Power System Protection Coordination Calculation

This study covers protection relays settings calculation for standby power system generators in the company’s data center.

Table of Contents

OBJECT DESCRIPTION

This study covers protection relays settings calculation for standby power system generators in the company’s data center. The standby power system will have 8 synchronous generators: MarelliMotori MJH630 LB4, connected to a 15 kV internal power supply system. Each generator is 3,2 MVA. Generators will be started in the case the power supply is lost from MVA/MVB or both feeders. In island operation, the network is grounding through a grounding transformer. The grounding transformer feeder closes as soon as the first generator circuit breaker is closed. Main feeders cannot work in parallel. 

Possible operation modes:

  • island operation, a single generator is running,
  • island operation, all generators are
 

Generator relay will have these functions:

  • dI>(87), generator differential,
  • Idir>(67), directional overcurrent,
  • T> (49), thermal overload,
  • Q< (40), generator under excitation,
  • Ih> (50H/51H), harmonic overcurrent,
  • P(32), power protection,
  • I2> (46), negative sequence,
  • U> (59), overvoltage,
  • U< (27), undervoltage,
  • f> (81H), over frequency,
  • f<(81L), under
 

Selected relay AQ-G257.

The network single-line diagram can be found in the downloadable pdf file in Annex No. 1. Calculations are performed with a power system modeling software EA-PSM. Learn more about the software.

LOAD FLOWS CALCULATION RESULTS

Single bypass generator apparent power is 3,2 MVA. Prime mover mechanical power is 2,6 MW. Reactive power that generator can supply at rated conditions is 1,92 MVar lagging or 1,12 MVar leading, based on generator capability curve, depicted in Figure 1.

EA-PSM Energy System Calculations software

Figure 1 Generator capability curve

According to this data, the maximum current from the generator at nominal voltage is 124 A. Cumulative current from all generators working in parallel is 958 A.

SHORT CIRCUIT CALCULATION RESULTS

Short circuit acronyms explanation:

  • K3 – three phases short circuit,
  • K2 – two phases short circuit,
  • K1 – one phase short circuit,
  • K11 – two phases with ground short

 

Short circuit currents when all generators are connected to bus MVG1A and fault is located at one of the generators incomers, are depicted in Figure 2.

Figure 2 Short circuit currents: case A

Click on the image to enlarge

The short circuit current when a single generator is connected to bus MVG1A is depicted in Figure 3.

Figure 3 Short circuit currents: case B

Click on the image to enlarge

Short circuit current when generators are connected to MVG1B is assumed the same as depicted above. Short circuit currents when a fault is located at other places are not depicted in the report, but due to small impedances, currents are almost the same.

CURRENT TRANSFORMERS SIZING

Current transformers should not saturate during short circuit events to provide accurate readings. Assumptions for current transformers sizing are depicted in Figure 4. The transformation coefficient for all transformers is assumed 150/1.

EA-PSM Energy System Calculations software

Figure 4 Assumptions for current transformers sizing

According to calculations, saturation can be avoided if the accuracy limiting factor of the current transformers is at least 15. Therefore, recommended current transformer type is 5P15 or 5P20. For generator 150/1 10P10 transformer is selected, according to the generator datasheet.

GENERATOR RELAY COORDINATION

Generator differential

The estimated current transformer is 150/1 10P10. Differential relay characteristic is depicted in Figure 5.

Figure 5 Differential relay characteristic

Click on the image to enlarge

Current transformer error CTE = 10%, relay measurement error REM = 0,5%, safety margin SM = 5%. 1st slope calculation:

1𝑠𝑡_𝑠𝑙𝑜𝑝𝑒= 𝐶𝑇𝐸1 + 𝐶𝑇𝐸2 + 𝑅𝐸𝑀1 + 𝑅𝐸𝑀2 + 𝑆𝑀 = 10% + 10% + 0,5% + 0,5% + 5% = 26%

 

Base sensitivity calculation:

𝐼𝑑𝑖𝑓𝑓> = 0,5 ∝1𝑠𝑡_𝑠𝑙𝑜𝑝𝑒= 13% (130 𝑚𝐴)

 

1st slope picks up calculation:

𝐼𝑏𝑖𝑎𝑠_1𝑠𝑡𝑠𝑙𝑜𝑝𝑒 = 50% (0,5 𝐴)

 

2nd slope picks up calculation:

𝐼𝑏𝑖𝑎𝑠_2𝑛𝑑𝑠𝑙𝑜𝑝𝑒  = 300% (3𝐴)

 

2nd slope:

𝛼2𝑛𝑑_𝑠𝑙𝑜𝑝𝑒  = 100%

 

2nd differential current:

𝐼𝑑𝑖𝑓𝑓≫ = 𝐼𝑑𝑖𝑓𝑓> +∝1𝑠𝑡𝑠𝑙𝑜𝑝𝑒 (𝐼𝑏𝑖𝑎𝑠_2𝑛𝑑𝑠𝑙𝑜𝑝𝑒 − 𝐼𝑏𝑖𝑎𝑠_1𝑠𝑡𝑠𝑙𝑜𝑝𝑒 ) = 13% + 26% ∗ (300% − 50%) = 78% (0,78𝐴)

 

Settings are summarized in Table 1. 

Generator-differential-protection-settings

Table 1 Generator differential protection settings

Use for TRIP

Thermal overload

Thermal overload protection monitors generator temperature and trips in case the permissible temperature limit is exceeded. The generator overload curve according to the manufacturer is provided in Figure 6.

EA-PSM Energy System Calculations software

Figure 6 Generator overload curve

Estimated reference points for the overload curve are provided in Table 2.

 

Overload curve reference points

Table 2 Overload curve reference points

Recommended settings for this protection function are provided in Table 3.

Thermal overload protection settings

Table 3 Thermal overload protection settings

Use for TRIP

Generator overload and relay thermal curves are depicted in Figure 7.

EA-PSM Energy System Calculations software

Figure 7 Generator overload and relay thermal curves

Thermal overload relay curve is lower than generator overload curve, that ensures generator protection from thermal overload.

Directional overcurrent

Directional overcurrent protection is used as backup protection. Settings of this protection function are provided in Table 4.


Table 4 Directional overcurrent protection settings

Use for TRIP

Generator under excitation

Generator under excitation is calculated according to generator capability curve depicted at Figure 1. Settings of this protection function are provided in Table 5.

generator under excitation protection settings

Table 5 Generator under excitation protection settings

Use for TRIP

Harmonic overcurrent

In island operation, all harmonics will flow through the generators, causing them to overheat. Therefore, harmonic overcurrent protection is recommended. Settings of this protection are depicted in Table 6.

generator harmonic overcurrent protection settings

Table 6 Generator harmonic overcurrent settings

Use for ALARM

Power protection

Power protection is used to avoid reverse active power direction. Settings for this protection are provided in Table 7. Setting is calculated:

Pset = −5% *Pprime_mover/Sgen = -5% ∗ 2590𝑘𝑊/3200kVa = −4%

power protection settings

Table 7 Power protection settings

Use for TRIP

Negative sequence

Negative sequence of current unbalance function is used to protect generator from overheating, that can be caused by unbalanced operation. Settings for this protection are provided in Table 8.

Negative sequence protection settings

Table 8 Negative sequence protection settings

Use for TRIP

Overvoltage

Overvoltage protection settings are provided in Table 9.

Overvoltage protection settings

Table 9 Overvoltage protection settings

Use for TRIP

Undervoltage

Undervoltage protection settings are provided in Table 10.

Undervoltage protection settings

Table 10 Undervoltage protection settings

Use for ALARM

Over frequency

Over frequency settings are provided in Table 11.

Over frequency protection settings

Table 11 Over frequency protection settings

Use for TRIP

Under frequency

Under frequency protection settings are provided in Table 12.

Under frequency protection settings

Table 12 Under frequency protection settings

Use for TRIP

Learn to carry out such a study

If you would like to perform a power system coordination study, book your seat in an online course about power system coordination and selectivity at https://eapsm.net/product/low-voltage-protection-relay-coordination/

If you need any help with your projects contact us or arrange an online call.

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