Derating and uprating factors
What is the power rating of the specific VFD? An indicative value is provided in the technical catalog or brochure. This value shall refer to certain operation conditions, among others the temperature of cooling medium (air or liquid), content of anti-freeze additive (usually some type of glycol) for liquid cooled VFD, target switching frequency (if applicable), fundamental output frequency, duty cycle (preload versus overload) or voltage variation of the grid. Since all above quantities are generally project specific, also the actual power rating becomes project specific and can differ from the catalog value. This article introduces some of the most common derating and uprating factors for VFDs.
The overall rating factor is a multiplier of the individual rating factors. However, some rating factors do not need to be multiplied and MIN function can be used instead. The rating factor can generally be less than 1.0 (derating) or more than 1.0 (uprating). Derating is necessary when the ambient and operation conditions are unfavorable. On the other hand, uprating may or may not be possible. It depends on the VFD design and rating of all internal components.
Please note that meaningful power rating of a VFD shall be expressed as apparent power in kVA or MVA rather than active power in kW or MW. The active power is defined by the motor and its performance characteristic. In case of kW rating the reference power factor shall be stated.
Temperature of cooling medium
The temperature of cooling medium directly impacts the effectiveness of cooling system. Well cooled power section can handle higher power (current) rating. Less efficient cooling cannot evacuate the same amount of losses and the power (current) loading must be reduced. The components are dimensioned up to certain max. operating temperature which is basically the sum of coolant temperature plus temperature rise inside the component. The lower the coolant temperature the higher temperature rise might be allowed. The coolant is typically ambient air for air-cooled VFD and deionized water or water-glycol mixture for liquid cooled VFD. Relevant is always the temperature of the internal cooling medium (in case of multiple cooling loops). For example, if the liquid cooled VFD has a water/water heat exchanger, then the relevant temperature is the temperature of the cooling liquid circulating inside the VFD. Each VFD may be designed differently and the temperature difference across the heat exchanger is product or project specific. A project-optimized heat exchanger with smaller temperature difference between cold and hot side can help to increase the power rating of the VFD. The dimensions of the heat exchanger will increase (larger surface for heat transfer), but if it helps to avoid a jump in VFD frame size it can still be a more compact and more cost-effective solution.
An example of the rating factor based on cooling water temperature is shown below:
Anti-freeze additive
Liquid cooled VFDs in ambient with temperatures below zero degree C require some percentage of anti-freeze additive. Most common anti-freeze is monoethylene glycol or propylene glycol (anti-freeze shall be ideally pure and shall not increase the conductivity). The lower the minimum ambient temperature the higher content of glycol is needed. Glycol has lower specific thermal capacity and therefore lower cooling capability (pure water: 4.187 kJ/(kg*K), ethylene glycol: 2.380 kJ/(kg*K)). Therefore, the higher content of glycol the higher derating factor. Table below shows recommended glycol concentration based on minimum ambient temperature. Corresponding rating factor is shown. The specific thermal capacity is approximate. The value depends on glycol concentration as well as coolant temperature (which is variable).
The anti-freeze rating factor and coolant temperature-based rating factor are multiplied as both directly impact the cooling capability. However, cooling liquid with relatively low maximum temperature principally compensates (partially or completely) anti-freeze based derating.
Anti-freeze additives are especially common in installations of liquid cooled VFDs without raw water. In other words, the user has no cooling water on site and the VFD uses an air-water heat exchanger (also called ‘fin fan’, dry cooler or air-cooled cooler). This heat exchanger is located outdoors. Since the outdoors min. design temperature is often significantly below the freeze point, a considerable amount of anti-freeze must be added.
Example:
Customer intends to install a 12’000 hp continuous duty drive system for a compressor. VFD will be installed in a building with controlled temperature in the range +5°C to +30°C. The outdoor ambient temperature is specified in the range -40°C to +40°C. The compressor station is located remotely and there is no cooling water available on site.
For 12’000 hp a liquid cooled VFD is the preferred solution to minimize the air conditioning of the building (CapEx, reliability, OpEx). Due to -40°C approx. 52% glycol is required as additive. This leads to anti-freeze rating factor of 0.885. The maximum cooling water inlet temperature is sum of maximum ambient temperature (+40°C) plus the temperature delta across the air-water heat exchanger (typically 4°C to 8°C). Therefore, the water inlet temperature reaches up to 44°C…48°C. For many VFDs such high temperature causes a derating as well, e.g. 1% above 40°C resulting in rating factor somewhere between 0.96 and 0.92. The total rating factor is he multiplier of both previous factors, i.e. 0.885*0.96 = 0.85 or 0.885*0.92 = 0.81, respectively. Due to unfavorable ambient conditions the VFD loses 15% to 19% of its catalog power.
Switching frequency
The semiconductor losses consist of conduction losses, switching losses and losses in blocking state. The distribution depends on specific semiconductor, but generally the switching losses are much more dominant than other losses. It means that with increasing switching frequency the losses will increase and also the temperature rise inside the semiconductor (junction temperature) goes up. To maintain max. allowed operation temperature of the semiconductor the current must be reduced à derating factor.
The VFD is designed and dimensioned for certain switching frequency or range of switching frequencies. Although this parameter might not appear in the technical catalog, it is an important quantity and the manufacturer always takes it into account. The catalog rating assumes the typical target switching frequency. Sometimes the switching frequency needs to be adjusted outside of the typical range. Reasons might be e.g. high-speed application or untypical motor design.
With high-speed applications we mean motors directly driving the load (centrifugal or axial compressor, high-speed pump, various test stands) without speed increasing gear with motor fundamental frequency above 60 Hz (often above 100 Hz). To maintain reasonable output waveform a certain ratio between switching frequency and fundamental motor frequency is recommended (there are other solutions as well, but that is scope of dedicated article on high-speed drives). In short, high-speed drives might require higher switching frequencies than normal drives with standard motor frequency range.
Untypical motor design might also require boosting the switching frequency. Voltage source inverter based VFDs use motor stray reactance to smooth the motor current. Shall the stray reactance be lower than normal target, it might be necessary to compensate it with higher switching frequency.
There is principally also an uprating due to lower switching frequency. That might apply for motor drives with high stray reactance, with an output reactor or with lower rated motor frequency.
Motor output frequency
Lower motor fundamental frequency is usually not a problem. However, extremely low output frequency requires special considerations. The peak value of current has too long duration and as a result one has to dimension for peak current instead of RMS value. That means quite significant derating; the rating factor is approx. 1/sqrt(2) = 0.707 (simplified). Additional concerns e.g. due to cyclic thermal loading might further reduce the rating factor below 0.7.
This type of derating applies only for a specific (low) output frequency range and is usually to be considered only during starting and only when the operation at very low frequency exceeds certain time (otherwise cold start condition applies).
Duty cycle
The load cycle affects both peak load as well as the average loadability. Keep in mind that VFD generally has low thermal capacity (unlike transformers or motors) and overload of few minutes must usually be considered as continuous loading. Air cooled VFDs have somewhat longer thermal time constant compared to liquid cooled VFDs, but it is still way shorter than the time constants of electric machines. Duty cycle shall be properly considered for derating and uprating rules.
Grid voltage variation
Most VFDs are designed for fluctuation of supply voltage in the range -10%…+10%. If the positive voltage tolerance is more than +10%, it might be necessary to reduce the nominal no-load rectifier input voltage. The inverter output voltage is reduced accordingly and with given maximum current the output power is reduced.
This article mentions some of the frequent rating factors. It is not the intention to provide a complete list of rating factors, but rather explain the most frequent ones. Some manufacturers have specially designed VFDs suitable for harsh environmental conditions or heavy duty overloads.
We hope that after reading this article you better understand the effect of derating and uprating factors and also why the project specific rating might not be the same as catalog rating.
Summary
This article lists the most common derating and uprating factors impacting the dimensioning of a variable frequency drive (VFD). A short article on the same topic is presented in [2].
References
[1] Medium voltage AC drives, https://new.abb.com/drives/medium-voltage-ac-drives
[2] How to dimension drives for performance and reliability, https://www.abb-conversations.com/2017/08/how-to-dimension-drives-for-performance-and-reliability/
