Online Transient Stability Controls

For the last few decades the growth of the world’s population and its corresponding increased demand for electrical energy has created a huge increase in the supply of electrical power. However, for logistical, environmental, political and social reasons, this power generation is rarely near its consumers, necessitating the growth of very large and complex transmission networks. The addition of variable wind energy in remote locations is only exacerbating the situation. In addition the transmission grid capacity has not kept pace with either generation capacity or consumption while at the same time being extremely vulnerable to potential large-scale outages due to outdated operational capabilities.

For example, today if a fault is detected in the transmission system, the only course is to shed both load and generation. This is often done without consideration for real-time consequences or alternative analysis. If not done rapidly, it can result in a widespread, cascading power system blackout. While it is necessary to remove factors that might lead to a large-scale blackout, restriction of power flow or other countermeasures against such a failure, may only achieve this by sacrificing economical operation. Thus, the flexible and economical operation of an electric power system may often be in conflict with the requirement for improved supply reliability and system stability.

Limits of Off-line Approaches

One approach to solving this problem involves stabilization systems that have been deployed for preventing generator step-out by controlling the generator acceleration through power shedding, in which some of the generators are shut off at the time of a power system fault.

In 1975, an off-line special protection system (SPS) for power flow monitoring was introduced to achieve the transient stability of the trunk power system and power source system after a network expansion in Japan. This system was initially of the type for which settings were determined in advance by manual calculations using transient stability simulation programs assuming many contingencies on typical power flow patterns.

This type of off-line solution has the following problems:

  • Planning, design, programming, implementation and operational tasks are laborious. A vast number of simulations are required to determine the setting tables and required countermeasures, such as generator shedding, whenever transmission lines are constructed;
  • It is not well suited to variable generations sources such as wind or photovoltaic farms;
  • It is not suitable for reuse and replication, incurring high maintenance costs; and
  • Excessive travel time and related labor expense is required for the engineer and field staff to maintain the units at numerous sites.

By contrast, an online TSC solution employs various sensors that are placed throughout the transmission network, substations and generation sources. These sensors are connected to regional computer systems via high speed communications to monitor, detect and execute contingencies on transients that may affect system stability. These systems in turn are connected to centralized computers which monitor the network of distributed computers, building and distributing contingencies based on historical and recent information. If a transient event occurs, the entire ecosystem responds within 150 ms to detect, analyze, determine the correct course of action, and execute the appropriate set of contingencies in order to preserve the stability of the power network.

In recent years, high performance computational servers have been developed and their costs have been reduced enough to use many of them in parallel and/or in a distributed computing architecture. This results in a system that not only provides a benefit in greatly increasing the availability and reliability of the power system, but in fact, can best optimize the throughput of the grid. Thus not only has system reliability improved or remained stable, but the network efficiency itself has increased without a significant investment in new transmission lines. This has resulted in more throughput within the transmission grid, without building new transmission lines.

Solution and Elements

In 1995, for the first time ever, an online TSC system was developed and introduced in Japan. This solution provided a system stabilization procedure required by the construction of the new 500kV trunk networks of Chubu Electric Power Co. (CEPCO) [1-4]. Figure 1 shows the configuration of the online TSC system. This system introduced a pre-processing online calculation in the TSC-P (parent) besides a fast, post-event control executed by the combination of TSC-C (child) and TSC-T (terminal). This online TSC system can be considered an example of a self-healing solution of a smart grid. As a result of periodic simulations using the online data in TSC-P, operators of energy management systems/supervisory control and data acquisition (EMS/ SCADA) are constantly made aware of stability margins for current power system situations.

Using the same online data, periodic calculations performed in the TSC-P can reflect power network situations and the proper countermeasures to mitigate transient system events. The TSC-P simulates transient stability dynamics on about 100 contingencies of the power systems for 500 kV, 275 kV and 154 kV transmission networks. The setting tables for required countermeasures, such as generator shedding, are periodically sent to the TSC-Cs located at main substations. The TSC-Ts located at generation stations, shed the generators when the actual fault occurs. The actual generator shedding by the combination of TSC-Cs and TSC-Ts is completed within 150 ms after the fault to maintain the system’s stability.

Customer Experiences and Benefits

Figure 2 shows the locations of online TSC systems and their coverage areas in CEPCO’s power network. There are two online TSC systems currently operating; namely, the trunk power TSC system, to protect the 500 kV trunk power system introduced in 1995, and the power source TSC system to protect the 154 kV to 275 kV power source systems around the generation stations.

Actual performance data have shown some significant benefits:

  • Total transfer capability (TTC) is improved through elimination of transient stability limitations. TTC is decided by the minimum value of limitations given by not only thermal limit of transmission lines but transient stability, frequency stability, and voltage stability. Transient stability limits often determines the TTC in the case of long transmission lines from generation plants. CEPCO was able to introduce high-efficiency, combined-cycle power plants without constructing new transmission lines. TTC was increased from 1,500 MW to 3,500 MW by introducing the on-line TSC solution.
  • Power shedding is optimized. Not only is the power flow of the transmission line on which a fault occurs assessed, but the effects of other power flows surrounding the fault point are included in the analysis to decide the precise stability limit. The online TSC system can also reflect the constraints and priorities of each generator to be shed. To ensure a smooth restoration after the fault, restart time of shut off generators, for instance, can also be included.
  • When constructing new transmission lines, numerous off-line studies assuming various power flow patterns are required to support off-line SPS. After introduction of the online TSC system, new construction of transmission lines was more efficient by changing the equipment database for the simulation in the TSC-P.

In 2003, this CEPCO system received the 44th Annual Edison Award from the Edison Electric Institute (EEI), recognizing CEPCO’s achievement with the world’s first application of this type of system, and the contribution of the system to efficient power management.

Today, benefits continue to accrue. A new TSC-P, which adopts the latest high-performance computation servers, is now under construction for operation in 2009 [3]. The new system will shorten the calculation interval from every five minutes to every 30 seconds in order to reflect power system situations as precisely as possible. This interval was determined by the analysis of various stability situations recorded by the current TSC-P over more than 10 years of operation.

Additionally, although the current TSC-P uses the same online data as used by EMS/ SCADA, it can control emergency actions against small signal instability by receiving phasor measurement unit (PMU) data to detect divergences of phasor angles and voltages among the main substations.


The online TSC system is expected to realize optimum stabilization control of recent complicated power system conditions by obtaining power system information online and carrying out stability calculations at specific intervals. The online TSC will thus help utilities achieve better returns on investment in new or renovated transmission lines, reducing outage time and enabling a more efficient smart grid.


  1. Ota, Kitayama, Ito, Fukushima, Omata, Morita and Y. Kokai, “Development of Transient Stability Control System (TSC System) Based on Online Stability Calculation”, IEEE Trans. on Power System, Vol. 11, No. 3, pp. 1463-1472, August 1996.
  2. Koaizawa, Nakane, Omata and Y. Kokai, “Acutual Operating Experience of Online Transient Stability Control System (TSC System), IEEE PES Winter Meeting, 2000, Vol. 1, pp 84-89.
  3. Takeuchi, Niwa, Nakane and T. Miura
    “Performance Evaluation of the Online Transient Stability Control System (Online TSC System)”, IEEE PES General Meeting , June 2006.
  4. Takeuchi, Sato, Nishiiri, Kajihara, Kokai and M. Yatsu, “Development of New Technologies and Functions for the Online TSC System”, IEEE PES General Meeting , June 2006.