Blackouts: A cascade of problems and ways to mend it

The scope for socio-economic development, industrialisation and poverty alleviation in the country's power sector is immense. 

On 4 October, the national power grid failure triggered a seven-to-eight-hour-long blackout across most of the country, starting at 2:04 pm. The biggest incident of national power grid failure (cascade tripping), prior to this, happened on 1 November 2014 - when the entire country plunged into darkness and was left without power for 17 hours. 

Let's take a look into what happens at the grid level and beyond that result in nationwide power outages. 

A transmission line is a key way to transmit generated power from one place to another. The Power Grid Company of Bangladesh (PGCB) reported around 71 incidents of power interruptions due to problems in the transmission lines from FY2016-17 to FY2020-21. 

As of September 2022, the PGCB has a 400kV line of around 1,397 circuit km, a 230kV line of around 4,022 circuit km, and a 132kV line of around 8,190 circuit km, as well as another line of 187.57 circuit km. 

But the substation capacity has not increased substantially. 

The generation, transmission and distribution of electricity are regulated by The National Load Dispatch Centre (NLDC). The NLDC's job is mostly done over phone calls from the power stations and distributors, the latter of which is a higher risk in power synchronisation. 

A cascade of problems: The causes behind power failures 

Cascade tripping refers to the tripping of an unbalanced power grid or the tripping of safety mechanisms and isolating a portion of the system to guard against equipment damage. When there is a frequency imbalance or another unbalanced circumstance, cascade tripping happens. 

The frequency of the machine decreases as demand exceeds available power. The grid will trip if this frequency falls below a predetermined threshold. 

There will be an abrupt loss of electricity as a result. Such circumstances will also arise if demand is lower than supply. This is sometimes known as a grid breakdown or a blackout. The grid's synchronicity needs to be kept up to prevent this. 

Following a short circuit or other disturbance, one group of generators may accelerate relative to another group, resulting in instability and loss of synchronicity. Synchronous generators, located thousands of kilometres apart, must operate steadily and in synchronicity under an infinite number of load and power transfer conditions, equipment outages and power disturbances. 

In an electrical system, a cascade is a dynamic, unplanned series of events that, once started, no amount of human intervention can stop. Transmission lines, generators and automatic load shedding are sequentially tripped by power swing voltage changes in an expanding geographic area. As the cascade expands, the fluctuations become less intense until equilibrium is reached and the cascade comes to an end.

Blackouts have enormous effects on the electrical power sector, as is obvious. The limitations of the grid are being reached in some places.

Many of the global grids are intensively utilised and running close to capacity. The grid becomes unstable and susceptible to system-wide disturbances like cascade tripping or blackouts when abrupt bulk transmission happens.

India's Northern Region Grid had a significant disturbance on 2 January 2001. 

The entire region went dark and over 1,500 MW of generation were lost, causing a significant hardship for the general public and a loss of 700 crore rupees. The system's limp return to a state that is close to normal took 16 to 20 hours. The last major grid disturbance, prior to 2001, in the region was in January 1997.

The role of 'frequency'

Frequency is crucial to a grid's stability. There must be split cycle synchronisation between generators located thousands of miles apart, and there must be coordination between thousands of transmission lines carrying power across vast swaths of the nation. Supplies must be coordinated, but so must supply and demand, and they must be coordinated everywhere.

Our power system frequency varies routinely on a normal day between 48.9 and 51.2 Hz, and can go as low as 48.7 Hz and as high as 51.5 Hz. This is a serious obstacle to system reliability and also causes severe economic loss.

There is no practice in Bangladesh to hold regulating reserve and frequency control participation from any of the generators in the system, with the governors largely remaining inactive even during major supply and demand imbalances. As a result, the system frequency of Bangladesh's power system is very unstable (50±1.2 Hz), which makes it insecure and unreliable. 

There have been grid failures, including a major countrywide blackout event on 1 November 2014. This latter incident provoked swift responses that led to the quick activation of governors for several generating units for the main response in order to stop system frequency variation within a range of 50–0.5 Hz (free governor mode of operation or FGMO).

The ultimate goal of these measures is to stabilise system frequency in a tighter range by implementing all levels of control (i.e. AGC) in the future. Maintaining a frequency higher than 50 Hz, often close to 51 Hz, using expensive oil-based generation has also been part of the poor practice. 

Oil generation is kept on in anticipation of load picking and has been costing the system several hundred million dollars per year. The absence of a frequency control ancillary services allocation mechanism is a problem in most developing countries.

Power Grid India, which continues to rely on largely improvised mechanisms for frequency control ancillary services, took note of this. Both extremes under and over-frequency issues require a fresh start as straightforward as FGMO. To determine the efficacy of these techniques, an eight-hour trial of frequency regulation with 10 power plants was performed on 6 August 2016. 

The National Load Dispatch Centre in Dhaka served as the test's location and primary supervisor. 

Other developing nations that deal with a comparable set of difficulties will find a lot of the findings and lessons fascinating and pertinent. The frequency of the system is directly related to the network's actual power balance. When the system generator is running normally, it operates synchronously and generates enough energy, including actual transmission losses to supply all load needs simultaneously.

The synchronous operation of the generator represents a stable system state. Accordingly, once a generator is synchronised with the network, electromechanical forces start to build up inside the device and tend to keep it running at the same speed as the rest of the network. You can control how much power the generator generates by altering the torque from its prime speed after the generator's speed has been synced with the rest of the system. 

To achieve this equilibrium, each generator's speed and frequency must be precisely controlled. Therefore, the optimum way to operate the system would be to direct the machine operator to modify each generator's water gates and steam valves to a setting that exactly matches the real power balance while maintaining a consistent speed and frequency. 

The reality, sadly, is not quite as neat and tidy.

The consequences of unstable system frequency 

The entire power system is severely harmed by unstable system frequency (from the generator to the consumer level). A notable adverse effect is the hampering of system stability and security. 

In Bangladesh, frequency relays for the first stages are set as low as 49.0 Hz due to the country's somewhat unstable system frequency. A system blackout may result from a big plant trip or an interconnection, as was the case in November 2014.

For power plants, off frequency increases vibration, causes overheating and damages turbine blades and shafts. Under-frequency affects other consumer equipment and lowers motor efficiency. The addition of major producing units (500 MW or more) and nuclear power reactors to the grid is almost unachievable with such frequency volatility. 

It is extremely challenging to integrate renewable energy into the electrical grid. The Bangladeshi power grid must address both of these concerns as it begins construction on a nuclear power plant and connects large-scale solar and wind energy sources. Private power producers (IPPs), it seems, are unwilling to invest in such an unstable system.

Higher rotation and more energy loss are implied by high frequency. In Bangladesh, system frequency tends to stay higher than desired, which results in significant energy loss. According to calculations, 2,300 to 3,200 MW of energy are lost every day owing to greater system frequency, costing roughly Tk1-1.5 crore per day.

Droop for power plants is typically defined in the range of 2-5% in the majority of power systems. A 5% frequency deviation results in a 100% change in valve position or power output, according to a 5% droop or regulation. The ramp rate (MW/min) for loading and unloading is used to specify how a plant's output changes over time. 

Initially, 250–320 MW of spinning reserve from 10 generating units with a total generation capacity of 1,600–1,800 MW (about 14% of capacity) were targeted for the primary response to test FGMO and assess its effects on system frequency. 

A total of eight hours of testing (from 9:00 to 17:00) were completed, and various metrics were logged in the SCADA archive. The system's operating range throughout the test was between 6,500 and 7,100 MW, with a systematic bias of 22–25 MW/0.1 Hz.

To stabilise frequency variation, the total capacity of the chosen plants was kept at roughly 1,400 MW out of 1,867 MW during the FGMO test. The spinning reserve ranged from 250 to 500 MW. 

For the first time, the FGMO trial proved beyond a reasonable doubt that Bangladesh's power system's frequency can be kept within the 50–0.5 Hz band with roughly 350–400 MW of spinning reserve for a contingency size under 300 MW.

The ways forward

To run the system economically and prevent a blackout, a particular protective mechanism (such as a contingency-based load shed, sub-frequency relays, etc.) must be in place for contingencies greater than 300 MW. 

Additionally, it will guarantee that users will receive quality power.

Although there are now some contractual and technological restrictions to using FGMO for all producing units, these can (and are being) overcome with just modest outlays of money. Out of 103 power plants, governors of 15-20 units can be turned on to conserve a significant quantity of energy, which might cost up to $500 million annually. 

The system might save more than $1 billion a year if some of the other transmission restrictions and dispatch inefficiencies were eliminated, making these actions some of the most cost-effective ones in the industry. 

Bangladesh may make significantly greater investments in interconnectivity and large-scale variable renewable energy sources by resolving the frequency issue. 

Smart grid technology is necessary to overcome the cascade of grid failure and to ensure sustainable socio-economic development.