5.1 Introduction

Electricity grids are quite complex and can be very large. A grid is simply a collection of generators, transmission lines, switchgear, distribution systems and cables and all the necessary control, protection and monitoring required to run it safely, efficiently and reliably.

However we can break a grid down into 4 basic components.

• Generators - These supply the electrical power
• Transmission - This is the tower structures and lines to link generators.
• Distribution - This is the poles and wires that take power from the transmission system to the end customer
• Load - The consumer of the electrical power.

5.2 Generators

These are traditional rotating machines that generate electricity but with the adoption of renewable energy these devices can be static such as an inverter.

All generators need a fuel. Common types of fuel are

• Coal - Used on large generators. In Australia the machines are typically from 230MW up to 750MW per unit with a power station having sometimes multiple units. Eraring Power Station in NSW for example has 4 units. Each unit is about 720MW.
• Natural Gas - There are generally two type; Large frame units and aero derivative. These can be from 20MW up to several hundreds of MW per unit.
• Hydro electrical - Potential energy is stored in the form of water at a height using gravity. Australia is a relatively flat country so there are limited locations for hydro electric. Tasmania has a large supply of hydro electric units and so too does the Snowy Hydro system.
• Wind - Kinetic energy in the form of wind is used. Turbines are found in the low MW range.
• Solar - Photovoltaic energy in the form of solar is used. Modules are installed in strings which form arrays in a solar farm. Sizes well into the hundreds of MWs are common.
• Geothermal - Heat energy from the Earth is used by exploiting deep wells to access geo thermal energy.
• Chemical - Strictly speaking batteries are not a fuel generation device. They store energy so would not be considered in this field.
• There are others like biofuels, diesel, oil, waste to energy etc

At the grid level, the generator is required to provide a stable source of power and dispatchable.

Generators have strict requirements to ensure grid stability. Too much power and the frequency goes up. Too little power and the frequency goes down. Too much excitation and the voltage will be out of range as well as too little excitation.

5.3 Transmission

The transmission system takes generation and transfers this power around the grid ready for tap off by distribution systems. Transmission systems are large and complex and carry significant power. They can be seen as large towers crossing the country. Generators will often have a terminal station attached to it. This is where the voltage of a generator is stepped up to transmission voltage levels (usually 132kV and above) and then switched to outgoing lines or feeders.

These feeders will interconnect to other parts of the grid often forming a mesh network so that there are multiple paths to load centres far away or interconnections to other terminal stations or substations.

Transmission systems are costly and are vulnerable to weather events such as wind and lightning and occasionally birds like parrots. Often such transmission lines will carry an earth conductor at the top of the tower (or multiple earth conductors) to shield the power conductors below. These earth conductors may have embedded fibre optic cores within them as an effective way of spanning hundreds of kilometers to send data over fibre. Since fibre optic is immune to electrical interference, this practise is common nowadays.

5.4 Distribution

The distribution system takes power from the grid and drops the voltage down to reticulate around cities and towns for eventual end consumer connection. A substation where there is at least 1 transformer installed and this will take the transmission voltage from the transmission line and step it down to a distribution voltage level typically 11kV and up to 132kV but most commonly 11kV, 22kV, 33kV and 66kV. 11kV is commonly seen in residential areas.

At this point the transmission authority ends and the distribution authority will take over and provide the poles and wires to send the distribution system close to the end load. Then a final customer connection is achieved by typically dropping the voltage again by a local transformer from the distribution voltage down to the customer voltage. In Australia this is 230V/400V (but normally operated at 240V/415V).

Distribution systems are costly due to the vast quantity of infrastructure needed and are vulnerable to all sorts of hazards like weather, cars crashing into poles, ageing poles, cables dug up, fauna and flora attack, bushfires etc.

5.5 Consumer Connection

The distribution system will generally pass out the front of a business or premise and a cable is connected to tap off the line. This tap is wired to a point of connection and then to the customer's distribution or power box. The supply is metered and fused and then available for an electrician to wire into the premise as required.

5.6 Dispatch

The grid is managed by an overall agent or in some countries, the government controls it. Generators are often considered as generating entities and are just another business that sells bulk electricity.

The grid manager or operator has the role to dispatch generators to match the load in the system. In a commercial market like that operated in the eastern part of Australia, there are multiple generators that will bid in the market.

The grid operator will through a number of tools, determine the future grid demand in the short and medium term (minutes to hours) so that generators may bid for supplying the power required. Prices are usually set in $ per megawatt hour and short term price fluctuations can occur and will benefit smaller and nimble generators. Dispatch is also in effect a type of contract that the generator must then comply with otherwise there would be penalties to failing to meet the contract whether it be too little or too much. The grid is dynamic so rapid and sudden changes can and do occur. Significant disturbances can cause sudden instability that needs to be corrected quickly (sometimes less than 1 second). For example a large load like a smelter that is taking 1000MW has its main feed line trip off. The grid at the point of disconnection will have 1000MW of excess generation. Due to electrical theory the grid doesn't absorb this 1000MW instantaneously. What happens is this excess power is not converted but remains in the mechanical system and as a result will speed up a turbine until this is arrested. Typically a large steam generator has very fast turbine control valves that can respond in much less than 1 second and back off the energy. Aero derivative turbines are slower because there is no fast valve like a steam control valve. The fuel is already in the engine either combusting or about to be combusted so these take a little longer. Static converters like inverters can respond extremely quickly and often the fastest responders.

If the opposite happens and a large generator trips off the grid, then the opposite happens. Lets say we have a 700MW generator that trips. We lose 700MW and by the opposite process, the load on the grid will pull down the grid frequency quickly. Generators need to respond quickly to arrest this. The problem here is that often getting the 700MW is harder because you need more fuel to respond to this which is a far slower process. In this case, large turbines can open up their control valves very quickly and respond but the additional demand on the boiler will limit the amount of energy (time). Static converters can supply this extra power very quickly if the energy source is there. This is like firming up a grid. In the older days, many spinning machines would have some energy stored in their inertial rotation and there would have been many generators to provide this firming. Secondly the grid operator would not allow new generators onto the grid that exceeded a certain size as the loss of one very large machine would risk losing the whole grid. There are other ways to arrest failing frequency such as load shedding. This is turning off certain, large customers to protect the grid.

More to come!