Fluke 435 I, 435 II, 437, 437 II Service Guide

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Page 1

Common power

Unbalanced load: kVA

TOTAL

= kVA1 + kVA2 + kVA

kVA

Red

Black

kVA

ø1

ø2

ø3

quality factors

affecting transformers

Application Note

Commercial buildings commonly have a 208/120 V transformer in a delta-wye configuration to feed receptacles. Single-phase, non-linear loads connected to the receptacles produce triplen harmonics, which add up in the neutral. When this neutral current reaches the transformer, it is reflected into the delta primary winding where it causes overheating and transformer failures.

Another transformer problem results from core loss and copper loss. Transformers are normally rated for a 60 Hz phase current load only. Higher frequency harmonic currents cause increased core loss due to eddy currents and hysteresis, resulting in more heating than would occur at the same 60 Hz current.

Transformers supplying nonlinear loads should be checked periodically to verify operation

within acceptable limits. Transformers are also critical to the integrity of the grounding system.

Factors

1. Transformer loading (kVA)

Start by measuring kVA and determine wether the transformer load is balanced.

Connect voltage probes on

•

Phase 1 and Neutral and clamp current probe on same phase. Repeat for Phase 2 and

3. Use a single phase power

•

quality analyzer to read kVA of each phase and sum all three for total transformer kVA. Or, connect all four current

•

clamps and all five test leads for the three phase power quality analyzer to read kVA for each phase and the total.

Compare actual load kVA to

•

nameplate kVA rating to determine % loading.

When using a single phase analyzer on a balanced load, a single measurement is suffic

ient. Transformers loaded at less than 50 % are generally safe from overheating. However, as loads increase, measurements should be made periodically. At some point the transformer may require derating.

Figure 2. Harmonic spectrum.

2. Harmonic spectrum

The harmonic spectrum of the secondary (load) current will give us an idea of the harmonic orders and amplitudes present:

In a transformer feeding sin

•

gle-phase loads, the principal

n is the

onc

ill add arith-

ye transformer

harmonic of c

. The 3rd w

3rd metically in the neutral and

irculate in the delta primar

c of a delta-w The good news is that the delta-w rest of the system from the 3rd (though not the 5th, 7th or

ye tends to isolate the

other non-triplen harmonics).

e 1

Figur

. Measuring transformer load (unbalanc

From the Fluke Digital Library @ www.fluke.com/library

ed) using a single phase power quality analy

The bad new transformer pa w

zer

ith additional heat

s is that the

ys the pric

Page 2

Table 1: Measurements at the distribution transformer

Measurement

. kVA Transformer loading. If loading exceeds 50 %, check for harmonics

2. Harmonic spectrum 3rd harmonic (single-phase loads)

3. THD Harmonic loading within limits:

-factor Heating effect on transformer from harmonic loads

4. K

5. Ground currents

In a transformer feeding three-

•

phase loads which include drives or UPS systems with 6pulse converters, the 5th and 7th harmonic will tend to predominate. Excessive 5th is of particular concern because it is negative sequence. It will tend to produce counter-torque and overheating in polyphase motors. Harmonic amplitudes normally

•

decrease as the frequency goes up. If one frequency is significantly higher in amplitude than lower frequencies, we can suspect a resonant

Look for

and possible need for derating.

Harmonic orders/amplitudes present:

•

5th, 7th (primarily three-phase loads)

e of higher order harmonics

Resonanc

•

Effectiveness of harmonic trap filters

•

Voltage %THD < 5 % Current %T

•

HD < 5-20 % (Table 2)

Objectionable ground currents are not quantified but are prohibited by the N

eutral-ground bond in place

N ESG (Electrical Safety Ground) connector to ground electrode (typically building steel) in place

condition at that frequency. If such a condition is detected, be sure to take readings at capacitor banks to see if the caps are experiencing overcurrent/overvoltage conditions. Before-and-after harmonic

•

spectrum measurement is extremely valuable to determine if harmonic mitigation techniques, like trap filters, which are tuned to specific frequencies, are sized properly and are working as expected. Different harmonic frequencies

•

affect equipment in different ways (see below).

Harmonic Sequences

Name F 2nd 3rd 4th 5th 6th 7th 8th 9th

180 240 300 360 420 480 540

Frequenc Sequence +—0+—0+—0

Rule: If waveforms are symmetrical, even harmonics disappear.

Eff

Sequence Rotation Effects (from skin effect, eddy currents, etc.)

Positive Forward Heating of conductors, circuit breakers, etc. N Zero None Heating, + add in neutral of 3-phase, 4-wire system

Harmonics are classified as follows:

1. Order or number: Multiple of fundamental, hence, 3rd is three times the fundamental, or

3. Sequence:

ects of Harmonic Sequences

egative

0 Hz

1 Odd or even order: Odd harmonics are generated during normal operation of nonlinear loads. Even harmonics only appear when there is dc in the system. In power circuits, this only tends to occur when a solid state component(s), such as a diode or SCR, fails in a

onverter c

Positive sequence. Main effect is overheating.

•

Negative sequence. Create counter-torque in motors, i.e., will tend to make motors go

•

backwards, thus causing motor overheating. Mainly 5th harmonic.

Zero sequenc

•

Reverse

ircuit

e. Add in neutral of 3-phase, 4-w

Heating as above + motor problems

ire system

. Mainly 3rd harmonic.

3. Total Harmonic Distortion

Check for THD of both voltage and current:

For voltage, THD should not

•

exceed 5 % For current, THD should not

•

exceed 5-20 % (see Odd Harmonics table)

IEEE 519 sets limits for har-

monics at the PCC (Point of Common Coupling) between the utility and customer (EN50160 is the European standard). IEEE 519 is based on THD measurements taken at the PCC. Technically, the PCC is the primary of the utility supply transformer (although there are cases where the PCC is at the secondary if the secondary feeds a number of customers). In practice, these measurements are often made at the secondary of the customer’s main transformer, since that is the point most easily accessible to all parties (and also since that is generally a Low Voltage measurement).

Some PQ practitioners have

broadened the concept of PCC to include points inside the facility, such as on the feeder system, where harmonic currents being generated from one set of loads could affect another set of loads by causing significant voltage distortion. The emphasis is on improving in-plant PQ, rather than on simply not affecting utility PQ.

3a. Voltage THD

THD has a long history in the industry. The underlying concept is that harmonic currents generated by loads will cause voltage distortion (E=IZ) as they travel through the system impedance. This voltage distortion then becomes the carrier of harmonics system-wide: if, for example, the distorted voltage serves a linear load like a motor, it will then create harmonic currents in that linear load. By setting maximum limits for voltage distortion, we set limits for the system-wide impact of harmonics.

2 Fluke Corporation Common power quality factors affecting transformers

Page 3

Table 2: IEEE 519 limits for harmonic currents at the point of common coupling

CR=Isc/IL <11 11-17 17-23 23-35 >35 TDD

<20 20-50

00 10.0 % 4.5 % 4.0 % 1.5 % 0.7 % 12.0 %

50-1

00-1000 12.0 % 5.5 % 5.0 % 2.0 % 1.0 % 15.0 %

000 15.0 % 7.0 % 6.0 % 2.5 % 1.4 % 20.0 %

Short circuit ratio (Isc/I

SCR=

Available short circuit current at PCC

Isc =

Maximum demand load current (rms amps)

IL=

TDD = Total demand distortion Note: IEE E allows these limits to be exceeded for up to one hour per day, while IEC allows

them to be exceeded for up to 5 % of the time. The concept of

ties,

ILis calculated by a

(information a Transformer rating could be used and would be the most conservative estimate (i.e., it would result in the lowest SCR), since it assumes that the transformer would be used at full capacity.

IL, maximum demand load current, is key to using Table 2. For existing facili-

vailable in billing records). For new installations,

Voltage distortion, however, depends on source impedance, i.e., on system capacity. It was quite possible for the first (or second or third) customer to inject significant harmonic currents into the system and not cause voltage THD to exceed 5 %. The entire responsibility for harmonic mitigation could fall on the last customers unlucky enough to push V-THD over 5 %, even if their particular harmonic load was relatively small-literally the straw that broke the camel’s back.

3b. Current T

To restore some fairness to this

(All percentages are % of IL, maximum demand load current)

Odd Harmonics

4.0 % 2.0 % 1.5 % 0.6 % 0.3 % 5.0 %

7.0 % 3.5 % 2.5 % 1.0 % 0.5 % 8.0 %

)

ing the maximum demand current for 12 consecutive months

verag

ILmust b

e estimated.

For equipment manufacturers, IEC 1000-3-2, published in 1995, is the applicable standard. It specifies maximum current levels out to the 40th harmonic. Its expected effective date is projected to be early 2001. To certify for CE, a requirement for the European market, manufacturers will have to meet this standard. This edict will have a major effect on power supply design.

For the facility, IEEE 519 is the standard (EN 50160 in Europe). The limits set in IEEE 519 for harmonic currents depend on the

size of the customer relative to the system capac

ity. (See Table 2.)

situation, standards for maximum current harmonics were added,

e current harmonics were

sinc under the control of the local

ility and equipment manufac

fac turer (rememb

er, harmonic “loads” act as “generators” of harmonics). This emphasis on the mitigation of current harmonics at the load, including the not-too-

Table 3

Inspection of Transf

Check for N-G bond. A high impedance N-G bond will cause

Check for g integrity of connection to building steel these connections, so they should be as low (exothermic weld). impedance as possible.

Check for tightness of all conduit connections. will tend to act as a “choke” for higher

rounding c

ormer Ground Explanation

onductor and

distant requirement that the load generate virtually no harmonics, has become the prevailing regu-

y philosophy

lator

. It puts the

burden of responsibility on the

Measure for grounding conductor. always be some ground current due to

ground currents on the Ideally there should be none, but there will

local site and on the equipment manufacturers

The SCR (Short Circuit Ratio) is a measure of the electrical size of the customer in relation to the utility source. The smaller the customer (higher SCR), the less the potential impact on the utility sourc

e and the more generous the harmonic limits. The larger the customer (smaller SCR), the more stringent the limits on harmonic currents.

3c. TDD and THD

TDD (Total Demand Distortion) is the ratio of the current harmonics to the

maximum load (IL). It dif- fers from THD in that THD is the ratio of harmonics to the

taneous

load. Why TDD instead

instan-

of THD? Suppose you were running a light load (using a small fraction of system capacity), but those loads were nonlinear. THD would be relatively high, but the harmonic currents actually being generated would be low, and the effect on the supply system would in fact be negligible. So who cares? TDD acknowledges this, and allows harmonic load to be referenced to the maximum load: if harmonic load is high at maximum load, then we have to watch out for the effect on the supply source. So where does that leave current THD as a useful measurement.

The closer the current THD reading(s) is taken to conditions of maximum load, the

closer it appr

voltage fluctuation.

ault currents will return to the source via

onduit is not itself g

If the c

frequencies and limit fault current (remember that fault currents are not just at 60 Hz but have high-f components).

normal operation or leakage of protective components (MOVs, etc.) connected from phase or neutral to ground. However, anything above an amp should be cause for suspic but experienc a feel for possible problems).

oximate

ion (there is no hard and fast rule,

ed PQ troubleshooters develop

s T

rounded, it

3 Fluke Corporation Common power quality factors affecting transformers

Page 4

A final word on measuring

480 V

208 Y/120 V

Neutral

Grounding electrode nearby, preferably structural metal

THD: the one place not to apply the specs is at the individual harmonic-generating load. This will always be a worst-case distortion and a misleading reading. This is b

ecause as harmonics travel upstream, a certain amount of cancellation takes place (due to phase relationships which, for practical purposes, are unpredictable). Measure at a PCC, or at the source transformer.

4. K-factor

K-factor is a specific measure of the heating effect of harmonics in general and on transformers in particular. It differs from the THD calculation in that it emphasizes the frequency as well as the amplitude of the harmonic order. This is because heating effects increase as the square of the frequency.

A K-4 reading would mean that the stray loss heating effects are four times normal. A standard transformer is, in effect, a K-1 transformer. As with THD, it is misleading to make a K-factor reading at the load or receptacle because there will be a certain amount of upstream cancellation; transformer K-factor is what counts. Once the K-factor is determined, choose the next higher trade size. K-factor rated transformers are available in

standard trade sizes of K-4, K-13, K-20, K-30, etc. K-13 is a common rating for a transformer supplying office loads. The higher ratings tend to be packaged into PDUs (Power Distribution Units) which are spec

ially designed to supply computer and other PQsensitive installations.

5. Ground currents

Two prime suspects for excessive ground current are illegal N-G bonds (in subpanels, receptacles or even in equipment) and socalled isolated ground rods:

Subpanel N-G bonds create a

•

parallel path for normal return current to return via the grounding conductor. If the neutral ever becomes open, the equipment safety ground becomes the only return path; if this return path is high impedance, dangerous voltages could develop. Separate isolated ground rods

•

almost always create two ground references at different potentials, which in turn causes a “ground loop” current to circulate in an attempt to equalize those potentials. A safety and equipment hazard is also created: in the case of lightning strikes, surge currents travelling to ground at different earth potentials will create hazardous potential differences.

Transformer grounding

The proper grounding of the transformer is critical. (Table 3.3.) NEC Article 250 in general and 250-26 in particular address the grounding requirements of the SDS.

A ground reference is estab-

•

lished by a grounding connection, typically to building steel (which, in turn, is required to be bonded to all cold water pipe, as well as any and all earth grounding electrodes). Bonding should be by exothermic weld, not clamps that can loosen over time. The “grounding electrode conductor” itself should have as low a high-frequency impedance as possible (not least because fault current has high frequency components). Wide, flat conductors are preferred to round ones because they have less inductive reactance at higher frequencies. For the same reason, the distance between the “grounding electrode conductor connection to the system” (i.e., N-G bond at the transformer) and the grounding electrode (building steel) should be as short as possible: in the words of the Code, “as near as practicable to and preferably in the same

...”

area The neutral and ground should

•

be connected at a point on the transformer neutral bus

. Although permitted, it is not advisable to make the N-G

ond at the main panel, in

b order to maintain the segregation of normal return currents and any g

round currents

. This point at the transformer is the only point on the system

G should be bonded.

where N

Figure 3. Transformer grounding.

4 Fluke Corporation Common power quality factors affecting transformers

Page 5

Solutions

There are a number of solutions for transformer-related PQ problems:

Install additional distribution

•

transformers (Separately Derived Systems) Derate transformers

•

Install K-rated transformers

•

Used forced air cooling

•

1. Separately Derived System (SDS)

The distribution transformer is the supply for a Separately Derived System (SDS), a term which is defined in the NEC (Article 100). The key idea is that the secondary of this transformer is the new source of power for all its downstream loads: this is a powerful concept in developing a PQ distribution system. The SDS accomplishes several important objectives, all beneficial for PQ:

It establishes a new voltage

•

reference

taps which allow the secondary voltage to be stepped up or down to compensate for any voltage drop on the feeders. It lowers source impedance by

•

decreasing, sometimes drastically, the distance between the load and the source. The potential for voltage disturbances, notably sags, is minimized. It achieves isolation. Since

•

there is no electrical connection, only magnetic coupling, between the primary and secondary, the SDS isolates its loads from the rest of the electrical system. To extend this isolation to high frequency disturbances, specially constructed “isolation transformers” provide a shield between the primary and secondary to shunt RF (radio frequency) noise to ground. Otherwise, the capacitive coupling between primary and secondary would tend to pass these high-frequency signals right through.

. Transformers have

A new ground reference is

•

established tion of the SDS is that it “has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.” (NEC 100) The opportunity exists to segregate the subsystem served by the SDS from ground loops and ground noise upstream from the SDS, and vice versa.

rated transformers

2. K-

Figure 4. Typical K-factor in commercial building.

Harmonics cause heating in transformers, at a greater rate than the equivalent fundamental currents would of their higher frequency. There are three heating effects in transformers that increase w frequency:

Hysteresis. When steel is

•

mag all line up, so that the North poles all point one way, the South poles the other poles switch with the polarity of the applied current. The higher the frequenc often the switching occurs, and, in a process analogous to the effects of friction, heat losses increase. Eddy currents. Alternating

•

mag whirlpools of current that create heat loss. This effect increases as a square of the

. Part of the defini-

. This is because

ith

netized, mag

netic fields create localized

netic dipoles

. These

y, the more

frequency. For example, a 3rd harmonic current will have nine times the heating effect as the same current at the fundamental. Skin effect. As frequency

•

increases, electrons migrate to the outer surface of the conductor. More electrons are using less space, so the effective impedance of the

onductor has increased; at

c the higher frequency, the conductor behaves as if it were a lower gauge, lower ampac higher impedanc

The industry has responded with two general solutions to the effects of harmonics on transformers: install a K-factor rated transformer or derate a standard transformer. Let’s look at pros and cons of the K-factor approach first. K-factor is a calculation based on the rms value, %HD (harmonic distortion) of the harmonic currents, and the square of the harmonic order (number). It is not necessary to actually perform the calculation because a harmonic analyzer will do that for you. The important thing to understand is that the harmonic

e w

ity,

ire.

5 Fluke Corporation Common power quality factors affecting transformers

Page 6

order is squared in the equation

100

20 40 60 80 100

Transformer Capacity (%) After Derating for Electronic Load

Switched-Mode Power Supply Load (% of Overall Load)

and that is precisely where the high- frequency heating effects, like eddy current losses, are taken into account.

K-rated transformers are

desig

ned to minimize and accommodate the heating effects of harmonics. K-rated transformers do not eliminate harmonics (unless additional elements like filters are added). They accommodate harmonics with techniques such as the use of a number of smaller, parallel windings instead of a single large

inding: this gives more skin for

w the electrons to travel on. The primary delta winding is up-sized to tolerate the c

irculating third harmonic currents without overheating. The neutral on the secondary is also up-sized for third harmonics (typically sized at twice the phase ampacity).

Application issues with K-factor transformers

K-rated transformers have been widely applied, but there are certain issues with them. Many consultants do not see the need for using transformers with a rating higher than K-13 although K-20 and higher might be supplied as part of an integrated Power Distribution Unit (PDU). Also, early applications sometimes overlooked the fact that K-rated transformers necessarily have a lower internal impedance. Whereas a standard transformer has an impedanc 5-6 % range, K-rated transformers can go as low as 2-3 % (lower as the K In retrofit situations, where a standard transformer is being

ed by a K

replac of equivalent kVA, this may require new short circuit calculations and re-sizing of the secondary overcurrent protective devices.

6 Fluke Corporation Common power quality factors affecting transformers

e typically in the

-rating increases).

-rated transformer

3. Derating standard transformers

Some facilities managers use a 50 % derating as a rule-of-thumb for their transformers serving single-phase, predominantly nonlinear loads. This means that a 150 kVA transformer would only supply 75 kVA of load. The derating curve, taken from IEEE 1100-1992 (Emerald Book), shows that a transformer with 60 % of its loads consisting of SMPS (switched-mode power supplies), which is certainly possible in a commercial office building, should in fact be derated by 50 %.

The following is an accepted method for calculating transformer derating for single-phase loads only. It is based on the very reasonable assumption that in single-phase circuits, the third harmonic will predominate and cause the distorted current waveform to look predictably peaked.

Use a

true-rms meter to make

these current measurements:

1. Measure rms and peak current

of each secondary phase.

(Peak refers to the instanta-

neous peak, not to the inrush

or “peak load” rms current).

2. Find the arithmetic average of

the three rms readings and the

three peak currents and use

this average in step 3 (if the

load is essentially balanc

this step is not nec

3. Calculate Xformer Harmonic

Derating Factor:

xHDF = (1.414 * IRMS) / IPEAK

4. Or, since the ratio of

Peak/RMS is defined as Crest

Factor, this equation can be

ritten as:

rew

DF = 1.414 / CF

If your test instrument has the

capability, measure the CF of

each phase directly. If the load

is unbalanced, find the aver-

age of the three phases and

use the average in the above

formula.

Since a sine wave current waveform has a CF=1.414, it will

ve an xH

DF=1; there will be no derating. The more the 3rd harmonic, the higher the peak,

ed,

essary).

Figure 5. Transformer derating curve (IEEE 1100-1992)

the higher the CF. If the CF were

2.0, then the xH

DF=1.414 / 2 =.71. A CF=3 gives us an xHDF =.47. A wave with CF=3 is about as badly distorted a current waveform as you can expect to see on a single-phase distribution transformer.

Caution: This method does not apply to transformers feeding three-phase loads, where harmonics other than the third tend to predominate and CF is not useful as a simple predictor of the amount of distortion. A calculation for three-phase loads is available in ANSI /IEEE C57.110. However, there is some controversy about this calculation since it may underestimate the mechanical resonant vibrations that harmonics can cause, and that accelerate transformer wear above and beyond the effects of heat alone.

4. Forced air cooling

If heat is the problem, cooling is the solution. Break out the fan, turn it on the transformer and use forced air cooling. Some experi-

ed hands fig

enc 20-30 % on the up side. In any case, it can only help.

ure that’s worth

eping your world

Fluke. K

up and running.

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