SOLAR PV IN NEW ZEALAND - Transpower New Zealand · serve New Zealanders to generate their own...

S O L A R P V I N N E W Z E A L A N D

D I S C U S S I O N D O C U M E N T D E C E M B E R 2 0 1 7

CONTENTS

01SUMMARY

05OUR RESEARCH SCENARIO: WIDESPREAD SOLAR PV ACROSS NEW ZEALAND

10ENSURING A BALANCED POWER SYSTEM

11MANAGING PARTLY CLOUDY DAYS

12RESPONSE TO SUDDEN EVENTS

14STABILITY IN THE POWER SYSTEM AFTER A SUDDEN EVENT

15MANAGING VOLTAGE

17LOOKING AHEAD

TRANSPOWER NEW ZEALAND LIMITED

SOLAR PV IN NEW ZEALAND

SUMMARY

In New Zealand, our abundant

natural resources enable us to

have an electricity system that is

primarily renewable – an object

of admiration around the world.

As a country, we’re committed to

a future with even higher levels

of renewable generation.

At Transpower, we’ve been

investigating how our national

power system can enable New

Zealanders to use new

technologies to generate and

store their own renewable

electricity.

This report considers how the National Grid will respond to a very significant increase in electricity generated from solar photovoltaic (PV) panels – which we are seeing more and more on the rooftops of homes and businesses.

We carried out this research because rooftop solar PV electricity is different than electricity from other sources. Solar energy is converted into electricity by an inverter – there is no turbine-spinning generator. Unlike most generation today, where energy produced at one location is fed into the grid and can be managed and coordinated simply, solar PV is spread out across New Zealand communities in small quantities, and the impacts are less easily managed or coordinated at a system level. It’s possible to predict solar PV production to a large degree, based on location and seasonal patterns of solar irradiance, but predicting how overall power system operation will need to adjust, when significant numbers of electricity users are generating their own solar PV power, is not quite as simple.

Some countries with significant amounts of solar PV have experienced difficulties managing their power systems, especially where solar PV has displaced large thermal (coal and gas fired) power stations.

Although New Zealand is less reliant on thermal generation (up to 85% of our electricity is already provided by renewable sources), we sought to understand what issues could cause potential difficulties and ensure the National Grid will enable electricity consumers to use the technologies they choose.

By ‘stress-testing’ the power system against a scenario in which there is a substantial amount of solar PV in New Zealand, we know it is already in a good position to enable an increase in solar PV in our communities now and in the future.

We have also developed a roadmap so we know when further research will be required, and when we might need to review the standards and market arrangements for power system operation with our regulator so we can provide further value to electricity consumers.

We need to be agile and responsive to our evolving industry and energy environment, and this solar PV report is part of that approach. In our perspective document Transmission Tomorrow, we predicted that the role and operation of the National Grid will change in the future. You can read more about this, and about our other investigations (such as our research into battery storage) on our website.

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TRANSPOWER NEW ZEALAND LIMITED 2


The issues we investigated

We wanted an understanding of five potential issues to ensure the National Grid will serve New Zealanders to generate their own power through solar PV.

�� Ensuring a balanced power system during the eveningIf lots of people are using solar power during the day, as the sun goes down and they switch to power supplied from the grid the rate of demand on the grid will increase sharply – much more sharply than we see today. Can enough electricity from other sources be appropriately supplied to meet this demand?

�� Partly cloudy daysThe impact of rapid cloud movement across the sun results in sudden changes in the amount of solar PV electricity generation at any moment. How material is this fluctuation across the whole New Zealand power system?

�� Response to sudden events1

How will the power system response be different in the situation of emergency events than in the present environment? Presently, events are managed in a way that has minimal impacts on electricity consumers. Internationally, the reduction in the inertia of the power system from the displacement of thermal generation by new forms of electricity generation has been a factor in recent blackouts.

�� A stable power systemPower systems are complex. If disturbed by a failure of part of the system, can it recover to a steady, stable state if there is substantial solar PV in the mix?

�� Grid voltage managementIf there is lower demand for electricity from the grid and more local supply from solar PV, will some regional parts of the grid be harder to manage? If so, what are the solutions?

1 Examples of sudden events include the loss of a large generator off the power system, or unexpected disruption to the power flow between the North and South Island.



Our findings

We found that the existing New Zealand power system is an enabler: the core transmission network can accommodate significant solar PV in addition to the existing generation mix and present demand for electricity.

This is due to the inherent capability of the New Zealand power system to accommodate two-way power flows from north-to-south and south-to-north, and for hydro generation to provide flexibility to cover short-term variations in electricity generation from other sources. Both these features of the power system aid our capability to accommodate high levels of solar PV.

We have concluded that further review and studies are warranted when solar PV approaches 1000 MW – about 16 times what we have today – when about 5% of electricity will come from distributed solar PV. If the mix of technologies used to generate electricity in New Zealand changes significantly before that point, we may need to review our finding also.

Our studies highlighted the importance of compliant inverters. Inverters are the devices used to convert solar irradiance to electricity suitable for use in homes or for export to the grid. There is a standard for inverter requirements (AS/NZS 4777.2:2015). If inverters are sold and used in New Zealand that do not meet this standard, it will impose additional costs and limitations on the capability of the power system in the future.

In brief, the specific findings of our five investigations were:

�� Ensuring a balanced power system – the existing generation can ramp up to cover a winter evening peak with 4000 MW of solar PV.

�� Partly cloudy days – the distributed effect of fast-moving cloud cover across New Zealand has a negligible effect on the power system, even with 4000 MW of installed solar PV.

�� Response to sudden events – there is reduced capability to respond to system events. However, the potential magnitude of such events is also reduced. Standards for solar PV inverters could help improve capability in this area. We anticipate this reduced capability to become an issue as we approach 2000 MW of solar PV.

�� Power system stability – the power system remained stable. As solar PV increases, we will monitor any trends using existing systems and carry out further studies.

�� Grid voltage management – we anticipate issues as we approach 2000 MW of solar PV. These can be mitigated through operational measures, such as switching out transmission circuits, as happens at present over night when electricity demand is at its lowest. However, such sustained removal of parts of the grid from service can reduce reliability and resilience. Therefore, grid investment in appropriate voltage control equipment may be more appropriate to enable voltage management and maintain existing reliability and resilience when solar PV is reaching this rate.

All five effects would likely be mitigated or delayed by the uptake of storage technologies such as batteries.

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Looking ahead

During 2018, we will undertake further studies to consider the impact of the coordinated and complementary operation of solar PV and storage technologies. The findings of those studies will be used to guide our plans for further technical review of the impact of distributed solar PV.

In the following pages, we have summarised our report in six sections. First we describe our research scenario and why we choose it, then in the next four chapters we summarise the four different studies we carried out to investigate our five potential issues. In section six, we present our roadmap for future research and action. Our full technical reports and a summary of those are available on our website.

Although services such as peer-to-peer trading and demand response programmes for residential consumers show signs of future potential, many battery services cannot presently be monetised, and developing ways to do this will increase options to realise value for end-consumer battery owners.

The outcomes of these pricing changes and any subsequent market design and Electricity Code changes may alter the way transmission and distribution services are delivered and charged, and how consumers are enabled to participate in the market.

OUR RESEARCH SCENARIO:



WIDESPREAD SOLAR PV ACROSS NEW ZEALAND

TODAY, SOLAR PV

ACCOUNTS FOR LESS

THAN 1% OF NEW

ZEALAND’S

ELECTRICITY

GENERATION, BUT

THE RATE OF

INCREASE IS RAPID.

How and where solar PV will increase is hard to forecast, but we expect that a range of factors will drive growth. These factors could include differing sunshine hour rates, planning for new suburbs, and demographic and economic factors related to the age and wealth of different communities.

To effectively stress-test the power system, we needed to assume a future scenario where New Zealand has a very substantial proportion of solar PV connected into local electricity networks across the country. For our research, we assumed the distribution of solar PV that is effectively as if every residence in New Zealand had the equivalent of a 3kW solar PV array on the roof. We considered the impacts as being spread over the entire power system according to population density.

This worked out to approximately 4000 MW of installed solar PV, slightly less than half the currently installed capacity of all the other forms of electricity generation in New Zealand. This would generate up to 3500 MW of electricity at midday on a mid-summer day that was fine and sunny across all New Zealand (based on a typical day in January 2015), and up to 3000 MW of electricity at mid-winter (based on a typical day in August 2015). That represents as much as 75% of the electricity used today in New Zealand in the middle of a typical sunny day in mid-summer and up to 60% of the electricity used in the middle of sunny mid-winter day. It would be approximately 20% of the total electricity that New Zealand uses in a year – about 8000 GWh.

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Figure 1 shows the amount of PV generation produced on cloudless day across New Zealand, for our selected typical day in each season, in our scenario with 4000 MW of rooftop solar PV. The differences are due to variations in solar irradiance by season. For our study, we have assumed all solar panels are clean, operating and brand new.

Rooftop solar PV is a distributed form of electricity generation. The electricity generated is directly related to the energy available from the sun at the location of the rooftop, and at the time it is generated. It is not controllable, but is reasonably predictable at any specific location. Electricity from rooftop solar PV is used where it is generated – reducing the amount of electricity required to be generated at remote locations and then transmitted and distributed through electricity networks.

Figure 2 shows the impact that solar PV would have on the generation mix, on a typical, cloudless winter day. As electricity generated by PV generation increases and decreases during the day, generation required from other sources decreases and increases.

Figure 1

PV GENERATION IN NEW ZEALAND (4 GW INSTALLED)

Figure 2

TYPICAL WINTER DAY GENERATION MIX

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Hydro Thermal Geoth Wind Other PVGen Total Gen

13%



In our studies, we wanted to identify what to watch for in New Zealand as the amount of solar PV increases. We also needed to understand what action we might need to take to accommodate significant solar PV while providing a secure and reliable supply of electricity. Further, we sought to determine how the performance and nature of solar PV will interact with power system operation.

Our scenario sought to achieve this by ‘stress-testing’ the core power system with a very high level of solar PV penetration. The purpose of the research was to understand the impacts of solar PV, so assumed no other technologies were mitigating the impacts of solar PV. We are aware that high levels of solar PV will first have an impact on distribution networks. We have not sought to investigate these, as our aim is to

understand the impacts on the central transmission grid that forms the backbone connection of our power system.

Figure 3 shows how this rooftop solar PV was allocated across the country based on population and dwellings. This meant the impacts of solar PV on the power system were geographically spread across New Zealand, which was important for our study. The actual output of each rooftop solar array was adjusted for the solar irradiance at its location from information used in the GREEN Grid project, based on data supplied by NIWA. As a result, a rooftop array in Nelson, for instance, provides more electricity on a cloudless day than one in Southland.

There are many ways New Zealand meets its need for electricity. Most (60-

65%) is generated at hydro power stations, using water to spin a turbine, which is connected to a generator that supplies electricity into the grid. Steam from geothermal fields in the centre of the North Island is also used to spin turbines, generating 14% of our electricity. Likewise, gas and coal is burnt to spin turbines to generate electricity.

Electricity from rooftop solar PV panels is produced differently. There is no spinning turbine or generator. The electricity produced by a rooftop solar panel is converted by a device, called an inverter, to be suitable for connection into a household mains supply.

Electricity from generators driven by spinning turbines have features that help grid operators ensure a reliable electricity supply and meet essential

600 MW

(15%)

Northland

280 MW

(7%)

Bay of Plenty

120 MW

(3%)

Gisborne – Hawkes Bay

480 MW

(12%)

Wellington

320 MW

(8%)

Marlborough

160 MW

(4%)

Canterbury

120 MW

(3%)

Otago

200 MW

(5%)

Southland

40 MW

(1%)

West Coast

160 MW

(4%)

Tasman

200 MW

(5%)

Manawatu – Wanganui

120 MW

(3%)

Taranaki

320 MW

(8%)

Waikato

880 MW

(22%)

Auckland

Figure 3

ROOFTOP SOLAR PV ALLOCATION BASED ON POPULATION IN OUR SCENARIO

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quality standards for that electricity supply. Many of the features that aid the reliable operation of the power system are absent from electricity produced by rooftop solar PV.

In developing our scenario, we didn’t include solar PV installed on commercial or industrial buildings, given the large amount of solar PV already in our scenario, which is sufficient to identify where challenges may arise to the national power system. We are mindful of this and anticipate solar PV at these sites will increase in future. We also did not allow for the possibility of large solar farms, connected directly to the grid, such as are seen internationally. These are a future possibility, although New Zealand has few suitable areas (farms elsewhere typically require extensive tracts of bare land and very high levels of solar irradiance).

As a starting point, our scenario assumes that solar PV would supplement the existing mix of electricity generation types, with thermal generation from gas and coal displaced first by solar PV when there is excess electricity generated. Thermal generation typically has higher operating (fuel) costs than hydro, wind and geothermal and its displacement mirrors what has occurred in practice overseas. We also assumed the demand for electricity was the same as it is today.

Importantly, in our scenario we have not assumed any impact from other new technologies. For instance, batteries including charging electric cars, can reduce the impact of solar PV on the power system by storing electricity at

times when solar PV generation is at its greatest and supplying this stored electricity at times when demand for electricity is high. Storage technologies can mitigate the impacts of solar PV on the power system, particularly if electricity market arrangements evolve to maximise the opportunity for storage and variable generation (such as solar PV) to complement each other. For our study, we wanted to assess the limits of the power system to absorb the impacts of solar PV so have not factored in these potential mitigations.

The future mix of technologies used to generate and, increasingly, to store electricity will be different from what we have today, although we can be confident that cheap, renewable hydro and geothermal generation will continue to play a strong role.

We have based our studies on how electricity is used today, using sample days from the past two years in our analysis. We acknowledge that, with high levels of solar PV, when and how we use electricity may change. This may encourage greater electricity use during the day to maximise the benefits from solar PV.

We established the amount of electricity that still needs to be supplied through the electricity transmission grid from existing generation sources to meet demand during the day. We assumed any solar PV generated electricity nets off or cancels out the equivalent amount of electricity demand. Figure 4 shows the electricity demand that needs to be supplied through the transmission grid in our



scenario. The blue line shows current daily demand, while the other three lines depict demand with solar generation of 2000 MW (red line), 3000 MW (orange line) and 4000 MW (green line).

Some readers will recognise this as the “Duck Curve”. It is where the daily net demand for electricity through the power system dips during the day due to increasing amounts of distributed solar PV generation, then rises sharply in the transition to evening with increasing electricity demand and end of daylight.

The next five sections describe

our research, which uses our

scenario to stress-test the power

system and understand how it will

respond to potential challenges of

incorporating solar PV. For further

details about the scenario, please

refer to our technical reports,

available on our website.

Figure 4

DAILY ELECTRICITY DEMAND THROUGH TRANSMISSION GRID IN OUR SCENARIO

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with 2GW PV Penetration with 3GW PV Penetration with 4GW PV Penetration Average August National Demand

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ENSURING A BALANCED POWER SYSTEMCAN ENOUGH ELECTRICITY FROM OTHER SOURCES BE

APPROPRIATELY SUPPLIED TO MEET THE SHARP INCREASE IN

DEMAND AS THE SUN GOES DOWN AND AS THE EVENING PEAK

APPROACHES?

Power systems operate by balancing demand for electricity with supply in real-time. In our distributed solar PV scenario, on a typical day electricity from solar PV will displace a significant amount of existing grid-connected generation. Later in the day as the sun goes down and PV generation reduces, this disconnected generation must reconnect and rapidly increase its output to meet electricity demand, which also typically increases in the evening. We wanted to understand how the power system would cope with this transition.

We needed to understand exactly how quickly generation would need to connect to the grid and increase its output. We used the average profile of electricity demand for different PV generation levels, and identified when the evening rate of increase in the demand for electricity from grid-sourced generation is at its greatest.

We considered examples in spring, summer, autumn and winter, based on information provided by generators in the wholesale electricity market about their ability to increase output and the total electricity each can supply. Our winter case shows the sharpest increase, as the highest demand for electricity is on winter evenings. In our winter case with solar PV, the rate of increase required collectively from other forms of generation to meet demand reached a maximum of approximately 300 MW over a five-minute period. It represents a 1% increase every minute in the amount of total electricity supplied to New Zealand – approximately three times the rate presently required to meet demand on a winter evening.

We found that in each season there was sufficient capability within other generation options presently available to cope with this rate of rapid increase in electricity supply from the grid in the transition from a sunny day into the evening.

One factor we did identify in our ‘wet’ winter case was that, due to greater ability to utilise hydro generation, the power system may struggle to meet peak evening demand due to insufficient thermal generation being offered. This is not a new issue for the power system. The availability of hydro generation can make it less likely that large thermal generation stations are available to generate electricity just for short periods covering high demand over the evening peak.

Our conclusion is that we can operate a balanced power system with up to 4000 MW of solar PV with the existing grid-connected generation mix, because there is sufficient capability to cover the reduction in solar PV in the evening as well as the increase in electricity demand.

If New Zealand does have a high proportion of distributed solar PV in the future, it will be important to power system operation that other resources can quickly increase output and both compensate for the reduction in solar PV and meet the increase in electricity demand in the late afternoon and evening. These future resources may include not only electricity generation, but also new electricity storage and demand management technologies.



Rapid cloud movement across the sun, casting shade over solar PV panels, has the potential to quickly vary the output of solar PV. We wanted to understand if this localised effect would have a significant impact on the balancing of supply and demand at power system level.

If the power system is balanced, the frequency of the electricity across the grid is a stable 50 Hertz (Hz). Small second-by-second imbalances as demand for electricity changes cause power system frequency to fluctuate around 50 Hz. These small changes are managed by selected generators contracted to be “frequency keepers”. As frequency keepers, the generators make small, continual changes in output to maintain system frequency at a stable 50 Hz and ensure frequency is kept within acceptable ranges (49.8 Hz to 50.2 Hz).

Our studies showed that, for our scenario of 4000 MW of distributed solar PV with the current mix of other generation types, local sudden variations in electricity from solar PV due to cloud movement on sunny days would result in the system frequency going beyond the acceptable range for just 33 minutes in a year – that’s 0.006% of the time. System frequency is currently outside this range for 10-15 minutes each year.

Overall, we found the effects of cloud movement shading solar panels on sunny days to be negligible. This is in part due to the flexibility of the current generation mix.

We note that new technologies such as storage and demand response could potentially play a role in helping manage system frequency.

MANAGING PARTLY CLOUDY DAYSHOW MATERIAL IS THE FLUCTUATION OF SOLAR PV

GENERATION CAUSED BY CLOUD MOVEMENT TO THE

NEW ZEALAND POWER SYSTEM?

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HOW WILL THE POWER SYSTEM RESPONSE BE DIFFERENT IN

EMERGENCY EVENTS THAN IN THE PRESENT ENVIRONMENT?

RESPONSE TO SUDDEN EVENTS

Sudden events can occur on the power system which instantly and significantly alter the balance of generation and demand. Examples of these events include the instantaneous shutdown of a large generator or partial shutdown of the High Voltage Direct Current (HVDC) link that connects the North and South Islands. If the power system is not re-balanced quickly it can shut down, resulting in a widespread loss of supply or blackout.

Some sudden events are sufficiently likely or credible that we have performance objectives set to ensure such events have minimal impact on electricity consumers. Today, the response and recovery from these events are aided by grid-connected generators driven by spinning turbines. These provide a large rotating mass that adds inertia to the power system – dampening the effect of a sudden imbalance – allowing time for the system operator to rebalance the power system after a sudden event.

In our study scenario, a significant amount of this grid-connected generation is displaced during the day by distributed solar PV (which does not have the same capabilities as electricity provided by spinning turbines), so it was critical to investigate how the power

system would recover from credible sudden events.

The response required to a sudden event depends on how much electricity was being delivered into the power system by the specific generator or HVDC link just before the event. Our assumption is that, at times of high solar PV generation, larger generating units would be shut down or delivering a reduced quantity of electricity. Likewise, the transfer on the HVDC link between the North and South Island link would be lower due to locally supplied electricity from solar PV meeting the demand for electricity.

We found that the power system’s ability to recover from credible events was not significantly impaired by high levels of solar PV, because the magnitudes of these events were smaller and did not require as much capability from other generators with spinning turbines to restore the system to a balanced state.

We also studied more extreme events to determine the response of the power system with high solar PV. Examples are the loss of two or more generators at once or the sudden total shutdown of the HVDC link. A mitigation for these more extreme and rare events today is a system that automatically disconnects



electricity consumers to help rebalance supply and demand quickly to avoid a widespread blackout. Our studies found that in our scenario the settings of this automatic scheme would need to be reviewed before we reached 3000 MW of solar PV to ensure it would still work correctly. This is due to less grid-connected generation being available to dampen the immediate effects of the imbalance from an extreme event.

Sudden imbalances also occur due to the sudden loss of demand, perhaps a failure of a power system connection to a major city due to damage to electricity transmission lines. Today we automatically disconnect selected generators to bring the system back to balance. Our studies found that with less grid-connected generation at times of high solar PV this might be harder to achieve.

However, the inverters on each solar PV installation have the capability to reduce output when there an imbalance with more generation than demand for electricity. This can help restore balance to the power system as well.

Overall, we found that the New Zealand power system can accommodate a significant amount of PV generation before encountering challenges when responding to the impact of sudden events affecting the balance of generation and demand.

In a future with more than 3000 MW of solar PV, with our current generation mix and electricity demand, there is reduced capability to respond to system events. The displacement of other generation by solar PV means the magnitude of these events is also reduced. Standards for solar PV inverters can also assist manage the issues.

We expect to explore this issue further before we reach 1000 MW of solar PV, or if the generation mix changes significantly from today.

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STABILITY IN THE POWER SYSTEM AFTER A SUDDEN EVENT CAN THE POWER SYSTEM RECOVER TO A STEADY, STABLE

STATE IF PART OF THE SYSTEM FAILS?

Grid-connected generators with spinning turbines have the ability to help restore the power system to balance after a sudden event that affects supply and demand. There is the potential for sudden events to cause some generating units to become unstable and disconnect from the grid. This can result in other generators disconnecting and potentially cause a power system blackout.

We carried out studies to investigate if grid-connected generators still respond to a sudden event in a stable manner when most of the demand for electricity is being met by solar PV in the middle of the day. Our concern

was that, with fewer other grid-connected generators operating at times of high solar PV, there is less inertia to dampen any unstable responses.

In our studies, we found that power system response remained stable. We will monitor any changes and trends from systems we currently have in place. We will carry out wider detailed studies before we approach 2000 MW of installed solar PV or if there is a significant change in generation mix. We will also monitor changes in the way the system behaves through our power system monitoring equipment.



MANAGING VOLTAGE

Managing voltage under normal conditions

The voltage of electricity supplied through the power system varies, depending on how much electricity is flowing through the lines and cables. When the grid is heavily loaded, running near its maximum capacity (for example, on a winter evening), voltages across the power system are lower and need to be boosted to stay within the correct limits. When the power system is lightly loaded and has excess capacity (for example, in the early morning), voltages are higher and action is taken to bring them back within limits.

Power system operators manage voltages in a number of ways. There is equipment specifically installed on the grid to raise or lower voltages, such as devices known as capacitors or STATCOMs. Other equipment may have capability to manage voltage in addition to its primary role, such as transformers and grid-connected generators. Another option, when the

grid is lightly loaded, is to switch off circuits on some transmission lines.

Our scenario with a substantial amount of solar PV has two impacts on power system voltage management. It reduces the amount of electricity needed to be supplied through the transmission grid during the middle of the day and afternoon, and it reduces the number of grid-connected generators available to assist with voltage control.

On a sunny day, our scenario creates conditions on the power system like those presently experienced overnight in the early hours of the morning, but during the middle of the day and afternoon.

Our studies found that with the existing combination of equipment available to manage voltage, up to 2000 MW of solar PV could be managed. Above this point, operators would

WILL VOLTAGE BE HARDER TO MANAGE?

WHAT ARE THE SOLUTIONS?

To ensure a reliable, safe supply of electricity to homes and businesses, it is essential that the voltage of electricity supplied across, and delivered through, the power system is managed. Voltage management also protects the equipment supplying and distributing electricity.

We expect that significant amounts of solar PV will pose challenges for the management of voltages within local distribution networks, as has been observed internationally. This impact is not part of our studies, as we are investigating the impact on the electricity transmission grid.

We considered three areas of study for our scenario with high solar PV:

�� What is the impact of voltage management when the power system is in a stable, normal condition?

�� How does the power system respond to a sudden change in voltage?

�� How is solar PV generation impacted by a sudden change in voltage?

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need to resort to switching off some transmission lines to help voltages stay within limits. Although this is an acceptable practice, if used regularly it reduces the reliability of supply. Therefore, investment in additional, dedicated voltage control equipment is likely to be required. Our studies assume that the uptake of solar PV happens at a consistent rate across New Zealand. If solar PV were installed with significant regional variations, the impacts on voltage management in some regions may occur earlier than we identified.

Power system response to sudden changes in voltage

The sudden unexpected loss of a generator, transmission or distribution line can cause a spike or dip in voltage across the power system. Grid-connected generators with spinning turbines act automatically to help dampen the impact of any spike or dip.

Our studies showed that, with fewer grid-connected generators in operation on a sunny day, these disturbances would be felt more widely across the power system. We will investigate this further in future studies, including researching the capability of future solar PV inverters to mitigate this impact.

Impact of sudden changes in voltage on solar PV generation

We assumed that the inverters used in each solar PV installation were one of three commonly available types, which we had tested and modelled. In our scenario, each of the three types were assumed to be installed in the same proportions that they are currently installed in New Zealand today.

We found that one type of inverter disconnected when there was a sudden dip in voltage. If all inverters of this type disconnected, the sudden widespread loss of a large amount of solar PV in response to a widespread voltage dip would create a power system imbalance – a sudden event as discussed on pages 12–13. The potential size of this voltage-driven imbalance could be greater than the credible sudden events we considered in our research. If this risk – the sudden loss of PV generation due to a voltage dip – becomes credible in the future, it will impact the operation of the power system with a need for added ancillary services and costs to supply electricity. If not treated as a credible event, there will be a higher risk of a failure of the power system.

The type of inverter that disconnected in response to a sudden voltage fluctuation did not comply with the current standard requirements for inverters, AS/NZS 4777.2:2015.

Our studies therefore confirmed that it is very important for solar panel inverters to comply with the standard to ensure we can manage our power system at reasonable cost and/or avoid reduced reliability in future.



LOOKING AHEAD

TODAY’S TRANSMISSION GRID AND POWER SYSTEM CAN ENABLE AND

ACCOMMODATE A SIGNIFICANT AMOUNT OF ROOFTOP SOLAR PV GENERATION.

Our full technical reports are available on our website, and we welcome your feedback and engagement as we plan and prepare our country’s power system to enable the technologies of the future.

We will carry out further studies on the impacts on the standards and market arrangements for operating the power system when we approach 1000 MW of installed solar PV or if the way we generate or use electricity materially changes from the assumptions in our studies.

One thousand MW of distributed solar PV is about 16 times that currently installed in New Zealand today. With 1000 MW of solar PV, we would expect solar PV to be providing 5% of New Zealand’s total electricity needs over a year, and up to 15% of the electricity supplied on a sunny winter day.

From our research, we have created a roadmap to summarise the potential issues that we may need to address as levels of rooftop solar PV increase.

Figure 5

SOLAR PV INTERGRATION ROADMAP

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PV

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W)

�� Reduced capability to respond to sudden events on the system may require intervention – additional services may need contracted from the electricity market. Research will be needed ahead of this time.

�� Rapid cloud movement causes system frequency to exceed acceptable limits 33 mins per year.

�� Reduced capability to respond to sudden events on the system is evident

�� The settings for the system that manages credible extreme system events warrants review

�� Voltage, which has steadily become more difficult to manage, reaches a tipping point and investment in voltage control equipment likely to be needed.

�� Re-check power system stability study – operational measures may be needed to mitigate potential issues.

2000 MW (32x today)

3000 MW (48x today)

4000 MW (64x today)

= 5% of NZ’s power over a year

�� Re-check our assumptions and scenarios

�� Detailed studies to further investigate power system stability following impacts on the standards and market arrangement.�� Further exploration of the

impact of sudden events on the power system.

62 MW Today

1000 MW (16x today)

The most notable finding is the importance of ensuring that inverters installed in New Zealand comply with the requirements standard (AS/NZS 4777.2:2015). We expect that significant issues will arise with large-scale adoption of solar PV if non-compliant inverters cause PV generation to disconnect if it experiences a sudden dip in voltage.

The next phase of our investigation into how the power system and transmission grid can enable new technologies will explore battery storage in combination with rooftop solar PV.

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