ECE 576 POWER SYSTEM DYNAMICS AND STABILITY Lecture 39 Dynamic Security assessment in Power Systems Professor M.A. Pai Department of Electrical and Computer Engineering © 2000 University of Illinois Board of Trustees, All Rights Reserved

Dynamic Security Assessment The problem of Dynamic Security assessment (DSA) is concerned with the ability of the power system to withstand severe contingencies both from an angle stability and a voltage stability perspective. Due to limited generation and the need to make efficient use of transmission facilities, DSA has slowly grown into an important area of research and development. While algorithms for steady-state security assessment are well understood such is not the case with DSA.

DSA . s.s.op. point Dynamically stable Dynamically unstable s.s. security region of post-fault system s.s. security region i.e. for critical contingencies the op. point is within this region Delays in getting new plants / transmission lines on line make the existing system vulnerable to disturbances. In cases where there is a lag between demand and generation the problems listed make things worse (e.g. California deregulation).

DSA (contd) Through SSA (Steady State Security Assessment) the contingent state may be steady state stable but not dynamically stable. Look at the problem in another way. contingency P-V curve Stability region at B may shrink as loading is increased. B’ the contingence state may not be not dynamically stable. Limited operation and operating practices have been a bottleneck.

DSA (contd) In the 1980’s due to environmental concerns, the need to fully exploit the capabilities of transmission facilities was the primary focus. Hence, two technical problems, (i) The system may be operating close to its stability limit in the traditional sense, i.e., synchronous (angle) stability, and (ii) The system may be operating close to the voltage stability limit. There was not much addition to generation. For (i) a disturbance may render the system synchronous unstable. For (ii) a similar disturbance may initiate voltage collapse. In some instances inappropriate relay settings for current operation conditions may result in security problems.

What is DSA? (Dynamic Security Assessment) DSA deals with unexpected changes in the configuration of a power system due to severe loading conditions and/or contingency. Events such as loss of generation and/or transmission facilities. A system is said to be dynamically secure if the current state can survive a set of postulated conditions and be transiently stable and/or maintain a desired voltage profile.

DSA Although individual utilities adhere to mutually agreed upon criteria, the risks of frequent interruptions upon service are increasing. In deregulated systems, market power may be a factor. The system is operated closer to its maximum capability. Reliance on hydropower from remote locations increases the risk of spontaneous oscillations due to longer transmission lines.

DSA (contd) Tight interchange schedule between utilities makes it difficult to sustain acceptable voltage levels. A procedure for systematic analysis of possible contingencies online and preventive control strategies are needed. Synchronous (angle) stability analysis. Thermal problems (overload). Voltage problems. Maximum loadability of the system. Frequency problems.

Current tools Direct methods using Lyapunov based energy functions. Simulation methods including parallel processing and simulation by special hardware. Eigenvalue methods for voltage security and low frequency oscillations. Direct computation of point of voltage collapse. Probabilistic / AI / expert systems. Artificial neural network approach. The preventive control techniques use sensitivity analysis , LP and NLP approaches.

Sensitivity Approach Using TEF The parameter is chosen as the system loadability For online application, have analytical expressions for the sensitivities. Two more assumptions Transfer conductances are neglected. Only self-clearing faults with no line switching are assumed so that the pre-fault network is equal to the post-fault network.

Math Model The differential equations during the faulted and the post-fault states are with zero transfer conductances is where assumes different values for

DSA The energy function is given by where where is the post-fault stable equilibrium point which, for the faults with no line switching is the same as the pre-fault stable equilibrium point.

DSA (contd) The energy margin EM is can be approximated either as where is the controlling u.e.p. or is crossing of PEBS boundary.

DSA Using Sensitivity Take as the parameters and develop the sensitivity function as resulting in sensitivity differential equations. Alternatively, can be taken as the parameter with the sensitivity function defined as where In steady state This results in only n sensitivity differential equations.

The sensitivity of EM to is given by DSA The sensitivity of EM to is given by and is computed by the economic dispatch criteria. We may assume Hence since in the pre-fault state .We can write SEM functionally as

DSA (contd) The dynamic sensitivity equations are (Derivation omitted) The D.E’s in the faulted and post-fault state together with SEM and (1) then constitute the composite energy margin sensitivity model. Equation (1) is a set of time varying differential equations since in (1) are functions of time. The initial conditions are

DSA (contd) The sensitivity information SEM obtained at can be used to reschedule the generation. Extension of this basic approach to the case where energy margin is . where is the controlling u.e.p is discussed in the literature. In addition to other variables have been used. DSA in practical networks using sensitivity concepts computes maximum of interface power limits on a large network. The controlling u.e.p. method is used in computing instead of the PEBS method.

SIME SIME (single machine equivalent) method using sensitivity.(Pavella) Generally, when a system looses stability, it splits into two groups. Generators in each group can be combined into a single generator so that we have a two generator system. This can be done by simulating the system online. This in turn can be converted into a single machine infinite bus system (SMIB). The SMIB has swing equation of the form

SIME (contd) Compute sensitivity of the energy margin (EM) to changes in the pre-fault loading conditions for a given contingency and a given In the case of the SMIB system, EM = where and are decelerating and accelerating areas, respectively. Critical group Rest of the system

SIME (contd) EM is first expressed in terms of the system parameters and δ only. Then compute the quantity which will be a complex expression involving several partial derivatives including . For a given operating condition, contingency and clearing time is computed at using either the fault-on trajectory or a trajectory approximation. This sensitivity information can be used to find limiting value of which will make EM = pre-specified value.

DSA for voltage stability (Overbye and Demarco) It is based on the following concepts The current operating point of the system is a stable one and constitutes the “high power flow” solution in the terminology of multiple power flow solution techniques. It sits at the bottom of a potential “well”. The potential energy function is the same as the one used in transient stability studies with structure preserving model.

DSA “Low power flow” solution in which voltages at one or more buses are very low constitute the u.e.p.’s. As the load increases, the difference decreases linearly and the point of voltage collapse is reached when this difference is zero.

DSA Just as computing the u.e.p.’s in the angle stability case requires some special techniques, same is the case for computing the appropriate “low power flow” solution. a High power flow solution b e Low power flow solution e a b

Operator intervention Flow Chart for DSA TEF filter Critical contingencies Fast simulation Display of info to operator Indices Margins Corrective action NO Yes Operator intervention

DSA Performance requirements for on-line DSA Desired frequency of automatic execution of DSA is essentially the same as that of the static security assessment. Suggested numerical values of the frequency varies from once in every 30 seconds to once in 30 minutes. The real world events that require a new assessment include unit status changes, transmission configuration changes, unit load changes, and any changes in operator selected control options (e.g. selecting a new generation tripping scheme). A preliminary list of 100-200 contingencies is appropriately screened to identify a reduced set of 10-30 contingencies to be studied in detail. For screening TEF is used.