Abstract : Based on the SVC power supply system of a 35kV substation, this paper first introduces the equipment load conditions of the Jiuzhou Electric test station, and then describes the basic principle and composition of the TCR+FC static var compensator system. Practice has proven that after the system is put into operation, it has effectively mitigated power quality issues caused by harmonic currents, voltage fluctuations, and flicker generated by nonlinear loads.
Keywords : SVC; TCR; FC; harmonics
1 Introduction
In recent years, with the continuous increase of high-power nonlinear loads, the reactive power impact and harmonic pollution of the power grid have shown a continuous upward trend. The lack of reactive power regulation means that the bus voltage varies greatly with changes in operating mode, leading to increased line losses and reduced voltage qualification rate. In addition, with the development of the power grid, the issue of system stability has become increasingly important.
Static var compensators (SVCs) are devices that rapidly regulate reactive power and have been successfully applied to compensate for impulsive loads in industries such as metallurgy, mining, and electrified railways. Their characteristics include: fast regulation speed, continuous and phase-by-phase reactive power regulation, reliable operation, and wide applicability. Therefore, they play a significant role in stabilizing power system voltage and improving power quality.
In addition, the use of TCR static dynamic var compensator (SVC) is very effective in eliminating reactive power impacts generated by symmetrical loads such as rolling mills and other large motors. It significantly improves grid voltage fluctuations and power factor, making it a new type of energy-saving device with high technical content and significant economic benefits.
2. Overview of Kyushu Electric Test Station
The Jiuzhou Electric Test Station, with a building area of approximately 3,000 square meters, is currently a leading domestic test station capable of handling 5MW and 35KV medium- and high-voltage converters. It includes a full-load test platform for 10kV and below 5MW high-voltage frequency converters, a full-load test platform for 35KV and below 5Mvar high-voltage grid dynamic reactive power compensation devices, and full-load test platforms for 1.5MW full-power wind turbine converters and 1.5MW doubly-fed wind turbine converters. It can centrally conduct intelligent load performance tests on products such as frequency converters, soft starters, SVC reactive power compensation devices, and wind turbine inverters at medium and high voltage levels. The operation of these devices not only generates high-order harmonics that are integer multiples of the power frequency, but also generates side-frequency harmonics that vary with the output frequency, resulting in severe voltage distortion and serious pollution to the power grid. Severe voltage fluctuations, high-order harmonics, and excessively low power factors deteriorate the power supply environment of the test station, shorten the service life of the equipment, affect product commissioning efficiency, and increase main transformer losses and line losses. For these reasons, reactive power compensation of the power supply lines is necessary.
3. Basic Principles of TCR+FC Type SVC
Figure 1 shows a simplified single-line diagram of a TCR+FC type SVC. The TCR (Thyristor-Controlled Reactor) is a thyristor-controlled reactor, and the FC (Fixed Capacitor) consists of several sets of fixed capacitors. In each single-phase branch, the compensating reactor and the thyristor valve work together.
The TCR+FC type SVC controls reactive power by controlling the current flowing through the reactor using thyristor valves. Based on changes in the load reactive power QV, the reactive power QTCR (inductive reactive power) of the reactor is adjusted. When the load reactive power QV increases, the reactive power QTCR generated by the TCR decreases; when the load reactive power QV decreases, the reactive power QTCR generated by the TCR increases. That is, regardless of the changes in the load reactive power, the sum of the two must always be constant, i.e., QV + QTCR » constant. This constant is equal to the value of the capacitive reactive power QC emitted by the harmonic filter bank, ensuring that the reactive power QN taken from the grid is constant or zero, i.e., QN = QC - (QV + QTCR) = constant (or 0). This achieves a constant grid power factor and almost no voltage fluctuation, thus achieving reactive power compensation and suppressing system voltage fluctuations and flicker caused by load fluctuations. The key is to accurately control the firing angle of the thyristor to obtain the required current flowing through the compensation reactor. The thyristor converter and control system can achieve this function by collecting the reactive current and voltage values of the bus, synthesizing the reactive value, comparing it with the set constant reactive value, calculating the firing angle, and then using the thyristor triggering device to make the thyristor flow with the required current.
Figure 1. Wiring diagram of TCR+FC type SVC
4. 35KVSVC System Design
4.1 Design of the Thyristor-Controlled Reactor Branch
Figure 2 shows the principle and current waveform of a thyristor-controlled reactor. Th1 and Th2 are two anti-parallel thyristor groups, which conduct during the two half-cycles of the power supply voltage wave, respectively, controlling the firing angle α to be adjusted within the range of 90° to 180°. The inductive reactive power generated is maximum at 90° (i.e., thyristor short circuit), and zero at 180° (i.e., thyristor open circuit). Since the reactor is a purely inductive load, the current lags the voltage by 90°, absorbing inductive reactive power. A TCR phase-controlled reactor consists of a thyristor valve string and a reactor connected in series. The wiring configuration is mostly delta-connected to eliminate the influence of the third harmonic on the system. Furthermore, in practical engineering, when the capacity is large, the reactor is divided into two equal-capacity parts connected in series across the thyristor valve string. This protects the thyristors in case of reactor failure. The thyristor valve consists of multiple thyristor pairs connected in series to obtain a rated voltage of 35KV and withstand overvoltage conditions during normal operation. The high-voltage thyristor valve here uses thyristors manufactured by ABB, which can withstand the maximum overcurrent/overvoltage level of the SVC device and high dv/dt and di/dt levels. It also works with the reactor to achieve good dynamic response. At the same time, the microcomputer monitors the operating status of the TCR thyristor in real time and can directly calculate, display, alarm, and trip on the low-voltage side.
Figure 2. TCR wiring principle and current waveform diagram
4.2 Design of FC Filter Branch
The common filtering method is to configure passive filters, which can both filter out harmonics and compensate for a certain amount of reactive power. For SVC devices installed in power systems, the system harmonic content is relatively low and relatively stable, with the main harmonic source being the SVC device itself. Therefore, the design is relatively simple, generally requiring only a single-tuned filter of order 5 or higher, with a high-pass filter installed if necessary. However, for SVC devices compensating for impulsive loads, the filter design must consider not only the harmonics generated by the device itself but also the large number of non-characteristic harmonics and even-order harmonics generated on the load side. Sufficient design experience is crucial to achieving the highest cost-effectiveness in filter design. Furthermore, filters are not in a true resonant state during actual operation. To prevent excessive harmonic currents, considering that the system is generally inductive, the filter is usually designed as an inductively tuned filter, thus avoiding the possibility of resonance with the system.
4.3 Control System Design
The control system is the core of the entire SVC device, mainly composed of independent hardware modules such as an A/D acquisition system, a DSP processing unit, a pulse phase-shifting circuit, and an electro-optical conversion device. It also includes a monitoring system and a protection system. The DSP-based fully digital controller features fast dynamic response and high control precision. Its control algorithm employs instantaneous reactive power theory, using instantaneous values instead of traditional effective values to analyze the three-phase power system. After vector transformation, it calculates the compensation admittance and trigger angle for each phase, enabling the high-voltage reactive power compensation device to quickly generate or absorb reactive current, achieving reactive power balance, maintaining system voltage stability, improving the system power factor, and reducing system losses. The controller also features flexible control modes, including simultaneous three-phase control, phase-by-phase control, and three-phase balancing. The multi-functional automation interface provides remote operation and automation system interface capabilities, enabling unattended operation.
The thyristor triggering method adopts high-level energy extraction photoelectric triggering technology. This technology includes thyristor trigger board power supply technology, which provides working power to the trigger board. Its energy is taken from the thyristor side of the high-voltage main circuit and is independent of the secondary control circuit power supply, ensuring strict isolation between the main circuit and the control circuit, and is not affected by the control circuit power supply during operation. The thyristor triggering technology is used to convert the photoelectric trigger signal sent by the control system into an electrical signal and send it to the high-voltage main circuit thyristor to control its conduction. During operation, it has no direct electrical connection with the control circuit. At the same time, it also has static voltage equalization, dynamic voltage equalization, BOD and other protections, which enhance the anti-interference capability of the system and achieve effective protection of the thyristor.
4.3.1 Monitoring System Design
The SVC monitoring system can perform the following functions: monitor the device startup process; acquire and display the system's operating status in real time; determine system fault status online; record important system operating information; transmit data to remote computers; analyze the system's long-term operating status and generate voltage quality and harmonic content analysis reports; and record and display waveforms during system commissioning to assist installers in quickly checking the system status. These functions provide strong support for the normal operation of the SVC. Therefore, the SVC monitoring system is developed based on an industrial control computer with a built-in data acquisition card and using LabVIEW virtual instrumentation. In addition to data acquisition, the monitoring system is also responsible for communication with other devices and remote data transmission. The use of a high-performance industrial control computer makes it easier to complete such complex tasks simultaneously.
This system uses a PCI-6220 data acquisition card to sample and process three-phase voltage and current signals. The sampling frequency is set to 10kHz, and other parameters can be changed according to the actual application, but the default settings are generally sufficient. During software processing, Fast Fourier Transform (FFT) and harmonic distortion analysis are applied to obtain power quality parameters of the power grid and perform related system calculations. The monitoring system interface is shown in Figure 3. Display items include active power, reactive power, and power factor of three phases A, B, and C; fundamental amplitude, frequency, and RMS value of voltage and current; and DC component monitoring parameters. Clicking the drop-down menus for harmonic monitoring and power factor curves allows selection of the corresponding waveform to be monitored.
Figure 3 Power Grid Monitoring Main Interface
4.3.2 Protection System Design
The SVC protection system can receive all fault signals and important real-time data from the valve group side. It can analyze the faults based on the data and alarm limits to identify the specific fault type and fault branch. Then, it transmits these alarm messages and real-time data from the valve group side to the DSP processing unit through a customized communication protocol. The DSP processing unit performs the corresponding processing. In the event of a serious fault, the protection system can issue a lockout signal to block the trigger pulse and issue a switching signal to control the I/O unit to disconnect the high-voltage equipment in the main circuit, thereby greatly improving the safety factor of the SVC system.
5. Conclusion
After the 35KVSVC device was put into operation, actual measurements showed that the interference of the test equipment on the power quality of the power supply system had been suppressed to a low level.
(1) The harmonic absorption effect is significant. The harmonic currents injected into the 35KV side test point of the system meet the national harmonic standard requirements. The total harmonic distortion rate of the 35KV bus voltage caused by each test equipment is reduced to 2.5%, which is lower than the national standard limit (3%).
(2) The voltage fluctuation of the 35KV bus caused by various experimental devices is 1.6%, which is lower than the national standard allowable value (2%).
(3) The 35KV high voltage reactive power compensation device can improve the average power factor of the entire test station system to over 0.96. Not only will it no longer be subject to fines from the power industry, but it will also receive a certain amount of reward every month.
The expected compensation target was achieved, and the pollution caused to the power grid by three-phase asymmetry and nonlinear loads was also resolved.
The 35KV high-voltage reactive power compensation device, from design and manufacturing to commissioning and operation, has achieved and exceeded performance targets. The system is safe, reliable, and well-designed. These achievements mark another major breakthrough for Jiuzhou Electric in the field of high-voltage technology, and are of great significance for its expansion into the 35KV VVC market.
About the author:
Wang Guoqiang, Power Grid Division, Harbin Jiuzhou Electric Co., Ltd., No. 162 Haping Road, Nangang District, Harbin, Heilongjiang Province
Postcode 150081 Telephone: 13936307357 [email protected]
