Protection of VFD transformers
In the introduction to this series we have explained the differences between a distribution/power transformer and a VFD transformer [1]. Due to different design, construction and operation of VFD transformers the protection requirements shall be adapted as well. While many end users, system integrators, EPCs and consultants have deep knowledge on protection of power transformers they often tend to transfer the same approach to the VFD transformer. Of course, some principles are the same or similar. However, copying the protection concept of power transformer to a multi-winding VFD transformer likely calls for a problem. Continue reading to learn why and what could be the better alternative.
Remark:
We focus on VFD input transformers in this article as almost every medium voltage VFD requires a dedicated input isolation transformer. Most VFDs feed the motor directly. However, in some cases there is also an output transformer, i.e. transformer between VFD and motor. Such transformer usually has less windings (often just a simple 2-winding design). However, it operates at variable frequency. The specific topics around output transformers will be covered in a separate article.
Multi-winding VFD transformers
VFD input transformers usually incorporate a multi-winding design [2]. Multi-pulse transformers are especially popular for VFDs with diode rectifiers. As the rectifier is passive, the (almost) only way how to mitigate harmonics is to increase the pulse number and use phase shifting transformer for cancellation of certain harmonic orders [3].
12-pulse transformer with 3 windings (e.g. Yy0d11) is quite common. However, it might not be sufficient to comply with harmonic requirements defined by the standards such as IEEE 519, IEC61000, GB 14549 or other international or national standards and regulations. Therefore, transformers with 18, 24, 30 or 36 pulses are commercially available from all major manufacturers.
Figure 1 shows the cover of a 12-pulse liquid filled transformer with 3 x HV bushings and 6 x LV bushings. Figure 2 illustrates a 36-pulse transformer with 18 x LV bushings. Such solution can be realized as one active part (“single core”) or two 18-pulse active parts inside one common tank liquid filled) or one common enclosure (dry type) [4,5].
Overcurrent protection of VFD transformers
Overcurrent protection is a basic protection that shall always be included, regardless what type of transformer you have or what application it is. The implementation of this protection is easy. It only requires current measurement on the primary side of the transformer and suitable transformer protection relay [6, 7]. The current transformers or sensors can be mounted e.g. in the input switchgear as shown in figure 4 [8].
However, in multi-winding transformer designs the challenge arises how to detect an overcurrent in a reliable way. The higher the number of secondary windings the lower the current seen on the primary side when just one secondary winding has a fault.
In order to be more specific we have selected four transformer configurations:
– 12-pulse / “3-winding” transformer (e.g. Yy0d11)
– 24-pulse / “5-winding” transformer (e.g. Yd11.30d0d0.30y1)
– 36-pulse / “7-winding” transformer (e.g. Yd11d11.20d11.40y0d0.20d0.40)
– 30-pulse / “16-winding” transformer (e.g. Y3x(d1d0.18d0.06d11.54d11.42))
Note that for configurations “A” to “C” each secondary winding is displaced against all other secondary windings to achieve the phase shifting effect and harmonic cancellation. Configuration “D”, due to large amount of windings, does not have each individual winding displaced. Instead, three windings always have the same displacement, each of them supplying a cell associated with different motor phase.
Now we come back to overcurrent protection. Generally the bigger amount of secondary windings the more difficult it is to detect a fault at no load or partial load. The fault current also depends on the transformer short circuit impedance (voltage) zk. For a simple 2-winding transformer the fault current in per unit system [pu] is simply inverse proportional to the short circuit impedance in case of 3-phase fault. For phase to phase fault the short circuit current is factor sqrt(3)/2 times lower.
For example: 2-winding transformer with 0.1 pu short circuit impedance (zk = 0.1 pu, i.e. 10%) the 3-phase short circuit current is 10 pu and phase to phase short circuit current is 8.66 pu. The grid impedance was neglected to make things simple. In reality, the grid impedance helps to limit the fault current so actual fault value is lower.
Now for a multi-winding transformer the worst case is following: Transformer has a fault in one of the secondary windings while operating at light load or no-load. Such case is nothing artificial – it can easily happen e.g. when restarting the VFD with a rectifier diode fault.
Fault is in just one secondary winding while other secondary windings are at no load. The current seen on the primary side of transformer is calculated as per formula below:
Note that we still neglect the grid impedance. Such impedance limits the fault current and make it further more difficult to detect. From above formulas is clear that with increasing number of windings the fault detection becomes challenging. It can be practically compensated by lowering the short circuit impedance, but from practical reasons the short circuit impedance cannot be too low. Moreover, the IEC 60076-5 defines minimum recognized short circuit impedance values. Also note that in a multi-winding transformer the individual impedances HV-LV1, HV-LV2,…, HV-LVn are not exactly the same. Some of them will be slightly higher than the others.
Now let’s come back to our four configurations “A” to “D” as per figure 5. We will calculate the fault current in per unit of nominal primary current. Considered is both 3-phase and phase to phase fault. This time the grid impedance is considered in order to be more accurate.
A) 5 MVA transformer, zk (uk) = 6%, Ssc = 200 MVA
B) 10 MVA transformer, zk (uk) = 8%, Ssc = 150 MVA
The first table is an optimistic scenario considering stronger grid and low transformer impedance of 6%. The second table is more “difficult” as the grid is weaker and transformer short circuit impedance is higher. This reflects partly the reality as the % short circuit impedance increases with transformer power rating (see again IEC 60076-5 or concrete transformer designs). The fault values above consider nominal voltage in the grid. In case of undervoltage condition during the fault the short circuit values drop further.
What is clear is that the multi-winding transformer configuration “D” cannot be sufficiently protected without additional measures. Configuration “C” seems feasible in most cases, but might need little more attention as well. Sound manufacturers have additional features how to cover potential “white spots” and ensure a reliable protection under all circumstances.
Differential protection
Differential protection is seen by protection experts as the best protection method. In power and distribution transformers the differential protection is frequently used. It has superior sensitivity and very fast reaction.
Differential protection for a 2-winding or 3-winding transformer can be easily done. However, as the number of windings increases, this protection method becomes less practical and at some stage virtually impossible (configuration “D” in our comparison).
There will be a separate article on differential protection of VFD transformers coming up soon! Refer also to [9] for more details.
Arc back fault
What is an arc back? This term is used by IEEE. It is a consequence of a diode failure inside the rectifier. When the transformer continues in operation, the fault current includes significant DC component compared to a symmetrical 3-phase fault. The peak current of arc back is up to 1.5-times higher than the AC fault. Since the forces are quadratic proportional to the current, the peak force is 2.25-times higher causing a severe mechanical stress. This is very critical for the winding fixation and overall mechanical integrity.
Inrush current
Inrush current is a known phenomena associated with energizing of transformer. The transformer core is temporarily saturated causing high reactive current (excitation current) flowing from the grid into the transformer. The inrush current is characterized by unipolar pulses with decaying magnitude. Depending on system parameters the first peak can be as high as 30-times the nominal primary current! This creates a challenge for the transformer protection: it shall be sensitive enough to detect fault in one of the secondary windings causing relatively low current on the primary side and at the same time it shall not create false overcurrent trips during transformer energization. Of course, most modern relay recognize the inrush current (e.g. based on its frequency spectrum) and mask this event.
Another point related to inrush current is the mechanical stress for the transformer windings. Note that distribution transformers are energized rather seldom. In contrast, VFD transformer might experience much higher frequency of energizing events. In case of doubts consult the transformer manufacturer if the transformer design is suitable for intended frequency of energizing events. A possible solution to tackle the inrush current is a pre-magnetization system. We will post a dedicated article on transformer pre-magnetization soon.
Overvoltage protection of VFD transformers
Transient overvoltages have two main sources:
(1) atmospheric overvoltages, i.e. lightning strikes
(2) switching overvoltages
Atmospheric overvoltage is a result of lightning strikes. The waveform is characterized by steep rise time and a tail. In laboratory conditions, for test purposes, the lightning impulse is typically defined by a waveform defined in IEC 60076-3 with 1.2 µs rise time.
It was discovered that fast switching overvoltages caused by operation of vacuum circuit breakers (VCB) create even more severe overvoltages. The danger of high frequency switching transients is well known and mentioned in international standards such as IEC 60076-1. Yet, the topic is sometimes forgotten during system integration.
Captured transients in real installations revealed voltage rate of change exceeding 500 kV/µs and frequency of overvoltage transients up to 100 MHz. The voltage distribution across the windings is more critical for these high frequencies.
Generally surge arrestors shall be used. However, to reinforce the overvoltage protection, they might be combined with RC snubber or other additional measure. Consult your VFD or transformer manufacturer for their best practice.
Summary
VFD transformers differ from power and distribution transformers.While some basic principles are the same, others need specific considerations and adjustments. This article mentions some of the challenges that need to be managed to achieve reliable protection of the transformer and whole drive system. Additional articles will follow focusing more on particular items such as e.g. differential protection or pre-magnetization strategies. You will find them in this series.
At the end let us state that there are solutions for all mentioned challenges, whether this is a software/hardware feature in the VFD, set of design rules or a smart drive system concept.
More questions? Contact us through the form below:
References
[1] VFD transformers: Introduction, https://mb-drive-services.com/vfd-transformers-introduction/
[2] VFD transformers: Multi-winding design, https://mb-drive-services.com/vfd_transformer_design/
[3] Network harmonics – Introduction, https://mb-drive-services.com/net_harm-introduction/
[4] Trasfor – custom built dry-type transformers and reactors, http://trasfor.com/
[5] Variable speed drive transformers, https://new.abb.com/products/transformers/special-application/variable-speed-drive-(vsd)-transformers
[6] Transformer protection and control relay RET615, https://new.abb.com/medium-voltage/distribution-automation/numerical-relays/transformer-protection-and-control/transformer-protection-and-control-ret615-iec
[7] RET 670 – transformer protection relay, https://new.abb.com/substation-automation/products/protection-control/transformer-protection/ret670
[8] IEC air insulated switchgear UniGear ZS1, https://new.abb.com/medium-voltage/switchgear/air-insulated/iec-and-other-standards/iec-air-insulated-primary-switchgear-unigear-zs1
[9] M. Bruha, M. Visser, J. Von Sebo, E. Virtanen, P. Tallinen, “Protection of VSD transformers”, PCIC Europe, Vienna, May 2017
