Detection of saturation and compensation of CT secondary current distortion by considering the change of the normal structure of the power system

Master's Thesis in Electrical Engineering

Power Trend

Abstract

In this thesis, while investigating the phenomenon of saturation in protection CTs, the problems related to the detection of this phenomenon and the compensation of CT secondary current have been addressed, and in order to solve the problems raised, methods have been introduced and the results obtained in the software environment and modeling methods have been compared.

In order to reveal the phenomenon of saturation based on On:

third order derivative,

discrete wavelet transform,

progressive morphology,

and mathematical morphology

have been used.

for the compensation of the secondary tortuous current, the following methods are also used:

least squared error,

flow estimation Magnetization,

and artificial neural network considering the structural changes of the sample network (and as a result the change of short-circuit strength at the CT installation location) have been used to train this network.

After the implementation and comparison of the mentioned methods, the mathematical morphology method and the least square error are proposed as the most appropriate method to reveal the saturation phenomenon and compensate the CT secondary current.

In addition to the cases As mentioned above, an attempt has been made with changes in the method of applying mathematical morphology (for detection) and the least square error method (for compensating the secondary eddy current), to make it possible to use the aforementioned methods in online conditions.

Key words: current transformer, detection of CT saturation phenomenon, third order derivative, discrete wavelet transform, progressive morphology, mathematical morphology, compensation of secondary eddy current, least square error, estimation of magnetizing current, artificial neural network

According to the development of the industry and the extent and complexity of power systems, the level of short circuit in the power system increases, which increases the role of protective relays and interface equipment in preventing Damage to pressure equipment in power systems. These relays need to receive the correct information to function properly, and therefore, in case of distortion in the received signals, expecting the desired performance from them is considered pointless. Current transformer (CT) [1] is one of the most important elements as a relay interface, which is used to obtain a current signal corresponding to the primary current and with smaller amplitudes. Although CTs use iron cores to maximize the bonding flux between the primary and secondary windings (and minimize the leakage flux), they are prone to saturation due to the non-linearity of the magnetic core. At points above the knee of the magnetization curve, the core magnetic current will increase significantly for the initial current changes. Since the secondary current of CTs is obtained from the difference of the primary transformer current and the magnetization current, under saturation conditions, the secondary current with a constant ratio does not follow the primary current and in addition to increasing the conversion ratio error, distortion will appear in the output signal. When a fault occurs, due to the DC component of the fault current (which is usually not included in CT design), the saturation phenomenon will occur, and one of the ways to limit this effect is to use a CT with a higher rated specification or use special algorithms to correct this phenomenon. Since the use of CT with higher rated specifications is not economically viable, software compensation of the phenomenon of CT saturation in power systems is a suitable solution to the problem, which will lead to cost reduction and increase the reliability of the power system; Especially, such an algorithm can be easily applied in the structure of numerical relays (as an information preprocessor). Therefore, the purpose of this project is to detect the saturation phenomenon and compensate for CT secondary current distortion using signal processing methods. 1-1- Overview of the work done. In [1-3], the problems caused by the occurrence of saturation in current transformers have been investigated.

In [4], a method for detecting saturation in current transformers is presented based on the fact that the current changes rapidly when the saturation starts. This method detects CT saturation due to a sudden decrease in the amount of current, but it is not very successful if an anti-aliasing low-pass filter is used. In [5] and [6], a method for detecting current transformer saturation based on the third-order derivative of the secondary current is presented. In these articles, the effect of the anti-aliasing low-pass filter is considered.

In [7], an algorithm for calculating the core flux from the secondary current and then its compensation is proposed. This algorithm calculates the core flux well and detects CT saturation in different conditions. However, in this method, the assumption that the residual flux is equal to zero at the beginning of the calculations is used, which is not a suitable assumption in real conditions.

Another method for detecting saturation by calculating the average error and variance of the current range is proposed in [8]. The error value is determined with the assumption that if a sinusoidal current is complete, the sum of that current with a coefficient of its second derivative must be zero. In [9], an impedance method is proposed to detect saturation in a current transformer for busbar differential protection. This method is based on the first-order differential equation of the impedance of the power system source at the relay location, and it uses the busbar voltage and the secondary current of the current transformer to calculate the impedance. Changes in this impedance are used to determine the state of the current transformer. Also discussed are the effects of residual flux in the core, size of magnetization inductance and different fault modes. In [10], a detection method using symmetrical components for differential protection is proposed. In [11], another method for detection using artificial neural network and genetic algorithm is proposed. In this method, a neural network is used to detect saturation and a genetic algorithm is used to find the optimal structure of the neural network in terms of the number of layers and the number of neurons in each layer. In [12], a new hybrid method using the second derivative of the output current of the current transformer and the zero crossing rule is presented. In [13], a compensation method is proposed in which, after estimating the magnetizing current of the CT core, this current is added to the measured secondary current, so that the secondary current is obtained. This algorithm works well for different fault and system conditions, but (as in [7]) it is based on the assumption that the residual flux is zero before the fault occurs. The algorithm proposed in [14] compensates for the distorted secondary current and the residual flux level does not have an adverse effect on it. This algorithm uses a second-order differential equation to detect the moment of saturation.

An alternative method is to use an artificial neural network to estimate a function that corrects the current transformer secondary current that is distorted by saturation. This method has been used in many articles [15-19]. Dependence on the secondary capacity of the current transformer, failure to consider all the factors that can affect the saturation, and the non-optimal structure of the neural network are some of the defects that can be seen in these articles. In [20], the artificial neural network, whose number of neurons and layers of this network is optimized by genetic algorithm, has been used in order to detect and compensate saturation.

1-2-    Thesis structure

In this thesis, after the initial introduction of the project in this chapter, the introduction of current transformers, its equivalent circuit, the core model and finally the investigation of CT saturation phenomenon and the effect of existing parameters on it have been discussed in the second chapter. In the third chapter, the techniques used in the thesis to reveal CT saturation are described, and the fourth chapter includes the stages of CT modeling, the sample network (part of Iran's electricity network) and the implementation of the methods reviewed in the third chapter. After comparing the implemented methods and determining the appropriate method to reveal the saturation phenomenon in the fourth chapter, the CT secondary current compensation methods are reviewed in the fifth chapter and the results of the implementation of the CT secondary current compensation methods and the selection of the appropriate method are presented. In the sixth chapter, it is dedicated to the description and implementation of the proposed methods of the thesis to reveal the saturation phenomenon and compensation of CT secondary current in online conditions, and finally, in the seventh chapter, the summary, conclusions and suggestions are presented.