ABC of power system stability
In an electrical power system, loads1 are switched on and off continuously and the power system must maintain a balance between supply and demand at all times. The ability of a power system to maintain this power balance is key to system stability. Electrical energy, once generated, cannot be practically stored within a supply grid and must be consumed the instant it is generated in real time. Available storage technologies involve the conversion of electricity into other energy forms, e.g. chemical energy in batteries. Historically, battery storage technologies have generally been prohibitively expensive on a grid scale, though the recent installation of a 100 MW battery system in Australia illustrates that this might be about to change.
Power system stability can be viewed in terms of voltage or frequency. With regard to voltage stability, the system voltage should be maintained within acceptable limits (±10% of the nominal voltage) under normal operating conditions. At more than 10% deviation, the voltage will eventually “collapse” and cause instability in the power system. This article focuses mainly on frequency stability.
The frequency of a synchronous power system should be maintained as close to 50 Hz (or 60 Hz in North and South America) as possible. A synchronous power system is one where all power producers and consumers are connected to each other through transformers and transmission and distribution lines. Figure 10 below illustrates the balance between power supply and demand, where the balance is measured in terms of frequency. From the figure, we observe that an instantaneous increase in demand causes a drop in frequency, whereas a drop in demand increases the frequency. Excessive imbalance causes the acceptable frequency limits to be exceeded, resulting in instability and potential damage to important system components, unless load shedding or other mitigation measures are implemented immediately.
Figure 10 - Illustration of the balance between supply and demand
in a power system2
It is important to understand the relationship between frequency and the operation of synchronous generators. Generators in a power system are directly connected to a turbine by a shaft, and they convert mechanical energy from the rotating turbine into electrical energy (Figure 11). There is rotational energy stored in all synchronous generators connected to the system, more specifically in the rotors and connected turbine shafts. The power system frequency has a strong connection to the rotational speed of the connected synchronous generators. In principle, all generators in a synchronous grid rotate at the same average speed.
Figure 11 - Main Parts of a Turbine-Generator, here using a Kaplan turbine3
Assume that at one moment, a power system is in balance, that is, electricity generation and consumption are equal. What happens if more loads (power consumption) are suddenly added to the power system, while generation remains the same? In order to maintain the balance between generation and consumption, the generators will compensate for the load increase by using their rotational energy. The generator rotation will decelerate as a result and the system frequency will likewise drop. In order to restore the nominal system frequency after the change in load, more mechanical power is added to the turbines, increasing the generators’ rotational speed, and hence grid frequency. Generators that can increase their generation when frequency decreases are called primary controlled units. Not all connected generators are primary controlled. The primary controlled units must have sufficient “spinning reserve”, which is the instantaneous available capacity from the online generators.
The above scenario assumes that a load increase triggered the frequency change. A similar course of events will occur if a generating unit in the system stops or fails while the consumption remains constant. Then the remaining generators will have to increase their generation. If the failed unit is too large, the available spinning reserve will be insufficient to cover the lost generation. In this case the frequency will drop below the acceptable limits and cause load shedding or black out to protect system components. This scenario was played out in reality on 9th January 20184 when the loss of a generating unit in Kenya caused widespread blackouts in Uganda and Kenya whose synchronous power systems are interconnected. On the other hand, a load decrease or generation increase will cause the other generators to compensate by reducing their generation through increased rotational energy, hence an increase in system frequency.
Power system stabilizing functions and sequences
Primary control is not the only action performed to stabilize a power system. Figure 12 shows the four different control actions that are needed for system stability, in the order and time scales in which they are implemented. “Spinning reserve” described above is part of Turbine Control, and “non-spinning reserve” is part of System Control Centre Action. Non-spinning reserve means power plants that are not rotating (offline), but are ready to start when needed. In Uganda it takes 5 minutes for hydropower and 30 minutes for thermal power to start producing power5 . This overall system control is very complex but due to the different time scales, the different control actions are virtually de-coupled from each other and can be viewed separately (Andersson, 2007)6
Figure 12 - Different time scales of power system controls (Andersson, 2010)
The first three control actions are located at each power plant, including plant protection, voltage control and turbine control. The overall frequency control is done by the System Control Centre Action which is responsible for production planning and operation, as well as automatic safety systems in the grid, such as load shedding. This action is performed by the System Operator which is UETCL in the Ugandan power system.
(1) An electrical load is a component in the power system that consumes electrical power.
Hence, the opposite of a generator, which supplies power.
(5) UETCL GDP (Grid Development Plan) 2015
(6) ANDERSSON, G. 2007. Dynamics and control of electric power systems. ETH Zurich.