Application of online detection method for distributed voltage wiring anomaly in smart grid

introduction

Smart power grids are the intelligent power grids. They are based on an integrated, high-speed two-way communication network. Through the application of advanced sensing and measurement technology, advanced equipment technology, advanced control methods and advanced decision support system technology, they achieve the goals of making the power grid reliable, safe, economical, efficient, environmentally friendly and safe to use.

The basic characteristics of my country's smart grid are to achieve informationization, automation and interaction in technology. The most basic work is to establish a collection system for various parameters in the operation of the power system, such as online monitoring of load power, online monitoring of transformer operation, online monitoring of SF6 gas in GIS room, etc. However, online monitoring work has just started, and much work needs to be done urgently.

0.4KV ring network power supply is the most basic and widely used power supply network architecture in the power system, and its importance is self-evident. The quality of 0.4KV ring network power supply directly affects the service quality of the power supply industry. However, it should be noted that due to its wide use, this basic system often has abnormal phenomena such as reverse phase sequence, wrong phase connection, loose wiring or phase loss caused by leakage fuse during operation due to various reasons. Such defects occur frequently, but often due to various situations such as scattered environment and unprofessional users, the time of discovery is delayed, the power supply quality is affected, and the normal power consumption is delayed. It has a negative impact on both the power system and users.

The PT secondary metering voltage circuit involves the billing problem of the watt-hour meter. The use of cables with too thin a diameter will cause the metering voltage to drop, which will directly affect the billing. Moreover, abnormal conditions such as reverse phase sequence and phase loss in the metering circuit will cause the meter to slow down or even stop rotating, resulting in a significant reduction in the revenue of the power supply company.

In the power system and some large electricity users, there is currently no better automatic detection method for the above two situations. The main method is to send special personnel to conduct inspections, which is inefficient and costly. Of course, for metering systems involving electricity bills, load power monitoring instruments are installed in the system for monitoring some important users. However, due to the high cost, it cannot be widely promoted.

In view of the above situation, in the current power system and the future smart grid, we hope to have a distributed online monitoring method for the voltage phase sequence of the 0.4KV ring network power supply and metering system, which is low cost and has no monitoring blind spots. When the voltage loop wiring in the system is abnormal, it can be found in real time, and its abnormal signal can be instantly sent to the relevant automatic monitoring system. The monitoring system quickly judges and gives processing instructions, notifying the relevant teams and personnel of the processing tasks and abnormal occurrence points for processing. The rapid processing of such abnormalities has important practical significance for improving the power supply quality and service quality of the power system.

1. Implementation principle and characteristics of distributed voltage wiring abnormality online monitoring technology

Distributed voltage wiring abnormality online monitoring technology is not only a technical means, but also a management means. By establishing distributed voltage wiring abnormality, the stubborn problem that has long plagued power system management departments has been effectively solved.

Of course, for the 0.4KV ring network power supply circuit and PT secondary metering circuit, it is not easy to establish distributed online monitoring, because there are many devices involved. As long as there is a connection, there may be abnormal phase sequence or phase loss. If too many online phase sequence online monitoring components are used, the cost will be a considerable burden. Even for a 35KV substation system, it is estimated that dozens or even hundreds of online phase sequence abnormality detection components will be used. In other words, if real-time monitoring is to be achieved, a large number of online phase sequence monitoring components must be used. Then, the development cost of phase sequence monitoring components is a very important indicator. After careful analysis, it is not difficult to find that in order to make online phase sequence monitoring technology widely used, the developed monitoring components must meet the following standards: 1. Low cost, 2. Easy installation (installation without power outage), 3. Maintenance-free, 4. No impact on the monitored circuit, 5. Alarm signals can be collected and transmitted remotely, etc.

There are ready-made mature technologies for monitoring abnormal voltage circuit wiring. The corresponding purpose can be achieved by applying the negative sequence filter principle. The principle is also very simple. In order to make the developed components suitable for the requirements of the 400V ring network power supply system and correctly reflect the abnormal voltage wiring of the PT secondary metering circuit, in addition to selecting appropriate parameters, an adaptive circuit is also added to the negative sequence filter circuit. In addition, due to the asymmetry of the load in the 400V ring network power supply, the power supply is often unbalanced, causing the negative sequence filter to malfunction and cause false alarms. In order to avoid this phenomenon, a floating threshold should be added to avoid the influence of the unbalanced three-phase voltage. In terms of principle implementation, we can use vector diagrams for detailed analysis.

Figure 1 Three-phase negative sequence voltage filter

According to the above vector diagram, the voltage amplitude between mn and n when various voltage wiring abnormalities occur can be analyzed, and the following conclusions can be drawn:

The online phase sequence monitoring element developed can detect the abnormal voltage circuit of the 380V ring network power supply system and can also be used for online detection of the PT secondary metering voltage circuit (phase-to-phase voltage is 100V). It is necessary to further analyze these two situations.

According to the working principle of the negative sequence filter, from the above formula, it can be concluded that under the standard voltage input, the relevant output voltage Umn value can be calculated, see Table 1

It can be found from the table that when the 380V voltage circuit is connected in reverse phase sequence, the Umn output voltage is the largest; when the B phase of the PT secondary metering voltage circuit is missing, the Umn output voltage is the smallest.

The developed online phase sequence monitoring element must be able to give an alarm for all the above 9 situations, and can stably withstand a maximum high voltage of less than 700V. This is mainly because when there are system abnormalities such as lightning transient overvoltage and operating overvoltage, the phase sequence monitoring element should not be damaged. When the system is normal, the phase sequence monitoring element can resume normal operation.

The design of the online phase sequence monitoring element should be able to avoid the Umn output caused by the unbalanced three-phase voltage. According to the empirical value, the unbalanced voltage output of the 400V ring network system is estimated to be around 15~20V. Considering the margin, multiply it by the reliability factor K, which is generally 1.5 times. In other words, the online phase sequence monitoring element should be able to avoid the unbalanced output voltage below 30V when working. Considering the three-phase imbalance caused by severe load asymmetry, the online phase sequence monitoring element needs to use a floating threshold to ensure that no signal is sent incorrectly.

Considering the importance of the ring network power supply circuit and metering circuit, if the online phase sequence monitoring element itself is abnormal, it should not affect the main circuit, and relevant protection measures must be taken. There are two situations that affect the main circuit: 1. Due to the failure of the online phase sequence monitoring element, the main circuit is short-circuited and grounded; 2. Due to the selection of the internal chip of the online phase sequence monitoring element, a high-power chip is selected, which affects the amplitude of the metering voltage, resulting in under-metering and loss of electricity billing.