Everything you always wanted to know about power systems but were too afraid to ask
Part 1 of Malcolm Metcalfe's Power System Primer
There are two distinctly different methods used to balance supply and demand. These are:
- Balancing supply/demand in an isolated system (one that is not interconnected with the larger grid. Examples are local systems to power a remote location).
- Balancing supply/demand in an interconnected system, where a utility is a part of a major interconnection of many utilities.
Case 1 – Isolated Systems
Case 1 was used when utilities were generally not interconnected and continues to be used in isolated communities where small power plants are implemented to meet local needs or to power mines or raw material production in isolated parts of the world. There has also been a resurgence of isolating grids that the industry has termed “microgrids,” for which case 1 applies when the microgrids are isolated.
Generators that provide electricity are generally known as “synchronous generators.” A synchronous generator will create a specific number of electrical cycles per revolution. This is a function of the number of electrical poles on the generator rotor. The figure shown here is a drawing of a “two-pole” generator. The two poles are on the rotor (one at each end of an electromagnet) that rotates within the bounds of the stator, which is fixed.
The rotor poles make up a single electromagnet that has a north and south pole at each end of the rotor. A single rotation of the rotor will create ONE electrical cycle. Every generator has an even number of poles. For every north pole magnet, there is a south pole magnet. To use this two-pole generator for 60 Hz power would require that it turn 60 times each second – or 3,600 RPM. Many of the steam turbine generators that are nuclear or coal-powered, operate at 3,600 RPM.
The photo shown here is the rotor from the Hoover Dam. There appear to be approximately 50 poles on the rotor, meaning that the generator would create 25 cycles for each rotation. To create 60 cycles, the generator would need to rotate 2.4 times every second (2.4 x 25 = 60 cycles). This translates to 2.4 x 60 rotations = 144 RPM.
The operating speed of a 60 Hz generator can be calculated simply, using the formula:
Generators can be connected in parallel, but they MUST turn at a proper RPM in order to all generate the exact same frequency. The voltage on all connected generators will be synchronized, and if any angular difference occurs between any generators, power will flow to the lagging machine — pulling it into sync.
In this way, there can be any number of generators connected to a common system, and they will all be essentially “locked” in sync. If one speeds up, the others all follow. The speeds will be locked together based on the number of poles on each machine. A two-pole generator will turn at 3,600 RPM, and a 72 pole generator will turn at 100 RPM — and they will be locked together like gears in a gear chain.
When generators are connected to a load, the generators deliver power, and if the supply and demand are not exactly equal, the entire system will speed up or slow down. The system can be balanced simply by managing the generator or the load capacity.
Generators are equipped with a governor. The governor is a simple proportional controller with a few stabilizing systems. The governor will have a speed setting, and a proportional gain setting, that in “utility lingo” is called speed droop. Most generators in North America are set to operate at 5% droop, meaning that a 5% change in speed will result in a 100% change in capacity. A generator that is running at “speed no load” and 60 Hz will increase its power output to 100% when the frequency falls to (60 Hz - 5%) 57 Hz.
The droop setting can typically be adjusted. Where a system has only one generator, the operators may define the droop setting to be zero (or very close to zero). This results in the system maintaining a relatively constant frequency, regardless of the load. The governor will respond to any error, with a continuous increase or decrease in power output to fully restore system frequency.
Where multiple generators are connected together, only ONE can operate at zero droop, as the system would be unstable if two or more generators were operated at zero droop. One generator may measure the frequency as 60.0 Hz, while another might measure it at 60.01 Hz. The machine targeting 60.01 Hz would slowly ramp down, and the one set to run at 60.0 would ramp up until one of the two reached an upper or lower limit.
Isolated utilities often run one generator at zero droop, and the generator is said to be on speed control. Operators monitor the capacity of the speed control generator, and when it reaches a high or low limit, other generators are adjusted, resulting in the speed control generator returning to mid range, where it has ample regulation capacity in both up and down directions.
As the systems became larger, this method becomes more difficult to administer. A large system would experience proportionally larger changes. A single generator may not be able to deliver enough capacity to be effective.
As a bridge between Case 1 and Case 2, New Zealand established a related process that was effective for larger systems. Instead of using zero droop, they set several generators at 1-2% droop, with the remainder operating at 5% droop. This meant that the frequency would deviate slightly, but changes would be equally shared among the lower droop machines, known as “frequency keepers.” Time error is then monitored and managed by setting the speed of the frequency keepers.
Case 2 – Interconnected Systems
The use of low or zero droop generators works well to manage the supply/demand balance in isolated systems, but where many utilities are interconnected, the system has several significant disadvantages:
- A zero-droop generator would need to be extremely large in order to meet the flexibility needs of an entire interconnection. There can only be one zero droop generator on the grid.
- Utilities want to maintain their own load and not have an uncontrolled amount of power flowing in or out of their grid. The zero-droop system would be almost impossible to dispatch effectively.
The principal goal is to establish an effective means of measuring the difference between supply and demand for each utility or area in an interconnected system. It turns out that this may be easier than most people expect.
This figure shows a single utility connected to a major grid. The utility is generating 850 MW, but the total load, including all losses, is 870 MW. The utility is importing 20 MW from its interconnection(s) to make up the difference between supply and demand. The import is not controlled; it is simply an EXACT means of measuring the difference between the supply inside the utility and the demand. If the generation in the utility is increased or decreased, the capacity measured on the interconnection tie line(s) will increase or decrease, tracking any generation or load changes in the utility. In cases where a utility has many tie lines to other utilities, the sum of the tie-line capacities, known as the NET INTERCHANGE (NI) is the difference between the supply and demand within the utility. This is a key value that makes balancing a relatively simple task to complete.
Wrapping It Up
In summary, there are two distinct methods used to provide balance between supply and demand. Where the system is isolated and relatively small, a single generator set to operate at zero droop can be very effective, but when the system is synchronously interconnected with a large grid, the balance is controlled simply by managing the net interchange. The synchronous connection refers to an AC connection between the systems. With the invention and roll out of microgrids, utilities must ensure the speed droops are updated and relevant for the current configuration. It is also important to note that a system that is interconnected only with DC does not follow the same rules because the flow on the interconnection is controlled directly by the DC system.
The automatic generation control (AGC) system manages the supply and demand system but also provides support for the system frequency. That system will be explained in another article.