The extensive application of nonlinear loads in power grids brings a series of problems harmonic wave.
The main hazards of harmonic wave include:
Increase the load of power facilities, reduce the power factor of the system, reduce the effective capacity and efficiency of power generation, transmission and electrical equipment, resulting in equipment waste, line waste and power loss;
Causes the resonance of reactive power compensation capacrent or over voltage;
Harmonic wave will change the action characteristics of the protection relay, cause the malfunction of the relay protection facilities, and cause the automatic devices such as relay protection to work disorderly.
In order to eliminate or reduce harmonic wave in the power grid, passive filtering and APF solutions have been proposed. This article mainly discusses the latter. Active power filter (APF) uses modern power electronic devices to actively generate a harmonic wave that is equal in magnitude and opposite to the harmonic wave to counteract the harmonic wave generated by the system.
The application industries of APF include electroplating enterprises, water treatment equipment, petrochemical enterprises, large shopping malls and office buildings, precision electronic enterprises, power supply systems of airports/ports, medical institutions, etc. Load currents vary from a few hundred amps to several thousand amps. Usually filter manufacturers use traditional CT to measure the load current, but they face different difficulties due to different on-site conditions.
In the reconstruction project, since the busbars or cables have been installed, a lot of dismantling and installation work must be done to use the through-center CT;
In some switch cabinets, the internal space is narrow, the distance between the busbars just meets the insulation requirements, and the CT size is larger than the gap, making it difficult to install. Or the CT size is suitable, but the on-site construction is very difficult for engineers;
The size and shape of the busbars vary greatly. Different projects need to purchase CTs of different sizes, which brings many difficulties to the procurement work. Some projects even require customized CT, which is not only costly, but may also affect the construction period;
Some projects have a small load when they are initially launched, and gradually increase the load over time. In such a project, if a CT with a small range is selected, the CT needs to be replaced later due to saturation problems. If a CT with a large range is selected at the beginning, the current is very small at the beginning, which will affect the measurement accuracy and effect of filtering.
The Rogowski coil is a toroidal coil uniformly wound on a non-ferromagnetic material,containing no iron core.
The output signal is the derivative of the current with respect to time. Through a circuit that integrates the output voltage signal, the input current can be truly restored.
Rogowski coil does not contain ferromagnetic material, no hysteresis effect, almost zero phase error; no magnetic saturation phenomenon, so the measurement range can be from several amperes to hundreds of kilo-amps of current; simple structure, and the difference between the measured current There is no direct circuit connection; the response frequency bandwidth is 0.1Hz-1MHz. Compared with the traditional transformer with iron core, the Rogowski coil has a wide measurement range, high precision, stability and reliability, wide response frequency band, measurement and relay protection functions, small size, light weight, safety and environmental protection requirements.
Rogowski coils can be used in relay protection, thyristor rectification, variable frequency speed regulation, resistance welding and other occasions where the signal is seriously distorted, as well as electric furnace, short circuit test, lightning signal acquisition and other occasions with high current.
In the engineering application of APF, the installation problem is the biggest problem for filter manufacturers. The Rogowski coil solves the problem of filter manufacturers due to its advantages of opening, flexibility, high precision, wide measurement range and small position error.
