How to safeguard your mains supply and your power drive system?

For reliability and safety reasons, you will want to protect your Medium Voltage vfd drive from disturbance from the grid, from short-circuits, overloads, etc …. But also, your mains supply must be kept healthy and safeguarded against negative impact of harmonics caused by your vfd drive.

First things first, getting the basics right:

• • Series-Connected H-Bridge (SC-HB) Topologies and Modular Multilevel Converter (M2C or MMC):

• • • M2C drives also allow for redundant power cells, enabling you to bypass faulty cells without affecting operation. The topology of the 12-pulse transformer with direct front end (DFE), shown on the diagram above, requires the same kind of protection means as for the 3L-NPC or 5L-ANPC drive topology. However, the fault energy in case of a short circuit in one semiconductor, feeding into the fault is very small thanks to the distributed energy storage.

Drives are nonlinear consumers causing harmonics

As already mentioned in our chapter “power quality and drives” we know that drives are nonlinear consumers: they draw non-sinusoidal current out of the mains supply.

Please find here below the basic diagram of a low voltage source inverter representing the 3 building blocks: the rectifier, the intermediate circuit and the inverter. The rectifier bridge connected to the capacitor only conducts current when the AC-voltage is higher than the capacitor voltage. So, the AC-input current waveform contains humps and will not follow the sinus wave of the grid. See for yourself for the line current (I Line) of this nonlinear consumer in the diagram below.
By the way, you can also see that we have 6 pulses in the rectified voltage (Udc) over one period of the mains voltage. This type of input stage is therefore often referred to as a 6 pulse Direct Front-End (DFE).

While the line current is clearly not sinusoidal, we can reconstruct this distorted non-sinusoidal waveform via many smaller sinusoidal waveforms at different frequencies and magnitudes. Thanks to a mathematical method, called Fourier Analysis, we can identify the frequencies and magnitudes of each of those waveforms, called harmonics. For the 6-pulse DFE we will find, apart from the fundamental frequency (50 or 60Hz), an important waveform at 5 times the fundamental frequency (250 of 300Hz), called the 5th harmonic. Further on, we find the 7th and the 11th harmonic.
In the picture below you can see the distorted wave form of a 6-pulse drive fed by a transformer wye/wye. We have represented the 5th up to the 11th harmonic in the time diagram.  With the spectrum analysis you can identify the frequency and magnitude of each of the present harmonics.

The 12-pulse medium voltage source inverter below is made up of two 6 pulse rectifiers supplied by a three winding transformer. The intelligent combination of secondaries with a phase shift of 30°el, cancels harmonics (the fifth and the seventh) on the primary side.
By the way, the number of pulses determines the harmonic number by the formula:  pulses* 2 +/- 1. For a 12-pulse drive this results in 12-1 =11th and 12+1 = 13th, 12*2-1= 23rd and 12*2+1=25th, etc … thereby eliminating all harmonics below the 11th.

Harmonics are often discussed in terms of a THD (total harmonic distortion) percentage. This percentage value describes how badly the waveform is distorted and deviates from a pure sinusoidal waveform. A waveform that is highly distorted will be more flat-topped and have a higher THD percentage value.
The following two formulas are used to quantify the harmonics present in a system:
THDV is the total harmonic distortion of the voltage waveform.
THDI is the total harmonic distortion of the current waveform.

Total harmonic distortion of the current (THDI) is the ratio in percentage of the total RMS (Root Mean Square) value of the harmonics over the RMS value of the current at the fundamental frequency. See formula below and practical example for a 6-pulse direct front end.

The voltage distortion is a consequence of the current distortion multiplied by the impedance at the point of measurement of the voltage referred to as the point of common coupling (PCC).

U_dist= I_{dist *  Z     ( indeed again ohms law ;-).}

If your supply system is rather weak, it will have a high impedance (Z) =>You can’t have as much current distortion (I_dist) to keep the resulting voltage distortion (V_dist) at the point of common coupling within acceptable limits.
A typical use-case is the setup of the supply by the transformer in normal conditions and the switch-over to the back-up generator in case of loss of mains power. The impedance Z of a generator is about 3 times larger than the impedance of the normal supply via the transformer. If the voltage distortion may not appear as a problem when supplied by the transformer, a serious issue might arise in case of supply in generator mode. Below you see an example of a severely distorted mains voltage with multiple zero crossings. Such a distorted voltage waveform can cause unstable operation of sensitive electronics and issues with welding applications relying on zero crossing detection. The distorted line voltage might also introduce harmonic currents in other linear loads such as motors. In this use case, the harmonic analysis must be done based on a generator source, in addition to the traditional utility source analysis.

In fact, the impedance Z is a measure for the strength of the supply. But there is a much better definition for system strength used in the relevant standards such as IEEE519 and IEC 61000-2-4: the short circuit ratio (SCR). It is an indicator of the strength of a network bus short circuit current (I_sc) versus the current demand of a device I_L (=maximum average load current in 15- 30- minute interval over 12 months) at the point of common coupling (PCC= is the point where the utility connects the consumer).
The value of the short circuit current should be easily obtained from the utility. It can also be calculated with the aid of nameplate data of the transformer of the utility.  Look for the power (P in kVA => multiply by 1000), the secondary voltage (U_sec in volts) and the short circuit impedance in % (U_k also sometimes called “impedance” but always in %). Determine the secondary current (I_sec) by the first formula below and obtain the network short circuit current by multiplying with 100 divided by the short circuit impedance U_k.

The following table extracted from table 10-4 of IEEE 519 (1992) advices limits on the individual current harmonics according the short circuit ratio.  The higher the short circuit ratio, the higher the allowed total demand distortion (TDD). Indeed, with a high short circuit ratio, the expected voltage distortion (at PCC) caused by the nonlinear consumer becomes smaller.
By the way, a high network short circuit means a low source impedance Z. Coming back to Oms law
U_dist= I_{dist *  Z}  : if  your supply system is rather strong, it will have a low impedance (Z) =>the current distortion (I_dist) allowing the resulting voltage distortion (V_dist) at the point of common coupling, to stay within acceptable limits, can be higher.

As mentioned above, the voltage distortion should be kept within safe limits at the point of common coupling. The table below, extracted from table 10-3 of IEEE 519, advices limits according to the context.

Special applications use sensitive electronics in places like hospitals, airports, data centres, or laboratories.
Dedicated systems are exclusively dedicated to the converter load.

The IEEE 519 is sometimes a bit mis-used in specification of drive requirements. Some specs impose limits to the current harmonics of the individual drive by defining the point of common coupling (PCC) of the drive on the primary (PCC3) of the converter transformer and not on the primary of the utility transformer (PCC1). The short circuit ratio will be lower when the PCC is situated right upstream of the drive (PCC3) and thereby imposing lower limits than originally advised by the IEEE 519. You might actually end up addressing a problem that doesn’t exist. In fact, IEEE 519 has not been developed for individual assessment of nonlinear consumers but for guidance in the design of power systems with nonlinear loads.

The IEC 61000-2-4 has a slightly different approach: the manufacturer of the variable frequency drive is to deliver the current harmonic level THC, under rated conditions, as a percentage of the rated RMS current for each order up to the 40th. The Power Drive System shall be assumed to be connected to a point of common coupling (PCC) with a short circuit ratio of RSC = 250 and with initial voltage distortion less than 1%. IEC 610000-2-4  introduces the in-plant point of coupling (IPC) to a supply network where other loads may connect. By focusing on the in-plant point of coupling (IPC), it ensures that the coordination of interference limits is effectively managed, thereby maintaining the integrity and reliability of the electrical systems. These reference levels, while not direct limit values, are categorized in classes 1 to 3. 

 IEC 61000-3-6 or GB/T 14549 can be used for the assessment of emission limits for distorting loads in MV power systems. In that context, there are quite some sophisticated software tools available on the market that analyse complex networks with multiple elements and various configurations of the grid. 

Harming harmonics and why should you care?

High levels of harmonic distortion can cause a wide range of issues:

• Nonlinear currents increase the true root mean square (TRMS) value of the current, leading to additional heating and potentially adverse effects.
• Premature failure and reduced lifespan of devices because of overheating of transformers, cables, circuit breakers, direct-on-line powered motors.
• Erratic trips of fuses and circuit-breakers due to additional heat.
• Unstable, unreliable operation of back-up generators and sensitive electronics relying on pure sinusoidal waveform.
• Flicker.(repeated fluctuations in light intensity caused by variations in RMS voltage)
• Possible resonance with power factor capacitors.

Rather than focusing on fear, let’s concentrate on safeguarding.

As a prevention measure, we would recommend regularly performing harmonic measurements (included in your power quality campaigns) at well selected points. You should measure at points where you can expect negative impact from too high harmonic distortion. Think about power factor capacitors, filters, cables and transformers and check for harmonic currents and voltage distortion to be within acceptable limits.Verification of possible resonance would require a harmonic distortion analysis.
In case of the installation of a new power drive system you could include the evaluation of the potential impacts on the distribution system into your specifications. This assessment typically encompasses a detailed analysis of the distribution system design and impedance, similar to a short-circuit study evaluation.

For the effective root cause handling of the harmonic distortion caused by the medium voltage drives as nonlinear consumers, we refer to the chapter topologies of drives.

If we lost you along the line, please contact us for a free video call

In all of the situations mentioned above, we are here to assist you. Simply reach out to us for a free, no-obligation consultation.
As you move forward, we can support your decision-making process by providing independent advice on any of the following tasks:

• Prepare and present the project plan.
• Define and manage the requirements for the Power Drive System (PDS).
• Evaluate the composition and components of the Power Drive System.
• Organize and coordinate internal and external pre-bid meetings.
• Create and distribute the invitation to bid in collaboration with Engineering, Procurement, and Contracting compagnies (EPC), and/or vendors.
• Assess preferred manufacturers for transformers, inverters, and motors.
• Review offers and identify any deviations from the PDS specifications.
• Assist in the selection of vendors for transformers, variable frequency drives, and motors.
• Clarify any outstanding issues related to the PDS.
• Define the requirements and test plans for both witnessed and non-witnessed factory acceptance tests (FAT).
• Establish the requirements for site acceptance tests (SAT).
• Perform on-site validation of the site acceptance tests for the components of your Power Drive System (PDS).