Case Study
TERNA S.p.A.
By Gabriele Ribichini, Managing Director, Dewesoft S.R.L., Italy 

Circuit breakers are an important and critical component of power delivery systems. They prevent damage to the system’s components and wiring when the current in the electrical circuit supersedes design limits. The Italian transmission system operator Terna S.p.A. wanted to check the switching function of an old synchronous high-voltage circuit breaker. Dewesoft data acquisition systems and Power Analysis software did the job.

high voltage switchers

Terna S.p.A. is a transmission system operator (TSO) based in Rome, Italy. It operates through Terna Rete Italia, which manages the Italian high-voltage transmission grid. A network of 74,669 Km of lines with 888 electrical substations, and 4 control centers managing 320 billion kWh every year.

The Synchronizer

Circuit breakers are mechanical switching devices capable of making, carrying, and breaking our grid current in normal and abnormal conditions. During abnormal conditions such as when lightning strikes a transmission tower, circuit breakers isolate the faulty components of the system to prevent additional damage. Ideally, in the closed conditions, a circuit breaker should function as a perfect conductor to ensure optimal current flow. 

A synchronizer is widely used in high voltage (HV) circuit breakers to avoid that the contact is opened or closed when the current flow is high. Opening a high current flow would lead to electric arcs and reduce contact lifetime.

The synchronizer, knowing the actuator delays, observes the sinewave timing and gives a command to the breaker in a magic instant, calculated in order to have a null instantaneous current when the contact moves.

The same actuation is performed on all three phases, with different timing since the three sine waves are shifted by 120 degrees.

A brand-new installer is calibrated to work perfectly and preserve contacts during operations, but unfortunately, during years it happens that contact opening/closing time varies and results in actuation at the wrong time.

Purpose of the Test

The purpose of this functional test was to analyze an older synchronous HV circuit breaker to check for possible deviations from the initial setup.

The Measurement Equipment Used

The Dewesoft SIRIUSi-HS acquisition module ready to measure
Figure 1. The Dewesoft SIRIUSi-HS acquisition module is extremely compact and robust, designed for field operations.

The module integrates signal conditioning functions, such as AC or DC coupling selection, programmable gains, offset compensation (deriving from transducers), programmable LP filters, excitation of current probes, and much more, including self-recognition of the probes current and sensitivity to use.

Among the various features, the galvanic isolation at 1600V channel to channel and channel to the ground (GND) is noteworthy.

Connecting the Instrument to the System

The three voltages were connected on the secondaries of the voltage transformers with a conversion ratio of 1:3800, while the three currents were intercepted downstream of the CTs with a conversion ratio of 1:800.

Connecting the Dewesoft instrument to the power system with current camps
Figure 2. Connecting the instrument to the power system.

The two command signals (opening and closing) sent by the synchronizer to the switch were also connected, signals at 110Vdc.

Carrying Out the Test

A typical circuit breaker closure test can be divided into three distinct moments: the energization of the HV busbars, the closing of the no-load circuit breaker, and delivery on load.

The graph of circuit breaker shows three distinct moments
Figure 3. The circuit breaker - three distinct moments: 1) the energization of the HV busbars, 2) the closing of the no-load circuit breaker, and 3) delivery on load.

Energization of the HV Busbars

The energization of the high-voltage bars is measured by a sequence of high-frequency discharges with time history.

A sequence of high-frequency discharges with a time history measures the energization of the HV bars
Figure 4. A sequence of high-frequency discharges with a time history measures the energization of the HV bars.

Closing the Switch

Analysis of Closing Times

The execution of the switch closure test shows the command signal that rises to 117V at a certain instant.

The switch closure test - the command signal is the green signal at the top
Figure 5. The switch closure test - the command signal is the green signal at the top.

From the trend of the current signals, we are able to measure the activation delay equal to 163ms.

The detail of the current signals in the instant of closure highlights a very nervous trend, probably due to the triggering of discharges with current peaks up to 900A.

A detail of the current signals in the instant of closure highlights a nervous trend
Figure 6. A detail of the current signals in the instant of closure highlights a nervous trend.

The absence of a load downstream of the switch is probably the cause of the important distortions of the current signals following closure.

Distortions of the current signals following closure - comparing the three voltages and the three currents
Figure 7. Distortions of the current signals following closure - comparing the three voltages and the three currents.

Behavior Analysis of the Synchronizer

A detailed analysis of the switch switching instant highlights that the closure of the three phases does not occur at regular intervals and in incorrect instants.

The analysis shows that phase 12 (red) is closed when the instantaneous voltage V12 is about 24kV, followed by phase 4 (after 4.33ms) when the instantaneous voltage V4 is about 234k €, and finally, phase 8 (after 1,89ms) when the instantaneous voltage V8 is -317kV.

Analysis of the switch switching instant highlights - the closure of the three phases does not occur at regular intervals and appears in incorrect instants
Figure 8. Analysis of the switch switching instant highlights - the closure of the three phases does not occur at regular intervals and appears in incorrect instants.

This behavior causes electric arcs, clearly shown as peaks on the currents graph above. 

Return on Load

Upon delivery, the current signals assume a regular shape
Figure 9. Upon delivery, the current signals assume a regular shape.

Phasor Analysis

A phasor analysis highlights the mismatches and imbalances in the three phases.

The following figure shows the trend of the first harmonic voltages and currents (h = 1, f = 50.011Hz) with the switch closed and without load.

Phasor analysis
Figure 10. Phasor analysis – this can be repeated for any harmonic.

Harmonics Analysis

While a harmonic analysis highlights the presence of important distortions, which contributed to the achievement of PF 0.534

Harmonic analysis reveals distortions graph
Figure 11. Harmonic analysis reveals distortions.

Other

Many other parameters are calculated by the DewesoftX Power Module, the standard, customizable power analysis screen is shown below.

The DewesoftX standard power analysis screen
Figure 12. The DewesoftX standard power analysis screen.

Conclusion

Following the tests carried out, it is clear that the synchronizer under test is not working correctly; the closing of the contacts does not take place at the right moment causing important discharges with currents up to 900A.

Furthermore, the closure of the three contacts does not occur at regular intervals, the closing interval between phase 12 and 4 is 4.33ms while between phase 4 and 8 the interval is 1.89ms, a clear symptom of malfunction in case of steady-state periodic three-phase.

The analysis of phenomena with Dewesoft data acquisition systems allows easy verification of operation correctness. It is possible to automate this verification with integrated sequences in DewesoftX that guide the operator in the field who does not necessarily have to be competent in the technology used.

A regular synchronizers verification is easy and enough to identify those with aging problems that require some maintenance to come back to normal operation. Correctly working synchronizers are important to preserve expensive contacts of HV circuit breakers.

References