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11REFCL management of harmonics

All electricity distribution networks suffer from distortion of the pure 50Hz sinusoidal voltage waveform caused by non-linear customer loads. These distortions are described in terms of mathematically equivalent harmonic voltages and currents. Network harmonic voltages can cause harmonic currents to flow in earth faults, so they are therefore relevant to powerline fire risk.

One REFCL technology, the GFN, has the capability to inject a synthesised voltage waveform to cancel network harmonic voltages and reduce harmonic currents flowing in faults. This capability was explored in Kilmore Tranche 4 tests. The tests confirmed the capability was effective but revealed that further GFN development would be required to realise the potential benefits of this feature in addressing fire risk from network earth faults.

11.1Summary of findings and recommendations

The effects of network harmonics on fire risk in a REFCL-protected network were explored in the KMS test program. The following conclusions were drawn:

  1. In some circumstances, harmonic currents generated by customer loadscan flow in a network earth fault and they may have the potential to increase fire risk.
  2. It is likely the contribution of harmonic currents to fire risk is second order.
  3. Tests demonstrated that:
  4. Harmonics do not significantly increase fire risk from GFN diagnostic tests to confirm the presence of a fault and identify the faulted powerline.
  5. GFN measurement of low-order network voltage harmonicsis consistent with that performed by other network voltage monitoring devices;
  6. The GFN has the capability to effectively reduce low-order harmonic components of earth fault current;
  7. Further product development would be required for the GFN harmonic compensation capability to provide its full potential fire risk benefits; and
  8. The GFN canitself produce some harmonic fault current when it displaces network voltages – this is likely to be a property inherent in all ASC-based REFCLs.

It is recommended that industry work with the GFN manufacturer to further develop the GFN harmonic compensation capability to reduce fire risk in network earth faults.

11.2Fire risk from harmonics in network earth faults

Many electrical devices have non-linear voltage-current characteristics. When 50Hz network voltages supply customer loads that include such devices, the network load current contains harmonics, i.e. components at frequencies that are multiples of 50Hz. When harmonic currents flow in the series impedances of powerlines, they create harmonicsin network voltages. A typical frequency spectrum of a KMS21 test network voltage is shown in Figure 47 – harmonic voltages of between one and 30 volts extend in frequency up to 1600Hz.

In a single phase to earth fault, harmonic network voltages produce harmonic components of fault current that increase the total rms fault current. If a REFCL is used to cancel the 50Hz component of fault current, the harmonic currents will remain largely unaffected and their presence can significantly increase the magnitude of residual fault current and thereby potentially increase fire risk from ground ignition.

This is illustrated by Figure 48 which shows the 50Hz voltage on the faulted conductor has been reduced by the GFN from its pre-fault value of 12,700 volts to about 65 volts. However, the post-fault harmonic voltage magnitudes remain of the same order as before the fault, i.e. between one 1 volt and 30 volts[1]. The harmonic components of fault current are of a similar magnitude to the 50Hz component of fault current.

Figure 47: KMS Test 651 frequency spectrum of pre-fault White phase voltage

Figure 48: KMS Test 651 (400Ω fault) frequency spectrum of White voltage and fault current immediately post-fault

Ignition tests indicate that total rms of fault current is likely to be the most meaningfulmeasure of the environmental energy release that drives fire risk. Harmonic components increase the total rms value of fault current in accordance witharoot-sum-of-squares formula:
This means that an additional harmonic component equal in size to the 50Hz component would increase the total rms fault current by 40%, not 100%.

The fire risk contribution of harmonic components of fault current is therefore only likely to be significant if the harmonics are of a similarorder of magnitudeto the 50Hz component.

11.3Increased ground ignition fire risk from harmonics

High impedance faults are realistic representations of known fire causes, e.g. a live high voltage wire falling into dry grass on dry ground. When the fault impedance is high, the ignition mechanism is ground ignition rather than bounce ignition.

In a high impedance fault, both the 50Hz and harmonic components of fault current are reduced to low levels by the fault impedance. The 50Hz component is reduced more because of the high impedance presented by the resonant neutral-earth connection (the parallel combination of the ASC coil and the combined capacitance of the network and the ASC tuning capacitors), as well as by the RCC compensation to cancel the 50Hz component of voltage on the faulted phase. This is illustrated by Figure 49 for a 40,000 Ohm earth fault. The fifth harmonic of the fault current is 1.2mA which is consistent with the pre-fault value of fifth harmonic voltage (49 volts) divided by the fault resistance (40kΩ). The 50Hz component of fault current is 2.5mA which less than one-hundredth of the 320mA that the 40kΩ fault resistance alone would produce with an applied 50Hz voltage of 12,700 volts.

Figure 49: KMS Test 634 (40,000Ω fault) fault current with RCC compensation

In ‘wire on ground’ ignition tests with the GFN in service, once the GFN detected and compensated for the fault, no ground ignitionsoccurred. The tests outlined in Section 6 above confirm that fault current of 0.5 amps is required for ground ignition in a worst-case earth fault. For a relatively hightotal harmonic voltage of, say, 500 volts to generate this level of harmonic current the fault impedance would have to be not more than 1,000 Ohms.

Even in this worst-case situation, there are two considerations that indicate the fire risk from harmonics would at most be second order:

  1. It is difficult to postulate realistic circumstances in which just three metres of conductor on the ground would have a fault resistance as low as 1000 Ohms without assuming higher levels of soil and fuel moisture content than would be consistent with worst-case fire weather and fuel conditions.
  2. Detection of such a fault would be relatively fast, both in the initial instance and in the GFN fault-confirmation test, so the harmonic current flow would last at mostabout fiveseconds - the period between the fault and the first steps of the fault-confirmation test. A fault current of 0.5 amps generally takes longer than this to produce ground ignition – often tens of seconds.

Whilst there is no guarantee that a particular combination of factors capable of starting a fire from harmonic currents can never occur, the probability of this combination is considered to be relatively low, i.e. fire risk from harmonic currents is likely to be a second order risk.

11.4Fire risk from harmonics in diagnostic tests

In high-impedance faults, the key factor in fire risk is the current produced in diagnostic tests such as the fault-confirmation test. The GFN performs such tests by injecting a 50Hz voltage. This leaves harmonic components of fault current largely unaltered as can be seen from Figure 50 - the fault-confirmation test increased the 50Hz current by twenty-five times from 2.5mA to around 65mA (shortly afterwards the FCTre-detected the fault at a current of 132mA), but the fifth harmonic current remainedunchanged at 1.2mA. It can be concluded that harmonic currents do not significantly increase fire risk in diagnostic tests.

Figure 50: KMS Test 634 (40,000Ω fault) fault current during GFN fault confirmation test (Stage 1, Step2)

This leads to the conclusion that in high impedance faults, harmonics are unlikely to contribute significant fire risk from ground ignition.

11.5Increased bounce ignition fire risk from harmonics

The ignition mechanism in low impedance faults tends to be bounce ignition. The REFCL response required to prevent fires from this cause is fast reduction in the voltage on the faulted conductor to less than 1,900 volts in 85 milliseconds. For a relatively high level of network harmonics of perhaps 500 volts, the root-sum-of-squares addition means the total voltage at 85 milliseconds would only be increased by 3.4% to 1,960 volts compared to the 1,900 volts 50Hz component. The effect of this increase on bounce ignition fire risk would likely be immaterial.

11.6Measurement of harmonics by the GFN

The GFN samples the network voltage waveforms with 16-bit precisionat a rate of 100 samples per 50Hz cycle. To measure harmonics the GFN applies a discrete Fourier transform to groups of 100 samples.GFN documentation does not identify the window function (if any) appliedprior to the transform calculation to minimise spectral leakage.

Theory predicts this arrangement should be capable of measuring harmonics up to about 2 kHz, i.e. to the 40th harmonic of the 50Hz fundamental. However, the voltage waveforms seen by the GFN are produced by anelectromagnetic voltage transformer on the substation’s 22kV busbars. The limited bandwidth of this type of transformer may affect measurement accuracy at higher harmonic frequencies.

A manually synchronised spot check was performed to compare measurements on two selected low-order harmonics supplied by three devices:

  1. The GFN supplied by the substation 22kV bus voltage transformer;
  2. An Elspec supply quality meter in the zone substation also supplied by the substation 22kV bus voltage transformer; and
  3. The Gen3i data acquisition system at the test site supplied by a wide-band capacitive voltage divider.

The three sets of readings are shown in Table 23.

Table 23: comparison of harmonic measurements - red phase voltage 8th October 2015 at about 1:20pm

Harmonic / Time / GFN[2] / Gen3i / Elspec
5th (250Hz) / 1:23:45 pm / 0.2 (30-50 volts) / 48 volts / 48 volts
7th (350Hz) / 1:18:30 pm / 0.1 (10-30 volts) / 25.2 volts / 27.1 volts

Within the limitation imposed by the single significant figure display of the GFN, its displayed readings were consistent with the measurements taken by the other two devices. This spot check also indicated that the limited bandwidth of the busbar voltage transformer was not a material factor in accurate measurement of harmonics up to 350Hz[3].

11.7Compensation of harmonics by the GFN

The GFN has the capability to include low-order harmonic components into its RCC-injected compensation voltage to cancel corresponding harmonic components of fault current. This capability was tested in the Kilmore test program. Test 723 best illustrated its effectiveness as shown in the three fault current frequency spectra set out in Figure 51, Figure 52 and Figure 53 together with the fault current waveforms associated with each.

With no harmonic compensation, Figure 51 shows that the third harmonic and fifth harmonic components of the fault current are of the same order of size as the 50Hz component. Those two components increased the total rms fault current by 94% above the 50Hz value.

After manipulation of the GFN harmonic compensation settings, Figure 52 shows the third harmonic component of fault current has been reduced by 90%. However in the 23 second period since the first record, the fifth harmonic component of fault current has increased by 37% to 0.14 amps.

After further manipulation of the GFN harmonic compensation settings, Figure 53 shows the fifth harmonic component of fault current has been reduced by 70%. At this point, the third and fifth harmonic components of fault current were increasing the total rms fault current by just 6% above the 0.117 amps 50Hz value.

Though Test 723 demonstrates an effective capability to compensate low-order harmonic components of fault current, this was done by manual manipulation of GFN settings. To reduce fire risk by minimising fault current caused by network harmonics, the GFN must include a dynamic capability to measure and cancel the ever-changing levels of low-order harmonics on the network.

It is recommended that industry work with the GFN manufacturer to enhance the harmonic compensation capability of the GFN to minimise fire risk from network harmonics.

Figure 51: KMS Test 723 (400Ω fault) fault current with 50Hz compensation only

Figure 52: KMS Test 723 fault current with third harmonic compensation

Figure 53: KMS Test 723 fault current with both third and fifth harmonic compensation

11.8Generation of harmonics by the GFN

The response of a GFN to an earth fault, i.e. voltage injection by the RCC to cancel the voltage on the faulted phase, can itself produce harmonic components of fault current.

11.8.1How a non-linear ASC coil generates harmonic earth fault currents

The mechanism is as follows:

  1. Harmonics in themagnetising current drawn by the ASC coil

When the GFN compensates the 50Hz component of earth fault current, it effectively applies a synthesised pure 50Hz sinusoid voltage across an iron-cored inductor (the ASC coil) that is in resonance with the capacitance of the network in parallel with the capacitors in the ASC unit itself. The iron core of the coil is not linear and it will draw some harmonic currents from thepure 50Hz source voltage applied by the RCC. The harmonic current components generated by the coil’s non-linear characteristicswill flow in boththe coil and the RCC inverter.

  1. Harmonic voltage produced on the neutral of the transformer and on all network conductors

If the RCC were an ideal voltage source, the harmonic currents flowing between the RCC and the ASC coil would not affect network voltages. However, the non-zero source impedance of the RCC[4] means they produce harmonic components of neutral voltage. The resonant combination of the coil and the combined network plus ASC capacitance will have very high impedance at 50Hz but lower impedance at frequencies other than 50Hz – the actual value will depend on the total system (network plus ASC) damping. This impedance forms a voltage divider with the RCC source impedance to produce a harmonic voltage across the ASC coil, i.e. at the neutral point of the transformer. For lower harmonic frequencies, this harmonic voltage will appear between every powerline conductor in the whole network and earth.

  1. Harmonic current flow in an earth fault

When a single phase earth fault is present, the harmonic voltage on the faulted conductorwill produce a harmonic current in the circuit loop comprising:

  • The substation transformer winding between the neutral-ASC coil connection and the faulted phase of the substation 22kV busbars;
  • The faulted phase conductor from the substation 22kV busbars to the fault location;
  • Through the fault to earth; and
  • Through the earth back to the ASC coil-earth connection at the substation.
  • Factors that determine the magnitude of harmonic fault current

Two main factors will influence the magnitude of the harmonic component of fault current caused by the GFN response to the fault:

  1. Harmonic neutral voltage:The magnitude of the harmonic voltage across the ASC coil is likely to be close to the magnitude of harmonic magnetising current necessary to support the 50Hz voltage applied across the coil by the RCC multiplied by the effective source impedance of the RCC. Other factors are likely to be second order.
  2. Earth fault loop impedance:two impedances will determine the magnitude of the harmonic component of fault current produced by the harmonic voltage across the coil:
  3. Transformer winding impedance: the substation transformer will offer some impedance to the harmonic current flow. Transformers can have complex internal magnetic configurations which may influence this impedance. For example, most large substation transformers (such as the one at KMS) have three limbs to the internal iron core. The internal distribution of magnetic flux this produces means that lower impedance may be offered to the flow of triplen[5] harmonic currents on the network and higher impedance to harmonics at other frequencies. The detailed modelling that would be required to confirm this was beyond the scope of this project. However, the test results did indicate a qualitative difference between the behaviour of triplen harmonics and that of other harmonics in the fault current.
  4. Fault resistance: the fault resistance impedes current flow equally at all frequencies. A high-impedance fault will greatly reduce harmonic current flow, whereas a low-impedance fault can produce high harmonic currents.

In summary, the non-linearity of the ASC coil can create some harmonic fault current even if there are no harmonics in pre-fault network voltages.

11.8.3Confirmation in simulations

Simulations of the response of a GFN with a non-linear ASC iron core were used to confirm the mechanism for generation of harmonic fault currents by a GFN. These were not exact simulations but served to verify the concept. The model used is shown in Figure 54.

Figure 54: conceptual model of GFN response to an earth fault with non-linear ASC coil

The non-linearity is introduced by the Zener diodes D5 and D6 (±15,000 volts withstand) and D7 and D8 (±10,000 volts withstand). The source impedance of the RCC is modelled by R14 and L4 - (50Ω+j50Ω), equivalent to about 50 milliohms on the low voltage side of the ASC.

The concept simulation results are set out in Table 24.

Table 24: simulation of non-linear ASC - effect on fault current