Power limiting requirements
The Teleport can run various strategies that keep a site within its grid connection limits. This page describes the requirements a site must meet to use them.
Physical connection limit
The physical connection limit should be big enough to absorb sudden uncontrollable changes in load. This can be assessed in two steps:
- Response time: how long does it take between an uncontrollable change and the correction from controllable assets being effective?
- Load volatility: what’s the biggest uncontrollable change that can happen in the response time? The physical limit should be larger than that
As the load volatility calculation is in most cases based on statistics, violating instantaneous (physical) grid limits cannot be fully ruled out!
Response time
The duration between an uncontrollable change and the correction from controllable assets being effective can be approximated by the sum of:
- Meter refresh interval
- Meter readout roundtrip
- Setpoint writing roundtrip
- Asset setpoint response time
The total response time can be smaller than 500ms for best-in-class installations, but larger than 30 seconds for worst-in-class.
Meter refresh interval
Control strategies require a meter at:
- the point of common coupling (PCC)
- each controllable asset (PV, wind turbine, BESS, ..), which is often built-in and can be read out by the Teleport through the same interface used to control the asset
Next to that, we highly recommend using a dedicated meter for uncontrollable load. The alternative of deriving it by taking the difference between the meter at the POC and the sum of the controllable assets will cause spurious transients, because meters are not time-synchronized. Those transients could double the load volatility estimated below!
The refresh interval of a meter can be found in its datasheet. 100ms to 500ms is pretty common. Note that aggregating systems (like PV loggers) typically have a high refresh interval (1-10 seconds) as they query the underlying assets in one by one.
The refresh interval of the slowest meter should be used in the response time calculation.
Roundtrips
Roundtrip times depend on the connection type between the asset and the Teleport. To get a good estimate they must be determined on site. For a crude calculation the following values can be used. They are more conservative than average, but larger values are definitely possible:
- direct TCP connection over ethernet: 50ms
- direct connection over RS485: 200ms
- string of n assets on an RS485 string: n * 200ms
- aggregating systems like loggers with n underlying assets: 50-200ms + n * 200ms
The roundtrip of the slowest meter should be used in the “meter readout roundtrip” term of the response time calculation. The roundtrip of the slowest controllable asset should be used in the “setpoint writing roundtrip” term of response time calculation.
Asset setpoint response time
This varies a lot between assets. PV and BESS are frequently rather quick (< 200ms), wind turbines and EV are slow (many seconds). To be conservative, the response time of the slowest asset should be used in the response time calculation.
Load volatility
Determine the biggest uncontrollable change that can happen during the response time calculated above. As physical connection limit is symmetrical, take the maximum of
- an unexpected change in the direction of more consumption
- an unexpected change in the direction of more feed-in
The following table can help in the assessment, where you would sum up the effect of the sources relevant for the direction.
| Source | Typical variability | Direction |
|---|---|---|
| Solar | cloud cover can cause a drop of 30% of nominal power in a second and 75% of nominal power in 30 seconds. Larger sites have somewhat smaller variability. | more consumption |
| Wind | drop in wind can cause a reduction of 5% of nominal power per second or 50% per minute. cut out ramp rate of 10%/ minute. Larger sites have somewhat smaller variability. | more consumption |
| Battery | no significant impact | neither |
| Generator | no significant impact | neither |
| EV | 100% of nominal power in a second for a single charger (one vehicle disconnecting), the larger the pool the lower the percentage. | more feed-in |
| Uncontrollable load | really depends on the site. Can be 100% of nominal, can be less then 10%. | both directions |
The volatility must be corrected for power quality deficiencies (eg. phase balance and power factor).
Note that assets experiencing a fault can cause larger power deviations than assumed. This could still cause a limit to be crossed!
Contractual connection limit
As a general rule of thumb, the contractual 15-minute connection limit should be 5 x Load volatility x Response time (in seconds) / 900.
Controllable assets
All controllable assets must meet the following requirements:
- A fall back to an output of 0W in case of communication loss must be enabled