Designing for a high penetration of DER - regonline.com fileAgenda • Background 1. Growth in...

Designing our networks for a high penetration of distributed energy resources

ByMark Hibbert

Technical Director - Aurecon

Agenda• Background

1. Growth in distributed energy resources2. Current design parameters

• Types of embedded generation1. Synchronous rotating machines2. Asynchronous rotating machines3. Asynchronous inverter interfaced.

• Impacts1. Thermal2. Fault level3. Power Quality4. Voltage

• Mitigation• Conclusion

Background

Background• Embedded generation is not new, but its nature is changing• Previously it was largely comprised of synchronous rotating machines• Today we have a much larger proportion of inverter interfaced

generation, so it requires some depth of knowledge of the various types.

• Inverter interfaced generation is growing consistently.

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Background

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Background

Background• Distributed solar installations are growing at around 50,000 to 60,000

installations per year.

• If the average size is assumed to be 5 kW, this equates to 250 to 300 MW of new generation every year.

• This is not likely to stop. Solar systems are getting cheaper. It is already cost effective for commercial businesses to offset their energy usage with solar PV generation.

• So the effective integration of DER into the network is a priority.

• Distribution design for existing networks accounts for expected loads and maintains LV voltage within +/- 6%. i.e. from 254.4 V to 225.6 V.

• The 240 V standard is the legacy we have to live with in most of our existing networks. Based on this standard we have a 12% voltage range allowed.

• Just keep in mind, we have 16% (4% extra) voltage regulation available if the 230 V standard is applied.

Background

• LV network design usually allows 10V drop from the distribution transformer to the end of the LV.

• The distribution transformer contributes to the voltage drop/rise based on its impedance, i.e by about 4% at full load/100% penetration.

• And the medium voltage distribution network will also see a voltage rise of around 1% for every 20% of solar PV penetration.

Background

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11 kV Distribution Feeder Voltage Profiles for Light and Heavy Loads

Full Load Voltage Light Load Voltage Absolute Regulation %

Distribution Transformer tap 3 (Nominal) Distribution Transformer tap 4 (2.5% boost)Tap 2 (2.5% buck)

Background

Types of Embedded Generation

Types of embedded generation• There are three main types of generators

1. Synchronous rotating machines2. Asynchronous rotating machines

3. Asynchronous inverter interfaced generation.

Types of embedded generation

A salient pole machine has a rotor with obvious pole pairs. (salient means; outstanding, noticeable, striking, or prominent), and is most often found in lower speed machines, although 4 pole salient pole machines are reasonably common.The construction method is relatively straight forward, and provides for improved ventilation over cylindrical rotor machines, but suffers from greater mechanical losses than cylindrical rotor machines. Salient pole machines have a shorter axial length but greater diameter when compared with cylindrical rotor machines.

Salient pole synchronous machines


Salient pole synchronous machines


Round rotor synchronous machines

A round rotor construction is typically chosen for high speed machines, (2 and 4 pole machines), as it results in greater mechanical efficiency. Usually they are longer and of smaller diameter than a similar rated salient pole machine. The electrical performance is similar however, and no factors accrue in the equations determining performance that relate to the construction method.


Round rotor synchronous machines


Permanent magnet synchronous machinesA permanent magnet machine has no connections to the rotor, thus making it cheaper and simpler to construct. They are generally inverter interfaced.

Generally these have been restricted to smaller units, but with improvements in magnet technology capacities of several MW are now common. High speed operation is common in co-gen applications where the prime mover is a small gas turbine.

For ratings up to 10 MW they are finding their way into wind turbine generators with an inverter interface to the network.


Permanent magnet synchronous machines


Synchronous machine equivalent circuitRegardless of the construction, all synchronous machines have the same equivalent circuit, as shown below.


Asynchronous rotating machinesInduction motors are the workhorse of industry

Very simple in construction, the most common type of induction machine is the squirrel cage motor, as shown adjacent.

Squirrel cage machines are asynchronous. They must be driven at a speed greater than synchronous speed. They also consume reactive power.


Asynchronous rotating machinesExamples of induction machine cage rotors


Asynchronous rotating machinesWound rotor induction machine Induction machines can also be

constructed with a wound rotor. Usually the terminals are brought out to slip rings which enable the rotor circuit to be modified. Most commonly a set of resistors are used to improve low speed torque. In more recent times the rotor circuit is interfaced via a power electronic converter, allowing both speed and power factor to be controlled.


Asynchronous rotating machinesExamples of induction machine wound rotors


Asynchronous rotating machines

Induction machine equivalent circuit

The induction machine equivalent circuit is similar to that of he synchronous machine, except that the rotor resistance is modified by the reciprocal of the slip


Asynchronous rotating machinesDoubly fed induction generatorsThe most common form of induction generator today is the doubly fed induction generator (DFIG) which has a power electronic converter connected to the rotor circuit, as shown below


The full power converter system places a converter between the grid and the generator. The generator could be a permanent magnet synchronous generator, a standard round rotor or salient pole synchronous generator, or a squirrel cage induction generator. Regardless of the machine and its parameters, the fault contribution is limited to the maximum converter current


Wind turbine generated energy using voltage source converters, shown below:-


Inverter interfaced generationThere are two main types of inverter used for converting solar PV generated energy for grid connection. These are the full bridge converter, and the neutral clamped inverter.


Inverter interfaced generationThe neutral clamped inverter.


Inverter interfaced generation§ Modern inverters are equipped with controllable power factors (except for the small

kW scale applications) and are also equipped with grid synchronisation features and anti-islanding protection.

§ Grid synchronisation is achieved using either Fast Fourier Transform (FFT) techniques to monitor and compare frequency performance, or with Phase Locked Loop (PLL) systems.

§ Anti-islanding protection can be either passive or active, or both.


Inverter interfaced generation§ There are a number of methods that can be used to achieve anti-islanding protection.

Active anti-islanding is achieved by frequency drift methods, voltage drift methods, or grid impedance estimation methods using either harmonic injection, PQ variation, or inverter current angle manipulation techniques within a phase locked loop.

§ The various frequency drift algorithms attempt to shift the converter frequency sufficiently to trigger the OUF protection. If connected to the grid this will not be achieved, due to the stiffness of the grid source.

Impacts of embedded generation


Thermal§ Not likely to be an issue for residential solar provided we have sensible

inverter limits.

§ Thermal limits need consideration for anything bigger, and typical 1 to 2 MW synchronous machine embedded generators will require consideration of their thermal impact on the existing network.

§ The growing trend to install 1 to 2 MW embedded solar systems will need similar consideration.


Fault level§ Not an issue for inverter interfaced generation.

§ Needs consideration for anything involving directly connected synchronous or asynchronous machines.

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Power quality§ Modern inverters are generally quite clean from a harmonics perspective

§ Flicker needs to be considered for any intermittent generation.

§ The size of any generator needs to be matched to the system so that a trip does not cause excessive voltage fluctuations


Voltage§ Embedded generation of any type will impact the voltage

§ Generally the ratio of the generation capacity to the fault level should be 3% or less to avoid issues. This is for individual installations.

§ The collective effect of many small generation units can be significant

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Voltage (Volts), no PV Average Voltage (no PV)

Voltage (Volts), with PV Average Voltage (with PV)


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Max Regulation with no Solar influence


Solar Penetration 10% 20% 30% 40% 50% 60%

11kV Voltage Rise 0.53% 1.06% 1.59% 2.12% 2.64% 3.17%

Transformer VR 0.40% 0.80% 1.20% 1.60% 2.00% 2.40%

LV Voltage Rise 1.08% 2.18% 3.32% 4.49% 5.70% 6.93%

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Voltage rise vs solar PV penetration table


Mitigation

• New Parameters

• The distribution network of the 21st century will need to cope with increasing solar PV penetration. Now we understand the problem, we need to design to new parameters, including.

1. Sensible solar PV limits.

2. Re-design the MV and LV systems

3. Make full use of modern inverter capability.

Mitigation

• What about the overall voltage range allocation?

a) The 230 V standard has a 16% range

b) Before any network redesign is tackled, some thinking needs to go into how that 16% range is allocated.

c) Remember we now have 2 way flows, and in some areas the peak export will equal the peak load, so the old 3, 4, 5% allocation will no longer work.

Mitigation

• Sensible solar PV limits.• What is a sensible solar PV limit?

a) That depends on the ADMD, and the load profile. For a low ADMD of 3.5 kVA, if we have a 50% load in the middle of the day, that results in a mid-day load of 1.75 kVA. This will be supplied by a solar system before it starts to export.

b) Even if solar systems reach a penetration of 100% of distribution transformer capacity, then we can generate 3.5 + 1.75 or 5.25 KVA before we challenge the design load limit, (neglecting voltage issues). So setting a maximum inverter size of (say) 1.5 x the ADMD, will limit export to below the 3.5 kVA current limit in those areas. If customers had more internal load (e.g. a battery), then they may be able to utilise a larger inverter, so quantifying this limit as an export limit (of 3.5 kVA) is preferable.

Mitigation

• What about higher ADMD areas?

a) For an ADMD, of 8 or 10 kVA, practicality indicates that we probably should not encourage Solar PV systems greater than 5 kW. Most residential homes can only accommodate 5 to 6 KW anyway.

b) Commercial premises should be limited to no more than their mid day maximum demand. So quantifying the inverter size limit as an non exporting system related to mid day peak demand is preferable.

Mitigation

• What about distribution transformers?

a) Lower impedances, or oversizing transformers will provide a margin for growth, and will minimise the impact on the voltage. Most are underutilised anyway.

b) In some areas, tap changing distribution transformers may be required.

Mitigation

• What about the MV system?

a) Shorter feeders

b) Underground feeders

c) This means smaller substations situated closer together

Mitigation

Conclusion

• The distribution system of the future will be quite different. We will probably have

a) Smaller zone substations located closer together

b) Shorter all underground MV feeders

c) Larger (on average) distribution transformers, some with tap-changers

d) Shorter all underground LV feeders

e) Sensible PV limits

f) Inverters with voltage controlled power factors.

Conclusion

• The distribution system of the future will also need to consider

a) Future uptake of batteries

b) Future electric vehicle charging

c) And probably a whole range of new technologies that enable greater interaction between customer and utility.

d) Peer to peer trading may also be a factor

Conclusion

Will the future distribution network be more of a platform to allow peer to peer energy trading rather than a delivery system for energy generated by base-load generators?

Concluding Question

Thank You

Any Questions ?

Date post:	18-Aug-2019
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Designing for a high penetration of DER - regonline.com fileAgenda • Background 1. Growth in...

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