Q5Power System Planning

(a) Automatic Generation Control and Economic Load Dispatch

Automatic Generation Control (AGC) is the real-time closed-loop control mechanism used in system operation to maintain the system frequency at its nominal value (50 Hz in India) and to maintain scheduled power interchange on tie-lines between control areas, while economic load dispatch (ELD) is the technique of allocating the total demand among the online generating units in the least-cost manner. AGC operates continuously, adjusting the governor set-points of designated regulating units based on the Area Control Error (ACE), which is a combination of the frequency deviation and the tie-line power deviation from schedule.

• Load-frequency control (LFC): the primary control loop within AGC that senses frequency deviation caused by mismatch between generation and load, and adjusts governor output of regulating units to restore frequency.
• Tie-line bias control: extends LFC to multi-area systems by defining ACE as a weighted combination of frequency deviation and net tie-line power deviation, so that each control area corrects its own imbalance rather than relying entirely on neighbouring areas.
• Economic dispatch integration: within AGC, the total generation correction signal computed to null the ACE is allocated among participating units according to their incremental (marginal) fuel cost characteristics, so that the cheapest available units bear the largest share of the correction, subject to their ramp-rate and capacity limits.
• Overall objective: AGC combined with economic dispatch ensures that system frequency and scheduled interchange are held within tight tolerance while the generation cost is kept as close to minimum as physically possible at every instant, which is central to short-term system operation planning.

For long-term operation planning, the economic dispatch function is exercised ahead of time through unit commitment studies (deciding which units to start up/shut down over the coming day or week) combined with hourly or sub-hourly economic dispatch, so that the real-time AGC only has to perform fine corrections around an already near-optimal base-point schedule.

(b) State Estimation

State estimation is a computational procedure used in the control centre of a power system to determine the most accurate and consistent estimate of the current operating state of the network - defined by the complex bus voltage magnitudes and angles at every bus - using a set of redundant, and generally noisy or occasionally erroneous, real-time measurements (line power flows, bus power injections, voltage magnitudes and, increasingly, phasor measurements) together with the known network model (topology and impedances). Because raw telemetered measurements can contain random errors, be lost due to communication failure, or occasionally be grossly wrong (bad data) due to instrument/telemetry faults, a simple direct power-flow solution using raw measurements is unreliable; state estimation instead uses statistical techniques, typically the weighted least squares (WLS) method, to combine the redundant measurements in a way that minimizes the weighted sum of squared measurement residuals and yields the most probable true state of the system.

• Topology processor: builds the current electrical connectivity of the network from switch/breaker status information.
• Observability analysis: checks whether the available measurement set is sufficient to uniquely determine the state of the entire network, and identifies unobservable islands if not.
• State estimation algorithm (WLS): iteratively computes the bus voltage magnitudes and angles that best fit the redundant measurements in a least-squares sense.
• Bad data detection and identification: statistical tests (e.g., chi-square test on residuals) are used to detect the presence of gross measurement errors and identify/remove the offending measurement before finalizing the estimate.
• Output to applications: the estimated state feeds downstream real-time security-assessment applications such as contingency analysis, optimal power flow and automatic generation control.

A practical challenge in AGC and economic dispatch coordination is that the regulating units chosen to respond to ACE must have adequate ramp rate and must be kept away from their output limits to have margin to respond in either direction; the economic dispatch calculation must therefore allocate base-point generation not purely on lowest marginal cost but with due regard to which units are designated for AGC regulation duty, sometimes at a small cost premium, in order to maintain adequate frequency-regulation capability at all times.

State estimation is particularly valuable in situations where the number of available real-time measurements is less than what would be needed to solve a conventional power flow directly; by exploiting the redundancy of typically two to three times as many measurements as unknown state variables, the weighted least squares algorithm can tolerate the loss of individual measurements and still produce a usable estimate, which is essential because communication channels to remote terminal units are never perfectly reliable in a real operating network spanning thousands of kilometres.

The economic dispatch component of AGC is generally re-solved on a periodic basis (e.g., every few minutes) rather than continuously, since re-optimizing the full economic allocation at every control cycle would be computationally unnecessary; between re-optimizations, the AGC loop distributes the required regulation using pre-computed participation factors derived from the last economic dispatch solution, giving a practical balance between dispatch optimality and the fast response needed for frequency control.

The tie-line bias control component of AGC is particularly important in a multi-utility pooled system such as the Indian regional grids, since without it, any single area experiencing a local generation-load imbalance would rely entirely on neighbouring areas to correct the resulting frequency deviation through their own governor response, which is neither fair nor sustainable; tie-line bias control ensures each area is responsible for correcting its own net imbalance, measured through its individual Area Control Error, while still benefiting from the pooled system overall inertia and reserve during the first few seconds of any disturbance.

The accuracy of the state-estimation weighted least squares solution depends heavily on the assumed measurement error statistics (weights) assigned to each meter based on its known accuracy class; measurements from more accurate, modern digital instruments are given higher weight (trusted more) in the estimation than older or less precise analogue instruments, and increasing deployment of synchronized phasor measurement units, which provide highly accurate, time-stamped voltage and current phasor data, is progressively improving both the accuracy and the observability of state estimation in modern control centres.

In summary, AGC and economic dispatch together, and state estimation as the informational foundation for all real-time control-centre applications, represent the two pillars of system operation planning: one ensures the system is continuously balanced and cost-effectively dispatched, and the other ensures the operator always has an accurate, trustworthy picture of the actual system condition on which to base every operating decision.