PROCEDURE FOR MEASURING CAPACITANCE & TAN δ OF DIFFERENT OBJECTS IN 400 kV SWITCHYARD, USING C/TANDELTA KIT Abbreviation High Voltage Terminal of Power Supply Low Terminal of Power Supply Ground Terminal of the Power Supply High Voltage Terminal of Test object / Primary of Transformer or Reactor Intermediate Terminal of the Autotransformer Low Voltage Terminal of the Transformer / Secondary winding Ungrounded Specimen Test Grounded Specimen Test by guarding Test Object Capacitance Capacitance Bushing Main Capacitance Capacitance between Bushing Tanδ tap and Ground I.
HIGH LOW GND HV IV LV UST GSTg C C1 C2
Measurement on Transformers / Reactors:
Different Types of Transformers, used in Transmission are. 1. 3 Phase, 3 winding Transformers – Contains Primary, Secondary Tertiary 2. 3 Phase, 2 Winding Transformers – Contains Primary, Secondary 3. 3 Phase, Autotransformers – Contains Primary, IV, LV, Tertiary 4. 1 Phase, 3 winding Transformers – Contains Primary, Secondary Tertiary 5. 1 Phase, 2 Winding Transformers – Contains Primary, Secondary 6. 1 Phase, Autotransformers – Contains Primary, IV, LV, Tertiary Different Types of Reactors in Transmission are 1. 3 Phase Reactors 2. Single Phase Reactors Insulation quality normally to be checked is between 1. Bushing Conductor and tan δ tap. 2. Primary and Secondary (HV – LV) 3. Primary/IV and Secondary – HV+IV- LV 4. Primary and Tank ( HV – Tank) 5. Primary to Tertiary (HV – Tertiary) ; in case of 3 winding transformers 6. Secondary and Tank ( LV – Tank) 7. Secondary to Tertiary ( LV- Tertiary) 8. Tertiary To Tank The tank is at Ground Potential.
Preparation: 1. Remove earth connection to Neutral point of Primary. 2. Short all HV Bushing terminals together with the neutral bushing. (Also I V Bushing terminals incase of Autotransformers) 3. Short all LV bushing terminal together. 4. Short all Tertiary Terminals (in case of 3 winding transformers) Measures C Connection Mode Precautions / Note Selected in & Tan δ of Instrument Bushing (C1) 1, HIGH to HV; UST Keep tan δ tap closed for the 2. LOW to tan δ tap of bushings which are not selected for corresponding busing measurement, 3. GND to tank Bushing (C2) 1, HIGH to tan δ tap; GSTg Apply max of 500 V 2. LOW not connected. 3. GND to tank HV - LV 1, HIGH to HV; UST Connect Tertiary to Ground 2. LOW to LV 3. GND to tank HV- Tertiary 1, HIGH to HV; UST Remove ground connection to 2. LOW to Tertiary Tertiary and ground LV 3. GND to tank HV- Tank 1, HIGH to HV; GSTg Remove ground connection to 2. LOW to LV and Tertiary and LV Tertiary shorted together 3. GND to tank LV-Tank 1, HIGH to LV; GSTg Remove ground connection to 2. LOW to HV and Tertiary and HV Tertiary shorted together 3. GND to tank Tertiary – 1, HIGH to Tertiary; GSTg Remove ground connection to Tank 2. LOW to HV and LV Tertiary and HV shorted together 3. GND to tank Measurement on Reactors: Preparation 1. Remove earth connection to Neutral point of Primary. 2. Short all HV Bushing terminals together with the neutral bushing. Measurement on bushing of reactors is as mentioned in Transformer section. The only other measurement is with respect to tank; The procedure is as mentioned above
II.
Current Transformers.
In case of CTs the insulation quality normally to be checked is 1. For CT Bushing 2. CT Primary to tank Measuring tan δ of CT bushing alone is possible only if tan δ tap of CT is accessible. If tan δ tap not accessible, then only Primary to tank measurement is possible. Also this can judge the quality of CT, as Bushing insulation contributes the major part in the total tan δ value. Preparation 1. Access the tan δ tap of the bushing and remove it from ground. 2. Short all secondary terminals and connect to ground. 3. Open the earth switch on CT side as well as bus side Measures C & Connection Tan δ of CT Bushing
1, HIGH to Primary; 2. LOW to tan δ tap. 3. GND to tank 1, HIGH to Primary; 2. LOW not connected 3. GND to tank
Primary to Tank
Mode Selected in Instrument UST
Precautions / Note
Closing the earth switch on Bus side reduces interference. *However Series combination of 2 CB capacitance will get selected parallel. *It is to be noted that in 400 kV transmission systems, normally grading capacitor will also placed beside the CT. That means CT and CB on CT side has common terminal. In such cases a simple connection is enough to make measurement on CT as well as two numbers CBs at once. III.
CB Grading capacitors
Measures C & Connection Tan δ of CB Grading Capacitor of CT Side CB Grading Capacitor of Bus Side
GSTg
1, HIGH to Primary of CT; 2. LOW to Centre of the double interrupter 3. GND to tank 1, HIGH to 4 inch IPS of the CT on bus side; 2. LOW to Centre of the double interrupter 3. GND to tank
Mode Selected in Instrument UST
Precautions / Note
UST
Open earth switch on bus side and close earth switch on CT side
Open earth switch on CT side and close earth switch on Bus side.
IV.
Measurement on CVT
In CVT capacitance tan δ of individual stack can be measured by applying voltage between the corresponding terminals. For measurement on whole CVT it is the required to open the HF point and tan δ point from ground. Here under is the procedure for measuring C & tan δ of whole CVT Preparation 1. Short all secondary terminals and connect to ground. 2. Open HF Point from ground. 3. Open Tan δ tap of the CVT from ground if accessible Measures C & Connection Tan δ of CVT capacitance CVT capacitance
1, HIGH to HV; 2. LOW to Tan δ tap. 3. GND to tank 1, HIGH to HV; 2. LOW to not connected 3. GND to tank
Mode Selected in Instrument UST
Precautions / Note
GSTg
If Tan δ tap and HF point is not open-able.
If Tan δ tap is open-able.
The procedures of measuring C / Tan δ on various test objects in energized 400 kV Substation was probably first illustrated in India by earlier Tettex Representatives. This was carried out at Power Grid 400 kV Jabalpur Substation during 1998. During same demonstration the usage of Manual Tan δ kit (Tettex / L&T make C & Tan δ instruments type 2805/5281) was also illustrated. It is to be noted that model 2805/5281 has highest tan δ resolution of 0.000002. This resolution is achieved by increased measuring sensitivity. While the modern instruments provide tan δ resolution of 0.0001, which is also the present required specification. Hence while using manual C, tan δ kits, it is recommended that, the sensitivity can be kept to the prevailing condition to achieve the resolution, and not to increase the unwanted signals (e.g. Interference Current). The user manual of measuring bridge 2805 mentions the same in its page. However the observation is that Manual Capacitance and Tan δ Kit without Interference Suppressor has limited applications in substations. Even, there is a provision (Mains / Commutator modes) for canceling the errors occurred due to interference current; the system is ineffective under high interference prone switchyards. A manual Interference suppressor that cancels the interference current will be handy to use with such manual kits. This Suppressor cancels the current injected to the test object through air capacitor from High Tension (HT) lines and nearby High Penitential points of Switchyard.
