Instruments Transformers (CTs-VTs)
CURRENT TRANSFORMERS
We provide
commissioning works for medium and high voltage ct’s
start up and prior to initial energization services
complete maintenance and preventive program
services in case of damage, malfunctions or any other circumstance
All above are provided according to the latest applicable international standards and will be followed by relevant test reports.
Insulation resistance measurements (IR – DAR – PI)
These are cornerstone tests for CT condition monitoring. These tests indicate the insulating quality of a dielectric by applying a DC voltage and measuring the current flow to present the insulation value in ohms and trend this value over a time interval of 10 minutes. ATE uses a series of testers (up to 5kV) to perform the measurements
Current Transformers’ ac withstand test
In a medium voltage current transformer it is essential to perform an ac withstand test. Test duration 60 secs. ATE uses a series of testers (up to 60kV) to perform the measurements
Capacitance and tan δ measurements.
The measurement and trending of the dielectric loss angle of the CT insulation is an important tool in the condition monitoring of the unit. The values and the trend will show degradation of the insulation and can be used in predictive assessment to determine the best time for costly maintenance. ATE uses the MEGGER DELTA 4000 test unit to perform measurements to the highest accuracy.
DC winding resistance tests
Whenever we are commissioning or performing routine maintenance on current transformers we test the winding resistance of the CT windings to ensure that they are within the design specification. This ensures the continued performance of the protection circuit. Winding resistance is part of the CT burden and needs to be taken into account when establishing the CT knee-point voltage (Vk) requirement for a particular application. This should be known and the smaller it is the better.
CT’s Knee Point – Magnetization Curve - Burden
It is necessary to check the knee point voltage of CT’s when preparing to place them into service as an incorrect knee point voltage will surely affect the performance of the protection circuits (relays and circuit breakers) to which the current transformer is connected in fault conditions.
To define the knee-point voltage of a current transformer we may say that is the magnitude of the secondary voltage after which the output current ceases to follow linearly the input current. This means that the one-to-one or proportional relationship between the input and output is no longer within declared accuracy. The curve formed between output and input during testing called Magnetization Curve or Excitation Plot and one of the most critical characteristics of a Current Transformer under test (commission or maintenance). The curve also demonstrates the difference between a protection secondary winding with a metering one.
When testing, if a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage, up until the knee point. The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetizing current by 50%. From the knee point upwards, the magnetizing current increases abruptly even with small increments in voltage across the secondary terminals. The knee-point voltage is less applicable for metering current transformers as their accuracy is generally much tighter but constrained within a very small bandwidth of the current transformer rating, typically 1.2 to 1.5 times rated current. However, the concept of knee point voltage is very pertinent to protection current transformers, since they are necessarily exposed to currents of 10 or 30 times rated current during faults.
Transformer’s ratio and angle deviation
The function of a CT is to provide accurate secondary current values that are directly proportional to the primary current, thus it is important to verify the ratio and phase accuracies at commissioning and then on a routine basis.
For the IEC standard, accuracy classes for various types of measurement are set out in IEC 60044-1, Classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1, and 3. The class designation is an approximate measure of the CT's accuracy. The ratio (primary to secondary current) error of a Class 1 CT is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are also important especially in power measuring circuits and each class has an allowable maximum phase error for specified load impedance. Current transformers used for protective relaying also have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults. ATE uses current transformer EZCT 2000 manufactured by VANGUARD INSTRUMENTS Inc to perform the measurements
VOLTAGE TRANSFORMERS
We provide
commissioning works for medium and high voltage vt’s
start up and prior to initial energization services
complete maintenance and preventive program
services in case of damage, malfunctions or any other circumstance
All above are provided according to the latest applicable international standards and will be followed by relevant test reports.
Secondary winding insulation resistance measurements
These tests indicate the insulating quality of a dielectric by applying a DC voltage and measuring the current flow to present the insulation value. ATE uses a series of testers to perform the measurements
Current Transformers’ ac withstand test
In a medium voltage potential transformer it is essential to perform an ac withstand test. Test duration 60 sec. ATE uses a series of testers (up to 60kV) to perform the measurements
Capacitance and tan δ measurements
The measurement and trending of the dielectric loss angle of the VT insulation is an important tool in the condition monitoring of the unit. The values and the trend will show degradation of the insulation and can be used in predictive assessment to determine the best time for costly maintenance. ATE uses the MEGGER DELTA 4000 test unit to perform measurements to the highest accuracy.
DC winding resistance tests
Whenever we are commissioning or performing routine maintenance on voltage transformers we test winding resistance of the VT windings to ensure that they are within the design specification. ATE uses micro- ohmmeter WRM-40 manufactured by VANGUARD INSTRUMENTS Inc.
Transformer’s ratio and angle deviation
The function of a VT is to provide accurate secondary voltage values that are directly proportional to the primary voltage, thus it is important to verify the ratio and phase accuracies at commissioning and then on a routine basis. ATE uses 3ph turns ratio meter type ATTR-03B manufactured by VANGUARD INSTRUMENTS Inc.
