PX8000 Precision Power Scope with Features of High-accuracy Power Meter and Waveform Measuring Instrument

Yokogawa Technical Report English Edition Vol.58 No.1 (2015)

PX8000 Precision Power Scope with

Features of High-accuracy Power

Meter and Waveform Measuring

Instrument

Osamu Itou

Satoru Suzuki

Hiroshi Yagyuu

Kazuo Kawasumi

Yokogawa developed the PX8000 precision power scope, a high-accuracy power meter,

which can measure reactor losses in inverters, motors and the like, by analyzing waveforms.

The major specifications of the instrument are as follows: basic accuracy is 0.2%,

voltage measurement bandwidth is DC and 0.1 Hz to 20 MHz (

3dB, typical), and current

measurement bandwidth is DC and 0.1 Hz to 10 MHz (

3dB, typical). The PX8000 offers

the functionality usually provided by a waveform measuring instrument, such as a variety

of triggers, tracking, statistical processing, and waveform parameter calculation functions.

Furthermore, to improve measurement accuracy at low power ratios this product comes with

a de-skew function for correcting signal delays from the current sensor and a data latency

adjustment function. This paper describes the PX8000, focusing on a newly developed

element dedicated for power measurement and technology for phase correction.

INTRODUCTION

long with the development of power electronics

technology, microcontrollers are used for electronic

control in every area of electrical energy conversion. As

a result, non-conventional electric power measurement

technology and analysis functions are required for measuring

instruments.

The power meters generally used for measuring electric

power are suitable for measuring signals that slowly change,

but not for evaluating dynamic characteristics including

rapidly changing signals, nor for targets with high-frequency

signals. In contrast, oscilloscopes are designed for observing

high-frequency waveforms, and are therefore not suitable

for accurately measuring electric power. Power meters are

preferred for measuring electric power in terms of traceability

for which AC accuracy is required. The newly developed

PX8000 precision power scope shown in Figure 1 has the

advantages of both measuring instruments. The PX8000

can perform high-precision measuring of electric power in

devices with rapid signal uctuation in them or those driven at

high frequencies, which was difcult for existing measuring

instruments to do.

Figure 1 External view of the PX8000

DEVELOPING ELEMENTS FOR MEASURING

ELECTRIC POWER

The PX8000 was developed based on the DL850

ScopeCorder

(1)

, and existing waveform measuring instruments.

Although the DL850 can measure voltage, current (by using

a current probe) and electric power, it does not conform

to the accuracy standard required for power meters, and

does not have the function of directly inputting DC, nor

of matching voltage and current phases. The PX8000 is

designed in a manner where a high-precision electric power

measuring element, a set comprising a voltage module and

a current module, is inserted into two slots of the input

module section of the DL850. Some portions of the DL850

components, including the peripheral circuit of the CPU, eld

programmable gate array (FPGA) for waveform processing,

housing and rmware were reused. As a result, the number of

man hours required for development was halved. The majority

9 9

*1 Development & Engineering Department 1,

Yokogawa Meters & Instruments Corporation

*2 Marketing Department,

Yokogawa Meters & Instruments Corporation

PX8000 Precision Power Scope with Features of High-accuracy Power Meter and Waveform Measuring Instrument

Yokogawa Technical Report English Edition Vol.58 No.1 (2015)

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of the effort required to develop the PX8000 was made in the

development of the electric power measuring elements.

The market is demanding higher sampling rates and wider

bandwidth. Therefore, the sampling rate was determined to

be 100 MS/s (mega samples/second), which is 20 times higher

than that of existing models, and is a sampling rate required for

measuring inverters and similar components. In addition, the

PX8000 has achieved the bandwidth for voltage measurement

of DC and 0.1 to 20 MHz, which is 4 times wider than that of

existing models, and the bandwidth for current measurement

of DC and 0.1 to 10 MHz, which is twice as wide as that of

existing models.

Optical Transmission Technology for High-speed Sampling

To achieve the sampling rate of 100 MS/s, the isoPRO

technology, a high-speed and high-voltage isolation technology

using optical transmission, was used. This technology was

originally developed for the 720210, which is the isolation

module of the DL850 with a high-speed sampling rate of

100 MS/s and 12-bit width. This enabled high-voltage isolation

of 1000 Vrms. Figure 2 shows the optical transmission part

of the PX8000, and Figure 3 shows a block diagram of the

isolating part of the electric power measuring element.

Figure 2 Optical transmission part of the PX8000

Electric Power Measuring Element

The PX8000 is based on the DL850, but the following

specications have been modied to meet the requirements as

an electric power meter.



Improved noise resistance (tolerance to pulse noise)



Increased internal power consumption (max. 2.5 W per

element) caused by inclusion of a shunt resistor



Added zero-crossing detection circuit (a circuit for

detecting the center point of a wave)

To achieve these specifications, the following

countermeasures have been implemented.



Noise resistance was improved as follows: First, analysis

of noise flow routes was carried out. Then, based on the

results, several parts of the shields were strengthened

including the optical transmission part, and noise routes

that do not affect the circuits were ensured.



Measures against increased internal power consumption

were taken as follows: First, heat dissipation routes

were analyzed. Then, based on the results, the mounting

mechanism of the heat exhaust fans and heat dissipation

routes was improved.



To help ensure stable operation of the zero-crossing

detection circuit, an input rejection circuit was added in

front of it. Because the zero-crossing detection circuit

is AC-coupled, it is susceptible to voltage fluctuations

caused by internal calibration for measuring the offset in

the internal circuits. In particular when an input signal has

an offset, it takes time to stabilize, and the correct zero-

crossing point cannot be detected. To avoid this problem,

an input rejection circuit was added in front of the zero-

crossing detection circuit so that it is not affected by the

voltage uctuation caused by the calibration.

Opt-OUT

2 Gbit/s

Opt-IN

2 Gbit/s

FPGA

Opt-OUT

100 MHz

Opt-IN

100 MHz

EEPROM

Reset

Infrared-In

(Detector

using OEIC)

Infrared-Out

(LED)

PX8000

Main

Part

Analog

Circuit

10 b to 8 b

Converter

8 b to 10 b

Converter

50 MHz

Isolated side Non-isolated side

Data transfer (2 Gbit/s)

Sampling clock (100 MHz)

Transferring settings for

measuring elements

Infrared

isoPRO

Sampling clock

(100 MHz) and setting

for conversion

AD Converted data

Figure 3 Block diagram of the isolating part of the electric power measuring element