CONFIGURATION AND WORKING

POINT AND STATE

ESTIMATION BY

OBALEYE DANIEL OLUWATIMILEHIN

13400007007

28-10-2014

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

INTRODUCTIONThe 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.

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.

GENERIC SYSTEM

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.

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.

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.

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.

• 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 = −QcThe 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]∈

CAPABILITY LIMITS OF A GENERATING UNIT.

SCHEMATIC DIAGRAM FOR THE CHOICE OF THE WORKING POINT.

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.

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

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.

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.

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.

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.

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).

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.

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.

TYPES OF PREVISIONAL SCHEDULING CONTD.

(b) Medium-Term SchedulingThe 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.

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.

REAL-TIME SCHEDULINGThe 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.

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).

STATE ESTIMATION PROCEDURE

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.

REFERENCE

• ELECTRIC POWER SYSTEMS BY Fabio Saccomanno