GIS Same-Frequency Same-Phase Withstand Voltage Test System: Working Principle, Functional Features, Technical Specifications, and Application Scope
1. Application Scope of GIS Same-Frequency Same-Phase Withstand Voltage Test System
GIS equipment is increasingly used in power systems due to its small footprint, high reliability, and low maintenance workload, making it particularly suitable for urban substations. In recent years, with more GIS equipment being put into operation, its advantages have been fully demonstrated, but some issues have also arisen during the promotion and adoption of GIS equipment. A prominent issue is that after expansion or maintenance of GIS equipment intervals with double-busbar connections, to meet on-site test conditions and avoid breakdown caused by the superposition of test voltage and the reverse-phase operating voltage of the busbar at the disconnector gap of the busbar, which could endanger the operating GIS equipment, the entire busbar at that voltage level must be de-energized. This significantly impacts power supply reliability.
Currently, the main method for on-site AC withstand voltage testing of GIS both domestically and internationally is the variable frequency resonant method, which achieves resonance in the test circuit by changing the frequency of the test voltage, thereby raising the test voltage to the required value. This method does not require high power supply capacity, is convenient for on-site implementation, and its withstand voltage test effect is similar to that of power frequency testing. However, when testing GIS substations with double-busbar configurations using this method, since the frequency of the test voltage differs from the frequency of the operating voltage, the entire substation must be de-energized to avoid breakdown at the isolation gap of the withstand voltage circuit, which could cause damage to the GIS equipment.
To overcome the issues of the variable frequency resonant method in double-busbar GIS substations, F is set to power frequency 50Hz, and compensation by changing L or C is used to achieve power frequency resonance. Then, frequency and phase are coordinated through the power supply equipment to achieve the test purpose, adopting the GIS on-site AC withstand voltage test method based on same-frequency same-phase technology, ensuring that the test voltage and operating voltage are of the same frequency and phase.
2. Functional Features of GIS Same-Frequency Same-Phase Withstand Voltage Test System
1) Good test equivalence. The output of this device is a sine wave with low waveform distortion. The waveform distortion rate is <3%, which is different from other types of power supply cabinets (which output square waves that are shaped into sine waves). Therefore, this device does not require peak measurement during testing.
2) Fiber-optic control is adopted, completely isolating the high-voltage and low-voltage control circuits.
3) Small size, light weight, flexible handling, and very suitable for on-site use.
4) Simple and convenient operation, easy wiring, and can improve work efficiency by 50% (compared to generator set methods).
5) Safe and reliable, with multiple protections integrated into the device, including discharge breakdown protection, overvoltage setting protection, output short-circuit protection, startup zero-position protection, bridge arm amplifier circuit protection, and power curve protection. When any protection is triggered, the device immediately disconnects the test voltage output and cuts off the main circuit power to ensure the safety of test personnel, the test object, and the test system.
6) The signal source in this device is generated by a dedicated chip, controlled by a microcomputer, with high output frequency stability, reaching 0.01Hz.
7) The output voltage is controlled by a high-speed microcomputer from abroad, with output voltage instability <1%.
3. Technical Specifications of SP-150 GIS Same-Frequency Same-Phase Withstand Voltage Test System
1) General Operating Conditions
Altitude: ≤3000m
Ambient Temperature: -5℃ to 40℃
Relative Humidity: ≤90%
Solar Radiation Intensity: 0.1%W/cm2
Maximum Daily Temperature Difference: <25℃
No conductive dust
No fire or explosion hazards
No gases corrosive to metals or insulation
A reliable grounding point must be provided
Placement tilt angle not exceeding 5 degrees
Storage location: Indoor
Storage environment temperature: -20℃ to 40℃, relative humidity ≤90%
2) Performance Parameters
Control Box
Rated Power Supply: Single-phase AC 220V±10%, 50Hz
Power Input: 10W Max
Divider Sampling Voltage: 0---100V
PT Secondary Sampling Voltage: 0---100V
Operating Busbar Frequency Range: 50.0±0.5Hz
Busbar Voltage Measurement Accuracy: ±(1.5% reading + 1d)
Test Voltage Measurement Accuracy: ±(1.5% reading + 1d)
Phase Difference Measurement Accuracy: ±(1.5% reading + 1d)
Transformation Ratio Setting Range: 1~65535
Power Supply Cabinet
Rated Input Voltage: Three-phase AC 380V±10%, 50Hz
Single-phase Output
Rated Output Power: 150kW
Output Voltage: 0～350V
Rated Output Current: 0～428.6A
Frequency Range: 50.0Hz
Frequency Stability: 0.01Hz
Continuous Operation Time at Full Load: 360 minutes
Output Waveform: Standard sine wave
Output Sine Wave Distortion Rate: ≤1.0%
Output Voltage Instability: ≤1.0%
Output Voltage Measurement Accuracy: ±(1.5% reading + 1d)
Output Current Measurement Accuracy: ±(1.5% reading + 1d)
Noise Level: < 85dB
4. Working Principle of GIS Same-Frequency Same-Phase Withstand Voltage Test System
The high-power output of the power supply device adopts the principle of step-by-step amplification (see figure). Starting from the initial tiny power signal source, it undergoes multiple amplifications to achieve high-power output, meeting the test requirements.
The signal source uses a function generator to produce a standard 50.00Hz sine wave signal, which is voltage-regulated through a digital potentiometer. This process also involves frequency and voltage adjustment during the test. The power output frequency is the same as the signal source frequency, and voltage adjustment is implemented in the signal source part. The output of the signal source directly drives the "pre-amplifier" to produce initial power output. This "pre-amplifier" output is divided into two paths: one path generates a "synchronization power supply" with the same frequency as the test frequency, used for partial discharge measurement instruments; the other path drives the "bridge amplifier circuit" (see figure). The "bridge amplifier circuit" is the main part for generating high power, and it generates significant heat during testing, requiring a forced air cooling system for heat dissipation.
In the figure, Q1 to Q4 are four equivalent transistors, corresponding to the four bridge arms of the variable power supply. Each bridge arm consists of thousands of transistors connected in parallel, with effective current balancing measures. During the positive half cycle, the pre-amplified signal acts on the bases of Q1 to Q4 through driving transformers (T1 to T4), with Q2 and Q3 cut off; Q1 and Q4 conduct, and current flows from Q1 through the load to Q4, forming the positive half cycle of the sine wave. During the negative half cycle, the analog signal drives the bases of Q2 and Q3, with Q1 and Q4 cut off, and Q2 and Q3 conduct, and current flows from Q3 through the load to Q2, forming the negative half cycle of the sine wave, thus forming a complete sine wave across the load.
The "bridge amplifier circuit" requires a high-power DC power supply as its working power. The power for this device is directly taken from a three-phase 380V AC power supply, which is sent through a vacuum switch with overcurrent and instantaneous trip protection to a three-phase bridge rectifier circuit, converting it into pulsating DC. Then, a filter circuit consisting of filter inductors and capacitors smooths the pulsating DC into a smooth DC power supply for the "bridge amplifier circuit." Since the capacitance of the filter capacitors reaches tens of thousands of microfarads, direct closing may cause a large charging current that could trip the main power switch. A pre-closing circuit is used to charge the filter capacitors with a small current first, and when charged to about 80%, the main closing is performed, avoiding a large starting inrush current. After opening, the charge stored in the filter capacitors is slowly discharged through discharge resistors. When using this device, pressing the "Start" button on the control box automatically completes the fan start, pre-closing, and closing processes in sequence.
The main circuit of the entire device is controlled by the "fast overcurrent protection" section, which controls the DC working power of the "bridge amplifier circuit." In case of a fault, the working power can be quickly cut off to protect the subsequent circuits.
The output terminals (OUT1, OUT2) of the "bridge amplifier circuit" are connected to the low-voltage side of the intermediate step-up transformer, whose high-voltage side is connected to the test circuit. The test circuit and the intermediate step-up transformer will not cause an open circuit condition. Due to the presence of a large-capacity freewheeling circuit inside the power supply, there is always an energy release path in the entire test circuit. Therefore, regardless of an internal power supply fault or sudden external power failure, the test transformer or resonant circuit is not disconnected, which is completely different from traditional test transformers, and there is no overvoltage caused by forced current zero-crossing.
