The Association of Manufacturers and suppliers of Power Systems and ancillary equipment


Non Linear Loads

Short Answer:

There is no single ‘over-sizing factor’ suitable to cover all applications involving loads which generate harmonic distortion, commonly described as Non Linear Loads (NLL).

Each application consideration must begin by establishing an understanding of the operational characteristics of the proposed NLL equipment in terms of the level of harmonic current distortion plus identification of the total connected load’s peak and steady state kVA level. As there are many types of NLL’s resulting in many different operating characteristics it figures that there will be many over-sizing factors.

Long Answer:

There is a logical route which will provide an engineered solution for the nomination of a suitable generating set for any application where complex loads including NLL’s are to be considered. The following explanation describes a route which has been broken down into parts in an attempt to provide a progressive understanding of the issues involved.

Part 1: Fundamentals.

It must be accepted that when supplying a load with inherent current waveform distortion, the supply voltage waveform will become affected and so will also become harmonically distorted. Therefore, consideration must always be given to identifying an acceptable level of harmonic voltage distortion. Many types of electrical equipment have a finite tolerance to a distorted level voltage waveform above which equipment is likely to malfunction or component failure occurs.

Once the percentage levels of Total Harmonic Distortion (THD%) for both the current and voltage waveform have been identified, then the operational characteristics of the required electrical power supply can be fully considered, and so the generator nomination exercise can begin.

A fundamental approach involves considering the relationship between harmonic current distortion, harmonic voltage distortion and in turn power supply (generator) source impedance in terms of Ohms Law. Where: Supply source impedance R = V.harm.dist% / I.harm.dist%

This basic level approach simply provides a relationship comparison as a route towards identifying a suitable level of generator impedance (reactance actually), and so a ‘rule of thumb’ guidance regarding the level of ‘specialness’ required when comparing the required ‘special’ generator to normal industrial prime rated generating set’s engine & generator combination.

Consider the following examples using the basic level approach;

  1. I.dist is advised to be 30% req’d V.dist 10%, then source impedance = 10/30 = 0.3 Therefore a generator with a VERY low reactance values will be required (therefore HIGH level of generator over-sizing).
  2. I.dist is advised to be 10% req’d V.dist 10%, then source impedance = 10/10 = 1.0 Therefore a generator with a low reactance values will be required (therefore MEDIUM level of generator over-sizing).
  3. I.dist is advised to be 10% req’d V.dist 20%, then source impedance = 20/10 = 2.0 Therefore a generator with a typical industrial ratings and corresponding reactance values will be required (therefore LOW level of generator over-sizing).

To make a comparison between the above calculated factors with a reference ‘factor’, consider a prime rated class H generator as having a factor of 2.

The above explains the importance of gathering data regarding the characteristics of the NLL and then to also consider the needs of all connected loads before the quest for generator sizing /nomination can begin.

Part 2.1: Types of NLL’s and typical characteristics for current waveform harmonic distortion.

Some electrical loads draw continuous current over the period of each applied voltage half wave, but due to their complex current consuming behaviour which results in the generation of a complex current waveform with individual harmonic orders which combine to give the fundamental current waveform a high harmonic content.

Examples of such loads are discharge lamps.

Some electrical loads draw discontinuous current for each applied voltage half wave. Such loads incorporate power electronic packages. The action of this discontinuous current is to generate a pronounced level of harmonic distortion which can vary considerably as the absorbed kVA varies over the equipment’s operating range.

Examples of such loads are: UPS, VSD’s, Power Supplies, Motor Soft Start modules, full wave 3phase uncontrolled rectifiers.

Rectifier units comprised of a full wave three phase bridge - 6 diodes - draw current equi-spaced about the voltage waveform’s peak value, and zero current around each half wave’s zero crossover points, the result of natural commutation between the diodes associated with the other two phases.

This current consuming characteristic causes complex problems for an ac generator, but only harmonic distortion effect is considered in this article.

Obtaining data regarding the harmonic current distortion characteristics of a proposed load can be difficult, and so the following section provides key questions, and then suggestions regarding typical values should assumptions need to be made.

Part 2.2: A questionnaire to identify technical characteristics of the NLL.

Discuss the current consuming characteristics of the proposed NLL application using the following list in order to gain the required information:

  1. Number of pulses associated with the converter stage, 6 or12.
  2. Level [%] of harmonic current distortion generated by NLL.
  3. Are harmonic filters fitted to reduce level of current distortion?
  4. Max. Allowable level of harmonic voltage distortion.
  5. If UPS, then peak input under battery recharge condition if VSD, then motor starting peak kVA condition.
  6. Operating voltage and frequency.
  7. Operating power factor and efficiency

When actual data cannot be obtained then assumptions must be made and the following offers typical values for types of NNL’s.

TYPICAL LEVELS of CURRENT DISTORTION:

If the harmonic distortion characteristics are not known then the below typical values for NLL’s with harmonic filtration can be used, but the customer should be made aware that assumptions are now replacing real engineering data.

12 pulse - 14%

6 pulse - 30%

4 pulse - 45%....... 1 phase equipment

TYPICAL ACCEPTABLE LEVELS of VOLTAGE DISTORTION:

If no specified levels for system harmonic voltage distortion can be provided then the following typical levels can be assumed, but the customer should be made aware that assumptions are now replacing real engineering data.

UPS Systems and Admin building loads - 10%

Inverter Drives (VSD) and industrial equipment - 15%

Soft Start Systems which are short-term (bypassed) - 20%

Diode based rectifiers systems only - 20%

Part 3: Associated considerations when supporting NLLoads. Thermal considerations:

When the voltage and current waveforms become distorted by harmonic content then the generators stator assembly’s operating temperature will be increased. For this reason selected generators for applications involving NLL’s must always be selected against nothing higher than their published class ’F’ (105/40) continuous rating.

Excitation system:

To ensure the generators excitation system remains stable under the imposed levels of harmonic distortion created by the NLL, an isolated and so stable power supply must be incorporated within the generator to provide the AVR with an excitation power source. Ideally this should be based on a PMG pilot exciter; although a well-designed auxiliary stator winding can provide acceptable levels of performance.

NLL’s with harmonic filters:

Harmonic filters designed to reduce the effective level of system harmonic distortion are available specific to the characteristics of each type of NLL.

The latest technology is based on ‘Active-Filters’ which are active harmonic compensating systems with the ability to maintain the voltage to virtually a pure sine-wave. NLL’s operating in conjunction with active filter will operate at very close to 1.0pf, and so engine nominations need to take this into account. Well-designed traditional harmonic filter packages consist of a combination of capacitors and chokes as tuned LC circuit.

Low cost filters are likely to be capacitors only, and these can cause operational problems for the generators excitation system, due to a self-excitation effect should the total connected load become a very leading pf (kVAr lead) condition. Most generators can only tolerate a low value of leading kVAr, (Zpf lead capacitive current) and typically this is in the order of 25% of the generators industrial rated kVA.

Inclusion of any form of harmonic filter package therefore needs technical consideration to ensure problems do not occur during the NLL’s ‘start-up’, when the active power is virtually zero, and the generator may only be supplying only a leading pf load.

Part 4: Sizing the Generator

It is important to note that the NLLoad will always be the most susceptible part of the site equipment package should unacceptably high levels of supply system harmonic voltage distortion be allowed to occur….NEVER a malfunction of a correctly ‘sized’ generator.

Once the key information regarding operational parameters has been gathered for the proposed NLLoad and the normal (linear) loads, therefore guidance now being available regarding an acceptable level of system harmonic voltage distortion for the proposed application the real ‘generator sizing’ work can begin.

The ‘basic level approach’ covered in Part 1 has a formula which introduces the term R as a route to provide an indication of power supply (generator) source impedance. For an ac generator the parameter which best ‘describes’ source impedance for consideration of voltage waveform distortion is the sub-transient reactance X”d.

From the examples of R previously given, it can be seen that when the level of harmonic current distortion is high yet the required level of harmonic voltage distortion is low, then the source impedance of the supply system must be very low.

In practical terms this means that a generator must be selected with very low levels of reactance, thereby a very low level of sub-transient reactance X”d will be achieved.

Engineers use complex computations to consider the effect of V & I harmonic distortion levels, distribution system impedance, and the consequence of operational linear loads which inevitably will be sharing the electrical system. However, in an attempt to keep it simple the following uncomplicated two steps firstly produce a value of X”d, and then secondly relate this X”d towards a ‘Rule of Thumb’ route for generator nomination.

Step 1

X”d % = (V.dist % / I.dist %) x (Pn / 6) x 10

Where:

V.dist% is: Level of harmonic voltage distortion considered acceptable for the application.

I.dist% is: Level of harmonic current distortion at which NLL operates

Pn is: Number of pulses at which NLL operates

Step 2

X”d% now needs to be corrected to the base kVA level of the proposed NLLoads input kVA.

The simple ‘correction’ calculation to identify the NLL’s base kVA level is:

(NLL input kVA /Gen’s published kVA for published value of X”d ) x pub’d value of X”d

Example: 415 V 50 Hz system, a mixed load of some 450 kVA which includes a UPS identified as having a maximum input demand of 300 kVA 6pulse with 30% harmonic current distortion, and the advised maximum level of harmonic voltage distortion is 10%.

Source impedance considerations:

The generator must have a class F rating of at least 450 kVA, and so this a fundamental starting point for generator consideration. From the above Step 1 the generator will require: X”d = (10/30) x (6/6) x 10 = 3.3%

From a data file a generator is found with a class F rating (remember above comments on harmonic heating effect and need to keep generator operating within its designed class F limit) of 475 kVA, with an identified X”d of 12% for this base rating at 415 V 50 Hz

Correct this published X”d for the ‘F’ rating to an recalculated for X”d against a base rating associated with the UPS input kVA

UPS base level X”d = (300/450) X 12% = 8% ……

As the req’d gen must have X”d <3.3% this gen is unsuitable.

From a data file another gen is selected, this has a class ‘F’ rating of 850kVA and published X”d of 9%

(300/850) X 9% = 3.2%

A suitable generator design has now been identified.

Excitation system considerations:

The generator must be fitted with a suitable excitation system and AVR, specifying the need for an isolated power supply for the AVR by preferably a PMG pilot exciter, alternatively a stator auxiliary winding system.

Practical considerations regarding generating set design must now be considered. As the correctly selected generator is an 850 kVA design, yet the application is identified as 450 kVA, and so an appropriate engine for a 450 kVA generating set is all that is required. The miss-match of large generator to small engine forces the genset designer to consider special engine-generator base frame design, torsional compatibility, starter motor torque, generator’s inherently high fault current levels and an electrical protection system and on-site installation scheme that will duly consider the operational harmonic distortion levels and any resulting EMC contravention.

Based on the above calculation the following list provides reference values for typically encountered NLLoad applications.

Consider a 6 pulse unfiltered NLL with some 30% harmonic current distortion:

To operate with the harmonic voltage distortion in the region of 10% then X”d ~ 3.3%

To operate with the harmonic voltage distortion in the region of 15% then X”d ~ 5.0%

To operate with the harmonic voltage distortion in the region of 20% then X”d ~ 6.7%

Consider a 12 pulse unfiltered NLL with some 14% harmonic current distortion:

To operate with the harmonic voltage distortion in the region of 10% then X”d ~ 14%

To operate with the harmonic voltage distortion in the region of 15% then X”d ~ 21%

Part 5: Existing and Old Generators

Identifying the capability of existing / old generators to support newly installed NLL’s requires all the above outlined operational information to be gathered and taken into account for the proposed NLL. This information then needs to be considered against the existing generators value of sub-transient reactance [X”d] calculated against a ‘base level’ for the proposed NLL’s operating kVA.

If the NLL is likely to result in generated level of harmonic voltage distortion being above 7% then the generator must checked to identify if the existing excitation system includes a scheme to provide an isolated power supply for the AVR. If the existing system is a ‘shunt’ based scheme and so provides the AVR with power directly from the generators main output connections, then an up-grade kit of PMG pilot exciter - or similar scheme - must be fitted to ensure that the AVR will be able to maintain the generator’s output voltage at a stable level.

Footnote;

The above technical explanation has purposely been kept at a level that provides a simple explanation which hopefully will remove some of the mystery which often surrounds a generating set sizing exercise when complex combinations of mixed loads which include NLL’s are specified. The described calculations have purposely been kept simple. Their roots are the results of years of experience making complex calculations which lead to an observation that for the vast majority of typical NLL’s a ‘fast-track’ calculation could be used, and this experience is now shared.

FAQ 1008

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