All you need to know about Power Grid Breakdown.

In Pakistan, we observe quite frequent power outages either partial or major blackouts and these outages mostly occur in winter season and there is a reason behind this which will be discussed below.

After reading this simplified article, you will get to know about;

How does the Power Grid work
Reasons behind Power Grid Outage or blackout
What is a Black Start and why does it take too much time to restore power grid
Why do we have frequent power cuts even after power grid is restored

How does the Power Grid work?

A power grid is a system of generating stations, transmission lines, and distribution lines that are interconnected to supply electricity to consumers. It is a network of power plants, transmission lines, and distribution lines that are connected to deliver electricity to consumers. Power grids vary in size, with some serving only a single community or region, and others spanning entire country.

In simple words, a number of power plants located at different locations are interconnected through high voltage transmission lines. These power plants include Hydro Power Plants, Coal Fired Power Plants, Nuclear Power Plants, Wind Power Plants, Solar Farms etc.

These power plants are interconnected to make the Largest Machine called Power Grid. The high voltage transmission lines are then connected to substations from where the electric power is distributed to different types of loads like industrial loads and domestic loads.

In Pakistan, main power plants are located at northern and southern regions forming Pakistan national power grid. In northern region, we have mostly hydropower plants. Examples are Ghazi Barotha Hydropower, Tarbela, Neelum Jhelum Hydropower Plant and Mangla power plants. In southern region, we have Thermal Power Plants e.g. Guddu Thermal Power Plant in Sindh province.

Note: Power plants are operated according to power demand in real time.

Power Grid Outage

An ideal power grid is like an indefinitely running machine where no electrical and mechanical faults occur but on the other hand, faults in a real power grid are inevitable but the effect of any fault can be minimized.

Some of the main reasons that cause a power grid outage are;

Frequency Deviations
Severe Electrical Faults
Human Error
Intentional Attacks on Power System

In case of any severe fault in power grid, the fault location/point/faulty equipment must be isolated by the automatic circuit breakers in no time.

Blackout due to Frequency Deviations

In a power grid, frequency is kept constant by balancing Power Demand and Power Generation in real time. In Pakistan, NPCC (National Power Control Center) under NTDC is responsible for frequency and overall power control.

Since Three-Phase Synchronous Generators are used for power generation in power plants, the frequency is greatly affected by actual rotor speed of the generator.

where;

n_s is synchronous or actual speed of rotor (rotating part of generator) and P is no. of poles.

From above relation, we can see that frequency output from generator is directly related to speed of rotation of rotor.

Additionally, the more we apply electrical load on generator the more force (Torque) will be required to spin the rotor or the more it will be harder to rotate the rotor. Same is the case for short circuited output terminals. Now, due to direct relation of frequency with rotor speed, the frequency will also decrease in case of a short circuit or overload because in these conditions the rotor slows down.

What low frequency has to do with Power Grid outage?

Before going deep into this concept, we need to understand how Power Demand and Power Generation are balanced. As mentioned above, in Pakistan NPCC is responsible for balancing power demand and power generation in real time.

During peak hours when power demand is maximum, more power plants are needed to be connected/synchronized with the grid. If this is not done, the frequency would drop below 50 Hertz. If no power plants are available, then Load-Shedding is done to keep the frequency fix at 50 cycles/sec.

Now during off peak hours, particularly during night time when less number of consumers are using electricity, the frequency will gradually start to increase above 50 Hertz (because at light loads especially in winters, the rotor speeds up) which is also not desirable therefore, some of the power plants are taken out of the power grid. This way frequency remains fix at 50 Hertz.

Also, in winters when demand is less, some power plants are disconnected from grid for maintenance.

As mentioned above, the power demand and generation must be balanced to keep the grid frequency constant at 50 Hz.

Since load demand is unpredictable, what if a heavy load is suddenly connected to the power grid during off peak hours when some of the power plants are already out of the system for maintenance or frequency balancing?

As power plants take time from cold start to start producing electricity. Therefore, in this scenario. the grid frequency will drop down below threshold frequency and Under-Frequency relays will start to pick up, triggering respective circuit breakers and isolating their respective generators. More and more power plants will be disconnected from grid leaving an unbearable load on other remaining plants and this cascade tripping continues until total grid failure.

A heavy load can be shed but cascade tripping occurs in seconds and requires fast response.

Operating System Frequency Criteria (NTDC Grid Code)

NTDC (National Transmission and Dispatch Company) the system operator, coordinates with all generating stations to keep frequency fix at 50 Hz with the following allowable excursions.

Declared or Target System frequency must be 50 Hz.

Frequency Sensitive Mode shall be 49.8 Hz and 50.2 Hz such variation is permissible to allow frequency variations while ramping up generation and load pick-up.

Tolerance Frequency Band protected periods of operation of the system at the frequency in the range of 49.5 Hz and 50.5 Hz

Minimum/Maximum Acceptable Frequency Band shall be 49.4 Hz and 50.5 Hz

If frequency fall below 49.4 Hz, Under Frequency Relay operates.
If frequency goes above 50.5 Hz, Over Frequency Operates

The power grid breakdown in Pakistan on 23rd January, 2023 was due to low generation than load demand. The power generation was less than 7000 Mega Watts while power shortfall was nearly 6000 Mega Watts according to reports.

The reason behind low power generation is more than 12 power plants including Neelum Jhelum, power plants in Jamshoro, Guddu and Muzafargarh are not in service. At 07:34 AM, Pakistan was without electric power from Sindh to KPK.

Note: Some of base load power plants can be isolated to keep the system alive by taking timely action.

Also, the Over Frequency relays are used in generating stations to keep an eye on over frequency due to many reasons like sudden loss of heavy load can increase system frequency.

Power Grid Breakdown due to Severe Short Circuited Faults

Short circuits in transmission lines also decrease frequency of grid just like overloads. Extra High Voltage overhead transmission lines i.e. 220 kV, 500 kV carry bulk power from generating stations located at less populated areas to distribution substations.

As mentioned above, these transmission lines extract bulk power from all the power plants that are interconnected and synchronized with each other. Electrical faults in these transmission lines, if not isolated, can cause whole power grid to shutdown.
The causes of electrical faults/short circuit in transmission lines are:

Smog in winters can cause conduction through insulator string or moist air surrounding bare ACSR conductors

Direct Lightning Strokes on overhead conductors

Conduction through a tree or any other external object due to increase in line Sag

Physical damage to conductor

Blackout due to Human Error

Power grid failures are also caused either by negligence of humans or due to unintentional human errors. Below is an example of human negligence according to official NEPRA report.

''On 09 January, 2021 at 23:40 hours, on closing of a 220kV Circuit Breaker at Guddu switchyard, bolted fault occurred as the earth switch of related Isolator was left closed. This bolted fault was not cleared due to malfunctioning of the relevant Circuit Breaker and the subsequent breaker failure scheme. The above persistent fault reflected on 220kV and 500kV transmission lines which tripped on Distance Relays in zone 2 and Unstable Power Swing respectively.''

Power Grid Breakdown due to intentional attacks

Apart from equipment failures, blackouts also occur due to intentional attacks on power equipment like transmission lines and blowing up pylons. Such attempts were made by terrorists in Baluchistan province blowing up two 220 kV pylons.

What is a Black Start and why does it take too much time to restore power grid?

It is a complex task to bring power grid back online after a complete breakdown and one just cannot flip a switch and restore power. Because the machine that has just failed is the largest machine (The Power Grid) that was supposed to run indefinitely. The reason is, in many power plants, some electric power is required to produce electricity (about 5-10% of power is consumed by power plant itself). For example, in coal fired power plants, coal handling plant, compressors for ash handling plant and pneumatic instruments connected with boiler and so many pumps including feed water pumps and lubrication pumps. All these systems are necessary for a coal fired power plant to generate electric power smoothly. Electricity is also required for generators for excitation of their magnetic field.

For instance, using standby diesel generators for black start is not feasible because even 5% of power that power plants produce is huge. For example, a 1000 MW power plant 50-100 MW standby generator is required

Using Hydropower for Black Start

Hydropower plants called Base Load Plant do not need much of electricity to generate electric power. All they require is a separate DC generator for excitation of magnetic field. So the power from such plants is used for other power plants to build up.

Note: There are synchronous generators that use inbuilt pilot exciter which does not require external DC generator source.

The Synchronization Process

When a hydropower plants starts generating power, it is then transmitted to other power plants through transmission lines.

Let's say we have four different power plants in our grid at different locations:

1000 MW Power plant at Location 1: There is a hydropower plant (Running & isolated)

600 MW Power plant at Location 2: There is a coal-fired power plant (Dead & isolated)

800 MW Power plant at Location 3: There is another thermal power plant (Dead & isolated)

1500 MW Power Plant at Location 4: There is another hydropower plant (Running & isolated)

Now, above power plants at four different locations are to be connected through long transmission lines (same transmission lines that were in use to extract power from these stations) while all the substations are also isolated from the main transmission line because during this process we are not yet ready to supply power to load centers or cities.

Next step is to power different equipment at coal power plant at location 2 and location 3 using power coming either from Location 1 (if enough) or location 4.

Once auxiliary equipment at those thermal power plants at location 2 and 3 are powered up, they are ready to produce electric power but this step takes time.

Now let's say it's been hours and all the power plants are running alright but still isolated

The next step is to synchronize all the power plants with each other one after another for which all the power plants must meet these conditions:

Frequency must be same

Phase sequence must be same

Voltages must be same

For instance, to synchronize plant 1 and plant 2, all the synchronization conditions are fulfilled first under supervision of concerned departments (NTDC & NPCC in Pakistan) and then they are connected to each other. Now we have 1600 Mega Watts of power in our grid. Similarly, when plant 1 and plant 2 are interconnected, it's time for next power plants to be connected in parallel.

In above example of four different power plants, when all of them are interconnected in parallel, the total grid power will be 3900 Mega Watts (in reality, more power plants are interconnected).

Now hours after a complete breakdown, it's time to provide power to cities and other loads. This is done by connecting those isolated distribution substations gradually because there are big cities waiting for electricity to turn on their power hungry machines e.g. water pumps, heating devices etc. In simple words, the amount of power that used to be enough in normal days would not be enough after a complete power breakdown that lasted for hours.

Why do we observe frequent power cuts even after the power grid is restored?

The reason behind this is Forced Load-Shedding which is done in order to gradually supply power to the areas that are not yet energized. This is done until power demand normalizes.

Also note that it takes days for a power grid to become stable.

What happens during a Power System Breakdown?