Analysis of partial discharge signals from generator propagation in excitation transformer by EMTP

: This study presents the simulation experiment of the partial discharge (PD) signal transmitted by the generator in the excitation transformer. Firstly, voltage over the low-voltage winding of the excitation transformer was displayed. The degradation process of the insulation and the PD online monitoring method were described. Electrical circuit was established by the electro- magnetic transients program (EMTP) software, which was simulated PD signal from generator propagation in the excitation transformer. Results show that simulation model was right and benefited to fault diagnosis of PD signals in the excitation transformer.


Introduction
The excitation system is an important part of the power plant. The excitation transformer is a device for providing three-phase AC excitation power for the excitation system of large generator [1]. Its safe operation is the prerequisite for the stable operation of the generating unit and the power system [2]. The low-voltage winding insulation of the excitation transformer is subjected to the common effect of AC voltage, high-order harmonic and DC voltage component, which makes the electric field distribution extremely complex. Its internal insulation runs in this electric field environment, and it is very easy to partial discharge (PD) occur. Therefore, the online monitoring of the PD is of great significance for improving the reliable operation of the excitation transformer and the whole power system [3].
PD monitoring of high-voltage electrical equipment began in the early 1940s [4]. It is the most widely used method to monitor the PD pulse current signal of electrical equipment by Rogowski coil. This method can be used to detect the pulse current caused by PD apparent discharge in the electrical equipment lead wire or the grounding line through the Rogowski coil. Moreover, the characteristic quantity of PD is evaluated and analysed. The international electrotechnical commission (IEC) has developed the IEC60270 standard for this approach [5]. In the 1980s, some scholars used Rogowski coil current sensor to monitor the PD of motor stator winding [6]. Then it is widely used in the PD monitoring of power cable [7], overhead distribution network and excitation transformer.
The research shows that Rogowski coil current sensor is easy to be interfered by narrow-band periodic interference, white noise and random pulse signal when monitoring PD signal of electrical equipment. Narrowband periodic interference can be filtered by a certain bandwidth filter. White noise can be denoised by effective means such as wavelet, and random pulse signal interference can be classified by clustering and other methods [8]. Large amplitude PD signals are easily occurred at the slot, end and inside of the generator. The signal is transmitted to the windings of the excitation transformer and will be detected by the online monitoring system of PD. This is equivalent to the random pulse signal with larger amplitude, which affects the detection accuracy of Rogowski coil current sensor and interferes with the diagnostic result for online monitoring system of PD signal [9].
In this paper, a simulation test is carried out on the detection of impulse interference signals in the excitation transformer by Rogowski coil. First, the voltage waveform of the low-voltage winding of the excitation transformer is analysed, and the deterioration process of the insulation is expounded. The online monitoring method for PD of excitation transformer is put forward. The propagation characteristic of external pulse interference signal in excitation transformer is simulated by using EMTP software, and the signal collected by Rogowski coil is analysed. Finally, the external interference signal in the field is collected and tested. The results show that the PD signal in the excitation transformer and the external pulse interference signal can be identified by the waveform characteristics of the pulse signal.

Voltage analysis of low-voltage side winding
The excitation transformer is one of the most important electrical equipment in the power plant. In recent years, a new power plant generator excitation has been excited by rotating the traditional AC excitation mode to change the static excitation. The basic principle of generator static self-excited excitation system is shown in Fig. 1. In this way, the excitation transformer is connected on the generator outlet line in parallel, and the alternating current is rectified to DC power through the depressurisation and rectification device. Finally, the rectified DC input is applied to the excitation coil of the motor to generate the magnetic field needed by the generator.

Online monitoring method for PD of excitation transformer
Method for monitoring PD excitation transformer is installed on a current sensor in the low voltage of each phase on the bus. The current sensor is composed of Rogowski coil, of which the resonance frequency of the heart is ∼500 kHz, and the 3 dB bandwidth is ∼100 kHz. The installation of current sensors should be highly valued, because it is not only related to the safety and reliability of sensors, but also to the sensitivity and antiinterference ability of sensors. The field safety intention of the #1 excitation transformer in a power plant is shown in Fig. 2.
First, the 10 kV insulation heat shrinkable bushing is sleeved on the low-voltage bus of the excitation transformer to ensure the insulation of the pulse current sensor shielding housing and the excitation transformer low-voltage bus. The insulation between the J. Eng sensor shielded shell and the sensor coil and the output cable can tolerate the AC voltage by 4 kV. The pulse current sensor is installed on the low-voltage bus. Finally, a coaxial cable is used to connect the electromagnetic sensor signal to the control box of the PD monitoring system.

Source of impulse interference signal
The main source of pulse interference from the PD monitoring of the excitation transformer is shown in Fig. 3. On the one hand, the thyristor rectifier produces periodic pulse signal (i 2 (t)) when rectifying. The signal has large amplitude and a certain periodicity. It is hard window processing in PD signal processing, so this paper does not study its propagation characteristics.
On the other hand, large PD is easy to occur at the slot, end and inside of the large generator. As in Fig. 3, the pulse current i 1 (t) is propagating along the high-voltage bus, and the coupling capacitance between the high-and low-side windings of the excitation transformer is transferred to the Rogowski coil. This kind of pulse signal is not periodic, and it is random in the PD signal. The signal waveform has little difference from the PD inside the excitation transformer, so it makes great difficulty in judging whether PD occurs in the excitation transformer.

EMTP simulation model and result analysis
Based on the #1 generator system of a power plant, this paper uses the EMTP software to simulate the propagation characteristics of the pulse signal in the excitation system. The generator's highvoltage outlet line to the power system is shown in Fig. 4. The distance between the generator and the booster transformer is ∼100 m, and the distance from the generator to the excitation transformer is ∼30 m. The excitation transformer and the power system can be regarded as a capacitor. Fig. 5 is an EMTP simulation circuit for detecting the impulse interference signal in excitation transformer by Rogowski coil (based on Figs. 3 and 4). The bus and excitation transformer are in the ideal state during the simulation. U is a pulse voltage source, simulating the PD signal generated by generator. The rising time constant is 10 ns, the time constant of falling edge is 20 ns, the amplitude is 1 V (as shown in Fig. 6). The bus is an equal value circuit of single wire, and its resistance-to-ground resistance is very small, so it is ignored. The simplified model is shown in Fig. 5. R 0 is the unit resistance of the bus, whose value is ∼0.07232 Ω/m. L 0 is the unit inductance of the bus, whose value is ∼0.92103 μH/m. Also C 0 is the unit earth capacitance of the bus, which is ∼12.064 pF/m. The length of the bus is ∼100 m, of which the distance between point 1 and point 2 is 30 m, and the distance between the point 2 to the power plant is ∼70 m. Point 2 is connected to the high-voltage windings of the excitation transformer. When the high-frequency signal flows through the excitation transformer, the excitation transformer can be equivalent to the ideal distributed capacitance circuit, and the coupling capacitor is connected between the high-voltage winding and the low-voltage winding. C T is the equivalent capacitance of other parts of the power plant (∼0.5 μF), and the detection impedance is replaced by RLC equivalent circuit. The resistance, inductance and capacitance  values are 50 Ω, 0.005 mH and 0.003 μF, respectively. The monitoring points 1, 2 and 3 are the installation positions of the high-voltage windings of the generator, the excitation transformer and the detection impedance (ideal wide band Rogowski coil). The external pulse interference signal transmitted by the high-voltage bus is transmitted to the low-voltage winding side of the excitation transformer through the coupling capacitance between the highand low-voltage winding, and then is collected by the PD online monitoring system. Using EMTP software to analyse the circuit model, the voltage waveform of three points (interference signal acquisition current sensor) as shown in Fig. 7, the PD signal (visible external interference pulse signal) becomes an oscillating wave that increases first and then attenuates gradually after oscillation of bus and excitation transformer.

Validation experiments
In order to verify the simulation model proposed in this paper, the field verification test of the #1 excitation transformer in a power plant is carried out in this paper. The experiment was carried out in the laboratory, and a discharge pulse was injected into the generator and the excitation transformer, as shown in Fig. 8. The output voltage of the square wave generator is 10 V, the rising edge is 30 ns, the end of the square wave generator is connected to the capacitor of 50 pF, that is, the charge of injection of 500 pC (q 0 = u 0 C 0 ). The Rogowski coil installed on the low-voltage bus is used to collect the pulse signal. The time domain signal waveform and normalised power map collected by the excitation transformer A are shown in Figs. 9 and 10, respectively. The results show that if PD occurs inside the excitation transformer, the PD signal collected by the Rogowski coil is an attenuation oscillating wave with a gradual decrease in the value. The external pulse interfering signal collected by the Rogowski coil is the oscillation wave that increases first and then decreases gradually, which is very different from the PD signal. The spectrum of PD signal and external impulse jamming signal is concentrated around 400 kHz, which is consistent with the central frequency of Rogowski coil, and the spectrum of PD signal is regular. The spectrum of the external pulse interference signal is also distributed near the 650 kHz after excitation transformer winding, and there is also a significant difference between the spectrum of the PD signal and the spectrum of the PD signal. All these provide the necessary conditions for the effective recognition of the PD pulse signals.

Conclusions
This paper presents a method to study the simulation test of impulse interference signal in excitation transformer by Rogowski coil. First, the voltage of the low-voltage winding insulation of the excitation transformer is analysed, and the online monitoring method of the PD of the excitation transformer is put forward. A simulation circuit model for studying the propagation characteristics of external pulse interference signal in excitation transformer is established by using EMTP software. At last, the test of PD test was carried out in the field. The experimental results show that the simulation model proposed in this paper can effectively simulate the propagation characteristics of the external interference pulse signal in the excitation transformer. It has a certain guiding significance for the analysis of the PD signal of the excitation transformer. It also lays the foundation for the denoising and fault diagnosis of the PD signal of the excitation transformer.
Due to the limited space, this paper only simulated and studied the signal characteristics of the PD and the external random pulse interference of the excitation transformer. In the follow-up work, the PD signal of the excitation transformer and the random interference signal of the external pulse will be clustered to denoise.

Acknowledgment
The authors acknowledge the funding of Chongqing Municipal Education Commission Science and Technology Research Project (KJ1709200) and the Natural Science Foundation of China (51607019).