Anke Electrical Co., Ltd.

Abstract: This article proposes a method for real-time monitoring the insulation of DC systems under normal conditions using an asymmetric bridge, establishing a mathematical model for determining insulation resistance. Based on this, a comprehensive microcomputer DC insulation monitoring and fault location device has been developed. Actual applications have shown that the device operates reliably and accurately.

Keywords: Power Plant; Direct Current System; Insulation; Faulty Grounding; Monitoring

1 Introduction

The direct current (DC) systems in power plants and sub-stations are the power sources for secondary equipment. To ensure the reliable operation of secondary equipment, real-time online monitoring of the insulation condition of the DC system and predicting its weak insulation points is necessary. In the event of a single grounding fault in the DC power system, it must be addressed immediately; otherwise, a repeated grounding incident could lead to a major accident. Currently, the method used to locate grounding faults involves sequentially opening and closing DC feeders and manually searching for the grounded circuit. This approach is time-consuming, labor-intensive, unsafe, and may cause protective relays or automatic devices to malfunction or fail to operate, leading to accidents. Therefore, there is a need to develop a safe and automatic device that can both predict the insulation condition of the DC system and locate grounding fault points.

2 Basic Working Principle of the Device

1. Monitoring Methods for the Insulation of Power Grids from Ground under Normal Conditions

The DC insulation monitoring devices currently widely used in power plants and substation transformers rely on a balanced Wheatstone bridge. When an insulation in the DC system decreases, it disrupts the bridge's balance, and the degree of imbalance in the bridge reflects the insulation condition of the DC grid. However, when both positive and negative insulations decrease uniformly, these devices fail and are unable to automatically detect the insulation resistance value of the grid to the ground. Therefore, a new detection method needs to be explored.

A star-shaped resistor circuit is connected between the positive and negative busbars of the DC system and the earth, forming an asymmetric bridge voltage sampling circuit as shown in Figure 1.

In the image, RZ and RF represent the insulation resistance of the positive and negative poles of the DC system to ground, U is the voltage between the positive and negative poles, and R1, R2, and RJ are sample resistors.

R1 ≠ R2. By applying the Thevenin's theorem, the equivalent circuit for Figure 1 can be obtained as shown in Figure 2. In Figure 2:

Based on Equivalent Circuit Diagram 2, it can be obtained that:

In Figure 1, if the grounding terminal of RJ is disconnected, the voltage between the negative busbar and ground, U', is:

In the formula: UJ represents the voltage across the bridge resistor RJ; U'F is the voltage between the negative busbar and ground after the resistor is disconnected.

The RZ//RF reaction reflects the insulation condition of the DC system to the ground. By sampling UJ, U, and U'F, the insulation of the DC system to the ground can be continuously and real-time detected using formula (4). For the case where the insulation between the positive and negative poles uniformly decreases, formula (4) remains valid.

Post-Failure Ground Resistance Testing and Fault Locating Methods

When the DC system's ground insulation resistance (RZ/ / RF) drops below 25kΩ, it is considered that a grounding fault has occurred. A low-frequency signal is injected between the DC bus and the ground, and weak current sensors wrapped around each DC feeder branch transmit the signal back. Since each branch has a capacitance to ground and a resistance to ground, the signal contains both a capacitive current component (IC) and a resistive current component (IR). The resistive current (IR) that reflects the insulation condition can be detected using a phase-sensitive circuit. By sequentially detecting the resistive currents in each circuit, the maximum value (IRm) is found and compared with the set value (IRdz). If IRm > IRdz, the branch is considered a grounding branch; otherwise, it is believed that the fault occurs on the bus.

When a grounding fault occurs on the power grid, the synchronous voltage signal U_T must be derived from the positive busbar; when a negative grounding occurs, U_T is derived from the negative busbar; when both positive and negative grounds occur evenly, U_T can be derived from either the positive or negative busbar. Before the low-frequency signal is connected to the grid, the bridge resistor RJ is removed. The determination of grounding can be based on the magnitude of the positive and negative voltages to the ground. The grounding resistance R0 ≈ RZ || RF, and its value can be determined before the low-frequency signal is connected to the grid according to Equation (4). Due to the presence of a significant low-frequency current signal on the grounding fault branch, which disappears into the ground at the grounding point, the specific location of the grounding point can be determined by detecting the presence or absence of this low-frequency current signal.

Development and Application of Device 3

Developed based on the aforementioned principles, the integrated device for microcomputer DC insulation monitoring and fault point localization is composed of a main unit, a weak current sensor, and a detector, forming the system as shown in Figure 3. The device is controlled and data processed by an 80C196 microcontroller. In normal operation, the microcontroller controls the switching of bridge resistor RJ, samples voltages UJ, U, and U′F through the voltage sampling circuit, calculates the insulation resistance of the power grid to ground using a mathematical model, and displays the resistance values and the voltage between the positive and negative buses on a digital display.

When a single-point grounding fault with resistance less than 25kΩ occurs on the positive or negative side of the power grid, or when the insulation resistance between positive and negative drops uniformly to below 80kΩ, the microcontroller controls the relay to disconnect RJ and introduce the low-frequency signal source. This signal is coupled through an isolation transformer and capacitors to the synchronous voltage forming circuit between the positive and negative busbars and the ground, which is used to generate the synchronous signal required for the phase-sensitive circuit. The microcontroller controls a multiplexer to sequentially process signals returned from weak current sensors on each DC feeder line, using the principles described in Section 2.2 to identify the faulted branch or busbar. If a branch fault occurs, the digital display shows the grounding branch number and resistance value, and the alarm sounds a longer, rhythmic tone; if a busbar fault occurs, the digital display only shows the grounding resistance value, and the alarm emits a shorter, sharp tone. By comparing the magnitude of the voltage between the positive and negative busbars to the ground, the nature of the grounding line is determined: the red light indicates positive grounding, and the green light indicates negative grounding. After locating the grounding branch, a detector is used to find the grounding point. The detector uses a clamp-on magnetic circuit at its front end, followed by a signal processing circuit, amplification, and sensitivity adjustment circuit to drive the light-emitting diode (LED) and buzzer. Clamp the magnetic circuit onto the grounding branch and move it from the busbar end towards the load end; where the LED lights up and the sound occurs, and the LED turns off, is the grounding point.

Ankore Insulation Monitoring Selection

With the advancement of industrial technology, leakage current poses a significant threat to the safety of industrial production. To enhance the continuity and reliability of power supply, many critical production sites have adopted ungrounded power supply systems. The Ankorui AIM-T series industrial insulation monitoring instruments are primarily used in industrial fields such as mines, glass factories, electric furnaces, testing equipment, metallurgical plants, chemical factories, explosive hazardous areas, computer centers, and emergency power supplies in AC ungrounded systems. They are designed to monitor the insulation condition of the system to the ground in real-time, and to promptly alert when a grounding fault occurs, reminding relevant personnel to investigate and rectify the issue. The product design strictly adheres to national standards and regulations. The Ankorui AIM-T series insulation monitors are mainly applied in IT distribution systems in industrial settings, monitoring the insulation condition of the IT system to the ground and promptly alerting in the event of an insulation fault, to remind electrical maintenance staff to address the issue in a timely manner. The product range includes the AIM-T300 and AIM-T500 series, with both insulation monitoring devices being suitable for pure AC, pure DC, and AC-DC mixed systems, with the only difference being the applicable voltage levels.

IEC61557-8-2007: "Testing, Measuring, or Monitoring Equipment for Electrical Safety Protection of Low-Voltage Distribution Systems with Alternating Current Up to 1000V and Direct Current Up to 1500V: Insulation Monitoring Devices in IT Systems"

GB/T 18268.24-2010 "Electromagnetic Compatibility Requirements for Electrical Equipment Used in Measurement, Control, and Laboratory Applications - Part 24: Test Configuration, Operating Conditions, and Performance Criteria for Insulation Monitoring Devices Compliant with IEC 61557-8 and Insulation Fault Location Devices Compliant with IEC 61557-9"

Industrial IT Insulation Monitoring Instrument

The Ankelei Insulation Monitor is an instrument designed for monitoring the insulation condition of IT power distribution systems (ungrounded systems) to the ground. The product utilizes microcontroller technology, boasts high integration, compact size, easy installation, and combines intelligence, digitalization, and networking. It features a wide measurement range, rapid response speed, and allows for a large system leakage capacitance. Additionally, it offers a rich set of functions, including insulation fault early warning, fault alarms, event recording, interconnection, and customizable parameters. It can be used in IT power distribution systems in industrial settings such as mines, glass factories, electric furnaces, testing equipment, metallurgical plants, chemical factories, explosive hazard areas, computer centers, and emergency power supplies, to monitor the insulation condition to the ground in real-time. It promptly alerts when an insulation fault occurs, reminding staff to investigate and resolve the issue.

(2) Test Signal Generator and Insulation Fault Locating Instrument

(3) Coupling Instruments for Industrial IT Insulation Monitoring

For systems not grounded above 800V, the AIM-T500 Insulation Monitor requires the use of a coupling instrument for insulation monitoring. The ACPD series of insulation monitoring coupling instruments currently includes two models: ACPD100 and ACPD200. The ACPD100 is used for insulation monitoring of single-phase AC systems ranging from 0 to 1150V and DC systems ranging from 0 to 1760V that are not grounded. The ACPD200 is used for insulation monitoring of three-phase AC systems ranging from 0 to 1650V, or AC systems with DC components (such as rectifiers) ranging from 0 to 1300V that are not grounded.

5 Conclusion

This article introduces a new method for real-time monitoring of DC system insulation under normal conditions, effectively addressing the issue of being unable to properly monitor system insulation when both positive and negative insulation degrade uniformly. A mathematical model for determining insulation resistance is proposed. After a grounding fault occurs, short-term additional low-frequency signals are used to identify the fault point. Applications show that this device operates reliably and accurately.

Reference

Li Gang, Cai Xu. A novel integrated device for DC insulation monitoring and grounding point localization [J]. Jiangsu: Coal Mine Automation, 2001.

Ankorui Enterprise Microgrid Design and Application Manual, 2020, 6th Edition.