Harmonics Impact Over Protection System of Power Generator

Case Study

Harmonics Impact Over Protection System of Power Generator

Thanasak Chaiyavech Thaieei Available online March 2009

P o w e r Q u a l i t y

Case Study Power Quality www.leonardo-energy.org

Abstract A theatre encountered a situation where incorrect operation of the earth fault protection system of a back-up generator tripped the circuit breaker unexpectedly when dimmer loads were fully used. The generator was used to supply critical loads via an automatic transfer switch (ATS) and the problem was caused by high levels of third harmonic current produced by dimmers and an inappropriate installation of neutral and ground system. Consequently, current would would flow in both neutral and ground system even in normal load conditions causing errors to current detection of earth fault protection system. To resolve the situation, the existing 3pole ATS was replaced by a 4-pole type to separate neutral and earth currents and allow the earth fault protection system to operate reliably.

1. Introduction Figure 1 shows the line diagram of the electrical system of the theatre. Power is normally supplied from the local utility (Metropolitan Electricity Authority of Thailand; [MEA]) to the main distribution board (MDB). Essential loads, such as theatre lighting, are supplied from the Essential distribution board (EDB) which is supplied from the MDB or, if the main supply is not available, from a local local emergency generator. Switching is performed by a three-pole automatic transfer switch (ATS). In the Generator Control Panel (GCP), an earth fault relay (EPR) is installed to detect a short short circuit to ground ground condition and and protect the generator. (Note that the red line, marked 3L, represents all 3 phases, the white line represents the neutral, and the green line represents the protective earth or ground line).

Figure 1: An electrical line diagram of the theatre.

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Harmonics Impact over Protection System of Power Generator www.leonardo-energy.org

2. Problem and consequence Theatres use a lot of dimmers to control stage lighting. Typically, these these dimmers are single-phase devices using solid state switching to control power to the lamps. In normal circumstances, the theatre is supplied with electricity from MEA only without without any problem. However, when the the public supply is not available and the emergency generator is in use, operation of the dimmers causes the earth fault relay to trip even even though there is no short circuit to ground. Consequently the performance can can be interrupted. interrupted.

3. Estimated loss There is no equipment equipment damage in this event. However, each show show has an audience of approximately 1000 people, each paying 1,000 Baht (20 Euro) each. The lost revenue revenue due to a disrupted disrupted performance may may reach 1,000,000 Baht (20,000 (20,000 Euro). Euro). In addition, addition, the theatre’s theatre’s reputation reputation will be adversely affected.

4. Root cause of the problem Figure 2 shows the waveform and and harmonic spectrum measurement measurement of the dimmer current, revealing a significant third harmonic current component. Third harmonic currents, currents, in fact, all currents with a harmonic order that is an odd multiple of three, can cause major problems when single phase loads are supplied supplied from three phase phase feeders. The problem arises because the third harmonic current from each phase is in phase with those from the other phases, so they add in the common neutral; in the worst cases, the neutral current can be 1.73 times the phase current as shown in Figure Figure 3. These currents are are sometimes referred to as ‘Triple Ns’.

Figure 2: Dimmer load current showing harmonic content.

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Figure 3: The third harmonic from single-phase loads is in-phase in neutral line .

The earth fault relay, which is used to detect a short circuit to ground condition and is shown in Figure 4, will measure the instantaneous net I in Figure 4). current in the three phase lines and the neutral line (current I in In normal circumstances, circumstances, this current will be zero. zero. However, in the event of an earth fault, current, I, will no longer longer be equal to zero. The relay is designed so that, when I reaches a set threshold, the relay disconnects the supply to protect the generator and system.

Figure 4: Diagram illustrating operation of an earth fault relay .

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In this particular theatre, the low voltage circuit neutral bus and the protective earth or ground bus are connected together in the MDB. Moreover, the generator neutral line is also connected to ground. Because of this, the earth and neutral busses are effectively in parallel, so that any neutral current caused by unbalance can divide between the two busses. This means that that some of the load load current – the part part of the neutral current that flows in the protective earth bus – is not seen by the earth fault relay and it appears appears that there may be an an earth fault. To prevent the relay from reacting to this situation, it is set to activate when a difference of 20 -30% of full load current is detected; in this case, it is set to 600 A.

5. Problem analysis Three generator operating scenarios are discussed here: normal load, operation •

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with a short circuit to ground

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with third harmonic current from the dimmers.

5.1 Normal load operation The neutral current resulting from an unbalanced load will flow back to the generator via the interconnected neutral and protective earth conductors, as shown in Figure 5. The fact that some current is missing from the sum of the neutral and three phases is detected by the EFR and interpreted as an earth earth fault. To prevent prevent operation operation of the relay under this normal condition, the threshold is set to 20 – 30 % of the full load current – 600A in this case.

Figure 5: Diagram showing normal load circumstance.

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5.2 Short circuit to ground In the case of short circuit to ground (earth fault), the majority of the fault current will flow to the neutral point at the GCP via the earthing system as shown in Figure 6. There will be a relatively low current current in the neutral line. The phase fault current flows through the EFR but the return current does not, so the unbalance is correctly interpreted as an earth fault and the relay trips if the fault current is higher than the trip setting.

Figure 6: Diagram showing earth fault condition

5.3 With third harmonic current from dimmers When the dimmers are in operation, the third harmonic currents that they generate add and return to the generator neutral point via the parallel paths provided by the interconnected protective earth and neutral conductors, as shown shown in Figure 7. Because of the high level of the third harmonic current, the unbalance detected by the EFR is likely to exceed the trip setting and the load will be disconnected even though there is no real fault.

Figure 7: Diagram showing the third harmonic neutral current from dimmers

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6. Solution The problem is caused partly by the high neutral current, but primarily by the fact that there are two connections between the neutral and the protective earth conductors. The solution is to change change the ATS from a three-pole type to a four-pole type so that the neutral is switched along with the phases. This change confines the neutral current, including that due to the third harmonics, to flow only in the neutral line, and therefore, through the EPR sensor.

Figure 8: Electrical diagram with a 4-pole ATS

The following figures show the current flow for the three conditions described above when a four-pole ATS is fitted. In normal load operation, Figure 9, all the neutral current flows through the sensor of the EFR. Consequently, the EFR can reliably detect a true earth fault and the set point can be reduced to provide better protection.

Figure 9: Current flow for normal generator operation with 4-pole ATS

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In the event of an earth fault, Figure 10, the fault current is restricted to the protective earth system due to the use of a four-pole ATS.

Figure 10: Short circuit current flow during earth fault with 4-pole ATS

Again, when high third harmonic currents are present, Figure 11, the use of a four-pole ATS restricts the third harmonic current flow to the neutral return path. The EFR functionality is unaffected, unaffected, no matter how high the third harmonic current may be.

Figure 11: Third harmonic current flow with 4-pole ATS

7. Expenditure The difference in cost for a four pole ATS compared to a three pole ATS is about 600 Euro, excluding wiring cost.

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8. Conclusion When the neutral and protective earth are connected in parallel in a system, the behaviour behaviour of an EFR can be be unpredictable. The situation is made worse when high neutral currents, due to unbalanced loads or third harmonic generators, are present. Ensuring that the neutral and protective earth conductors are connected at only one point separates the current and makes operation of the relay reliable.

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Harmonics Impact Over Protection System of Power Generator

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