The CT ratio is not directly determined by the system voltage, but rather by the rated current of the specific line.
Therefore, it is impossible to give a single, fixed answer, but common CT ratio ranges can be provided based on different application scenarios.
Key Conclusion: The common ratio ranges are as follows：
In 33 kV power systems, depending on line capacity and load size, Generally
the most commonly used CT ratios are typically within the following range:
Common ratios: 50/5A to 2000/5A
Most typical ratios: 300/5A, 400/5A, 600/5A, 800/5A, 1000/5A, 1200/5A
How to understand and select a CT ratio?
Selecting a CT ratio primarily depends on two key parameters:
Line rated current (load current)
This is the primary factor in selecting a CT ratio.
Short-Circuit Current
CTS must withstand the enormous currents (up to tens of thousands of amperes) generated during a system short circuit without damage and provide reliable signals to protective devices. This determines the CT's accuracy limiting factor (ALF).
Typical Selections for Different Application Scenarios
Application Scenario Line Characteristics Common CT Ratios (Example) Remarks
User Dedicated Lines/Distribution Outgoing Lines: Small loads, typically tens to hundreds of amperes. Used for the 33kV main transformer circuit within a substation.
Capacitor Banks/Reactor Banks: Fixed capacity, stable current. 400/5A, 500/5A, etc. Calculated based on the rated capacity of the reactive power compensation equipment.
Busbar Protection: Aggregates currents from multiple lines, requiring consistent CT characteristics. 1200/5A, 1500/5A, 2000/5A, etc. Typically, the larger ratio is selected for the system.
Check the electrical schematic (primary system diagram): The ratio of each CT will be clearly marked on the drawing.
Check the CT nameplate: The CT itself will clearly indicate the ratio (e.g., 600/5A), accuracy class (e.g., Class 0.5 or Class 5P20), and rated capacity.
Consult the equipment inventory or design documents: These technical documents contain detailed specifications for all equipment.
LVZW-35 current transformer by Shaanxi West Power Tongzhong Electrical Co., Ltd. is a sensor used to measure large current in 35kV system. It is mainly used in substations, power systems and electrical equipment. The transformer adopts magnetic core type, which is characterized by high linearity, strong anti-interference ability, small size and simple structure. It is a current transformer with very high cost performance.
LVZW-35 current transformer offers numerous advantages:
1.Safe and reliable
Electrical isolation: The current transformer can effectively isolate the primary circuit of high voltage and high current from the secondary measurement and protection circuit of low voltage and low current, avoiding direct contact of operators with high voltage current, greatly reducing the risk of electric shock, and ensuring the safety of personnel.
Strong anti-interference: Its working principle is based on electromagnetic induction, which will not affect and interfere with the measured circuit, ensuring the accuracy and reliability of the measurement results, and can work stably in a complex electrical environment, providing a strong guarantee for the safe operation of the power system.
2.High measurement accuracy
The output signal accuracy can reach 0.5, 0.2 or even higher, which can meet the application occasions with high requirements for current signal accuracy. Whether in the normal operation state of the power system or in the transient process when a fault occurs, the current can be accurately measured to provide accurate data support for relay protection, metering, monitoring, etc.
3.Wide measurement range
It can be applied to different voltage and current levels, and can meet the needs of various power systems and industrial application scenarios. Modern current transformers usually have a multi-speed adjustable measurement range. By adjusting the transformer ratio, it can adapt to current measurements of different sizes, and has strong versatility and flexibility.
4.Small size and light weight
Made of insulating materials and fine wires, it has a compact overall structure, small size, light weight, and is easy to install and maintain. In electrical equipment with limited space, such as switch cabinets, distribution boxes, etc., current transformers can be easily installed without bringing too much burden to the equipment or taking up too much space.
For more information,pls contact me
Shaanxi West Power Tongzhong Electrical Co., Ltd.
Contact: Ms.Grace Liu (Director of Sales Department)
Email:xdtz04@westpowerelectric.com
Tel: +86 917 3661109 Fax: +86 917 6739416
Mobile: +86 18091765882(WhatsApp/Wechat/skype )
Website:https://www.xdtzelectrical.com
Add: Nanpo Village, Chencang Avenue Jintai District Baoji City, Shaanxi Province, China.
