When is transformer tap changer needed?
The input isolation transformer in a drive system may be equipped with a tap changer. Such device provides some flexibility but adds cost and potentially also complexity to the transformer design. Thus, the question is whether a tap changer is needed or not. The subject is addressed in the following text.
Note: This blog post is considerably shortened. Full article is available for our premium subscribers.
Introduction
Power transformers are often equipped with a tap changer to adjust the transformer ratio. Similarly, some VFD transformers are specified with a tap changer, too. As the tap changer adds cost and complexity to the transformer design, it might be questioned whether it is really needed or not.
What is a tap changer
Tap changer is a device that allows to adapt the transformer ratio (ratio of number of turns on primary and secondary side) and to regulate the voltage. It can be located either on the primary or secondary side of the transformer. The choice depends on the rated voltage and winding connection. Detailed description of a tap changer is out of the scope of this post. We just note that there are off-circuit tap changers (OCTC), also called de-energized tap changer (DeTC) and on-load tap changer (OLTC).
Off circuit tap changer has relatively simple design. It provides a connection to the selected tap in the winding. Before changing the tap of OCTC, the transformer must be de-energized (disconnected and grounded). In contrast, on-load tap changer allows to change the tap while the transformer is loaded.
Tap changer in VFD transformers
The aim of a tap changer is to adjust the transformer output voltage by adapting the transformer ratio. In drive systems the total voltage variation is a superposition of three or four effects:
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In the sum they can make up to almost 25% depending on grid strength and system design. Tap changer could therefore help to compensate part of the variation. However, as most of the tap changers are of de-energized type, any adjustment would require the application to stop, change the tap position and restart the system. Thus, compensating the voltage variation due to different loading is not possible with OCTC. This type of tap changer helps to compensate long term deviation of grid voltage. Another purpose is to have certain flexibility in system design due to tolerances or uncertainties. In such case the tap position may be “tuned” during commissioning.
Voltage variation within the drive system
It was just mentioned that the voltage variation can reach up to 20 to 25%, in extreme cases even a bit more. Let’s elaborate where it comes from and what factors play a role.
A) Fluctuation of grid voltage
Electric grid is a complex living system and each grid has certain voltage variation. Medium voltage grids typically have a continuous variation of +/-10%. Although the real variation is usually much smaller, this range shall normally be considered for the design of equipment.
B) Voltage drop across the input isolation transformer
The input isolation transformer has certain short circuit impedance. The value is often defined by the VFD manufacturer in order to limit the fault current. Knowing the impedance value and input power factor, we can easily calculate the voltage drop across the transformer at different loading. Calculation examples for AFE and DFE type of drives are available for premium subscribers.
C) Ripple of dc bus voltage
In case of voltage source inverters (VSI) the voltage in the dc link contains certain ripple. What is the typical range of dc ripple? How does it depend on VFD topology or actual loading? More information in the full article.
D) Voltage drop across output isolation transformer or reactor
If there is an output transformer or reactor, the voltage drop has one more component as part of the equation (more information in full article).
Why does the voltage variation matter?
Fluctuation of the voltage inside the drive system poses challenges for the system components and the control system of the VFD. The system voltage at high load tendentially reduces due to multiple voltage drops. To transfer the required power, reduced voltage must be compensated with higher current. Above certain level this would be a thermal problem, especially for the motor. Also, compensating reduced voltage with higher current increases the voltage drop even more. The wide variation of voltage is a challenge for the control and protection system (internal over- and undervoltages, impact on inverter modulation, accuracy of control etc).
Disadvantages of a tap changer
Most transformers in variable speed drive systems have a tap changer of the off-circuit type (OCTC), if any. Although the device is relatively inexpensive, it brings additional cost on the transformer or system and has limited effect with regards to voltage regulation. Those disadvantages include:
- Increased dimensioning power
- Increased technical complexity
- Regular maintenance
- Overall reliability
When is transformer tap changer needed?
There are many drive installation where transformer tap changer is not required. In other cases the presence of a tap changer may be beneficial or even recommended. In this section the different cases are sorted out.
This chapter is exclusive for our premium members. It presents cases where transformer tap changer can be eliminated, cases where it is optional for additional flexibility and finally cases where tap changer is recommended to have.
Conclusion
Transformer tap changer is used to compensate voltage mismatch by adjusting the transformer ratio. However, commonly used off-circuit tap changer has limited flexibility as it can only be operated in a de-energized system. Moreover, the realization of a tap changer on a multi-winding transformer may require more complex design. The article provides a general recommendation about when to use a tap changer based on system voltage variation.
References
[1] Transformer Handbook, ABB, Business Unit Transformers, publication no. 1LAC000010, Switzerland, 2004
[2] VFD transformers: Need for a tap changer, https://mb-drive-services.com/need-for-a-tap-changer/
