Adviser to grid-control engineer on clearing overloads
A unified control system of the Russian national grid operation ensures a
reliable and cost effective supply of high quality electricity to consumers across the country with
the most effective utilization of the resources of the grid being the world largest power system with
a centralized control.
The top control centre CDU together with regional ODUs and
district RDUs develop long term (month, year), short term (day, week)
plans for the national grid operation modes, provide operational control
of the current modes and technological emergency modes.
The control engineer’s main objective is to prevent local emergency processes
from developing into a system failure.
The control engineer on duty is to monitor the modes of operation for
compliance with the plans and schedules, to get them adjusted to the changes in the conditions of the
system operation, to guide the work on restoring power systems to a standard mode of operation in
emergencies and to examine and make decisions on the requests for taking out of and putting into
service the core equipment of power stations and power networks.
To assist control engineers in their performance of complex tasks on grid
control various applications have been developed and are being developed for planning, operational
control, simulation, on–line mode optimization. The line of these products has been extended to include
advisers to grid-control engineers. These products use some means of artificial intelligence.
The grid-control engineer adviser (CEA) is a set of applications designed
to provide a control-engineer with advice on actions in the circumstances which the particular adviser
(CEA) has been developed for and within the time realistically required for the implementation of the
advised solution (Spoon is good for dinner!).
The CEA functions are to be different for each level of control largely because
the objects of their control are different and consequently the tasks are different. Furthermore it may
be necessary to have several CEA applications at the same level of grid control (for instance, a CEA
for maintaining economical mode, a CEA for optimizing trippings, a CEA for clearing overloads etc.)
NPZ "Prioritet" has developed a new set of applications
"Adviser to Grid-Control Engineer on Clearing Overloads" (hereinafter CEA-OL). Its
key objective is an automatic developing of "use-proven" advice on clearing overloads in
the equipment of power system within a time length specified by the user without tripping the load.
The CEA-OL set of applications was developed in response to a request from the
Mosenergo (Moscow power system) control centre and put into service in late 1990s. The customer‘s
requirement was to receive a solution within 150 s.
While handling overload cases the control engineer can be guided by existing
instructions or go by his own experience. However network configurations and operation modes can differ
from those for which the instructions were meant or the experience acquired from.
Besides when an overload occurs a control engineer can refer to the achieve and
use any previous mode as a basis for carrying out simulation process or try to apply the instructions
that were recommended in the "achieve" mode for clearing an overload.
The probability that such actions will result in restoring a normal operation
mode is low due to the disparity between the desired operation mode and network topology in question
on the one hand and those taken as reference on the other.
The mode is defined by a dimension vector (2*N-1) – where N is the number of
network nodes. To select a mode near the existing one so as to be able to go by the given recommendations
means to determine the degree of proximity of two vectors with the given dimensions which is rather
problematic particularly considering the change of the network topology.
The more powerful the equipment is the costlier is a mistake. Therefore it is
of prime importance to have a fast operating system capable of testing prospective operations and, if
necessary, producing an adequate "advice".
The soft-based Adviser to Grid-Control Engineer on Clearing Overloads
(CEA-OL) works with a real power network using a logical unit, which makes it possible to work out
variants of actions for clearing overloads, test them and sort out by the customer’s criteria within
the time scale defined by the customer.
The automatic output of advice by a set of applications of CEA-OL type is based
on a fast calculations made in the logical unit of the power system operation mode in the circumstances
of some changes in power generation, load, topology of power network.
CEA-OL makes use of the power system description file in the format CDU (RR-file)
and/or TM-ТS (telemetering - telesignalling) data processed by the mode actualization application.
CEA-OL uses standard programs for calculating a steady-state mode.
CEA-OL operation modes
There are various modes of working out required actions for clearing overloads.
- automatic – when a special "EXPERT" application is initiated,
- compound – when some actions are to be performed by the operational staff, while others are
defined by "EXPERT",
- manual when the search of actions for clearing overloads is the operational staff work.
When started, CEA-OL calculates the power system operation mode and the network
is tested for existing overloads. The detection of overloads makes it possible for the
"Expert" module to go active. Before it starts some changes may be introduced manually into
the network topology and data (generation or load).
"EXPERT" module makes use of the following instruments while searching a solution to the overload clearing:
- changing the generation of active power
- switching branches, including from one system of bus-bars to the other.
"Expert" module settings
In the Setting window of the "Expert" module interface the following parameters are defined:
- time frame for solution search,
- search accuracy,
- search instruments,
- priorities of the instruments to be used,
- setting of the generation change limits,
- capability of involving some service programs, for instance for the calculation of the currents
of three-phase short circuits and comparing them with the breaking power of circuit breakers,
unloading programs when there is no solution etc.,
- capability of sorting out solutions by the criteria defined by the user (by the number of actions
for clearing unload, by the increase of the value of generation change, by the increase of excessive
value of currents of three-phase short circuits beyond the limit).
The search of actions for clearing an overload is terminated if:
- a solution is found, i.e. a string of actions bringing the overload down below
the defined value (the field Accuracy in the Setting window of "Expert");
- no solution – i.e. only improving solutions can be found - actions reducing the overload
(the actions, carrying the overload over onto other power system equipment, are discarded);
- search time has expired although a solution to the problem can be either found, not found,
or an improving solution can be found
The variant of the "Expert" module not finding a solution should not
be surprising. It is easy to envision a network branch with a load exceeding the capacity of the
feeding power line.
The obtained solutions can be looked through in the solution window and
then there are two options: accept one of them or reject all. In the event of the latter changes of
the settings are effected in the Setting window and the "Expert" module restarted.
When there are improving solutions (which means that no solution has
been found within the defined time scale), they may also be examined and one of them may be accepted
and the "Expert", module restarted after the settings have been changed, if necessary. In
such cases solutions for removing overloads may involve a long string of actions.
When there is a multiple of overload cases the process starts with the partition
of all overloaded branches into independent groups.
The "Expert" module is applied separately to each group.
The solution to the problem is the superposition of solutions for each group.
Application of CEA-OL for examining requests about putting the equipment out of service for repair purposes
When examining requests for putting the equipment out of service for repair
there is no time limitation for finding solutions. Therefore the best solution out of the multiple
ones offered can be selected to meet the criteria of the operation mode quality, chosen by the user
(reliability, the lowest losses etc.)
Furthermore there are ordinary several requests related to various equipment
switchover or tripping. If each request is examined separately it may cause a situation when the
changes in the network needed for meeting one request can be at variance with the conditions for
another request. These requests should be allowed at different time.
Applying CEA-OL makes it possible to examine opportunities for meeting several
requests at the same time period and to get solutions excluding possible collisions with all the
restrictions observed. The solutions can be sorted out by the criteria specified by the customer.
CEA-OL interoperation with the grid-control system
CEA-OL is easy to integrate into the existing grid control system.
SD.dll dynamic library provides functions for the operational staff (client)
to interact with the expert system. Furthermore the CEA-OL set includes SDTest.exe test application
for self testing. The client application should interoperate with the supplied library.
Figure 1. A simplified diagram of the grid-control system interoperation with CEA-OL
The Client application downloads the library, gets all the required data ready
and after that calls the library’s function for solution search transferring to it the prepared data
and parameters managing solution search. In the library the function checks the received data and then
initializes the start of the expert system. The time frame as well as the methods of search are
specified by the Client application through the parameters mentioned above. After the expiry of the
time length defined for the search the expert systems terminates its work.
Its work results can include the following:
- advice for clearing the overload
- advice for reducing the overload
- no solution
The produced solutions are stored and sent to the client application. After
that the expert system gets deintialized and the function terminates its work.
In the event no solutions can be found there is an instrument allowing the
tripping of the minimum possible load bringing the overload down to the tolerable level.
In 2008 CEA-OL interoperating with SK-3000 complex was installed the Moscow
regional network control centre under the name СPPROP (CD) (Support System for Making Decisions by
Conclusion: The soft-based CEA-OL set can significantly facilitate the
work of both grid control engineers and the staff handling requests for taking the equipment out of
service for repairs, for maintaining the grid in the mode of the highest availability and, most
important, for excluding a chance of staff unintentional actions damaging the power system. It allows
the operational staff to make calculations-based decisions on removing overloads and to conduct
simulation of the system operation when investigating the reasons for the occurrence of undesired
modes of operation.