The past year saw the reality of power blackouts hit people around the world. From the US to the UK, Italy and some countries in the Middle East, power blackouts caused the general populace untold misery and huge losses to businesses.
At a time when efforts are on to declare the right to electric power as a basic human right, the consequences of all these blackouts only go to prove how dependent we are on electric power for our everyday needs. In fact, one of the indicators of the level of industrialisation of a nation is its per capita consumption of electric power.
What are electric power blackouts and what causes them? While a detailed technical explanation is beyond the scope of this article, an attempt will be made to explain the phenomenon in simple terms. In an electric power network, there are electric power generators and loads. Generators are machines that convert energy from steam, gas, water, etc. to electrical energy. Loads are any of the various electrical appliance/devices at home and in industry - these include air conditioners, electric lights, fans, all electrical appliances, motors, etc.
Electric power can be broadly classified into Direct Current (DC) and Alternating Current (AC). DC is the type of current that one obtains when a load is connected to, for example, a battery. The current in the load always flows from the positive terminal of the battery to the negative terminal. With AC, the current at the two terminals of a plug point changes its direction of flow 50 or 60 times every second - this is called the frequency and is 50 Hertz or 60 Hertz depending on the number of times the direction of power flow changes in a second. Hertz (Hz) is the general unit for defining oscillations. Due to practical and economic considerations, electric power is transmitted using AC.
In some countries (U.S., Japan, Saudi Arabia, etc.), the frequency of electric power is 60 Hz while in most other countries it is 50 Hz. The frequency is related to the speed of the generators and it is vital that all generators run at the same speed to maintain the frequency of the network constant. Now, when one switches on an electrical appliance such as an air conditioner, we are connecting a load to the network. The immediate reaction of the generators is to tend to slow down - the keyword here is tend since, under normal conditions, the generators do not actually slow down. All generators are provided with governors. These are devices that sense the tendency of the speed (or frequency) of the generators to slow down and take corrective action by increasing the amount of steam, gas, water, etc. (termed prime mover input) so as to maintain the frequency of the generators constant.
Thus, under normal conditions, in an electric network, there is always a balance between the amount of power produced and the amount of load connected to the system. This implies that the power generating capacity of a network (also called installed capacity in Megawatts or MW) is at all times more than the expected peak load demand. According to World Bank, the norm is that every electric utility should have an installed capacity that is 30 per cent more than the expected peak demand. While this is a laudable target, due to financial and other constraints, most utilities may not be able to maintain such a reserve of power. The basic idea behind having such a reserve of power is to cover eventualities when a generator or generators are not available due to some reason (fault or planned maintenance).
The picture changes dramatically when the available generating capacity is just about sufficient to meet the peak demand. Consider Could power cuts be a feature of the past?
a utility where there is no power reserve. There is still a balance between the generated power and the connected load. However, should the electronic protective equipment used for sensing and isolating faults detect a fault on a line or a transformer or a generator, the corresponding equipment will have to be switched off to prevent damage.
If the fault is on a generator, it will have to be switched off as well. This means that the power available on the network is lower than the connected load. All the generators on the network start to slow down - all this happens within the space of a few milliseconds. However, generators are not rated to operate at below their rated frequency. In other words, actions must be initiated to quickly disconnect some of the connected load so that the governors of the remaining generators can arrest the decrease in their speed and bring them back to their rated speed. This can only be done if we disconnect load with an equivalent power demand that is more than the deficit between generated power and connected load. This action is called load shedding and is initiated by electronic devices that measure the frequency of the power.
This shortage of power could also be caused in parts of the... network by the automatic switching off of critically loaded lines or transformers leading to the same phenomenon as described above.
However, if such preventive action is not sufficiently quick or proactive in its response, the frequency of the network slows down further causing more generators to be switched off (or tripped). This further exacerbates the problem with the ultimate result that the complete electric network collapses. In other words, we experience a power blackout. Since Generating Plants are fairly complex and involve rotating machines with critical and sensitive parts, the restoration process is usually time consuming as our readers may have very well noticed last Monday.
What we have described above is something that occurs due to the laws of electrical science. Now, the question is can something be done about it?
It appears technology has a panacea to eliminate power blackouts or at least mitigate their effects. In a recent article published by the Massachusetts Institute of Technology (MIT) on ‘’Ten Emerging Technologies that will change our world’’, one of the technologies that is highlighted is Advanced Power Grid Control. This technology, through Wide Area Monitoring, Protection & Control, essentially protects a complete network from collapse. Current monitoring systems can identify major disruptions, but in many cases are not accurate or timely enough to provide grid operators with the information they need to prevent blackouts before they occur. They are also hampered by the fact that readings taken across a large area cannot be synchronised to the necessary level of accuracy to be used in a predictive diagnosis capacity.
Hitherto, all measures employed to prevent power blackouts were realised at different local areas - these include localised load shedding, voltage control, judicious manual switching, etc without network-wide co-ordination. Also, when faults developed on lines, transformers, generators, etc., only the respective electronic protective devices operated to clear the fault and disconnect the affected element. There was no co-ordination to realise a network-wide Protection System. It is precisely this gap that Wide Area Monitoring (WAM) / Wide Area Protection & Control (WAP & C) attempts to fill.
What is WAM / WAP & C? We will attempt to give you a brief description here. WAM / WAP & C provides system-wide Monitoring, Protection & Control and comprises the following four main components: Phasor Measurement Units (or PMU’s); GPS Time Synchronisation; High Speed Communications; Software Algorithms.
PMU’s are devices that measure the currents and voltage phasors with very high accuracy. They are placed at critical points in a network.
GPS Time Synchronisation: For a network-wide Protection System, it is imperative that the current and voltage phasors measured by the PMU’s are properly time stamped to ensure that, when performing calculations, the phasors that correspond to the correct instant in time are used. This high accuracy time stamping (with errors less than 1 microsecond) is achieved by using time synchronisation signals derived from a Global Positioning System (GPS). The readings from the various PMU’s in a network can then be used to paint an extremely precise picture of grid conditions at any given moment in time.
High Speed Communications: Since PMU’s generate up to a 100 readings per second, a High Speed Communication network is required to transfer the data measured to a System Protection Centre (SPC). Most utilities have already begun to introduce fibre optic cables and other high-speed communications technologies in their networks. WAP uses these systems to deliver high volumes of sensor data back to the SPC in real time.
Software Algorithms: Recent advances in computing performance have enabled development of a number of completely new mathematical algorithms to analyze the flood of incoming data. The software assesses grid conditions (e.g., voltage stability, frequency stability, line temperature, power oscillations, etc.), identifies potential problems, and can even suggest corrective actions. The actual control of the physical equipment on the grid is still handled through the SCADA/EMS, but the actions that operators take can now be guided by the greatly improved information they receive from WAM / WAP & C.
At the SPC, calculations are continuously made to ascertain the power margin i.e., the power reserve that a network has before the onset of system collapse.
As with most things in life, the crux of the problem is timing: if the system is too sensitive or operates earlier than desired, it will disconnect more loads than is necessary and if operates late, well, we all know what happens then. The fine balance required is realised through the highly advanced algorithms employed in WAM / WAP & C.
The main goals of a WAM / WAP & C are to: Detect and warn of potential instabilities due to abnormal voltage, frequency or thermal conditions; Start effective countermeasures in time; Prevent or eliminate cascade tripping due to domino effect; Avoid blackouts or limit and contain failures
WAM / WAP & C achieves these goals by converting the data collected into useful information for grid operators to enable them to: Island networks to limit and contain network collapses; Disconnect right amount of loads early enough; Automatically control generated power; control System voltage through; transformer tap changes and/or static VAR compensators; and control system power flow.
