protection relays, energy
meters and Human-Machine Interface (HMI). Adaptation of microprocessor
based Intelligent Electronic Devices (IEDs) with high speed peer to peer
communication among the primary equipment and the secondary devices in
electric power industry, is known in the art. Accordingly, it has become
necessary to standardize the communication protocol between the IEDs in
substations and power stations. International Electro technical Commission (IEC)
61850 stipulated (a universal platform), which allows implementation of plug-
and-play technology, including interoperability between the IEDs of different
manufacturers. IEC 61850 is a widely accepted solution for communication within
substations all over the world.
One of the greatest advantages provided by IEC61850 standard is the real time
communication for high priority messages. Traditionally interconnection between
the IEDs was made with wire by means of electrical circuit using communication
interfaces and proprietary communication protocols. Technology today provides
an improved solution, for example using an optical Ethernet network to perform
multiple tasks in shorter time. The IEDs can now exchange status and control
signals with other IEDs by exchanging data packets containing status and control

signals over a communication link using Ethernet on local area network. (LAN).
IEDs can also send reports and logs, file transfer of disturbance records to
clients.
The connections between the IEDs and the process equipment (eg. switchgear,
current and voltage transformers) when implemented using the communication
services is called process bus which is based on the standard IEC 61850-9-2
Light Edition (LE) for Sampled Values (SV) and IEC61850-8-1 for Generic Object
oriented Substation Events (GOOSE). A merging unit (MU) publishes
synchronized samples of the three phase and neutral currents every 250
microseconds. It also publishes and subscribes to GOOSE messages on the
process bus. This eliminates the need for thousands of copper wires in a
substation used for transmission of the status and control signals from and to the
breaker and switches. It is only needed that they be replaced with a few fiber
optic cables. The sampled values that is published can be directly be subscribed
to by IEC61850 compliant IEDs in a wide variety of protection applications -
including generator, transformer, transmission line, bus, feeder, capacitor bank,
and motor protection.
In general, substation automation system based on IEC61850 communication
protocol is well known. In the recent years, merging units have been designed
for the process bus.
Related prior art publications known to the present inventors:
WO 2010/118550A1 discloses an Intelligent process Interface and Substation
Automation System, which is enabled to issue a command signal for operating a
switching device of the substation; generate and transmit over a communication

network, messages comprising sampled values of a first and a second signal.
Time synchronisation for the channels is achieved by computing the total time
delay for both the input channels, and by a method of decimation and
interpolation, calculate the new samples based on re-sampling and time tagging
data.
US 2010/0153036 describes an IED with integrated Power metering and Merging
unit capabilities. According to this invention, a plurality of data in the form of
digital signals, and analog signals from protection and metering transformers,
are given as inputs. The sampled values are published over two different
communications interface. The metadata tags include the input signal, sampling
rate, primary and/or secondary timestamps, scaling values, calibration values,
provided by the transformer, and/or the location of the IED in the electrical
system. The IED performs metering and/or power quality calculations on the
samples, and the calculation results are formatted and transmitted via a master-
slave protocol to a requesting master. A grouping unit groups the merged
samples into default or custom groupings, which are formatted and transmitted
over a network via a publish-subscribe mechanism.
WO 2010/040409A1 teaches a method and apparatus for dynamic signal
switching of a merging unit in an electrical power system. According to this
invention, the merging unit is designed to connect two separate current
transformers (CTs ) secondaries, for example, a protection current transformer,
and a measurement current transformer, and dynamically switch between them
in order to publish the outgoing Ethernet stream of digitized samples during the
saturation condition of the former set of CTs.

US7930460B2 teaches a Universal measurement or protective device which
allows direct connection to measurement transducers. Furthermore, this device
provides for communication interface with both the process and the
station bus, and wherein the time synchronisation is based on IEEE 1588
standard.
CN101795019 discloses a Soft core based merging unit of photoelectric current
transformers, which consists of an FPGA (Field Programmable Gate Array) chip of
a main processor and a peripheral circuit. The samples are published after
interpolation calculation, digital filtering, digital integral and digital calibration
operation.
OBJECTS OF THE INVENTION
It is therefore, an object of the invention to propose a method for publishing the
current and voltages signals as synchronised sampled measured values
compliant to IEC61850 communication protocol for the process bus connecting
the primary equipment and secondary equipment in a power network of
electrical substations.
Another object of invention is to propose a method of synchronisation of the
sampled measured values with one pulse per second signal by means of a real-
time process.
A still another object of the invention is to propose a method of synchronisation
of the sampled measured values with one pulse per second signal by means of a
real-time process, in which any loss of the one pulse per second signal is

detected and also to smoothly re-synchronise with the one pulse per second
signal when available.
A further object of invention is to provide a method of configuring the logical
device, various logical nodes, data sets, control blocks for Generic Object Orient
Substation Events (GOOSE) and Sampled Measured Values (SMV).
A still further object of the invention is to propose a publisher-subscriber
communication process for the sampled measured values (SMV) including peer-
to-peer communication with Generic Object Orient Substation Events (GOOSE)
messages over the Ethernet.
SUMMARY OF THE INVENTION
The present invention provides with a methodology for implementing an IEC
61850 standard in a Merging Unit for the Process Bus in a substation automation
(SA) systems to make the device IEC 68150 compliant intelligent electronic
device (IED). The IED is made IEC 61850 compliant with the augmentation of
the communication protocol with the features of Configured IED Description
(CID) file.
The invention proposes a unique method to synchronise the current and voltage
samples published by the merging unit with a standard one pulse-per-second
(1PPS) signal that is made available in the substation with a Global Positioning
System (GPS). In the event of loss of 1PPS signal, the hardware clock takes over
within 3 seconds. In the event of re-appearance of the 1PPS signal, the merging
unit synchronises itself within 3 seconds. More particularly, the invention
provides a method of implementation of IEC 61850 communication protocol in

the merging unit for publishing of the SMV packets and publication and
subscription of GOOSE messages.
Accordingly, the present invention discloses a method for synchronizing the
published analogue signals, publication of the samples, publication and
subscription of the GOOSE messages based on the IEC 61850 standards. The
models that are configured in the merging unit are:, GOOSE-Subscribe and
Publish model, Sampled Values Publisher model, Time and Time Synchronization
model.
The merging unit is configured to perform as a Simple Network Time Protocol
(SNTP) Client so that the polled time from a SNTP Server is accessed and the
merging unit's (Real Time Clock) RTC time is updated with the new date and
time.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The invention is described with the help of Figures 1 to 5, where:
Figure 1: Shows the wiring diagram of the IEC61850 compliant Merging Unit as
connected to the process bus in an electrical power system, and to the station
bus.
Figure 2: Shows the modular block diagram of the Merging Unit shown in Fig. 1
Figure 3: Shows the functional flow diagram of the various processes of the
Merging Unit shown in Fig. 1.

Figure 4: Shows the flow chart diagram of the time synchronisation in the main
process of the Merging Unit described in Fig. 1.
Figure 5: Shows the flow chart diagram of the non-preemptive synchronisation
polling application process of the Merging Unit described in Fig. 1.
DETAIL DESCRIPTION OF THE INVENTION
The merging unit is a physical electronic device with a communication interface
for a substation automation system process bus IEC61850 standard. The device
provides an outdoor hybrid switchgear solution with intelligence built into the
primary equipment. The device captures the current and voltage samples derived
from a Conventional or a Non-conventional Instrument Transformers (NOT), for
the control and protection functions including control and supervision of the
switchgear viz. Circuit Breakers and Disconnector / Earthing switches. This device
is designed to be an integral part of the Substation Automation System (SAS)
that interfaces with the bay-level equipment using a fiber optic process bus. The
device accepts current and voltage transformer inputs, binary inputs, time
synchronisation pulses from the 1 Pulse per Second (PPS); samples the current
and voltage signals at a rate of about 80 samples per nominal frequency cycle
and publish multiple time synchronised SMV packets which are multicast over the
fiber optic network viz. the process bus; along with subscription and publish of
high-speed peer-peer communication as GOOSE messages.
Referring now to Fig. 1, the merging unit [106] of the present invention is shown
being connected to a process bus arrangement within an electrical system. MU
(106) is a powerful and compact electronic unit located in the switchyard wired
to field Current Transformers [101] and Potential Transformers [102]. The

device (106) acquires the current signals [107] and voltage signals [108] and
converts them into digital samples and publishes them as digital data on the high
speed process bus as sampled values [111] . The MU (106) is wired to
switchgear status contacts[103], and status signals [109] are published by the
MU (106) as the status of the breaker and the switches in the yard in the form
of digital messages called as GOOSE messages [112] as per IEC61850-8-1. The
MU also subscribes to the GOOSE messages [112] which are the control
commands from a plurality of protection relays [113] and the bay control units
for operating a switchgear [104] viz. breaker and the switches. Commands (110)
from the switchgear (104) are wired from the MU (106) to switchgear coils
[104]. 1 Pulse per Second signal [105] is an optical input to the MU (106) which
is used for time synchronisation of the MU samples.
Fig.2 shows a block diagram of the hardware within the merging unit (106). The
MU [106] consists of a plurality of modules. An auxiliary d.c. voltage [201] given
to the MU is first stepped down to 24 Vdc in a power supply module [202]. This
voltage energises a second, third, and fourth modules [203], [204] and [205].
The analog field inputs from the transformers [101], [102] connected to the MU
terminal blocks are first processed by the second module [203]. The filtered
signals are then accessed by the fourth module [205]. The digital field input
signals from the switchgear contacts [103] is processed by the third module
[204]. The time synchronisation signal is an optical signal fed to an optical-to-
electrical converter present in the third module [204]. The fourth module [205] is
a powerful microprocessor based single board unit with a communication
interface [206]. The message received over the communication interface [206] is
processed by a single board microprocessor unit [205] and the command for
opening or closing the switchgear (104) is issued. The switchgear contacts [104]
are wired to the terminal blocks present on the MU (106).

The processing functions performed by a fifth module [206] is described in Fig 3.
The Block [301] represents three main processes performed by the fifth module
[206]. One of the process is the time synchronisation of the internal clock with 1
PPS signal from the external GPS clock, that is used for sampling and data
acquisition. The input pulse train signal with a rise time of 200 ns is polled by the
application and the internal clock is adjusted precisely. When there is a loss or
absence of the pulses in the 1 PPS signal [105], the process detects the same
within a time period of 70 microseconds. It is assumed that the time
synchronsiation signal is lost and the last computed time ticks is updated as the
sampling clock. If the 1 PPS signal is not available within the next three
seconds,, the sampling time clock is switched to an internal real time clock
without any apparent drift in the sampling time. Upon re-appearance of the 1
PPS signal later, the process dynamically switches to the 1 PPS signal without
any apparent delay.
The second step of the process is defined by the block [302]. The first thread
comprises of precise sampling of each of eight signals from current and voltage
transformers (101,102) every 250 micro seconds and formatting the message
frame as defined in the protocol for publishing the samples. Digital signals from
the switchgear status contacts are polled every millisecond and this data is
formatted for publish as GOOSE messages. This process also polls for GOOSE
messages which is subscribed to and decrypt the message for the binary data.
This binary data activates the digital outputs of the MU [106] for the control
action to the switchgear (104).
1 PPS signal [311] is polled for rising edge every 999930 microsecond for a
period of 140 microsecond and the internal sampling clock is synchronised by the
function defined in [312]. Field data from a current and voltage transformers

[321] are wired as analogue inputs, sampled at the rate of 80 samples per
nominal cycle frequency [322], formatted in a message frame [323] and
published within 300 microsecond [324]. The status signals from the switchgear
[331] are polled every 1 millisecond as digital inputs [332], formatted for
transmission [323] as GOOSE publish messages [324]. GOOSE messages with
information for control action on the switchgear [344] are subscribed to,
decrypted in the block [343], the process activates the corresponding binary
output hardware for control action [342] which is wired to the field switchgear
[341].
The third process is step for the time and date synchronisation of the internal
real-time clock and is performed by the function [352] in a Simple Network Time
Protocol (SNTP) server [351] program. This step is mainly used for the time
stamping of GOOSE events.
Fig. 4 shows the flowchart of the Real-time Data Acquisition application process
as defined by block [302].
Step 401: Initialise the various Inter-Process Communication (IPC) channels and
the hardware. IPCs are necessary for sharing of data between various processes.
Step 402: Poll for PPS_IS_VALID (digital input) wired from the time
synchronsiation application. Whenever the pulses are precisely polled to within 1
second, this digital signal is maintained at logic 'high' by the time synchronisation
process.
Step 403: If the PPS_IS_VALID signal is logic 'high', synchronise the sampling
time with CLOCK_PPS ticks.

Step 404: If the PPS_IS_VALID signal is logic 'low', synchronise the thread with
CLOCK_RTL ticks.
Step 405: Poll for PPS pulse signal from the optical port in the merging unit
hardware. This action is required when the one second pulse train resumes after
being absent for some time.
Step 406: If 1 PPS signal (rising edge) is sensed, set Refound_PPS (digital
output) signal to logic 'high'. This action is performed whenever there has been
a loss in the trace of the one second pulses and is resumed after a gap of time
interval. Another digital output signal is made logic 'high' in order to inform the
time synchronisation process that the pulse train has resumed.
Step 407: Acquire the field data from Analog and Digital Inputs. Read GOOSE
subscriber message from IPC.
Step 408: Condition the data for Sampled Values and Binary signals in the
appropriate format.
Step 409. Write Sampled Values and GOOSE Publishing data to the IPCs.
Step 410. Set / Reset digital outputs for Field data control action based on
GOOSE subscribed data.
Fig.5 is the flowchart of the Real-Time 1 PPS synchronisation module as shown
in block [312]. This is the first application process that starts on powering the
merging unit. The internal clock of the merging unit is first synchronised with the
1 second pulse train of the 1 PPS signal from the GPS receiver. Subsequently this
process is activated every second.

Step 501: Initialise the digital input and output ports of the merging unit [106].
Step 502: Poll for two consecutive rising edges of the time synchronisation
signal (1 PPS) from the optical port. This is necessary in order to compute the
precise second clock tick interval of the 1 PPS signal and update the sampling
clock.
Step 503: Set the CLOCK_PPS time of the internal clock and set PPS_IS_VALID
(digital output) as logic 'high'. The sampling clock is set with the updated clock
ticks which is very precise and also the digital output signal is made logic 'high' in
order to indicate to the other processes that the 1 PPS signal is available and
valid.
Step 504: Enter the inactive state until interrupt. This process is put into
hibernation so that other real-time and other fast processes continue.
Step 505: Poll for the next rising edge of the PPS signal between 999030 us and
1000070 ps.
Step 506: If PPS pulse signal is sensed within the next 100070 microsecond,
update CLOCK_PPS time and set PPS_IS_VALID signal. Resume healthy time
synchronisation function.
Step 507. If there is no PPS signal, update the CLOCK_PPS based on the old
values and increment consecutive _misses.
Step 508. Check if consecutive_misses is three. If the pulses do not resume in
the next three seconds, the merging unit assumes that the pulse train in absent.

Step 509. Reset PPS_IS_VALID (digital output) to logic Now'. This is to indicate
the other dependant processes that the precise clock is absent.
Step 510. Poll for Refound_PPS (digital input) every 1 second. This digital input is
wired from the real-time data acquisition process define in Fig. 4. When this
digital input becomes logic 'high', it incidates that the 1 PPS signal has resumed
and the pulse train is available. If Refound_PPS is set, reset all counters and
resume time synchronisation application from start.

WE CLAIM
1. A IEC 61850 compliant Merging Unit [106] with 1 PPS time
synchronization, comprising:
- a power supply module [202];
- a processing module [205];
- an analog input module [203];
- a digital input and output module with optical- to-electrical converter
[204];
characterized in that the unit is enabled to implement a precise time
synchronisation of the published samples as per IEC61850-9-2 (LE)
including publication and subscription of the GOOSE messages as per
IEC61850-8-1.
2. The Merging Unit [106] as claimed in claim 1, wherein the power supply
module [202] supplies 24V D.C. voltage to the processing module [205],
analog input module [203] and digital input and output module [204].
3. The Merging Unit [106] as claimed in claim 1, wherein the processor
module [205], comprises means for 1 PPS time synchronisation, real-time
data sampling and data acquisition and wherein a simple network time
protocol (SNTP) is incorporated in block [301].
4. The Merging Unit [106] as claimed in claim 1, wherein the device is
enabled to communicate with similar intelligent electronic devices (IEDs)
[113] via 100 / 1000 Mbps optical Ethernet network and is configured to
be fully compliant with IEC61850-9-2 (LE) communication protocol
standards.

5. The Merging Unit [106] as claimed in claim 1, wherein the GOOSE
subscribe and the publish model provides fast and reliable system-wide
distribution input and output data compliant with IEC61850-8-1
communication protocol standards.
6. A method of integrating IEC 61850 communication protocol in the Merging
Unit as claimed in claims 1-5, comprising the steps of:

- time synchronising the internal clock with 1 PPS signal [312];
- accessing wired field data from current and voltage transformers as
analogue inputs in real-time [321], sampling the inputs at the rate of
about 80 samples per nominal cycle frequency [322], formatting the
message frame [323] and publication within approximately 300
microsecond [324];
- polling the status signals from the switchgear [331] as digital inputs
[332], and formatting [333] for transmission as GOOSE publish
messages [334];
- subscribing to the GOOSE messages [344], decrypting the message
for binary outputs [343], and activating the corresponding binary
output hardware for control action [342] sent to the field switchgear
[341]; and
- time and date synchronising the internal real- time clock for GOOSE
messages [352] from the SNTP server [351].
7. A method as claimed in claim 6, wherein data is continuously exchanged

between the various modules through inter process communication.
8. A method as claimed in claim 6, wherein the field signals from current and
voltage transformers [102],[102] are wired to analog input module [203]
where they are processed.
9. A method as claimed in claim 6, wherein the field signals from switchgear
status contacts [103], and control signal to the switchgear [104] are wired
to digital input and output module [204] where they are processed.
10.A method as claimed in claim 6, wherein the 1 PPS optical signal is
connected to the optical to the electrical converter in the digital input and
output module [204] where it is processed.
11. A method for time synchronisation with one pulse per second signal, and
dynamically switch between clocks when the signal is lost and reappears,
without any change in the time delay in publishing of the sampled values
in IEC61850-9-2 message format.
12.A method for publishing the sampled values in the IEC61850-9-2 (LE)
message format while synchronizing in time with dynamic switching along
with publishing and subscribing to GOOSE messages as substantially
herein described and illustrated with reference to the accompanying
drawings.

The invention relates to a IEC 61850 compliant Merging Unit [106] with 1 PPS
time synchronization, comprising a power supply module [202]; a processing
module [205]; an analog input module [203]; a digital input and output module
with optical- to-electrical converter [204]; the unit is enabled to implement a
precise time synchronisation of the published samples as per IEC61850-9-2 (LE)
including publication and subscription of the GOOSE messages as per IEC61850-
8-1.