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Unbalance detection & protection schemes

The main purpose of an unbalance detection scheme is to remove a capacitor bank from the system in the event of a failure and fuse operation. This will prevent damaging overvoltages across the remaining capacitor units in the group where the operation occurs.

10 unbalance detection schemes for removing failed capacitor bank from the system
10 unbalance detection schemes for removing failed capacitor bank from the system (on photo: Medium-voltage reactive power compensation; credit: avalon.rs)

Removing a capacitor bank in such case protects against a situation that can be immediately harmful to the capacitor units or associated equipment.

Let’s describe how unbalance happens on an example and describe ten schemes for detecting:

  1. General about unbalance detection

Unbalance schemes for grounded wye capacitor banks:

  1. Unbalance relaying for grounded capacitor banks
  2. Summation of intermediate tap-point voltage
  3. Neutral current differential protection (grounded split-wye capacitor banks)
  4. Voltage differential protection method for grounded wye capacitor banks

Unbalance schemes for ungrounded wye capacitor banks:

  1. Neutral voltage unbalance protection using ungrounded wye connected capacitor banks
  2. Neutral voltage unbalance protection method using capacitive voltage divider
  3. Neutral voltage unbalance detection method using three PTs
  4. Neutral current unbalance detection method for ungrounded split-wye capacitor banks
  5. Neutral voltage protection method for ungrounded split-wye connected capacitor banks
  6. Neutral voltage unbalance protection method for ungrounded split-wye connected capacitor banks

1. General about unbalance detection

Consider the capacitor connection shown in Figure 0. When all the four capacitors are in service, the voltage across each unit will be V/2. If one of the fuses is open, then the voltage across the upper branch is 2⁄3V and the lower branch is 1⁄3V.

Such a voltage increase in any capacitor unit is unacceptable.

The unbalance in the voltage has to be detected and the unit must be isolated before significant damage occurs.

There are many methods available for detecting unbalances in capacitor banks, but there is no practical method that will provide protection under all possible conditions.

Open fuse and voltage distribution in a series group
Figure 0 – Open fuse and voltage distribution in a series group

All unbalance detection schemes are set up to signal an alarm upon an initial failure in a bank. Upon subsequent critical failures, where damaging overvoltages are produced, the bank would be tripped from the line.

Typical detection schemes associated with grounded and ungrounded wye banks are discussed below. Since the delta connected banks are so seldom used and ungrounded wye banks serve the same purpose, delta configurations are not evaluated.

The failure of one or more capacitor units in a bank causes voltage unbalance. Unbalance in the capacitor banks is identified based on the following considerations:

  • The unbalance relay should provide an alarm on 5% or less overvoltage and trip the bank for overvoltages in excess of 10% of the rated voltage.
  • The unbalance relay should have time delay to minimize the damage due to arcing fault between capacitor units. Also, the time delay should be short enough to
    avoid damage to sensors such as a voltage transformer or current transformer.
  • The unbalance relay should have time delay to avoid false operations due to inrush, ground faults, lightning, and switching of equipment nearby. A 0.5 second delay should be adequate for most applications.

Unbalance schemes for grounded systems:

1. Unbalance relaying for grounded capacitor banks

In Figure 2, a grounded capacitor arrangement is shown with neutral current relay.

Neutral current sensing using a current transformer
Figure 1 – Neutral current sensing using a current transformer

For a grounded wye bank or each wye of a grounded split-wye bank, the allowable number of units that can be removed from one series group, given a maximum %VR on the remaining units, can be calculated using the following formula [1]:

Units removed from one series group

If F is fractional, use the next lower whole number. The relay is then set to signal the alarm upon failure of F units. The neutral-to-ground current flow IN and relay setting upon loss of F units for this scheme is determined by the following formula [2]:

Neutral-to-ground current flow

The relay would further be set to trip the bank upon loss of F+1 units. The neutral-to-ground current flow and relay setting can be determined using F+1 in place of F.

The percent of overvoltage for any number of units removed from a series group can be determined by the following formula [3]:

Percent of overvoltage for any number of units removed from a series group

where:

  • Vph = Applied line-to-neutral voltage [kV]
  • Vc = Rated voltage of capacitor units [kV]
  • VR = Voltage on remaining units in group [%]
  • F = Units removed from one series group
  • IN = Neutral-to-ground current flow [A]
  • IU = Rated current of one unit [A]
  • S = Number of series groups per phase
  • N = Number of parallel units in one series group
  • F = Number of units removed from one series group

A typical unbalance protective scheme consists of a current transformer with a 5 A secondary using a burden of 10–25 Ω connected to a time-delayed voltage relay through suitable filters.

The advantages of this scheme are:

  1. The capacitor bank contains twice as many parallel units per series group compared to the double wye bank for a given kVAR size which reduces the overvoltage seen by the remaining units in a group in event of a fuse operation.
  2. This bank may require less substation area and connections than a double wye bank.
  3. Relatively inexpensive protection scheme.

The disadvantages of this scheme are:

  1. Sensitive to system unbalance, which is a significant factor for large banks.
  2. Sensitive to triple harmonics and will generally require a filter circuit.
  3. Will not act when there is similar failure in all the phases.
  4. It is not possible to identify the phase of the failed capacitor unit.

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2. Summation of intermediate tap-point voltage (grounded wye capacitor banks)

Figure 2 shows an unbalance protection scheme for a grounded wye capacitor bank using capacitor tap point voltages. Any unbalance in the capacitor units will cause an unbalance in the voltages at the tap points.

 Unbalance detection using summation of intermediate tap-point voltage in a grounded wye capacitor bank
Figure 2 – Unbalance detection using summation of intermediate tap-point voltage in a grounded wye capacitor bank

The resultant voltage in the open delta provides an indication of the unbalance. The changes in the neutral current magnitude and voltage are given by equations 2 and 3 above.

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3. Neutral current differential protection (grounded split-wye capacitor banks)

In this scheme shown in Figure 3, the neutrals of the two sections are grounded through separate current transformers.

The CT secondaries are connected to an overcurrent relay, which makes it insensitive to any outside condition, which may affect both sections of the capacitor bank.

Unbalance detection in a grounded split-wye capacitor bank using two CTs
Figure 3 – Unbalance detection in a grounded split-wye capacitor bank using two CTs

The advantages of this scheme are:

  1. The scheme is not sensitive to system unbalance and it is sensitive in detecting capacitor unit outages even on very large capacitor banks.
  2. Harmonic currents do not affect this scheme.
  3. For very large banks with more than one series group the amount of energy in the capacitors will decrease. This will lower the fuse interrupting duty and may
    reduce the cost of fuses.
By splitting the wye into two sections, the number of parallel units per series group is decreased, thereby increasing the overvoltages on the remaining units in the series group in the event of a fuse operation.

A double-wye type of capacitor bank needs more substation area and connections. A balanced failure in each wye does not provide any indication in this scheme.

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4. Voltage differential protection method for grounded wye capacitor banks

In this scheme, shown in Figure 4, two three-phase voltage transformer outputs are compared in a differential relay. Loss of capacitor unit in each phase can be detected independently.

The zero sequence voltage is present during the unbalance in the shunt capacitor bank

Voltage difference prediction method for a grounded wye connected capacitor bank
Figure 4 – Voltage difference prediction method for a grounded wye connected capacitor bank

The advantages of this scheme are:

  1. The capacitor bank contains twice as many parallel units per series group compared to a split-wye bank. The overvoltages seen by the remaining units in a group in the event of a fuse operation will be less.
  2. This capacitor bank may require less substation area.
  3. This scheme is less sensitive to system unbalance. It is sensitive to failure detection in the series capacitors.

The main limitation of this scheme is that the number of PTs required is six and extensive connections are also required.

Unbalance Detection in Ungrounded Capacitor Banks

In order to detect the unbalance in ungrounded capacitor banks, the voltage transformer or current transformer sensors are used along with appropriate relays in the secondary circuit. Six different schemes for the detection of unbalance in the ungrounded capacitor circuits are presented.

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Unbalance schemes for ungrounded systems:

5. Neutral voltage unbalance protection using ungrounded wye connected capacitor banks

Using a voltage transformer connected between the neutral and the ground, any neutral voltage shift due to the failure of a capacitor unit is sensed (see Figure 5) .

Neutral voltage unbalance protection for ungrounded wye capacitor bank using a PT
Figure 5 – Neutral voltage unbalance protection for ungrounded wye capacitor bank using a PT

The neutral voltage shift (VNS) due to the loss of individual capacitor unit can be calculated as:

Neutral voltage shift (VNS)

where F is the number of units removed from one series group. The percentage overvoltage for any number of units removed from a series group is given by:

Percentage overvoltage

The unbalance protective scheme consists of a time-delayed voltage relay with third harmonic filter connected across the secondary of the PT. The potential transformer may be a voltage transformer or a capacitive device.

The voltage transformer for this application should be rated for full system voltage because the neutral voltage can be expected to rise above the rated voltage during certain switching operations.

The advantages of this scheme are:

  1. The capacitor bank contains twice as many parallel units per series group compared to a split-wye bank. The overvoltages seen by the remaining units in a group in the event of a fuse operation will be less.
    2. This capacitor bank may require less substation area and connection in the power circuit.
    3. This scheme is less sensitive to system unbalance.

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6. Neutral voltage unbalance protection method for ungrounded capacitor banks using capacitive voltage divider

This scheme is similar to the PT scheme shown above (see Figure 6). A conventional inverse time voltage relay is connected across the grounded end capacitor.

The grounded capacitor is a low voltage unit, 2400 V or less, sized to provide the desired unbalance voltage to the relay. In the event of one phase open, the voltage in the neutral relay exceeds the short time rating and a limiter has to be used.

Scheme 6 has the same advantages and disadvantages as Scheme 5.

Neutral voltage unbalance protection for an ungrounded wye capacitor bank using a capacitor voltage divider
Figure 6 – Neutral voltage unbalance protection for an ungrounded wye capacitor bank using a capacitor voltage divider

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7. Neutral voltage unbalance detection method for ungrounded wye capacitor banks using three PTs

This scheme is shown in Figure 7. This protection scheme uses three lines to neutral PTs with the secondary connected in the broken delta and an overvoltage relay.

This scheme has advantages similar to Scheme 5. This scheme is sensitive to triplen harmonics and it is expensive.

Summation of line-to-neutral voltages with optional line-to-neutral overvoltage protection using three PTs
Figure 7 – Summation of line-to-neutral voltages with optional line-to-neutral overvoltage protection using three PTs

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8. Neutral current unbalance detection method for ungrounded split-wye capacitor banks

This scheme is shown in Figure 8. In this protection scheme, a current transformer is used in the neutral circuit to identify the unbalanced current.

An overcurrent relay can be used to provide an alarm or trip signal.

Ungrounded split-wye connected capacitor bank; unbalance detection method using neutral current sensing
Figure 8 – Ungrounded split-wye connected capacitor bank; unbalance detection method using neutral current sensing

Advantages of this scheme:

  1. The scheme is not sensitive to system unbalance.
  2. The scheme is sensitive to detection of capacitor unit outages and is not affected by the harmonic currents.
  3. This scheme contains only one CT and a relay.

Disadvantages of this scheme:

  1. The disadvantages of this scheme are an increase in the overvoltages per unit because there are fewer parallel units per series group.
  2. The scheme requires more substation area compared to a wye connected capacitor bank.

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9. Neutral voltage protection method for ungrounded split-wye connected capacitor banks

A schematic of this scheme is shown in Figure 9.

This scheme is similar to Scheme 8. The sensor is a PT. This scheme is not sensitive to system unbalance, but it is sensitive to unit outage and is relatively inexpensive. The split-wye may require more substation area.

Ungrounded split-wye connected capacitor bank; unbalance detection method using a PT
Figure 9 – Ungrounded split-wye connected capacitor bank; unbalance detection method using a PT

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10. Neutral voltage unbalance protection method for ungrounded split-wye connected capacitor banks

A schematic of this scheme is shown in Figure 10. The relay is 59N. This scheme is not sensitive to system unbalance, but it is sensitive to unit outage and is relatively
inexpensive.

 Ungrounded split-wye connected capacitor bank; unbalance detection method using a neutral voltage sensing method
Figure 10 – Ungrounded split-wye connected capacitor bank; unbalance detection method using a neutral voltage sensing method
10.1 Overvoltage and undervoltage protection

The relaying for the overvoltage and undervoltage are designated as:

  • 59 for overvoltage protection
  • 27 for undervoltage protection

These relays are normally set to coordinate with the system characteristics and with shunt capacitor banks on the system. Tripping for overvoltage typically occurs at 110% of the rated voltage. The low voltage tripping is set at 0.95 of the rated voltage.

In certain circumstances, the undervoltage relays are used to trip the capacitor banks when the system is re-energized.

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10.2 Voltage differential relays

The voltage differential relay is designated as 60 voltage or current unbalance relay that operates on a given difference.

These relays compare the voltage across the total capacitor bank with the midpoint voltage of the bank for each phase. If one of the capacitor units is lost, then the ratio of the two voltages will change. The change in the voltage will be proportional to the change in the impedance in the capacitor bank.

The voltage differential relays are set to alarm for greater than 0.7% but less than 1% change in the voltage ratio and will trip at greater than 2% change in the voltage ratio.

Voltage Differential Scheme for Grounded Single Wye SCB
Figure 11 – Voltage Differential Scheme for Grounded Single Wye SCB

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10.3 Voltage detection relays

The voltage detection relays use the midpoint voltage and are designated as 59–1/S and 59–2/S overvoltage relays. These relays are set to alarm for one capacitor unit out and will trip the circuit breaker for two capacitor units out.

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10.4 Neutral voltage relays

The neutral voltage relays measure the voltages developed by the neutral current through the capacitor bank and are designated as 59–1/P and 59–2/P overvoltage relays.

The neutral voltage relays need to filter the harmonics and only the voltage due to the fundamental frequency will be used to operate the relay. Loss of one capacitor unit is indicated by an alarm.

Loss of two capacitor units indicates the capacitor bank was tripped.

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Source // Power System by capacitors by Ramasamy Natarajan (Purchase hardcover from Amazon)

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

2 Comments


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    Nov 30, 2018

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    Nov 28, 2018

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