Neighbour's Solar and Storage Blog - Page 2 of 3

120 percent rule for solar breakers

Moustafa YoussefJune 15, 2020Blog

Most solar PV systems are connected to a property’s load panel or sub-panel. Solar inverters feed AC electricity to connected appliances, and any extra power is back-fed to the grid to power other appliances in the network. However, the Canadian Electrical Code limits the connecting capacity of multiple power sources feeding a distribution centre. In this article I will explain why such limitations exist and ways to increase a solar PV system’s capacity beyond CE Code 64-112’s 120 and 125 percent rules.

Why load side connections?

Load side connections are the simplest way to connect a solar PV system to an AC grid. The only upgrade required to a load panel is an installation of a bi-directional breaker as far away as possible on the bus from the other source breaker(s). However 64-112 limits the sum rating of the supply breakers at 125% of bus rating for homes, and 120% for non-dwellings, and I’ll give an example to show why.

What’s the point of CE Code 64-112’s 120 and 125 percent rules?

Consider a bus that has a rating of 225A and connected to a single source of energy (transformer, generator, or what have you) through a 200A breaker. No problem here. The bus is always protected because if more than 200A of current tried to reach it, the supply breaker will trip and interrupt the current. Is this still true when the bus is connected to another power sources? Below is a picture of a load panel being fed from two sources, namely a solar inverter and another power source, which could for example be a generator, a transformer or another panel board. Each power source is connected through a breaker that limits the current or power flow reaching the bus.

solar inverter interconnection — This is how most small grid-tried solar PV systems (<500kW) are interconnected to the grid. They are installed behind a breaker like any other appliance connected to the bus.

Suppose that a solar inverter is connected through a 100A breaker. Now if both sources were feeding the bus at their maximal capacity, they could be pushing a total of 300A into the bus, which exceeds the bus rating of 225A :

For a 225A bus in a home,CE Code 64-112 requires that the sum of the breakers feeding a bus can’t exceed (225+25%) 281A as per CE Code 64-112 (4). So if the main breaker that’s connecting the home to the grid has a rating of 200A, the solar breaker(s) can’t exceed a rating of 81A or 80A. So we can install one 80A breaker to tie-in one inverter or for example, 4x20A breakers to tie-in four separate inverters or distribution panels connecting several inverters.

CE Code 64-112’s 120 and 125 percent rules

64-112 (4) limits the sum of the sources feeding a bus to 125% of the bus’ rating for homes, and 120% for non-dwellings. If we want to install a greater output capacity, we can

buy a panel with a greater bus ampacity. This is the most expensive option, and its impact is limited. For example it’s not easy to find single phase panelboards with rating greater than 225A. We can also
reduce the rating of the main 200A breaker. This depends on whether or not the client can afford to reduce their loading capacity. Again this option is limited.

History of line vs load side connections in Canada

When Ontario spread-headed the adoption of distributed solar in 2009 with their microFIT program, they required output of solar inverters to be connected to a separate meter. A home would still have one run from a transformer, but it would split to two meters, one for consumption, the other for generation. Most jurisdictions today including Ontario are providing either net-metering or net-billing settlement systems that don’t require a separate meter. This load meter just has to be replaced with a bi-directional one to be able to measure exported energy.

Advantage of line side connections

Many installers and inspectors outside of Ontario are only familiar with load side connections because it is the most popular way of interconnecting relatively small systems. But CE Code 64-112 actually allows for line side connections (rule #1) before it allows for load side connections (rule #4). A line side connection will bypass any load-side connection limitation, including CE Code 64-112’s 120 and 125 percent rules.

CE_Code_64_112_Line_Side_Splitter_Solar_Grid_Tied — As we can see with a line side connection the panelboard is now fully protected and risks no overload. With a line side connection the panelboard no longer limits the interconnecting capacity of a grid-tied solar PV system. Furthermore, the line side equipment such as the meter socket doesn’t have to be upgraded at all, unless of course the capacity of the solar inverters exceeds the ampacity of the meter socket.

Line side connection on meter socket

If the bus bar on meter socket already has enough extra space for the new cables you want to install I would just install them there. It’s important to note to the inspector you are not actually increasing the capacity of power flow through the meter socket since solar is a negative load. So unlike a load side connection that increases the current through the bus, any current from the solar on line side will decrease the current from the grid and so the ampacity of the meter socket need not be greater than what it already is. Of course that’s assuming the size of the solar pv breaker is less than or equal to the rating of the main breaker on the home panel.

Is solar PV a reliable energy source?

Moustafa YoussefJune 10, 2018Blog

Solar PV is often criticized for not being a consistent or reliable source of energy. The sun doesn’t always shine when we need it, and it can be difficult or impossible to accurately predict short-term performance because it is strongly influenced by the weather. However if one were to take a step back and evaluate over the long-term they may find that there is a high degree of consistency in the performance of solar PV systems. In this article I am going to do some statistical analysis on production data one of the first systems we installed.

About the solar array

The system is located in Calgary. The array is flush mounted and made up of two sub-arrays of four modules, one facing SW with a true direction of 225 degrees; the other is facing SE with a direction of 135 degrees. Both sub arrays have a pitch of 4/12 or a tilt of 18 degrees. The system was commissioned in August 2014 so there are three full sets of annual data to analyze. The array experiences a little bit of shading and hasn’t experienced any serious power outages.

Annual energy production is highly consistent

Below are charts depicting the annual energy production of each module in 2015, 2016 and 2017. We can see that some modules consistently produce more or less energy than adjacent modules. This can be attributed to shading and surface temperature variation. Having said that the system’s total annual production is incredibly consistent. The system produced a total of:

2,287 kilowatt-hours in 2015,
2,218 kWh in 2016, and
2,141 kWh in 2017.

These numbers yield a mean of 2,216kWh and a coefficient of variation of only 3%. Assuming performance is normal we can be:

95% confident that next year’s energy production will be between 2,070kWh and 2,360kWh, or we can be
99.5% confident that it will be between 2,000kWh and 2,450kWh.

2015 solar production

2016 solar production

2017 solar production

Methodology

I collected four data points for each month for all months except for June, July and August which have only three data points each.
Data was not corrected for cell degradation. You may have noticed that the total and modular annual energy production has been decreasing year by year, but it’s too early to relate this to cell degradation.
Monthly averages are a mean of the monthly production data.
Monthly coefficients of variation are calculated in order to compare variation in production. A dataset with a higher COV means that it’s less predictable.

Results

In red below is the ratio of a month’s mean production to annual mean production, whilst the blue represents the month’s COV.

Observations

The summer months May, June and July each produced about one-seventh of the average annual production, which will be called ‘annual’.
May to September have an incredibly high consistency. For example the month of May has produced a total of 318kWh in 2015, 308kWh in 2016, 316kWh in 2017, and 319kWh in 2018, yielding a COV of only 2%.
Production from December up to and including February accounts for about 5% of annual.
The COV and monthly mean curve seem to be opposite one another. The higher the mean, the lower the COV and vice versa. Production is a lot more consistent between May and September than it is in the winter. But is this high winter variance really a problem?
Winter has a higher variance, but because there is very little production its influence is limited. For example, a COV of 32% for the month of January means that we are 95% confident that production to be between 0.5% and 2.3%. The bottom line is the amount of energy is negligible and will be absorbed by a design margin.
On the other hand, June makes up for about 15% of the energy and it has a COV of 3%. This translates to a 95% confidence that production is going to be between 14% and 15% of annual. That is a higher quality prediction, and an important one as well because of the amount of energy it accounts for.

Conclusion

We only analyzed four years of a system that is expected to stay in operation for 25 years. Therefore this analysis is still in its infancy. However I think it puts the question of reliability of solar PV in a good light. When looked at from a monthly or annual basis, solar PV seems to be a consistent energy source. Or at least one can say that this approach enables stand-alone system designers to efficiently communicate and justify generation and storage capacities. With probabilistic data a system’s size can be based on a client’s appetite for risk. Of course grid operators will still require short term forecasting as more solar is adopted across their jurisdictions, and a model based on observed data alone has its limitations.

I wrote an article about averages in the context of off-grid systems showing how it is favourable to use longer measuring intervals because they evolve to normal distributions with low variance i.e. higher quality.

May the power of averages be with you.

Click here for a .pdf of this article.

Solar in a deregulated power market

Moustafa YoussefJanuary 18, 2017Blog

Alberta has a market for the supply of wholesale electricity. Prices vary with supply and demand. All other provinces but Ontario have a utility that is responsible for generating as well as delivering electricity and maintaining its infrastructure. In Alberta these tasks are the business of private generators (e.g. TransAlta, Enmax Energy) and wires owners (e.g Altalink, Fortis, Enmax Power), and there is an independent body mandated by Alberta Utilities Commission, called the Alberta Electric System Operator, that manages the grid and price of power. In this blog post we’ll go over how power prices are determined, and how solar and renewables in general affect a deregulated power market.

Alberta has an energy-only power market, which means that generators only collect revenue from generating electrical energy. The price of wholesale electricity is determined in real time by a merit order as shown below. Much like with any supply-demand curve, prices (y axis) are put in ascending order alongside respective generator capacity (x axis), and the point to which the supply and demand curves intersect determines the clearing/market price and clearing capacity. This happens every minute ensuring that demand is always met and consumers are getting the lowest cost for electrical energy. Generators on the left of the intersection point will be dispatched because their offering price is less than what the market established. For example a natural gas power plant can put up 30MW for $15/MWh from 3pm to 4pm. If at 2:59pm the price of power happens to be higher then it will be dispatched.

Renewable generators have no fuel costs and therefore can afford to sell power at any rate. If they are generating power they want it to be sold immediately. Therefore they will always be the cheapest generators on the supply curve. On the other hand fossil fired generators such as coal and natural gas have considerable operating costs and will bid their capacity at a rate that’s economically sustainable. Baseload generators will be operating for most of the time and will be found more towards the left and centre of the curve while peaker plants as their name suggests are only dispatched when market prices occasionally spike.

Supply demand curves are not new. The genius of this system however is that it uses price signals to balance a highly sophisticated circuit in real-time. Generation-demand have to always be in balance otherwise power quality will deteriorate or worse cause power outages which are expensive and just unacceptable. Unlike a potato market where you get to meet potato farmers, look over a variety of potatoes or even consider substitutes, electricity is critical and its delivery must be anonymous and unconditional – power is transmitted to a pool/grid that everybody draws from. Electrons are electrons are electrons.

Like other energy commodities, consumption is independent of price. We flick switches, cook dinner, warm our homes whenever we need. System operators such as AESO rely on forecasting techniques to predict demand to ensure the system is prepared to meet demand both in real-time, as well as short and long term well being of the system.

Let’s take an example. Let’s say we have a grid that has a total of 250MW of generation assets – 100MW of solar, 50MW of coal and 100MW of gas, and their respective asking prices are $0/MWh, $30/MWh and $40/MWh. Let’s say that the demand is 100MW and the solar farm is generating 80% of its capacity or 80MW. Based on the generators’ merit order, 80MW of solar and 20MW of coal will be dispatched. Because coal is the last generator to be dispatched, the market price will equal its bidding price of $30, which all cleared generators will receive.

solar deregulated power market merit order

Merit order with 100MW of demand, 80MW of solar and 20MW of coal

Although the solar generator is willing to dispatch power for as low as $0/MWh, they receive the clearing price of $30/MWh. The owners of the solar farm will generate revenue of (80MW x $30/MWh) $2400 for every hour the system is dispatched. The gas generator will be standing reserve because its asking price is higher than the clearing price. If demand were to grow by 20MW, the coal generator will increase its dispatch by 20MW to make up for it. If demand were to increase beyond 130MW the gas generator will fire up and the clearing price will be $40/MWh, and then all three generators will receive $40/MWh.

Let’s suppose it’s getting darker and the solar farm is only putting out 20MW but demand is still at 100MW. The supply curve will be shorter because the solar farm is generating less power. The dispatchable coal and gas generators will now have to increase their output by 80MW. As can be seen below this causes the market price to increase to signal to the natural gas generator to dispatch their power. Although the solar generator is supplying less power, they are receiving it at a higher rate because the clearing price is higher – same goes for the coal power plant.

Merit order with 100MW of demand, 80MW of solar and 20MW of coal. Clearing price is now $40/MWh

How does solar affect a deregulated power market?

As we saw above, renewable generation facilities are always dispatched because of their low marginal costs and they will accept whatever the market price is, hence they’re sometimes referred to as price takers. However as solar and wind capacities increase so does their synchronized impact on the clearing price. For example, in Alberta we sometimes notice how times of high wind can cause the price of power to drop. Wind generators are starting to act more and more like price setters. Large utility generators such as Calgary’s Shepard combined cycle natural gas plant also behave as price setter because of the considerable space they occupy on the left side of the curve. If you go to ets.aeso.ca to monitor the grid’s load and clearing price, you can sometimes see how an expected shutdown of a large dispatched generator can cause the price of power to spike. In Alberta, the clearing price can be as high as $999.99/MWh, and as low as $0/MWh.

Example of a merit order showing different marginal costs

From energy-only market to energy and capacity markets

The Alberta power market has been in the news for the past couple of months as the province gears up for its plans to phase out coal by 2030 with renewables and natural gas. Some called into question whether Alberta’s energy-only power market will be able to attract needed investments in generation given the historic low power prices. Generators have been enjoying relatively high prices in the past which seemed to be a sufficient signal to build more capacity, but now that there is a glut of electricity AESO has recommended to introduce capacity payments which means that on-top of the revenue made on selling energy, generators will also collect revenue for being available.

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