Power System Restoration after Blackout ⚡🎡🚦

The power system restoration problem is practically as old as the electric system itself. Modern societies have become increasingly dependent on reliable electricity, therefore major disturbances in the power system have significant impact on both society and economy. Thus, disturbances should be resolved as quickly as possible. If a blackout occurs, the utilities responsible for the operation of the power system should restore the system back to normal operation as soon as possible using power system restoration processes.

Fast and robust processes are important for successful system restoration and to limit the adverse impacts of blackouts. Blackout occurrence is unpredictable and blackouts affecting large geographical regions or even entire power system are rare. Today, many power systems are planned and operated using an N−1 principle which means that the power system should withstand any single fault, including the most severe faults, determined by the system planning criteria.

Although system level blackouts are rare, the risk of a blackout occurrence cannot be ignored in system operation. For example, extraordinary events outside the power system planning and operation criteria may lead the system to operate beyond its operational security limits. Such high impact low probability (HILP) events are typically unpredictable chains of events with very low probability. However, if such a chain of events occurs, it will have high impact on the security of electric supply.

RESTORATION STRATEGIES

There are 3 main power system restoration strategies:

 

1. Top Down Restoration and
2. Bottom Up Restoration
3. Hybrid (above both combination)

In addition, these strategies may be used simultaneously as a hybrid strategy. Also, intentional islanding may be used to form an automatically disconnected islands with sufficient generation and load. The islands could then be used as starting points for the selected restoration strategy.

TOP DOWN STRATEGY

In Top Down strategy, the restoration starts from a strong voltage source such as a neighboring power system or a strong islanded system which, for example, has been intentionally disconnected from the power system just before the blackout. First, the sufficient high voltage transmission network is restored. After that, the restoration continues to the lower voltage transmission networks and distribution grids.

ADVANTAGES

The advantage of the top-down strategy is that the strong voltage source makes it possible to directly restore a high voltage transmission network that typically covers large geographical areas including both generation and consumption. This reduces the need for complex and time-consuming restoration procedures required in weak systems and consequently expedites the restoration execution.

DISADVANTAGES

The disadvantage of the top-down strategy is that it requires a strong voltage source. If such is not available, the strategy is not possible.

BOTTOM UP STRATEGY

Bottom up strategy; the initial energy is provided by, for example, a black-start generator which performs the initial transmission network restoration. At this stage, the system is weak (i.e. the short circuit current level is very low). After that, the island is expanded by connecting local load and generation. When sufficient generation is connected to the island (i.e. the short circuit current level is sufficient), the restoration may proceed towards the high voltage transmission networks.

ADVANTAGES

The advantage of the bottom-up strategy is that it enables power system restoration also when the top-down restoration is not possible.

DISADVANTAGES

The disadvantage is, however, that a black-start generator or other similar voltage source within the power system is required.

In addition, the management of the extremely weak network may make the bottom-up restoration more complex and slower to execute than top-down strategy.

The restoration process typically includes at least the following tasks:

1. The determination of the system status after a blackout,

2. The preparation of black-start and other generators for the start-up,

3. The preparation of the transmission network and other relevant power

grids for the energization,

4. The restoration of the selected transmission networks and power grids,

5. The formation of islanded power systems by connecting generation and

consumption,

6. The reconnection of islands to each other.

(These tasks can be executed sequentially or in parallel from different locations in the power system asper need.)

The verification of the restoration actions requires a variety of simulation tools:

 

1. Power flow analyses study, for example, the reactive power balance, steady-state voltages and thermal loading of the transmission lines.
2. In addition, time domain dynamic analyses are required to study, for example, frequency variations during load restoration and rotor-angle oscillations.
3. Both EMT simulations are required to study switching transients and harmonic interaction during restoration and short circuit current calculations are needed to evaluate the relay protection performance.
4. Furthermore, long-term dynamic simulations may be needed to study load restoration and the cold-load pickup effect.
5. The resources required during restoration should be allocated and the detailed instructions should be prepared, documented and trained.
6. Special attention should be given to the documentation format of the restoration plan so that it is easy to follow in case of an emergency.
7. In addition, the technical reasoning behind the selected restoration strategies and guidelines should be documented.
8. Dispatcher training simulators (DTS) or operator training simulators (OTS) have become an important tool for training dispatchers and controls center operators. A DTS/OTS makes it possible for operators to perform exercises on various emergency scenarios using the SCADA user interface and displays.
9. In addition, SCADA integrated machine learning and knowledge-based systems may also assist the operator in decision making during restoration execution. Since simulation models have a major role in restoration planning, it is essential that the models are properly validated to represents the phenomena of interest. For example, the analysis of North American 2003 blackout discovered that simulation models showed stable system operation while undamped oscillations were observed in the real system.
10. Regulation and legislation such as, specify mandatory and regular tests on black-start generators and its black-start function. Therefore, many TSOs perform such tests. However, some TSOs also perform field-tests to verify that black-start generators can restore the transmission network which may be used to supply start-up power for other generators that do not have the black-start functionality.

PHENOMENA AFFECTING RESTORATION

 

1. During the initial transmission network restoration, managing the system voltage is of especial concern.
2. Special attention should be given electrical phenomena causing overvoltages that may damage the grid equipment and delay or even prevent the system restoration using specific restoration paths.
3. The occurrence of these phenomena is mainly due to a weak system caused by low short circuit current levels due to few connected generators, lightly loaded transmission lines, insufficient inductive shunt compensation and exceptionally low system resonance frequencies.

The following phenomena are commonly reported causing overvoltages in weak systems:

1. Switching transients and harmonic resonance,
2. Sustained overvoltages,
3. Parallel line resonance,
4. Ferroresonance,
5. Synchronous generator self-excitation.

The possible triggers and implications related to the phenomena may compromise fast and robust restoration and therefore cause uncertainty for both restoration planning and restoring the system after a blackout.

Poorly damped high frequency switching transients may cause overvoltages in weak transmission networks, damage the grid equipment during restoration and consequently compromise fast and robust system restoration. Such transients are typical concerns during the restoration of transmission networks with voltages over 100 kV. The transients are mainly caused by the energization of capacitive elements such as non-loaded transmission lines.

Transformer energization in requires special attention during restoration since the transformer inrush current has significant harmonic contents.

Issues like Parallel Line Resonance, Ferroresonance, Synchronous Generator Self Excitation should be consider carefully during restoration plan.

TRAINING

Regular training of the control center operators is important. At least, the following aspects should be considered in the training.

 

1. Control center operators or dispatchers should be aware of the possibly encountered electrical phenomena and protection and automation system issues during restoration to be able to understand the system behavior and measurements.
2. Operators should understand what control actions may trigger the unwanted electrical phenomena.
3. Operators should understand what control actions may be used to manage and mitigate the unwanted electrical phenomena.
4. SCADA integrated training simulators such as a DTS or OTS are useful for restoration training. Although the simulators may not simulate the exact electrical response of the system, the possible issues and electrical phenomena may be simplified so that the system provides a realistic look and feel.