Transient Control Levels:

A Better Way to Voltage Ratings in Power Converter Applications

K.E. Crouch and F.A Fisher

General Electric Company

Pittsfield, MA

François D. Martzloff

General Electric Company

Schenectady, NY

Reprinted with permission from Proceedings, Annual Meeting, Industry Applications Society, October 1976.

Significance

Part 6: Textbooks, tutorial, and reviews

A tutorial paper presented to the IEEE Industry Applications Society, as a complement to a parallel paper

presented to the IEEE Power Engineering Society in 1976 (see the file “TCL Proposal”) and to the

international EMC community (see the file”TCL philosophy”). Includes a detailed description of the “Ring

W ave” and possible generator circuits to produce this novel (at the time) waveform.
TRANSIENT CONTROL LEVELS 

A BETTER WAY TO VOLTAGE RATINGS IN POWER CONVERTER APPLICATIONS 

K.E. Crouch F.A. Fisher F.D. Martzloff 

General Electric Company General Electric Company General Electric Company 

Pittsfield, Massachusetts Pittsfield, Massachusetts Schenectady , New York 

Abstract 

Failure and circuit upset of electronic equipment 

caused by transients has now been recognized as a significant obstacle to the development of new systems. 

While designers have recognized the problem and are 

taking steps to deal with it in their own systems, 

there is a lack of overall coordination beyond the confines of a given black box. The total environment must 

be considered in contrast to the present approach where 

electronic power and control equipment is all too often 

designed, built, and delivered before the transient 

threat is dealt with. 

The authors transient control 1 evel (TCL) phi losophy provides an approach toward coordination of the 

intrinsic capability of equipment to withstand overvoltages, the capability of existing or future protective devices for limiting overvoltages, and the known or assumed characteristics of surge voltages in power 

systems. Examples of this coordination, for general 

applications as well as for a specific power converter 

circuit, are given at the end of the paper. The paper 

also discusses the evolution of the TCL philosophy 

since its initial disclosure. 

INTRODUCTION 

Present standards do not offer sufficient guidance 

to designers and manufacturew of electronic equipment 

regarding what types of transients to consider and how 

to prove that equipment works in the presence of transients. This situation is perhaps under better control 

in the electric power field than it is in the fields of 

aerospace, general industry, housewares, and the mi 1 itary. For instance, the Working Group on Surge Voltages in AC Power Circuits Rated 600 Volts and Less, of 

the IEEE Surge Protective Devices Committee, has been 

collecting data on the occurrence of surges for several 

years. This group is now in the process of preparing 

a guideline document describing the transient environment [I]. The IEEE Power System Re1 ayi ng Comni ttee 

also has collected information on the surges found in 

power substations and has developed the Surge Withstand 

Capabil ity (SWC) test [2]. Transient insulation coordination is well developed with regard to high voltage 

apparatus. Insulation structures are designed to meet 

specific industry standard transient 1 evel s, and the 

equipment is subjected to proof tests. The design levels are coordinated with the capabilities of existing 

surge protective devices. Electronic and control 

equipment, on the other hand, is all too often designed. 

built, and delivered before the existence of a transient threat is recognized. If transients turn out to 

endanger the equipment, there may be na adequate surge 

protective devices. In fact, there may not be a satisfactory answer to the problem posed by transients when 

retrofit situations are involved. 

In an attempt to improve the situation, where existing standards do not offer sufficient guidance, the 

authors have proposed the concept of transient control 

levels [3], patterned after the basic insulation level 

(BIL) approach which has been utilized very success- fully in the electric utility field to coordinate the 

withstand capability of equipment, the surge-limiting 

ability of suppressors (surge arresters), and the overvoltage/overcurrents encountered in power systems. 

Therefore, it will be useful to briefly review the 

concept of insulation coordination, and the procedures 

developed to verify satisfactory performance, in the 

powerlutil ity industry. 

THE CONCEPT OF INSULATION COORDINATION 

As a result of the mature philosophy of insulation 

coordination, power equipment achieves its resistance to 

lightning-induced transients not so much from being designed to the threat posed by lightning, but by the threat 

posed by acceptance test. This acceptance test does not 

subject the equipment to transients having the complex 

waveshapes produced by 1 ightning. Instead, the equipment 

is subjected to transients which have elementary waveshapes that can be produced by basically simple test apparatus. Furthermore, the acceptance test does not subject the equipment to transients of the amplitude produced by lightning. However, it does subject the equipment to transients having an amplitude consistent with 

the capabilities of existing surge-protective devices. 

If it is intended that suppression devices be used to 

control the magnitude of the voltage transient, one 

should be sure there are suitable devices before the 

level is selected. 

The basic concept, which needs to be mutualTy accepted by both manufacturers and users of equipment, is 

that it is impossible to simulate all of the possible 

transient overvoltages a given product line might experience. However, by designing the equipment to a certain standard and controlling the occurrence of overvoltages by suitable protection, a much greater chance of 

successful operation in the real world is obtained. 

The challenge is to select a standard for the withstand capability of the equipment that will reflect the 

real world and yet be simple enough to be practical. 

The other difficulty is to trade off the specification 

of ultra-conservative standards that insure a very high 

reliability for the equipment, but impose on the manufacturer (and therefore on the user who pays for the 

equi pent) unnecessary costs. 

THE TRANSIENT CONTROL LEVEL CONCEPT 

Three basic parameters that relate to the amplitude 

of the surge, the waveshape of the surge, and the energy 

content of the surge must be considered. The amplitude and energy content depend, to some degree, on the nominal 

operating voltage of the system on which the surge may 

appear. Higher amplitude and higher energy surges are 

found on higher voltage systems. The waveshape and amplitude of the surge depend, to some extent, on the origin 

of the surge and how far away the point of observation is 

from the origin. Surges produced by arcing contacts tend 

to involve faster rates of change than surges produced by 

lightning. The latter, in turn, tend to have slower rates 

of change than surges produced by arcless switching of 

highly inductive circuits. Surges originating remote from 

an observation point tend to have slower rates of change 

than surges originating close to the point of observation. There is no "typical" surqe. 

On the premise that even an imperfect approach to 

transient coordination is better than none, two of the 

authors of this paper recently proposed the transient 

control level philosophy. In the paper [3] in which 

IAS 76 ANNUAL 
the philosophy was presented, a request was made for 

feedback about some of the details of the philosophy. 

Some of the feedback indicates that modifications to 

the test waveshapes and levels originally presented 

might be in order. The essential elements of the philosophy remain the same and are reiterated below. 

The amplitude and energy content of the surge are 

defined in terms of the voltage produced by the surge 

across an open circuit and the current produced by the 

surge through a short circuit. A specified open circuit voltage and short circuit implies a definite impedance of the source from which the surge originates. 

This impedance is usually reactive. Observations by 

Bull 141 and others suggest that a reasonable impedance 

for 115-VAC circuits is 50D paralleled by 50 UH (Figure 1). In the absence of other information, the au- thors suggest that a TCL philosophy be built around 

this impedance. This combination of elements has a 

breakpoint (wL = R) at 159 kHz, which may be difficult 

to implement in a simple surge generator. The important points about the source impedance are that the impedance should be in the order of 50n resistive at high 

frequencies and 50 pH inductive at low frequencies. 

Frequency mr 

Figure I. Inrpsdance of La, VoZtage Circuits 

(Prom reference 4) 

Consideration of waveshapes is of great importance. 

Two types of waveshapes enjoy some prominence. The first 

is the 1.2 x 50 ~s unidirectional voltage which is the 

basis of the BIL system of insulation coordination and 

whose historical origin comes from studies of the effects 

of lightning on high voltage power lines. The second is 

the highly oscillatory (1.5 MHz) SWC voltage waveform. 

The latter waveshape has its origins in the transients 

produced by switching operations in high voltage substations. Experience has proven that neither of these 

types of waveforms is a good representation of the types 

of transients found in 115-VAC circuits and other circuits of similar physical construction. The typical 

surge is oscillatory, but of a frequency lower than 

that specified for the SWC test. 

In the previous paper [3], the authors suggested a voltage test wave of the characteristics shown in Figure 2. That particular test wave had its origins in the 

Space Shuttle program and was originally viewed as a short duration surge predominantly unidirectional. It was not intended that the surge be oscillatory; but 

some degree of oscillation was considered a1 lowable. If 

treated as a damped oscillatory surge, Figure 2 implies an oscillatory frequency of about 50 kHz. After con- sidering other studies and feedback from readers of the 

original paper 131, it seems more appropriate to emphasize the oscillatory nature of the surge, rather than 

to downplay it, and to specify the surge in terms of 

its oscillatory frequency rather than in terms of the 

duration of the first quarter cycle. A specification 

that may more nearly meet the requirements of the industry is shown in Figure 3. 

The important characteristics include an i ni ti a1 

rate of change sufficient to stimulate the dv/dt ef~ 

fects in semiconductors and inductive components with 

an oscillatory decay, a decrement typical of that observed in low voltage circuits, and a duration comparable with clock cycles in digital equipment.. The duration of the original and revised waveforms have also 

been chosen after consideration of the failure modes of 

semiconductors. Failure characteristics tend to be different if the semiconductors are exposed to much shorter 

duration. Finally, the waveforms can be produced with 

simple test equipment. An example of a test circuit is 

shown in Figure 4. An example of the open circuit voltage and short circuit current produced by this circuit 

is shown in Figure 5. 

lr snould De understood that the TCL philosophy is 

not written around any particular test circuit. Any circuit that produces open circuit voltage and short 

circuit current of the characteristics shown in Figure 

3 and has an internal impedance similar to that of 

a) Open-circuit voltage 

t,= 0.25~~ A2~5O0IoA, 

122 5ps 

t3 Not specified 

Figure 2. First Proposal for TCL Wave Shape Figure 3. AZtemzative Proposal for TCL Wave Shape 

IAS 76 ANNUAL 
VOLTAGE 

0.25pF 

* LOW INDUCTANCE 

** LOW INDUCTANCE 8 NON-POLARIZED 

Figure 4. One ExumpZe of TCL Generator 

Figure 1 would be equally satisfactory. The calculated 

impedance characteristics of the circuit shown in Figure 4 are shown in Figure 6. If losses had been considered in the calculations, the height of the resonance peak would have been 

lo.ol 

c 0.1 I 10 

FREQUENCY - MHz 

Figure 6. CalcuZated Impedance of TCL Generator 

reduced to 100 or 200R. reflect the nature of the application and the consequences of a failure, as well as the exDosure of the 

power system. For instance, a low cost'domestic appliance, produced under strong competitive pressure, cannot be expected to offer tne same aegree of transient 

withstand capability as a critical industrial system. 

Table 1 

POSSIBLE TRANSIENT CONTROL LEVELS 

Open Circuit Short Circuit 

Level ~ol tage Level Current ~evel 

Number (volts) (amperes) 

1 60 2.2 

2 150 5.4 

3 300 11 

4 600 22 

5 1500 54 

6 3000 110 

7 6000 2 20 

It is unlikely that a single surge waveshape, such 

as that of Figure 3, can meet all of the needs of a mature TCL system any more than the 1.2 x 50 ps waveshape 

Voltage - 1000 Vlmajor div. meets all of the needs of the BIL system. For many apCurrent - 40 Almajor div. plications other waveshapes may be needed. The authors 

suggest that the 1.2 x 50 us unidirectional and the SWC Figure '. htput Chamcteristics Of TCL waveshapes be considered as part of the TCL family of 

For a standard to be practical, there must be a waveshapes. The family would then consist of: 

1 imited number of test levels reflecting system volt- 1. A relatively slow, low amplitude overvol tage, 

ages and application requirements. The authors propose representing switching transients and some 

that the levels shown in Table 1 be defined. The table fault clearing transients. This may be the 

is based on a geometrical progression and deliberately unidirectional 1.2 x 50 us waveform from the 

includes, but is not limited to, levels having the mag- BIL system. nitudes 600, 3000 and 6000 V specified in other standards [2,5, and 71. Intermediate levels, if needed, 2. A composite waveshape, including a fast rise 

should also be based on a geometric progression. time and oscillatory decaying tail. This will 

test some dvldt effects as well as the energy 

A concept requiring emphasis is that the defini- hand1 ing capability of suppressors. This 

tion of a series of levels from which a choice of a would be the waveshape shown in Figure 3. 

level may be made is a task separate and distinct from 3. A very fast burst-type oscillation, such as 

the task of choosing which level is appropriate for the current SWC test. This will test devices 

any particular application. This latter choice must against dielectric failures as well as electromagnetic interference. 

942 

35-A IAS 76 ANNUAL 
Depending upon the application, the degree of re1 i - 

ability required, and the available knowledge of the 

environment, one or more of these three tests could be 

specified for a particular system. 

EXAMPLES OF TCL SELECTION 

Two examples of TCL selection are presented: one 

leads to the selection of one of the proposed levels 

of Table 1 and the other does not. The first example 

deals with ground fault circuit interrupter (GFCI) devices. A combination o f f i e l d experience, qua1 ification testing [6], and a priori requirements has indicated that these devices should not functionally fail 

at 6 kV and should not experience nuisance tripping 

when exposed to 3 kV transients [7]. 

The type of transients to which these devices are 

most apt to be exposed are shown in Figure 5. A TCL 

of 6 kV (Level 7) would be appropriate for qualification testing of devices unprotected by circuit protection devices. However, protection devices that can 

clamp transients to 3 kV or less are available. A 

production test might be run at the 3 kV level on ininterrupters fitted with protective devices. The purpose of the test would be to demonstrate that the protective devices could withstand the current surge associated with a TCL of 6, rather than demonstrate that 

the GFCI could withstand 3 kV at its terminals. 

An example of a specialized system in which the 

proposed subdivision of levels shown in Table 1 may be 

too coarse in the static power converter shown in Figure 7. In a static power converter, the voltage transients are best separated into three frequency ranges or transient durations. High frequency transients 

(tens of microseconds duration) are basically lightning-related transients and some load switching transients. Medium frequency transients (hundreds of microseconds duration) include power system switching 

surge transients as well as SCR comutating transients. 

Low frequency transients (mil 1 iseconds duration) are 

confined to 60-Hz overvol tages and 1 eg fuse clearing 

vol tages . 

TRANSFORMER AC LINE LINE TO BRIDGE AND SNUBBERS DC FILTER 

685 V L-L FILTERS GROUND C 602 THYRISTORS LlrF 

I250 kVA 4pF FILTERS 12pHSO5pF, 26n 5n 

8 5% Z 15 fl 0 lrF 

Figure 7. Converter Bridge circuit Used in 

me-Clearing !7?ansient Tests 

The high frequency transients enter the system by 

capacitive coupling through the transformer and have 

relatively high source impedance (50 to 100.Q). The 

waveshape of Figure 3 is very appropriate for simulation 

of these effects. The medium and low frequency transients are inductively coupled or generated in the 

secondary winding and have fairly low source impedances 

(0.1 to 0.001n). Such waves primari ly stress insulation 

and semiconductors. The 1.2 x 50 ps waveshape would be 

appropriate for simulation of these effects. 

Basically, the TCL selected for the power converter 

will be an economic compromise between the cost of high 

transient withstand capability in the equipment and the 

cost of suppressor devices. Two 1 imi ting possibilities 

exist: 1) desi n the equipment (basically SCR reverse voltage ratings? to withstand the highest transient expected or; 2) design the equipment to withstand only 

the peak reverse operating voltage and clamp all transients to that level. 

Nominal peak reverse voltage for the bridge is 

1000 V. Commutating voltage spikes would nonnally reach 

1300 V. If these spikes were suppressed to 1200 V with 

a nonl inear varistor material , the varistors would be 

subjected to transient currents of 10 to 20A, and there 

would be about 50W of continuous power released in the 

varistors. Single pulse transients, such as these generated when a leg fuse is called upon to interrupt a 

short circuit, can reach 1600 to 1800 V. (Figure 8). 

Suppressing these transients to a 1200 V level would 

require that the suppressors handle 10 to 20 kA. High 

frequency transients, such as those produced by lightning or arcing contacts on switches, can reach 5000 

These transients can be easily suppressed to a 2000 V 

level with suppressors capable of handling about 100A. 

Upper trace : Line- to-1 ine voltage , 800 V/div 

Lower trace: Line current, 9.6 kA/div 

Sweep: 2 ms/div 

Figure 8. T~nsient Produced by Fuse BWng 

in Converter Circuit of Figure 7 

The proposed standard TCL's of 1500, 3000, and 

6000 (based on a 'JI0 progression) are not well adapted 

for this application. If SCR's of 1300 PIV were used, 

the transients associated with a 1500 V TCL would 

exceed the PIV and transients could only, with diffi-; 

culty, be suppressed to 1500 V in the first place. 

Transients could easily be suppressed to a 3000 V level, 

but the cost of SCR's with a PIV of 3000 V would be 

very high. Silicon controlled rectifiers with a PIV of 

1800 V are another possibil ity. These could be tied 

into a TCL progression based on 6d0. Appropriate 

levels, rounded to convenient values, would be 600, 

1000, 1500, 2000, 3000, and 4500 V. An e;momically appropriate TCL for the power converter might then be 

2000 v. 

ACKNOWLEDGMENTS 

Helpful comnents and suggestions on the TCL proposal have been received since the closing of the reference 3 paper. The feedback from Messrs. H.A. Gauper 

of Corporate Research and Development. General Electric 

Company, P. Richman of KeyTek Instrument Corp., A.W. 

Smoot of Underwriter's Laboratories, and P.D. Speranza 

of Be1 1 Telephone Laboratories are gratefully acknowledged. 

BAS 76 ANNUAL 35-A 943 
REFERENCES 

IEEE, SPD Committee, W.G. t3.4.4. "Guideline - Surges 

in AC Power Systems Rated 600 V and Less" - Project 

587.1. Inquiries and contributions concerning this 

project are invited; contact F.D. Martzloff, General 

Electric Co. 37-442, P.O. Box 43, Schenectady, NY. 

ANSI C37.90a-1974 (IEEE Std 472-1 974). "Guide for 

Surge Withstand Capabi 1 i ty ." 

F.A. Fisher, F.D. Martzloff, "Transient Control 

Levels - A Proposal for Insulation Coordination in 

Low-Voltage Systems, IEEE Trans. PAS-95, No. 1, 

Jan/Feb 1976, pp. 120-129. 

J.H. Bull, "Impedance of the Supply Mains at Radio 

Frequencies." 1st Symposium on EMC, Montreux, 1975. 

IEEE 75CH1012-4 Mont. 

MIL-STD-461A. "Electromagnetic Interference Characteristics Requirements for Equi pment." 

D.W. Nestor (Rucker Electronics), private comnunication (1972). 

Undenvri ters Laboratories, Inc., "Proposed Requirements for S~rge Tests of Ground Fault Circuit Interrupters. Me1 vi 1 le, NY , July 1975. 

IAS 76 ANNU1