DAC & VLF Voltage Sources Educational Session

May 26 2011

St Pete Beach, Fl

by

Henning Oetjen, Frank Petzold

HDW\

Group Company

Why do we need to talk about Voltage Sources ? (in context for field testing methods of underground power cables)

1. Suitable Voltage Sources are the prerequisite for any

type of field testing of Distribution and Transmission

type Underground Cables

2. Since DC test methods are in most situations not suitable

or permissible for testing Solid Dielectric Cables, AC Test

Methods using AC Voltage Sources are required

3

Termination

local issue Insulation

global issue

Joint

local issue

Insulation

local issue

VLF & DAC Voltage Sources are at the heart of

some of the most significant methods for

Cable Testing and Diagnostics

Selection of AC Voltage Sources

2 Criteria need to be addressed:

a. it is desirable for the AC source to generate 50/60Hz (Power

frequency range) Voltages, if the test methods requires to

replicate the same type of “effects” within the cable

insulation as are induced under operating frequency

b. AC Sources must be able to “produce” enough

power to charge not only “small” and “short” cables

(URD), but also “big” and “long” cables (Feeder, Network)

Problems associated with Selection

a. if 50/60 Hz are required, a substantial amount of

“reactive” power must be produced due to the capacitive

effect of the cable insulation (E= 1/2C x V2) [J] or [Wsec]

C = capacitance of cable in µF; V = test voltage[RMS]

b. ……worse, this reactive or “blind” power must be made

available 2 times in every cycle within a ¼ of a cycle

to raise the voltage from 0 to the desired test level

( ¼ cycle = 4 milli seconds for 60Hz!!)

c. ……even worse, this power must be also dissipated

2 times within ¼ cycle of every cycle

What are the Consequences of

E= 1/2C x V2 for 50/60 Hz?

In terms of

a. The rating of the power supply high

b. The weight and size of the power supply heavy / big

c. The capability to dissipate the energy “forced”

cooled Example Cable1 ,15kV Class, C= 0.5µF, Test voltage 25kV RMS

E = 156J or Wsec

P60Hz=40kW ( must have 40kW available every other 4 milli sec for

4 milli sec and must dissipate 40kW every other 4 milli sec

Cable2 , 35kV Class, C= 0.5µF, Test voltage 60kV RMS

E = 900J or Wsec

P60Hz =225kW(must have 225kW available every other

4 milli sec for 4 milli sec and must dissipate 225kW every 4 milli sec

E = Electrical Energy P = Electrical Power

Conclusions a. True 50/60Hz field tests of cables are to say the least for the

most part impracticle

b. Must develop AC method that requires less energy

Solutions a. Use a Very Low Frequency AC voltage VLF (0.1Hz)

b. Use a “resonant type” voltage source, which “conserves” most

of the energy and does not “dissipate” the energy

between pos. & neg. cycles

- const frequency –variable inductance

- const inductance – variable frequency,

const frequency means 50 or 60Hz,

variable frequency means power frequency range 20-300Hz (per IEC 62067, 650840)

c. Use a combination of VLF & Resonant Technologies

d. Use a combination of “Damped AC” & Resonant in a

“pulsed” application to limit the amount of energy to a few

cycles within the power frequency range

VLF Voltage Sources

2 Technologies available today

a. Sinusoidal 0.1Hz,

- makes use of the VLF technology to reduce “reactive” power

- most effective technology for small test capacitance @ fairly

modest test voltages

- only suitable wave shape to perform tan∂ measurements

- “Electrical Stress” dV/dt approx.600 x less compared to 60Hz

- Higher test capacitances are achieved by reducing the test

frequency from 0.1Hz to as low as 0.01Hz,

Caution: results not directly comparable between different frequencies!

Stress Level dV/dt very different & # of cycles different

Summary: - the only wave shape for VLF tan ∂ measurement,

- cost effective for smaller & lower voltage type cables

Time (seconds)

0 105

VLF Voltage Sources

a. Sinusoidal 0.1Hz,

State of the art Block Diagram according to

Time (seconds)

0 105

VLF Voltage Sources

sinusoidal 0.1Hz

Time (seconds)

0 105

All test voltage values in RMS!

The Growth Rate of Electrical trees depends on the Frequency

0

1

2

3

4

5

6

7

0 0.5 1 1.5 2 2.5 3

V / V 0

0.1

Hz d

issip

ati

on

fa

cto

r

Dissipation Factor tan ∂

of New and Serviced aged XLPE Cables

VLF Voltage Sources

b. Cosine rectangular 0.1Hz

Combination of VLF & Resonant Technologies

- produces equipment with high test capacitance at high voltages

(highest test capacitance 25µF @ 60kV RMS = 84kV Peak)

- only suitable wave shape to measure leakage current

(similar to DC test, due to 5 sec “plateau” phase )

- not suitable wave shape to perform tan∂ measurements

- “Electrical Stress” dV/dt approx. identical compared to 60Hz,

- Only feasible due to “energy saving resonant design”

(allows to provide the charging energy within a 2-12 milli second window)

Summary: - the only wave shape to measure leakage current

- cost effective to test larger and longer cables

- delivers the same “punch” as 60Hz dV/dt (withstand test)

T cosine shaped transition,

approx. 2 to 6 ms

Time (seconds)

0 105

T

VLF Voltage Sources

b. Cosine rectangular 0.1Hz, Block Diagram

T cosine shaped transition,

approx. 2 to 6 ms

Time (seconds)

0 105

T

13

Measurement of leakage current

approx. between 4 & 5 seconds

into the “Plateau Phase”

VLF Voltage Sources Specifics related to:

Cosine rectangular 0.1Hz

Relationship between Peak & RMS

between cos rect & sinusoidal

wave shape

T cosine shaped transition,

approx. 2 to 6 ms

Time (seconds)

0 105

T

VLF Voltage Sources Specifics related to:

Cosine rectangular 0.1Hz

T cosine shaped transition,

approx. 2 to 6 ms

Time (seconds)

0 105

T

The Initialization of Electrical Trees is a function of Frequency & Peak Voltage The Growth Rate of Electrical Trees is a function of the Frequency & Energy

DAC Voltage Source (Damped AC )

Combination of Resonant and Pulse Technologies

- DAC based field test equipment capable of High Test Capacitance @ HV

(highest test capacitance 13µF @ 350kV Peak)

- DAC wave shape to allow PD testing within power frequency range (20-300Hz)

- DAC wave shape simultaneously displays PDIV and PDEV

- DAC wave shape visualizes & estimates Dielectric Losses (tan ∂)

- DAC represents very low “risk” of cable due to short HV pulse exposure

Summary:

cost effective method to PD test larger, longer & higher voltage cables

16

Block Diagram DAC (Damped AC) Technology

DAC Voltage Source

DAC Voltage Source Specific performance characteristics

PDIV

can be detected simultaneously, because of

“pulse” application = non continuous power

PDEV

Dielectric Losses = tan∂ can be calculated

from attenuation of wave shape,

Δ indicated by “funnel”

0

Q /pC

PDEV PDIV

Ground noise level

U

U

Typical Hysteresis Effect of PD PDEV < PDIV

DAC Voltage Source

Comparison of basic PD Test parameters between

“continuous” & “pulsed” AC Voltage

Specific performance characteristics