CPR-D Collapse Prediction Relay - a. eberle - #2

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CPR-D Features

Routes into Collapse

Through liberalization and deregulation of the power markets the complex load dynamics in electrica power systems are sustainably modified by an additional driving force. Consequently the stability margins are thus more and more narrowed. If the power system gets into a highly stressed situation, the switching behaviour of the ULTCs of the trans­formers and the voltage regulators may aggravate the situation. Ultimately, they even contribute to the collapse of the whole power system. Yet a long time before the collapse occurs, however a so-called swing phase with special periodic and aperiodic superimposed oscillations can be detected (Fig. 1). It follows a drift phase with apparent silence, but of different time-scale for each collapse event. The duration length of this phase depends on the intensity of the preceding disturbance. In this phase the voltage, dependent on parameters, decreases and sometimes also increases. Shortly before the collapse event takes place the dynamics is abruptly changing again, resulting in the actual breakdown of the electrical power system (Fig. 2).

The Diagnostic Lens

Reaching the critical Hopf-point which indicates the stability limit, causes the system to undergo a transition into an unstable periodic cycle. This transition is called Hopf-bifurcation. In this context, the location of the Hopf-point fully depends on the underlying load dynamics and thus may vary its position.

The subsequent approach to the Hopf-point can be diagnosed at an early stage by the detection of the emergence of superimposed oscillations at specia frequencies (Fig. 3) and additionally by the evaluation of critical dynamics attached to these oscillations.

Conclusion: Collapses of electrical power systems can be predicted on time.

Blackouts can be avoided ...

What to do

The CPR-D Collapse Prediction Relay is continually measuring the exact frequency lapse in the power net. The results of these measurements play the role of the sensory input for the permanent surveillance and assessment of the system dynamics.

• Measurement of very low freque

Evaluation of the characteristics of harmonics

• Intelligent adaptation of parameters for

^{the measurement of frequencies}

• Asc ertainment of damping coefficients in

CPR-D Functions

• Detection of critical collapse-specific

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• Frequency-Relay (load shedding) ^ feuht record

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• Analysis and^essm e nt of system dynamics

If a dangerous situation is to be detected, the power system carrying company will immediately receive an alert message. This may initiate decisions of the operators to switch out the transformers by the regulating device to avoid further damage.

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Voltage Regulator ULTC Trans-former

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Fig. 3/1: Decreasing frequency before Hopf point H = green curve segment

Detection drift-process

System Power (MW)

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s. Fig. 3/2

System Frequency (pu)

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hypical Bus Voltage (pu)

Fig. 2: Drift phase (IEEE)

Fig. 1: Swing phase (IEEE)

Fig. 3: Hopf point H and limit point LP

Fig 3/2: Increasing frequency after Hopf point H and before limit point LP = red curve segment