Principles and Test Technology of Electronic Transformers

CHAPTE R 3

Principles and Test Technology of Electronic Transformers Chi Zhang⁎, Yu Cui†, Jianfei Ji⁎, Xingxin Guo⁎, Tuo Luo⁎, Xinyue Gong⁎ ⁎

State Grid Jiangsu Electric Power Research Institute, Nanjing, China †State Grid Jiangsu Electric Power Company, Nanjing, China

Chapter Outline 3.1 Overview of Electronic Transformers  63 3.2 Principles of Electronic Current Transformers  66 3.2.1 Active Electronic Current Transformers  66 3.2.2 Passive Electronic Current Transformers  69 3.2.3 Comparison of Different Principles  72

3.3 Principles of Electronic Voltage Transformers  73 3.3.1 Active Electronic Voltage Transformers  73 3.3.2 Passive Electronic Voltage Transformers  74

3.4 Test Technology of Electronic Transformers  76 3.4.1 Calibration of Electronic Transformers  76 3.4.2 Time Delay Test of Electronic Transformers  81 3.4.3 Polarity Test of Electronic Transformers  83 3.4.4 Heavy Current Test of Electronic Transformers  85

3.5 Application Issues of Electronic Transformers  85 3.5.1 Rogowski Coil Analog Small Signal Output Susceptible to Electromagnetic Interference  85 3.5.2 Waveform Drift Caused by the Integrator of a Rogowski-Based Transformer  87 3.5.3 Abnormal Transfer of Rogowski Coil Current Transformer Using Digital Integrator  87 3.5.4 Optical Fiber Current Transformer Environment-Dependent Accuracy  88 3.5.5 Polarity Calibration in the Field Test of Electronic Transformers  88

References  89

3.1  Overview of Electronic Transformers Current and voltage transformers provide current and voltage signals respectively for electric energy measurement, relay protection, as well as measuring and control devices. Their accuracy and reliability are closely related to the safety, reliability and economy of the power system. Traditional current and voltage transformers are electromagnetic-inducted, of which a series of IEC 61850-Based Smart Substations. https://doi.org/10.1016/B978-0-12-815158-7.00003-2 © 2019 Elsevier Inc. All rights reserved.

63

64  Chapter 3 inherent defects gradually emerge with the increase of capacity and voltage level in the power system. Since 1970s, researchers have been looking for a new way to realize the measurement of high voltage and current, which is expected to be safe, reliable, perfect in theory and superior in performance [1, 2]. Some kinds of transformers have attracted much attention and are researched for a long time. They are optical current transformers (OCTs) [3], optical voltage transformers (OVTs) [4], electronic current transformers (ECTs) using air core coil or lowpower iron core coil [5], and electronic voltage transformers (EVTs) [6], respectively. To date, ECTs and OCTs have been applied in the field with the realization of temperature stability and craft consistency. Compared with conventional electromagnetic transformers, electronic transformers are superior in the following aspects [7, 8]: (1) Excellent insulating property Magnetic fields of the primary and secondary sides of an electromagnetic transformer are coupled through the iron coil. Its insulating structure is complicated, and the cost grows rapidly with the increase of voltage level. In contrast, for electronic transformers, signals from the primary side are transmitted to the secondary side using optical fiber, whose insulating structure is simple, and cost grows slowly with the increase of voltage level. (2) Free of magnetic saturation and ferroresonance Iron coils are no longer used in electronic transformers, thus they are free of magnetic saturation and ferroresonance, which results in the desirable transient response and stability, ensuring the reliability of the system. (3) Antielectromagnetic interference The circuit in the secondary side cannot be open for an electromagnetic current transformer, and it cannot be short for an electromagnetic voltage transformer, otherwise it would be dangerous. For electronic transformers, optical fiber connects the two sides, which ensures the electrical isolation between them. Therefore, there are no risks of short or open circuits. Furthermore, since magnetic coupling does not exist, the transformer has antielectromagnetic interference. (4) Wide scope of transient response and high measuring accuracy In a normal situation, current flowing through a transformer is not large. However, the short circuit current grows fast. An electromagnetic current transformer is unable to realize a wide scope measurement because of magnetic saturation. It is also difficult for an electromagnetic transformer to satisfy the requirement of high accuracy measurement and protection. Nevertheless, electronic transformers have the wide scope of transient response. Their rated current can range from dozens of amperes to thousands of amperes. The overcurrent can be up to tens of thousands of amperes. Therefore, they meet the demand of both measurement and relay protection. Besides, they can avoid the complex structure of multiple channels in electromagnetic current transformers.

Principles and Test Technology of Electronic Transformers  65 (5) Wide range of frequency response The transducer of an electronic transformer has a wide range of frequency response. The real measuring range depends on the electronic circuit part. This kind of transformer is able to measure the harmonic waves in the high voltage lines. In contrast, the frequency response range of an electromagnetic transformer is narrow. Its response to high frequency signals is flawed. (6) Adapted to electric power measurement and the digitization, computerization, automation, and intelligentization of protection Microcomputer and digital electronic technologies have been widely used in the power system. Conventional electromagnetic transformers cannot be connected to the smart grid smoothly due to their weakness on interface. Electronic transformers, connected with optical fiber, can transmit signals quickly and accurately. The application of power electronic devices and digital electronic technology is capable of satisfying the requirements of precise measurement and quick action of protection in the situation of smart grids. In practical engineering application, electronic transformers are classified into two types: active electronic transformers and passive electronic transformers. Fig. 3.1 shows the classification. The principles, formation, and key technology are diverse for different transformers (see Fig. 3.1).

Fig. 3.1 Classification of electronic transformers.

66  Chapter 3

3.2  Principles of Electronic Current Transformers 3.2.1  Active Electronic Current Transformers Active ECTs mainly include transformers based on Rogowski coils and low-power coils, which need a power supply for transducers located in the primary side. The structure of an ECT is shown in Fig. 3.2, including the following four parts. (1) Primary transducer. The primary transducer lies at the high-voltage side, which consists of a low-power coil, two Rogowski coils, and a current power supply coil. The low-power coil transmits current signals for measurement and control while the Rogowski coils are used for protection. The power supply coil obtains electric energy from the power system and offers the power supply for the remote electronic modules. (2) Remote electronic modules. Remote electronic modules receive and process signals from the low-power coils and the Rogowski coils of which output signals are digital and optical. Remote electronic modules gain power from the laser in a merging unit or the power supply coil. The laser can supply power when a system current is less than 20 A. In addition, the power supply coil works to offer power when system current exceeds 20A. The two methods can be used for redundancy. (3) Optical fiber insulator. This is a kind of supporting insulator with the solid core and embedded optical fiber. Eight multimode fibers (e.g., 62.5/125 μm) are located in the insulator of which four are utilized (two for laser, two for digital signals), and the rest are for redundancy. The high-voltage end of the optical fiber insulator is connected to the remote electric unit, while the low-voltage end is connected to optical cables by welding.

Fig. 3.2 Diagram of an active electronic current transformer.

Principles and Test Technology of Electronic Transformers  67 (4) Merging unit. The merging unit provides remote electronic modules with laser power. In addition, it receives and processes signal data from current transformers and voltage transformers, synchronizes the current and voltage signals, and transmits signal data to secondary equipment in certain protocols. The merging unit employs multimode fibers of 62.5/125 μm for signal output. Electronic modules exist in the high-voltage part of an active ECT. Output signals from the electronic modules are then filtered, integrated, sampled in the following steps, and turned into digital signals. An electro-optical conversion circuit converts digital signals into optical signals, which are transmitted to the low-voltage side through optical fiber and used by relay protection and measurement devices. Electronic modules in the high-voltage part need a power supply, which is key technology for active electronic transformers and raises the question of power source insulation under the condition of high voltage. 3.2.1.1  Principles of Rogowski coil Rogowski coil, as a mature measuring element, is actually an air core coil of which the measuring conductors twine around the framework of nonmagnetic material with a uniform section [9, 10]. A sampling resistor is connected to the two ends of the coil, shown in Fig. 3.3. The coil itself is not connected to the current circuit directly, resulting in a desirable electrical insulation. The framework of a Rogowski coil is formed with nonferromagnetic materials so

Fig. 3.3 Rogowski coil.

68  Chapter 3 that it is free of magnetic saturation. Even when a direct current component is large in the signal to be measured, the transformer will not be saturated and keep linearity. Compared with conventional transformers, the Rogowski coil-based transformers have the advantages of high accuracy, wide measuring range, and large bandwidth. The Rogowski coil measures current based on the electromagnetic induction principle. Let the section area of the framework be A, inner radius be r1, outside radius be r2, average radius be R, number of windings be N, the current to be measured in the conductor be i1(t), and induced current in the coil be i2(t). According to Ampère’s circuital law, choosing a loop L around the framework of the coil whose radius satisfies the condition that r1 < r < r2, we can obtain the following equation. (3.1) ∫ B ⋅ dl = µ0 ∑ I = µ0 i1 ( t ) − N ⋅ i2 ( t )  In general, the induced current in the coil is much less than the current to be measured, that is, i2(t) > i1(t). Therefore, (3.2) ∫ B ⋅ dl = µ ⋅ i ( t ) 0

1

Where μ0 is permeability of vacuum. According to electromagnetic inductive law, when alternating current flows through a conductor, induced voltage in the coil meets the following condition: (3.3) e ( t ) = − dφ / dt = − d ∫ B ⋅ dS / dt

(

)

where S is the area surrounded by one winding. Considering an ideal coil model whose section area is constant, windings are uniform, wires are infinitely thin, and adjacent windings are infinitely closed, we can yield that, (3.4) e ( t ) = − N ⋅ A ⋅ dB / dt It can be derived from Eqs. (3.2), (3.3). e ( t ) = − µ0 ⋅ A ⋅ N / lc  di1 ( t ) / dt  = − M  di1 ( t ) / dt 

(3.5)

where A is the section area of the coil and N is the number of windings. Therefore, e(t) is the differential of i1(t). The i1(t) can be obtained from the integration of e(t). Eq. (3.5) shows that the inductive signal of the hollow coil is proportional to the differential of current to be measured. If the coil is connected to an integrator, the output of the integrator will be proportional to the current. 3.2.1.2  Principles of low power coil The low-power current transformer (LPCT) is a development of a conventional electromagnetic current transformer. The low-power coil is still an iron coil that is designed as high resistance, improving the saturation characteristic of a current transformer with large primary current and expanding its measurement range. In general, LPCTs owe a desirable linearity with 50%–120% rated current. The accuracy is 0.1/0.2 S, which is suitable for measurement and metering.

Principles and Test Technology of Electronic Transformers  69 In real application, requirements on data are different for protection and measurement. Thus, the protection coil and the measurement coil are separated, but they share one electronic data processing system. The Rogowski coil is generally used in protection because of its broad measuring range, while the LPCT is generally applied in measurement for its excellent dynamic response and high precision.

3.2.2  Passive Electronic Current Transformers A passive ECT mainly indicates a transformer based on an optical measurement principle, that is, an OCT. The power supply is unnecessary for the high-voltage part of this kind of transformer. The present OCT mainly measures current using the Faraday magneto optic effect, which can be classified into a magneto-optical glass type and an all-fiber type, according to the difference of sensing element. These structures are illustrated in Figs 3.4 and 3.5. 3.2.2.1  Faraday magneto-optic effect The Faraday magneto-optical effect, also known as the light wave magneto-circular birefringence effect, refers to the occurrence of rotation of polarized light when passing through a medium along the direction of an applied magnetic field or a magnetization direction of polarization. The Faraday magnetic field rotation phase ϴF and the magnetic field component along the light propagation direction satisfy the following relationship.

θ F = V .∫ Bdz L

Fig. 3.4 Diagram of a passive electronic current transformer.

(3.6)

70  Chapter 3

Fig. 3.5 Diagram of optical glass current sensor.

where B is the magnetic field strength, V is the medium Verdet constant, and L is the fiber length. The Faraday rotation direction is related to the light propagation vector direction and the magnetic field vector direction. The basic working principle of a magneto-optic glass type current transformer is the Faraday magneto-optical effect, that is, the external magnetic field applied to the optical medium causes the polarization plane of the polarized light passing through the optical medium to rotate. The Faraday magneto-optical effect can be considered as the mathematical expression of the working principle of magneto-optic glass type current transformer. The Faraday magnetooptical effect also shows that sensor sensitivity can be increased by increasing the number of optical paths around the current-carrying conductor with an appropriate optical path design. 3.2.2.2  All-optical principle electronic current transformer The magneto-optic glass type current transformer has the disadvantages of difficult processing, a fragile sensor, and high cost. In addition, the reflection phase shift introduced by the light in the reflection process can change the linearly polarized light into the elliptically polarized light, which will affect the system performance. With the development of optical fiber technology, an all-fiber current transformer overcomes the weakness of an optical glass type current transformer and is gradually applied to engineering practice. According to the structure of the optical fiber sensor, the optical fiber type current transformer is divided into reflective type and circular Sagnac, as shown in Figs. 3.6 and 3.7. The reflective fiber optic current sensor (R-FOCT) is designed with a common optical path, which possesses a high degree of structural reciprocity, excellent antivibration, and temperature interference suppression characteristic. It is free from the influence of gyroscopic effect and therefore becomes the main structure. The optical fiber type current transformer realizes the nonreciprocity phase angle caused by the Faraday magneto-optic effect in the optical loop by drawing digital closed-loop feedback technology of optical fiber gyro photoelectric signal in real time, and then

Principles and Test Technology of Electronic Transformers  71

Fig. 3.6 Structure of Sagnac optical current transformer.

Fig. 3.7 Structure of a reflective fiber optic current sensor.

obtains the external current information. Electromagnetic field-light wave coupling sensing technology and digital closed-loop feedback signal processing technology bring an all-optic current transformer the advantages of wide dynamic range, high measurement accuracy, and excellent insulation performance. The principle of R-FOCT is shown in Fig 3.7. Fig. 3.7 illustrates the transmission path of a light wave. (1) The light from the light source is polarized by the polarizer through the coupler to form linearly polarized light. The linearly polarized light is injected into the polarization maintaining fiber at 45 degrees and is transmitted uniformly in the X-axis (fast axis) and the Y-axis (slow axis) of the fiber. (2) After the two beams of light with included angle of 45 degrees pass through the λ/4 wave plate, they become a left-handed and right-handed circularly polarized light and enter into the sensing fiber.

72  Chapter 3 (3) In the sensing fibers, the two circularly polarized lights are transmitted at different speeds due to the Faraday magneto-optic effect of the conduction current. (4) After the specular reflection of the end face of the sensing fiber, the polarization modes of the two circularly polarized lights are interchanged. In other words, the left-handed light becomes right-handed and the right-handed light becomes left-handed. Afterwards, they pass through the conductive fiber again and interact with the magnetic field generated from the current, resulting in the doubling of the phase. (5) The two beams of light pass through the λ/4 wave plate again and return to linearly polarized light. The original light waves entering the wave plate along the X-axis and the Y-axis of the polarization maintaining fiber are emitted out of the wave plate along the Y-axis and the X-axis, resulting in the interference of light at the polarizer. (6) In the transmission process, the two lights pass through the X-axis and Y-axis of the polarization-maintaining fiber and the left and right modes of sensing fiber, which are only different in time. Therefore, the lights back to the detector carry only the nonreciprocity phase difference caused by the Faraday effect. An all-fiber current transformer is essentially an optical precision instrument based on the interference of polarized light. Control of light polarization is one of the key technologies.

3.2.3  Comparison of Different Principles Different types of ECTs have their advantages and disadvantages. Active ECTs are based on Faraday’s principle of electromagnetic induction, with the advantages of large measuring range, desirable linearity, nonmagnetic saturation, and so on. This current transformer takes advantage of the high insulation offered by fiber optic systems to significantly reduce the manufacturing costs, size, and weight. It also leverages the advantages of conventional current transformers, avoiding the complex path of the optical current transformer, phase difference of linear birefringence, block glass total reflection, and other technical difficulties. The practical technical barriers include the Rogowski coil structure, antielectromagnetic interference capability, electromagnetic compatibility, sampling linearity, accuracy performance, and stability issues. Passive ECTs are based on photoelectric sensing technology. The optical sensors in the primary side require no operating power and are the ideal solution for independent installation of transformers. They are currently under practical studies. The OCT also has some technical problems that are difficult to solve. For example, the magneto-optical effect may change with environmental factors. The sensor is troubled by a linear birefringence problem. The electrooptical effect is easily disturbed by the effects of the pop-up and thermo-optic effects. In addition, it is sensitive to temperature and vibration. In contrast, the Faraday magneto-optic principle OCT possesses an excellent dynamic quality and does not need power at the highvoltage side, leading to high reliability.

Principles and Test Technology of Electronic Transformers  73

3.3  Principles of Electronic Voltage Transformers 3.3.1  Active Electronic Voltage Transformers The active electronic voltage transformer (EVT) contains the primary side sensor (voltage divider), remote electronic module, fiber insulator, and merging unit. The analog signal output from the low-voltage arm of the voltage divider is used for analog-digital conversion and digital-optical conversion through the remote electronic module. The signal is transmitted to the merging unit through optical fiber. The outer insulation of the capacitor divider is made of silicon rubber composite insulator, which is lightweight. According to the principle of partial pressure of the primary side sensor, it can be divided into capacitive partial pressure and resistance partial pressure. The signal is processed by the electronic module and transmitted by the optical fiber, which is similar to an active ECT. The working principle of resistor divider EVT is shown in Fig. 3.8, and its core part is a resistor divider. The voltage divider consists of a high-voltage arm resistor R1 and a low-voltage arm resistor R2, and the voltage signal is taken out on the low-voltage side. U1 is the high-voltage input voltage, and U2 is the low-voltage output voltage. Since two resistors are connected in series, there is U2 = U1(R1 + R2)/R1 = U1k. The measured voltage and the voltage on R2 are k times in the amplitude, and the phase difference is zero. As long as R1 and R2 of the appropriate selection are properly chosen, the required partial pressure ratio can be obtained. In order to prevent overvoltage in part of low voltage and to protect the two side measurement device, a discharge tube or regulator S must be installed on the low-voltage resistance, so that the discharge voltage is just a little less than or equal to the maximum allowable voltage on the low-voltage side. In order to make the electronic circuit not affect the partial pressure ratio of the resistance divider, a voltage follower is added. The operating principle of the capacitance partial pressure EVT is shown in Fig. 3.9, and its core is a capacitive voltage divider. The voltage divider consists of a high-voltage arm capacitor C1 and a low-voltage arm capacitor C2, and the voltage signal is taken out on the low-voltage side. U1 is the measured primary voltage. UC1 and UC2 are the voltage on the voltage divider. Since

Fig. 3.8 Diagram of resistor divider.

74  Chapter 3

Fig. 3.9 Diagram of capacitor divider.

the two capacitors are in series, there are UC2 = U1C1/(C1 + C2) = U1k. As long as the capacitance of C1 and C2 are chosen properly, the appropriate partial pressure ratio can be obtained.

3.3.2  Passive Electronic Voltage Transformers A passive EVT mainly refers to the voltage transformer that uses the optical measurement principle, also known as the optical voltage transformer. At present, many researchers focus on the principle of Pockels effect of optical voltage transformers. In 1893, the German physicist Friedrich Pockels found that some crystals under the action of the electric field would change its anisotropic properties, resulting in additional birefringence effect. The refractive index of the crystal with a linear change in voltage is called linear electro-optical effect, namely Pockels effect. The Pockels effect exists only in noncentrosymmetric crystals. There are two working ways of the Pockels effect: one is that the light direction coincides with the direction of the measured electric field, called the longitudinal Pockels effect; the other one is that the direction of the light passing is perpendicular to the direction of the measured electric field, called the lateral Pockels effect. The following two types of electro-optic crystals are most commonly used as optical field measurement applications: (1) BGO crystal. With the change of temperature, the deformation is small, the uniformity is good, and there is no natural birefringence effect and thermoelectric effect. However, when the electric field is modulated, the induced optical axis rotates, the machining process is required to be high, the voltage of the crystal half-wave is high, and the sensitivity of the measured electric field is not as high as the LN crystal, which is suitable for the measurement of a high-voltage electric field. (2) LN crystal. With the temperature change, the deformation is large, and there is a natural birefringence effect. The optical axis does not rotate when the electric field is modulated. The processing is convenient, the voltage of the crystal half-wave is low, and the measurement sensitivity is high, which is suitable for measuring the electric field and the weak electric field in space.

Principles and Test Technology of Electronic Transformers  75 3.3.2.1  Longitudinal Pockels effect When a beam of linearly polarized light enters the electro-optic crystal in this electric field in a direction parallel to the applied electric field E, birefringence occurs due to the Pockels effect upon incidence of linearly polarized light into the crystal, so that two birefringent beams exiting the crystal arise difference of phase angle. The phase difference is proportional to the strength of the applied electric field. The phase changing can be transformed into light intensity changing using optical components, such as an analyzer, in order to achieve the applied electric field (or voltage) measurement.

δ=

2π 3 2π 3 n0 γ Ed = n0U λ0 λ0

(3.7)

In Eq. (3.7), δ is the applied electric field intensity, E is the wavelength of the light passing through the crystal, λ0 is the refractive index of the crystal, n0 is the linear electro-optic coefficient of the crystal, γ is the phase difference of the two beams caused by the Pockels effect, d is the thickness of the crystal in the direction of the applied voltage, and U is the applied voltage on the crystal. As can be seen from Eq. (3.7), this phase difference is proportional to the voltage applied to the crystal and not related to the crystal thickness, that is, the size of the crystal. 3.3.2.2  Lateral Pockels effect When the applied electric field E is perpendicular to the light passing direction of the crystal, the phase difference between the two birefringent beams is:

δ=

2π 3 l n0 γ U λ0 d

(3.8)

In Eq. (3.8), l is the length of the crystal pass direction, δ is the phase difference between the two beams caused by the Pockels effect, E is the applied electric field intensity, λ0 is the wavelength of the light passing through the crystal, n0 is the refractive index of the crystal, γ is the linear electro-optic coefficient of the crystal, d is the thickness of the crystal in the direction of the applied voltage, and U is the applied voltage on the crystal. In the longitudinal Pockels effect, the electric field is parallel to the direction of light. The total Pockels effect caused by this Pockels effect is the accumulation of Pockels effects caused by the electric field in the crystal along the beam direction. Since the voltage difference between any two points is equal to the integral of the electric field along the path between these two points, this integral is independent with the electric field distribution between two points. The longitudinal Pockels effect enables the measurement of the voltage applied directly across the crystal, and thus the measurement is unaffected by the adjacent electric field or other disturbing electric field. However, since the linearly polarized light beam is incident on the electro-optic crystal in the electric field in a direction parallel to the applied electric field E, it is required that the electrode

76  Chapter 3 is transparent and conductive to apply an electric field, which poses great difficulties in the actual production of the transformers. A phase delay will be caused by natural birefringence in a lateral Pockels effect, which is sensitive to the change of outside temperature. In order to overcome this shortcoming, two pieces of wafer are used to compensate and dispel the natural birefringence, raising higher requirements to the crystal processing and craft. In contrast, a longitudinal Pockels effect is free from the phase delay caused by natural birefringence. The half-wave voltage of the longitudinal Pockels effect is related to the electro-optic properties of the crystal, but not to the crystal size. Fig. 3.10 shows the working principle of the optical voltage transformer with a longitudinal Pockels effect. At present, optical current transformers and optical voltage transformers still lack the test of long-term operation, and their long-term stability and reliability have yet to be further certified. The influences of environmental temperature, vibration, and other external factors on the optical transformers also need to be verified in actual projects. All-fiber current transformers have gradually been applied in engineering. The EVT mainly applies the principle of partial pressure.

3.4  Test Technology of Electronic Transformers 3.4.1  Calibration of Electronic Transformers In the power system, the main concern for the user is the accuracy of the amplitude error and the angle error of the sampled data channel of the electronic transformer. Electronic transformers are different from conventional transformers, and so are the calibration methods. The accuracy calibration device for an ECT consists of a voltage regulator, a large current generator, a standard current transformer, an electronic transformer calibrator, a secondary converter, and related equipment. The accuracy calibration device for an EVT consists of a voltage regulator, a test transformer, a resonant device, a standard voltage transformer, a coupling inductance, an electronic transformer calibrator, a secondary converter, and related equipment. 3.4.1.1  Electronic transformer calibration method and system According to the technical specification from the State Grid Corporation of China, 220 kV and above protective devices should not rely on external time-sharing systems to achieve protection

Fig. 3.10 Diagram of Pockels effect voltage transformer.

Principles and Test Technology of Electronic Transformers  77 functions. An electronic transformer calibration system should be able to compensate the inherent delay of the merging unit. It does not need to use external synchronization signals, meeting the requirements from the State Grid Corporation of China that “direct sampling” protection does not rely on an external synchronization clock. In addition, some low-voltage stations adopt the external synchronization mode; therefore, the system should keep the external mode. Fig.  3.11 shows the electronic transformer steady state verification system. The system can perform steady-state accuracy verification on the analog output electronic transformers and digital output electronic transformers. It can also be compatible with the conventional electromagnetic transformer calibration test. The primary electricity can be current or voltage. The test object shows the general structure diagram of an electronic transformer. The calibration system includes a high-precision, multichannel, synchronous acquisition module, synchronization module, and synchronous data acquisition board. The host computer is the computer terminal with analytical software. The calibrator uses fast Fourier transform (FFT) to calculate the fundamental amplitudes and phase angles of signals from two different input sources to perform the corresponding error analysis. Simultaneously, the synchronization module sends sampling pulses to the highprecision synchronous acquisition module and the synchronous data acquisition board, and the main central processing unit (CPU) uses FFT algorithm to perform the Fourier transform on discrete sampled values (SVs). Let the sampling period of the synchronous pulse be Ts, and the sampling point be N, then the discrete Fourier transform in this interval is N −1

X ( n ) = ∑x ( k ) e k =0

−j

2π kn N

= Re ( X ( n ) ) + jIm ( X ( n ) )

( 0 ≤ n ≤ N − 1)

(3.9)

where X(n) is the nth harmonic complex coefficient of the periodic function in time domain. The phase of the fundamental signal is as follows,

θ = arctan

Im ( X (1) )

(3.10)

Re ( X (1) )

By extracting the fundamental parameters of the two input signals and obtaining the phase angles separately, the phase difference Δθ between the two signals is obtained. 3.4.1.2  Electronic current transformer calibration program Calibration methods are different for disparate types of ECTs. For the Rogowski coil and LPCT with analog output voltage signal, the calibration system directly receives output signals of the transformer to be tested, and the standard transformer and gains the ratio difference and angle difference, as shown in Fig. 3.12. The test system shown in Fig. 3.13 is applied on ECTs that

78  Chapter 3

Fig. 3.11 Electronic transformer steady state test system.

produce digital values through merging units and rely on external clock synchronization. The test system sends a synchronous sampling pulse to ensure the sampling synchronization. The test system shown in Fig. 3.14 is applied on ECTs that produce digital values through merging unit but do not rely on external clock synchronization. The calibrator compensates the inherent time delay of the ECT.

Principles and Test Technology of Electronic Transformers  79

Fig. 3.12 Diagram of analog output electronic current transformer calibration. Note: L1 and L2 are the terminals in primary side of the current transformer; K1 and K2 are the terminals in secondary side of the current transformer; T0 and K0 are the standard interfaces in electronic transformer calibrator; Tx and Kx are test interfaces in electronic transformer calibrator.

Fig. 3.13 Diagram of digital output electronic current transformer synchronization calibration (external clock synchronization). Note: L1 and L2 are the terminals in primary side of the current transformer; K1 and K2 are the terminals in secondary side of the current transformer.

80  Chapter 3

Fig. 3.14 Diagram of digital output electronic current transformer absolute time delay calibration (constant time delay mode). Note: L1 and L2 are the terminals in primary side of the current transformer; K1 and K2 are the terminals in secondary side of the current transformer.

If the standard transformer has a compensation winding whose head is connected to the primary polar point and the end is grounded, the symmetrical branch can be abandoned in the measurement. If output voltage of the standard transformer does not satisfy the input requirement of the calibrator, an induction divider circuit or standard resistor needs to be used to convert the output of the standard transformer into a small voltage signal, which is transferred to the calibration circuit and compared with that from the transformer to be tested. For the measurement and protection of the ECT, the digital output of the data message in each channel should be verified. For multicoil outputs of the ECT, each winding should be verified. 3.4.1.3  Electronic voltage transformer calibration program The EVT calibration method is similar to that of ECTs, except that the standard current transformer is replaced by a standard voltage transformer. Since the primary voltage needs to be improved to a high level in the test process, attention should be paid to the security issues.

Principles and Test Technology of Electronic Transformers  81

Fig. 3.15 Diagram of analog output electronic voltage transformer calibration. Note: A and X are the terminals in the primary side of the voltage transformer; a and x are the terminals in the secondary side of the voltage transformer; V0 is the standard input of the electronic transformer calibrator; Vx is the test input of the electronic transformer calibrator.

For the transformer that directly generates analog voltage signal, the output of the transformer under test and the standard transformer is directly transferred to the calibration system, as shown in Fig.  3.15. For a digital output transformer that relies on an external synchronous clock, a test system is shown in Fig. 3.16. For a digital output transformer that does not rely on an external synchronous clock, the test system is shown in Fig. 3.17.

3.4.2  Time Delay Test of Electronic Transformers In terms of direct sampling, the electronic transformers are required to output the rated delay correctly for delay compensation during protection sampling. The correctness of the rated time delay of the electronic transformer is directly related to the correctness of the sampling of the protection and is even related to the correctness of the protection action. Therefore, the delay of the electronic transformer must be tested and then compared with the rated delay of its output to verify its correctness. An electronic transformer rated delay test system is the same as a precision test system. Ignoring the transfer delay of a standard transformer, we can compare the phase difference between a measured electronic transformer and a standard transformer to calculate the rated delay of the transformer. In the electronic transformer calibration system, the sampling of the standard transformer is controlled by the sampling pulse of the system synchronization module, while the electronic transformer is independently sampled. Fig. 3.18 shows the data of the electronic transformer reaching the synchronization module at the time when the pulse is sent.

82  Chapter 3

Fig. 3.16 Diagram of digital output electronic voltage transformer synchronization calibration (external clock synchronization). Note: A and X are the terminals in the primary side of the voltage transformer; a and x are the terminals in the secondary side of the voltage transformer; V0 is the standard input of the electronic transformer calibrator; Vx is the test input of the electronic transformer calibrator.

Fig. 3.17 Diagram of digital output electronic voltage transformer absolute time delay calibration (constant time delay mode). Note: A and X are the terminals in the primary side of the voltage transformer; a and x are the terminals in the secondary side of the voltage transformer; V0 is the standard input of the electronic transformer calibrator; Vx is the test input of the electronic transformer calibrator.

Principles and Test Technology of Electronic Transformers  83

Fig. 3.18 Data reaching at the time when the pulse is sent.

The time difference between electronic transformer and the standard transformer is as follows, ∆θ ∆t = Ts (3.11) 2π where Δθ is determined by Eq. (3.10) and Ts is sampling period. Fig.  3.19 shows that the time when the data of the electronic transformer reaches the synchronization module is not the time when the pulse is sent out. The synchronization module records the arrival time t1 of the data and records the time t2 given by the next pulse to obtain the delay Δts between them. The arrival of the data at time t1 will be moved to the next pulse to be calculated. The actual value Δt will be obtained after calculating the corresponding delay by FFT and subtracting Δts. Table 3.1 shows the rated delay test data of an electronic transformer of which the rated output delay is 1761 μs. The delay data is recorded every second.

3.4.3  Polarity Test of Electronic Transformers Whether the polarity of the transformer is correct or not is directly related to the operation of the protection, measurement, and control devices. Therefore, the polarity of the transformer must be verified before it is put into commercial operation. Polarity tests need to be configured with DC or AC current generators and digital transformer analyzers.

84  Chapter 3

Fig. 3.19 Data do not reach at the time when the pulse is sent. Table 3.1: Test data of rated delay for electronic transformer No.

1

2

3

4

5

6

7

8

Time delay (μs)

1761.115

1761.111

1761.102

1761.115

1761.11

1761.12

1761.119

1761.116

The DC method can be used to measure the polarity of DC current electronic transformers, of which the principle is shown in Fig. 3.20. The DC current is injected into the ECT from the primary polarity end. The data processing system analyzes the SV current values from the merging unit and presents them in waveforms. If the SV current value is positive, it indicates that the optical fiber current transformer is connected in positive polarity. If the SV current value is negative, it indicates that the optical fiber current transformer is connected in reverse polarity.

Fig. 3.20 Diagram of polarity test for an electronic current transformer.

Principles and Test Technology of Electronic Transformers  85

3.4.4  Heavy Current Test of Electronic Transformers 3.4.4.1  Test principle The 63 kA current transient characteristics of an OCT are tested in Xi'an High Voltage Research Institute. The details are as follows: Three-phase optical fiber current transformer and high-precision current transformer are tested in series. The test block diagram and test environment are shown in Figs. 3.21 and 3.22, respectively. 3.4.4.2  Test results The results meet the requirements of a transient characteristics test, as shown in Fig. 3.23.

3.5  Application Issues of Electronic Transformers 3.5.1  Rogowski Coil Analog Small Signal Output Susceptible to Electromagnetic Interference At present, in the application of the low-voltage level, the protection and monitoring device is arranged in place where the distance between the device and the transformer is nearby. The electronic transformer produces analog signals that are transmitted to the integrated intelligent unit through the cable and converted to digital signals. Then they are transmitted to the protection and monitoring devices. The cable length is about 3–5 m, as shown in Fig. 3.24. Since the output of the Rogowski coil is a small signal of 150 mV, it is easy to be affected by electromagnetic interference from the operation of circuit breakers, causing the relay protection malfunction.

Fig. 3.21 Diagram of test principle.

86  Chapter 3

Fig. 3.22 Test environment.

× 104

Wave of current transient response

15

Current value

10

5

0

–5 440

460

480

500 520 Time (ms)

Fig. 3.23 Test result.

540

560

580

Principles and Test Technology of Electronic Transformers  87

Fig. 3.24 Diagram of low-voltage protection and measurement.

3.5.2  Waveform Drift Caused by the Integrator of a Rogowski-Based Transformer The output of a Rogowski coil is the differential signal of the original signal. An integrator is necessary to obtain the original signal. Theoretically, when the primary current is a sine wave, the transformer output should also be a sine wave. However, according to the field measurement, the whole sine wave will drift upwards or downwards. It seems that a low-frequency component signal is superimposed whose frequency is lower than the power frequency, as shown in Fig. 3.25, and it is caused by the zero drift suppression in the integrator, which needs to be studied in the practical application.

3.5.3  Abnormal Transfer of Rogowski Coil Current Transformer Using Digital Integrator In a Rogowski coil-based transformer with a digital integrator, the sampling element locates before the integrator. High-frequency current signals are turned into low-frequency current signals in sampling due to frequency aliasing. Although there is a low-pass filter in front of the sampling element, it is unable to prevent the process of frequency aliasing effectively because high-frequency signals are amplified in the Rogowski coil. After being transferred in the sampling element, high-frequency signals become low-frequency signals, which will be enlarged again in the integrator and then mixed with the original signal components. Therefore, the transformer works abnormally in the high-frequency region, which causes signal distortion. In contrast, the transformer with an analog integrator can avoid this problem. By increasing the order of the filter, reducing the cut-off frequency of the filter, and increasing the sampling frequency, the transient transfer performance of the transformer can be improved.

88  Chapter 3 Protection current (Rogowski coil) 100 50 0 −50 −100 80 60 40 20 0 −20 −40 −60 −80

5

10

15

20

25

30

5

10

15

20

25

30

Measurement current (LPCT)

Fig. 3.25 Waveform drift of Rogowski coil-based transformer.

3.5.4  Optical Fiber Current Transformer Environment-Dependent Accuracy The measurement accuracy of an OCT will change with the change of external temperature, pressure, and vibration. The fiber ring of the optical fiber transformer is connected to the preacquisition module using a polarization-maintaining fiber, which is sensitive to changes of external stress, temperature, and vibration. When the polarization maintaining fiber is squeezed or skewed, the transformer output will be severely distorted, which brings high requirement on the installation and operation.

3.5.5  Polarity Calibration in the Field Test of Electronic Transformers The traditional current transformer has the polarity of P1/P2, and S1/S2. If the polarity is reversed during the field test, it can only change the connection of S1 and S2. The electronic transformer outputs signals through the merging unit. In principle, there is only P1 access and S1 output. If the polarity is reversed, it cannot be changed by adjusting the secondary polarity of the packet. However, there are many ways to modify the polarity of the output data of the electronic transformer, such as the adjustment of the electrical unit, the merging unit, and the protection. In engineering applications, a simple and convenient method for configuring the polarity must be provided and the previously mentioned method of polarity verification must be used to verify the field-installed electronic transformers.

Principles and Test Technology of Electronic Transformers  89

References [1] D.A. Ward, Measurement of current using Rogowski coils, in: IEE Colloquium on Instrumentation in the Electrical Supply Industry, London: IET, 1993. 1/1-1/3. [2] P.P. Chavez, N.A.F. Jaeger, F. Rahmatian, Accurate voltage measurement by the quadrature method, IEEE Trans. Power Del. 18 (1) (2003) 14–19. [3] Y. Yamagata, T. Oshi, H. Katsukawa, et al., Development of optical current transformers and application to fault location systems for substations, Discussion 8 (3) (1993) 866–873. [4] S. Luo, Advances in optical voltage transformers, Adv. Technol. Electr. Eng. Energy (2000). [5] International Electrotechnical Commission, IEC 60044-7 Electronic voltage transformer[S], 1999. [6] International Electrotechnical Commission, IEC 60044-8 Electronic current transformer[S], 2002. [7] L.I. Jiu-Hu, X.U. Lei, S.N. Luo, et al., Application of electronic transformer in digital substation, Electrotech. Electr. 31 (7) (2007) 94–98. [8] Y.X. Gao, B.I. Wei-Hong, F. Liu, Design of power of electronic current transformer in high voltage side, Power Electron. 41 (10) (2007) 74–76. [9] C. Qing, L. Hong-Bin, Z. Ming-Ming, et al., Design and characteristics of two Rogowski coils based on printed circuit board, IEEE Trans. Instrum. Meas. 55 (3) (2006) 939–943. [10] C.R. Hewson, W.F. Ray, J. Metcalfe, Optimizing high frequency integrator operation of Rogowski current transducers, in: 2007 European Conference on Power Electronics and Applications, 2007, pp. 1–9.