23-09-2016, 02:19 PM
Abstract-T The paper concerns a high technology domain in
power systems related to their secure, reliable and economic
operating due to new conception, design and implementation of a
new devices including artificial distributed intelligence capable
to make local decision by solving promptly different kind of
faults. The whole supervision, control and monitoring process of
electric devices and equipments from electric power station and
distribution grids is based on automation - protection and data
acquisition and control equipments. The opportunity to control
the process by remote control devices is made available by the
monitoring and remote control of electric devices and
equipments. The continuous monitoring is completed for all the
components and levels. Taking into account the above
information and supported by suitable designs, it is possible top
obtain a better compromise between the probability of incorrect
running of the equipment and the protection devices.
GENERAL VIEW
The monitoring and remote control of electric devices and
equipments are more and more used by the energetics
operators and monitoring - control systems such as EMS
(Energy Management System), DMS (Distribution
Management System) ,i SCADA (Supervisory Control And
Data Aquisition) have been developed. The whole
supervision, control and monitoring process of electric
devices and equipments from electric power station and
distribution grids is based on automatization- protection and
data acquisition and control equipments. There is a close
relation between the equipments mentioned above; actually it
is not possible to have EMS or DMS functions if no SCADA
system providing technological flow-sheet data and the
opportunity to control the process by remote control devices,
is made available.
The main functions of a SCADA system designed for
electric installation and equipment monitoring include:
- Supervision and remote control of energetic installations
and networks.
- Warning.
- Post- damage analysis.
- Overall information of the operator referring to the
operated energetic system topology and condition by means
of operator - equipment interfaces (MMI: Man-Machine
Interface).
- Monitoring the networks load.
- Planning and supervision of maintenance and repairs
works to prevent power failure.
The basic element of the distributed data processing is the
connection of the informational centres by means of
communication channels. Their type and features depend on
the location where they are used, the types and functions
provided by the centres.
The access to the system information ensures the quality of
functioning both under usual circumstances and under
emergency conditions. The communication "connects" the
decentralized database. For instance, the parameters of a cell
are memory stored at the cell level and they are read by
request. Thus, at the station level, only the pre-processed data
become available ( it is not the instant values of the monitored
sizes that are transmitted, but the states reflecting the
compliance or the excess of such limits, compliance with
certain requirements). Communication is the basic element of
different protection charts coordination.
One of the facilities provided by the control charts is the
continuous self-monitoring. High protection availability is
obtained which is based on the easy current, voltage, auxiliary
tensions and command logics provided by the conventional
networks. The continuous monitoring is completed for all the
components and levels. Taking into account the above
information and supported by suitable designs, it is possible
top obtain a better compromise between the probability of
incorrect running of the equipment and the protection devices.
II. COMMUNICATION AT TRANSFORMATION
STATION LEVEL
In case of SCADA distributed systems, the following
specific situations referring to the data transmission have been
revealed:
Communication within the transformation station -
providing the station acquisition and command interfaces
inter-connection. The problem raised here is related to the
strong electromagnetic disturbances occurring within the
transformation station.
Remote transmission - ensuring the acquisition and
command EAC equipment inter-connection ofthe station with
the conducting level. Because of long distances (tens to
hundreds of km), the communication support provides low
rates and it is subject to many disturbances.
Leading level communication is usually based on high
speed local networks. The main issue here is related to the
capacity of the local network to meet the traffic and response time requirements as the application components from the
central point intensively uses network communication.
As a whole, the control protection, automation and
measuring systems consist of acquisition units UAD, digital
protection relays DPR, processing units UP at the cell and
station level as well as communication channels for
connecting the said equipments.
All these form a secondary sub-system based more and
more on multi-function digital equipments. The trend to
integrate functions that have been historically separated -
protection, control communication and measurement.
The secondary sub-system of a transformation station must
ensure:
1) Disconnecting the default parts from the network
whenever default occurs- default delimitation. Thus the
protection system must determine the default portion and to
suitably command the switches so that identify the default as
soon as possible.
2) The primary equipment must be correctly maintained so
that to be operational. The secondary sub-system must collect
information about the primary equipment condition and to
provide support for their maintenance.
3) The energetics dispatching must receive condition
information from station conditionfrom different levels (local,
territorial, national). The station secondary sub-system must
ensure the transfer of data to the control centers and
respectively to transmit the commands to the dispatching
technological process.
4) Local control. The secondary sub-system must provide
the local control function of the station, either as a back up
system in case of power failure or as an independent function
for stations not included within the remote control system.
Taking into account the above requirements, the main
functions of the station secondary sub-system are given
below:
- protection against the primary system defaults;
- controlling the primary equipment normal condition;
- automation;
- support for local commands;
- remote control;
- local measure and telemetering;
- network and primary equipment monitoring;
- automatic analysis of the data.
In case of EAC connection to the transformation station line
the following elements should be considered:
- maximum rate of information that can be handled through
the EAC and at the upper level;
- maximum rate of information conveyed in case of
modifications of the process conditions at different points and
they are taken over and transmitted by several EAC:
- the level of electro-magnetic disturbances of the electric
transformation stations and the necessity to immunize the
communication of these noises.
The physical distribution of points (EAC, computers at
station level) requires the serial communication from the very
beginning:
RS232C for EAC connections and a movable computer (
temporal connections for short distances). This standard offers
a small immunity against disturbances and can provide only
point to point connections.. The practical transmission speeds
range between 1,200-9,600 bit/ sec;
RS485 for a multi-point connection completion ensuring a
good immunity against noise and a practical distance between
points of several tens of meters. The transmission speeds
range between 1,200-14,000 bit/ sec:
Current loop - like RS485 can ensure multi-point
connections, well immunized against noises and with practical
communication speeds ranging between 1,200-9,600 bit/ sec;
Optical fiber - used to implement point to point and multipoint
connections with exceptional immunity against
disturbances and very high transmission speeds ( up to tens of
MBit/ sec). High transmission speeds require the use of strong
specialized communication processes.
The concrete requirements which communication inside the
transformation station must meet, are much varying and
mostly depend on the integration level of the secondary
system. In case that the switching line at the station level is
used particularly for SCADA functions and at a lesser extent
for protection and automation functions, it is considered that
the performances achieved by the traditional transmission -
physical wire support are sufficient. When the function
distribution is strong and particularly when the information
required to achieve the protection functions are transmitted
through the communication line, a more reliable transmission
support- optical fiber - is required, In such a case it is
necessary to take measures at the other levels of the
transmission chain too, in order to meet the specific
milliseconds time requirements.
The interfaces defining the information exchange between
two or more entities at the transformation station level are
presented in figure 1. The interface (9) provides the
connection between the control center and the station level, A
distinct connection (7) is provided for the system
configuration and service functions. The interfaces (1) and (6)
connect the level of the equipments of the same function
category- (8) control/ control, respectively (2) protection/
protection. Between the cell level and the technological
process there are the following interfaces: interfaces (4) for
analogue values acquisition (from current, voltage
transformers, etc); interfaces (5) ensuring the required
connections of the control functions; interfaces (10) for the
connections required by the protection functions. Table 1
presents the main requirements related to the performances
that the communication interfaces inside the station must
provide.