Configuration and working point and state estimation

1. CONFIGURATION
AND WORKING
POINT AND STATE
ESTIMATION
BY
OBALEYE DANIEL OLUWATIMILEHIN
13400007007
28-10-2014

2. PRESENTATION OUTLINE
• Introduction.
• Generic system
• Network Presentation
• Choice of the working point-constraints
• Capability limits of a generating unit
• Schematic diagram for the choice of the working point.
• Characteristic generic hydroelectric unit for operation
• Choice of the working point, through active and reactive dispatching
• Previsional scheduling
• Real-time scheduling
• State estimation
• Conclusion
• References

3. INTRODUCTION
The definition of the generic steady-state working point primarily requires:
• the system configuration and parameters are constant;
• load demands are constant;
• the three-phase electrical part is physically symmetrical with linear behavior.
If one would account for what actually happens in distribution and
utilization systems,
• an enormous amount of data, difficult (or practically impossible) to be
collected and subjected to significant uncertainties, would have to be
known;
• the overall system model would be excessively overburdened possibly in an
unjustifiable way, because many details may actually have effects that are
predominantly local;
• the previously mentioned hypotheses could appear unrealistic.

4. INTRODUCTION CONTD.
However, such inconveniences are of minor importance in
the overall behavior of the system; so it appears
reasonable (and convenient) that distribution and
utilization systems be considered only for behavior as
“seen” from the transmission network, by treating them
as equivalent circuits (more or less approximately)
consisting of:
• “equivalent loads,” directly fed by the transmission network
through nodes (called “load nodes”) to which distribution
networks are connected;
• connections among these nodes, to account for interactions
(e.g., at the subtransmission level) between distribution
networks.

5. GENERIC SYSTEM

6. GENERIC SYSTEM contd.
Figure 2.1 represents a generic system; which
indicates the block named “network” defined
within types of “terminal” nodes, which can be
classified as follows:
• generation nodes, which correspond to the synchronous generator
terminals (i.e., at the primary side terminals of step-up
transformers).
• reactive compensation nodes, which correspond in an analogous
way to synchronous and static compensator terminals.
• load nodes, i.e., nodes supplying the equivalent loads.
• possible boundary nodes, for connection with external systems.

7. NETWORK REPRESENTATION
In the generic steady-state, phase voltages and currents are, at
any given network point, sinusoidal, of positive sequence, and at a
“network” frequency equal to the electrical speed of the
synchronous machines. Also, at any given point, active and
reactive powers are constant. Specifically, by applying the Park’s
transformation with a “synchronous” reference (i.e., rotating at
the same electrical speed as the synchronous machines), the
following holds:
• each set of phase voltages or currents transforms into a constant
vector;
• the characteristics of the (passive) elements of the network and the
relationships between the mentioned vectors are defined by the
corresponding positive sequence equivalent circuits, both passive
and linear, with impedances (or admittances) evaluated at the
network frequency, and “phase-shifters” in the case of transformers
with complex ratio.

8. NETWORK REPRESENTATION
Therefore, the whole network is represented by a passive and
linear circuit, with “nodes” connected through “branches”. More
precisely, apart from the “reference” node for voltage vectors, the
following node types can be identified;
• terminal nodes (see Section 2.1.1), through which an outside
“injection” of current or power is generally performed.
• internal nodes, which refer only to network elements and do not
allow any outside injection.
A generic branch may be:
• a series branch, if it connects a pair of the above-mentioned nodes
(terminal and/or internal);
• a shunt branch, if it connects one of the above-mentioned nodes with
the voltage reference node.

9. CHOICE OF THE WORKING POINT-Constraints
At the scheduling stage, every future working point should be
chosen by considering not only the network equations, but also:
• conditions at the terminal nodes, which are defined by operating
characteristics and admissibility limits of the external equipment
connected to the network itself (e.g., generators, equivalent loads,
etc.).
• operating requirements (quality, economy, security).
• admissibility limits for each network equipment.
To meet quality requirements in this connection, the network
frequency should be kept at the nominal, e.g., ω = ωnom, whereas
node voltages (particularly terminal node voltages) must have
magnitudes not far from the nominal values, according to
“inequality” constraints; v ∈ [vmin, vmax]
Example; vmin = 0.90–0.95 vnom and vmax = 1.05–1.10 vnom.

10. • Moreover, at the terminal nodes, further constraints on injected
powers must be evaluated, concerning the characteristics of the
equipment external to the network.
• With load nodes, the active power Pc and the reactive power Qc
absorbed by a generic “equivalent load” may be assumed as input
data to the problem, so that “equality” constraints:
P = −Pc, Q = −Qc
The negative sign is the result of assuming P and Q as the generic
injected powers entering into the network.
• With generation nodes, the capability limits of each generating unit
should be evaluated, with inequality constraints (see Fig. 2.9):
P ∈ [Pmin, Pmax]
Q ∈ [Qmin,Qmax]

11. CAPABILITY LIMITS OF A GENERATING
UNIT.

12. SCHEMATIC DIAGRAM FOR THE
CHOICE OF THE WORKING POINT.

13. CHARACTERISTIC GENERIC HYDROELECTRIC UNIT FOR
OPERATION
• For a generic
hydroelectric unit, the
used water flow q
(disregarding losses)
depends on the active
power P supplied to the
grid, according to a
characteristic shown in
Figure 2.14a. Such a
characteristic may vary
with the operating
conditions of the plant
(e.g., water level in the
reservoir) and its
efficiency status.

14. CHOICE OF THE WORKING POINT, THROUGH
ACTIVE AND REACTIVE DISPATCHING.

15. ACTIVE AND REACTIVE
DISPATCHING
• Active Dispatching is the determination of the
active power share at different generation nodes and,
possibly, boundary nodes, which minimizes the overall
generation cost, accounting for constraints on generated
active powers and currents (with adequate margins, for
security requirements). Active dispatching also can take
advantage of adjustable parameters, if the system is
equipped with “quadrature”-regulating transformers to
modify the different branch currents in the network. To
obtain a satisfactory solution, the system configuration, and
more specifically the set of operating generators, may
require correction to meet the requirements of spinning
reserve.

16. ACTIVE AND REACTIVE
DISPATCHING CONTD.
• Reactive Dispatching implies the choice of
voltages (except at load nodes) by considering constraints on all
voltages (at terminal and internal nodes) and generated reactive
powers (at generation and reactive compensation nodes), and
providing sufficient reactive power margins. In reactive dispatching,
the role of adjustable parameters—corresponding to tap-changing
transformers, adjustable condensers and reactors, and “in-phase”
regulating transformers—is particularly important. To obtain a
satisfactory solution, the system configuration and, more
specifically, the whole set of operating compensators (and/or
generators themselves) may need to be corrected so that at the
corresponding nodes the required reactive power margins can be
achieved.

17. CHOICE OF THE HYDROELECTRIC
GENERATION SCHEDULE.
• The hydroelectric generation schedule has been
assumed to be preassigned up to now. Actually it
must be properly coordinated with the thermal
generation schedule, so that the most
economical overall solution may be obtained.
Such coordination must be performed “over
time,” based on:
• forecasting hydraulic inflows, and spillages for uses different
from generation;
• scheduling water storages in reservoirs and basins.

18. PREVISIONAL SCHEDULING
Data for the previsional scheduling basically concern:
• system components;
• load demands;
• different inflows available for generation.
Data about system components concern not only already
existing equipment, but also those on the way into
service. Thus problems relevant for the operational
scheduling also can overlap—particularly in the long
term, e.g., from 6 months to several years—with those
concerning the system development planning.

19. PREVISIONAL SCHEDULING contd.
Load demands are defined, in detail, by the variations with time (“load
diagram”), as active and reactive powers absorbed at each load
node. However, because of uncertainties in forecasting, it may be
preferable to accept simpler more easily predictable specifications,
by grouping loads at a single or a few “equivalent” nodes and/or
assuming step-varying load diagrams defined by mean values within
each time interval, etc., according to the following (risks resulting
from forecasting errors should be accounted for).

20. PREVISIONAL SCHEDULING contd.
• The different inflows available for generation
are typically constituted by water inflows (having subtracted
possible spillages) in the hydroplants. Similarly, we may consider
natural inflows of fuel or of motive fluid to possible geothermal
power plants, and so on. Hydraulic inflows are basically the natural
ones, caused by rain and snow and ice melting, and then depending
on meteorological conditions and water travel times. Moreover, in
the case of snow and ice melting, the inflows depend on the state
of snow fields and glaciers, and thus on the meteorological
conditions of the preceding winter. Further inflows may be added to
natural ones, such as those due to hydraulic coupling, i.e., caused by
the outflow from hydroplants located upstream and belonging to
the same valley, and those due to pumping.

21. TYPES OF PREVISIONAL
SCHEDULING
• long-term scheduling: T = 1 year;
• medium-term scheduling: T = 1 week;
• short-term scheduling: T = 1 day,
(a) Long-Term Scheduling
• With the long-term, the choice T = 1 year
appears particularly reasonable, because it
corresponds to the longest time interval by
which load demands and hydraulic inflows
vary more or less cyclically.

22. TYPES OF PREVISIONAL
SCHEDULING CONTD.
(b) Medium-Term Scheduling
The fundamental problems to be solved in medium-term scheduling concern
configuration and water storages; precisely, the typical goals are:
• determination of water storages at the end of subsequent days (for any value of
the storage at the beginning of the week), for each hydroelectric plant;
• determination of the operating schedule of units (unit commitment), specifically
thermal units.
The choice of a total interval duration T = 1 week, as assumed in the following, may
be efficient and meaningful, as:
• it allows a good link with the long-term scheduling;
• the operating schedule of thermal units (with possible startups and shutdowns)
must be chosen along a time interval including at least 1 week because of the
alternating working days and holidays, with significantly different load demands;
• considering a period of 1 week, it is possible to use data reliable enough, with
respect to the described goals.

23. TYPES OF PREVISIONAL
SCHEDULING CONTD.
• (c) Short-Term Scheduling
• The fundamental aim of short-term scheduling is to
accurately define the hydroelectrical and thermal
generation schedules and, more generally, the working
points of the system with reference, for instance, to the
next day.
The choice T = 1 day allows a precise connection with
medium-term scheduling, at least for water storages in
hydroelectric plants; whereas errors in forecasting load
behavior, etc. may be considered acceptable, without
adopting a shorter duration, because their effects can
be compensated by real-time scheduling.

24. REAL-TIME SCHEDULING
The goals to be achieved by means of Real-time Scheduling are:
• to check the actual working point with relation to quality, security,
and economy requirements, based on measurements performed on
the system during real operation;
• to determine necessary corrective actions (on control system set-points,
parameters, and system configuration itself), to obtain the
most satisfying working point.
• Real-time scheduling assumes that the operating state is a steady-state
that is kept unchanged for a sufficiently long time interval,
considering the unavoidable delay between the measurement
achievement and the corrective actions. This delay may be some
minutes, because of state estimation and dispatching (with security
checks, etc.), teletransmission of corrective signals, and their
subsequent actuation.

25. STATE ESTIMATION
• The “state estimation” may be performed
according to Figure 2.44. Analogtype
measurements to be converted into digital may
concern active and reactive generated powers,
active exchanged powers, voltage magnitudes,
active and reactive power flows and, rarely, active
and reactive powers absorbed by loads. Digital
measurements typically concern the status (open
or closed) of circuit breakers and disconnectors,
and the values of adjustable parameters
(discontinuous ones, such as transformer tap-changer
position).

26. STATE ESTIMATION PROCEDURE

27. CONCLUSION
• In order to get the needed and adequate
power output from hydropower generation
the Configuration and Working point and State
Estimation must be considered.

28. REFERENCE
• ELECTRIC POWER SYSTEMS BY Fabio
Saccomanno