pls - blondel monitoring all fases

7/30/2019 PLS - Blondel Monitoring All Fases

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SolutionsPowerLogicSolut ions

Volume 3, Issue 1 Beyond BlondelMonitoring all phases on a three-wire circuit

In this issue, we

discuss ground faultmonitoring with the

PowerLogic CircuitMonitor.

The Problem

An automotive manufacturer was in the start-up phase of a new produc-tion plant. The electrical system serving the new equipment was a 480Vthree-wire, ungrounded (delta) circuit. All of the new production equip-mentair compressors, motors, conveyors, lightswas operating fine.During final commissioning of the electrical system, however, electriciansmeasured unusual voltages.

Their readings showed that phase-to-phase voltages were about 480V onall three phases. Phase-to-ground readings, however, measured 130V,

550V, and 690V on the three conductors.(N ormal voltages to ground on a 480Vungrounded system are approximately277V.) Detection devices at the mainswitchgear showed there was a ground faul t(see sidebar on page 4) somewhere in thenew plant, but an exhaustive investigationproved fruitless. Electricians added to thepuzzle by discovering that the strange phase-to-ground voltage readings returned tonormal when the ground fault detection

system was removed from service. Eventhough plant equipment was operatingproperly, plant personnel were concernedthat the high voltages to ground wouldcause insulation failures and catastrophicelectrical faults.

This issue ofPowerL ogic Solut ionsdescribesground fault monitoringandneutral instabili ty,and recommends a way to measure groundfaults on ungrounded systems with thePowerLogic C ircuit Monitor fromSquare D Company.

The PowerLogic Circuit Monitor is designed to perform meteringfunctions; i t i s not a protec t ive re lay designed to pr ov ide c i rcu i tpro tec t ion. Think of the circuit monitor as device used for groundfault detection, not p r otection. Alarm/relay functions are includedwith the circuit monitor to provide alarm annunciation and, in caseslike this application, to be used in conjunction with existing groundfault protection systems.

Figure 1. In additionto its primary powermonitoring functions,the PowerLogic

Circuit Monitor canbe used for groundfault detection.


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Key Concepts and Terms

Neutral Instability. Neutral instability, a form offerroresonance, is a rare problem. It occurs onthree-phase ungrounded circuits that are

equipped with ground fault monitoring throughvoltage transformers (VTs, previously calledpotential transformers, or PT s) connected phaseto ground. The term itself seems to be amisnomer because a three-phase, three-wirecircuit has no neutral conductor. Neutralinneutral instability refers to the single imaginarypoint in a delta (three-phase, three-wire) circuitat which all three phases measure equal voltages.This point in a delta circuit is typically at

ground potential, except during a ground faultor neutral instability. (See figure 2.)

Neutral instability is caused by the interaction ofthe charging capacitance of an ungroundedcircuit and the nonlinear inductance of VTsconnected to ground. When conditions ofcapacitance and inductance are right, neutralinstability occurs. It can occur on a systemwhich has operated normally for some time. Itcan be caused by a ground fault detection ormonitoring design which has not causedproblems on other three-wire circuits.

The common characteristic of systems withneutral instability problems is that these systemshave VTs connected phase to ground somewhereon the circuit. Ungrounded systems normally donot have loads connected phase to ground; allloads, single-phase or three-phase, are con-nected phase to phase. In some ground faultdetection schemes, all three phases must bemonitored with respect to ground. When thedetection system includes VTs connected phase

to ground, neutral instability can occur.

PowerLogic

Solut ions

2

Monitoring of Ungrounded Circuits

Power monitoring of three-phase, three-wire accircuits is typically accomplished with twovoltage inputs and two current inputs to thecircuit monitor. This method of monitoringusesBlondels Theorem, which states that a circuitcan be monitored accurately with one lessvoltage/current sensing channel than there arecurrent-carrying conductors. A three-phase,three-wire circuit, therefore, requires two voltagesensing inputs and two current sensing inputs. Athree-phase, four-wire circuit requires three VTsand three current transformers (CTs).

Blondels theoremis based on simpleelectrical circuitprinciples. These

principles provethat the vector sumof voltages orcurrents for thecurrent-carryingconductors isalways zero. Thus,the voltage orcurrent on anunmetered conduc-tor can be accu-rately calculatedfrom the voltagesand currents on

the metered phases,for all power system parameters.

However, some customers want to monitor al lthree phasesof a three-wire circuit. I t isnt thatthey dont trust Blondel, but they are interestedin ground fault monitoring. During a groundfault, the grounded system becomes a current-carrying conductor and Blondels theorem isviolated to a small degree. For most applica-tions, this only causes a small error in powerreadings: ground fault currents are typicallylow (fewer than 10 amps on a 480V system)compared to phase currents. However, to

prevent ground faults from remaining on thesystem for long periods, they should be identi-fied and repaired quickly.

What other reasons are there to monitor allthree phases of a three-wire system? Onecustomer wanted to track voltages with respectto ground during utility faults. This customerreported losing several induction motors duringutility faults, each due to line-to-chassis failures.He suspected that line-to-ground voltage swellswere initiating the failures.

PowerLogic Solutions

is produced by

Square D Companys

Power Management

Operation. Each issue

presents a problem

that might occur in a

typical power system,

and offers guidance

on how to solve it.

Normal480

480

480

277

277

277

Ground Fault480

480

480

480

0

480

Neutral Instability480

480

480

130 ?

550 ?

690 ?

FromToAB

BC

CA

AG

BG

CG

ImaginaryNeutral Point

A

B

C



Figure 2. Typicalvoltage readingsfrom an ungrounded

circuit under normal,ground fault, andneutral instabilityconditions.


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3

L ine-to-ground readings will actually berepresented as line-to-neutral values. Since line-to-ground voltages drop to zero on the groundedphase and increase to line-to-line values on the

ungrounded phases, over- or undervoltagetriggers. Circuit monitor alarms can easily be set

to indicate ground fault conditions.

Conclusions

The unusual phase-to-groundvoltage readings at the new plantwere being caused by neutralinstability on a 480V ungroundedsystem. The instability wasintroduced by the voltage trans-formers serving the ground faultdetection system. These trans-formers were connected phase toground on the ungroundedcircuit, without regard for possibleneutral instability problems.

Square D recommended that thecustomer install resistors acrossthe VT s in the ground faultdetection system. The resistorsprovide damping in the LC(inductive-capacitive) circuit

formed by the transformer reactance and thesystem charging capacitance. Thus, the resistorschange the circuit characteristics to the pointthat no neutral instability occurs.

Circuit monitors can detectground faults whenconnected to a grounded instrument transformercircuit. But circui t m onitor s should never beused i nstead of a gr ound fault pr otect ion

system. The detection circuit should be equipped

with resistors to ensure neutral stability. Resistorsmust be sized according to the instrumenttransformer characteristics.

Circuit monitors detect ground fault conditionsby monitoring for abnormally low or highvoltages with respect to ground. Due to the lowamount of ground current, current variations areinsufficient to distinguish ground faults fromchanges in load current.

Ungrounded Source

VoltageTransformers

= Resistors

CM-2000

VaVbVc

Vn

R

R

R

R

Figure 3. Voltage transformers, connected to ground on an un-grounded system, require resistors to reduce neutral instabilityconcerns.

{

IG(small)

Ground FaultNo source ofground fault current

Ungrounded System

{

IG(large)

Ground FaultGrounded neutralpoint provides

source ofground current

Grounded System

IG

Figure 4. Ungroundedsystems are usedbecause inadvertentground faults do not

trip overcurrentdevices and shut downcritical processes.

Solution: Ground Fault MonitoringWith the Circuit Monitor

As stated earlier, the circuit monitor accurately

meters electricity parameters, but should not beused to replace overcurrent devices or ground-fault protection systems. However, the circuitmonitor can be used to monitor and alarm whenground faults occur.

Ground fault monitoring can be accomplishedas shown in figure 3. The resistors in the circuitserve to stabilize the voltage transformersconnected to ground and eliminate the VTs

as a source of neutral instability.

The resistors in figure 4 can be off-the-shelfheating elements, sized to meet the require-ments. The resistors provide damping load toreduce the adverse effectsof neutral instability. Toensure metering accuracy,the voltage transformersshould be instrumenttransformers (Square Dmodel 460R voltagetransformers, for ex-ample), not control power

transformers. Controlpower transformers canintroduce 5% meteringerror, plus several degrees of phase shift.

Transformers and resistors must be properlysized to ensure metering accuracy and neutralstability. Contact PowerLogic Technical Supportfor more details on your specific application.

Circuit monitors should be configured forsystem 40, as if monitoring a four-wire circuit.


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Ground Faults

Ground faults occur when energized conductors come in contact with

metallic parts normally at ground (zero voltage) potential. Common

causes of ground faults are nicks in wire insulation, failure in the

insulation in an electrical machine like a motor or heater, and contact

made between grounded parts and exposed energized conductors.

When ground faults occur on a circuit served by a transformer with a

grounded neutral point (grounded wye), high currents flow on the

faulted phase. These currents are usually sufficient to trip circuit

breakers or melt fuses. Even so, code requirements may call for a

ground fault protection system if expected ground fault current

exceeds a certain level.

Ground faults on ungrounded or high-resistance grounded transform-ers typically draw less than 10 amperes. Unlike faults between

phases, or between phase and ground on a grounded source, these

ground faults typically do not draw enough current to operate a circuit

breaker or fuse. If not detected and repaired, they can remain on a

circuit for considerable periods of time.

This ability to sustain a ground fault without tripping overcurrent

devices is the primary reason ungrounded or high-resistance

grounded systems are used. They serve critical processes that, when

unexpectedly shut down, cause high economic or human losses.

Most engineers prefer grounded systems, especially for serving

sensitive electronic equipment. As an alternative, they may consider a

high-resistance grounded system for critical loads. High-resistance

grounded systems offer some benefits of the grounded system, but do

not shut down critical processes during a ground fault. IEEE Standard1100, the recommended practice for powering and grounding

sensitive equipment, recommends solidly grounded systems,

especially when serving electronic loads.

PowerLogic

Solut ions

4

While the circuit monitor provides a readymeans of ground fault detection, dont neglectits real power: monitoring a myriad of othersystem parameters and helping you drive cost

reductions, productivity improvements, andpower quality enhancements.

Square D Companys Power M anagementServices can assist you with ground faultdetection and monitoring. In addition, weprovide power quality consulting services to

ensure that power disturbances dont disruptyour operation.

Test Your Power Quotient

Refer to volume 2, issues 1 and 2, in addition tothis issue, for answers to the following questions.Answers are below.

True or False

1. Neutral instability refers to current unbalanceon a 208/120 V wye circuit.

2. Total harmonic distortion is aterm indicating the magnitudeof harmonics present in avoltage or current signal.

3. High-resistance groundedsystems have some characteris-tics of both ungrounded andgrounded circuits.

4. Circuit Monitors should beused for protective groundfault relaying.

1997 Square D All Rights ReservedBulletin No. 3000HO9701 10M DL 4/97

POWERLOGIC, SQUARE D, and areRegistered Trademarks of Square D Company.

Answers

1.False,three-phaseungroundedcircuitsinaferroresonantcondition(pagetwo,thisissue).

2.True(page3,volume2,issue2).

3.True(page4,thisissue).

4.False,theyshouldbeusedfordetection,notprotection(page1,thisissue).

5.False,thevaluesaredifferent(page3,volume2,issue2).

6.True(page2,volume2,issue2).

5. There is no difference between THD and thdreadings.

6. By double-clicking on the title block in afour-cycle waveform capture, you can obtaina breakdown of the harmonic characteristicsof a voltage or current signal.

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