What is transient voltage dip (TVD) on an AC alternator? Why is it important? how can you reduce it?

What is transient voltage dip (TVD) on an AC alternator?

Transient voltage dip (TVD) is a measurement of the voltage drop when electrical load is applied to an AC alternator. When applying electrical load to an AC alternator, a voltage drop will always occur. The magnitude of this voltage drop depends on how big the electrical load applied is and how quickly the AVR and excitation system can react to correct it back to the pre-set value.

Why is it important to know about TVD?

TVD is an often overlooked consideration when selecting the correct diesel generator. When working with mains electricity, unless the load is very large the effect of load application on the voltage will be small, because the mains has almost unlimited power to bring the voltage back to the system level.

With a diesel generator, the size of the TVD and its ability to bring the voltage back to the pre-set level is determined by the size of the alternator and the alternators AVR and excitation system.

If the voltage falls too much, equipment can cut out or even be damaged by the voltage drop. being aware of TVD allows you to consider it in the application when sizing a diesel generator and take action to prevent it being a problem.

How big typically is the TVD?

Here is an example based on a 100kVA diesel generator with a Stamford UCI274C1 alternator running at 50hz. The below chart shows the relationship between the applied load and expected voltage dip of the alternator a the main terminals.

This alternator uses an SX series regulator (which is used for shunt / self-excited excitation systems).

As you can see for this alternator, if you apply 100kVA of load, you would expect to see a 14% voltage drop. Normally a 30% voltage drop is considered acceptable - but many modern electronics will not cope with a drop of such magnitude. 

An additional voltage drop will apply if the engine speed is also reduced because of a large real power (kW) element being applied. Read more about a diesel generators load.

How can you reduce the TVD?

There are three main ways of reducing TVD. 

1) Reduce the load

This is not always possible - but there are a few ways you can do it. You could split the load into two or more different sections and apply them separately with a time delay. For example, lights and sockets in one section and air-conditioning in another a few seconds later. This can be expensive as it might require some additional wiring and switch gear to achieve the desired outcome.

If you have induction motors you can fit star delta starters or soft starters to them. You may also be able to time these to start after a short delay from the rest of the loads in the system.

You can just turn things off before starting the generator that use a lot of power - you might not need them during a power cut, or you can turn them on gradually after the event.

2) Use a more advanced excitation system

Generators can come with different excitation systems - typically these are shunt/self excited, auxiliary wound or PMG systems. Because auxiliary wound systems have a second winding embedded in the main stator, they can't be retrofitted, but often PMG kits are available to retrofit to the alternator.

Below is the same 100kVA UCI274C1 alternator as the above example, this time fitted with a PMG. In this instance when applying 100kVA of load, the voltage drop is just 13% at 415V. While the difference appears small the AVR will react faster and potentially the visible voltage drop will be lower and shorter than with the shunt excitation system.

3) Increase the size of the alternator

Finally you can also oversize the alternator to improve the TVD. This is actually one of the most cost-effective methods. There are a few considerations when taking this approach, so ensure you talk to someone qualified to guide you through the process. In this example we will now use a 200kVA alternator (so double the original size) to reduce the TVD, along with the PMG option. The engine could remain the same in this instance, unless the amount of real power was also too high.

As you can see from this applying 100kVA at 415V will lead to an expected voltage dip of just 6%.