IEEE C37.99-2000 is the guide for protecting shunt capacitor banks (power and filter banks) installed in substations. It explains how the bank design shapes the protection, which threats exist (failed element, arcing, overvoltage, overcurrent, harmonics, energization) and which device answers each one. It is a guide of recommendations, not a test standard.
Short answer
IEEE C37.99-2000 is the protection guide for shunt capacitor banks used in substations, including filter banks and extra-high-voltage banks. It starts from one idea: protection begins in the design of the bank. The fuse type and the connection define what signal the relay will see and which failures can be detected. This page works as a map to locate each topic of the guide.
What it covers and what it leaves out
Clause 1 of the guide sets the scope:
- Covers: shunt power and filter banks and the protection methods used in many configurations. It also deals with bank design decisions, because they affect the protection, and with extra-high-voltage (EHV) banks.
- Does not cover: pole-mounted banks on distribution circuits or capacitors connected to rotating machines.
The edition dates from the year 2000. It cites standards of that period, such as IEEE 18-1992 for capacitors and IEEE 1036-1992 for their application. The figures in this series are those of that edition.
How the guide is organized
| Topic | Clause | What it answers |
|---|---|---|
| Unit limits | 4.1 | How much voltage, current and reactive power a capacitor can operate at without damage. See capacitor unit limits |
| Bank types | 4.2 to 4.4 | Externally fused, internally fused, fuseless (series strings) and unfused (series-parallel without fuses). See external fuses, internal fuses, fuseless, unfused and capacitor fuses |
| Connection | 5 | Grounded wye, ungrounded wye, delta, double wye and H configuration. See bank connection: grounded wye, ungrounded wye or delta |
| Grounding | 6 | Neutral grounded or isolated, single-point ground or peninsula ground. See bank grounding |
| Bank and system protection | 7 | Table 1: each threat with its device. See protection matrix: threat and device, external arcing and rack faults, overcurrent and breaker failure in a bank and bank voltage protection (59, 59B and 27B) |
| Unbalance | 8 | How the signal is measured and calculated for each bank type. See unbalance protection. Schemes: ungrounded wye, double wye, voltage differential, neutral current and H-bridge; also extra-high-voltage banks, inherent unbalance and measurement error, ambiguous indications and faults unbalance cannot see and alarm and trip times. Calculation: external fuses, internal fuses and fuseless and unfused |
| Filters and static var compensators (SVC) | 9 | Protection of filter banks and thyristor-switched capacitors. See harmonic filter protection and capacitors in SVC and TSC |
| Associated equipment | 10 | Switching device, energization control, surge arresters, sensors and control cables. See bank switching and switching transients in CTs, VTs and control circuits |
| System | 11 | Resonance, harmonics and telephone interference. See resonance in capacitor banks |
| Commissioning and operation | 12 | Preparing to energize and responding to an alarm or a trip |
The annexes provide the symbols, the bibliography, the equations for the effect of inherent unbalances, and the energizing current and frequency.
Bank protection and system protection
Clause 7 separates two groups, and they should not be mixed up.
Bank protection. It handles faults inside the bank. It can include fuses that disconnect a failed unit or element, trips that remove the bank when a fault could develop into a serious failure, and unbalance alarms.
System protection. It protects the bank from what the system imposes on it and protects the system from what the bank causes. It can include surge arresters, reactors against inrush and outrush currents, overvoltage or undervoltage relays, and disconnection of the whole bank when there is a large fault inside the installation.
Table 1 of the guide summarizes the conditions to be considered: failed element, element-to-case fault or fault at a terminal, arcing inside the bank, continuous overvoltage on elements after earlier failures, rack-to-rack fault in banks with two series sections, switching or lightning overvoltage, power-frequency overvoltage, harmonic overload, fault on the installation bus, nearby fault outside the bank, excessive inrush current, loss of system voltage, trip of the line feeding the bank, and breaker failure.
What to decide before choosing protections
The guide insists that almost every installation requires its own engineering analysis. These decisions condition the rest:
- Bank type. With external fuses, the fuse isolates the unit and unbalance measures the result. With internal fuses, fuseless or unfused designs there is no visible indication of a failed element, and detection depends on unbalance.
- Connection and grounding. They define whether zero-sequence current flows, what the neutral current and voltage transformers see, and what recovery voltage the breaker withstands when it opens.
- Parallel energy. A failing unit receives the discharge of the healthy units in its group. That value limits how many units can be placed in parallel.
- Voltage on the healthy units. The guide uses 110% of rated voltage as the reference for taking the bank out of service.
- Switching equipment. The breaker must be able to switch capacitive currents. See capacitive current switching.
How to use this guide on a project
For a new bank, the useful order is the guide’s own: unit limits, bank type, connection, grounding, bank protection, system protection and commissioning. For a bank that is already in service and tripping, start with the cause of the trip. See capacitor bank trips and the general guide to capacitor bank protection.
Selecting capacity and steps follows another path: how to size a capacitor bank.
What we do
The guide acknowledges that each installation needs its own analysis: natural unbalance, the measurement scheme, coordination with fuses and the effect of energization change with every bank. At Tiza Group we carry out that engineering design and studies work: we define the bank type, connection, grounding and protection scheme, and we quote the study according to the scope.
Related guides
- Capacitor bank protection
- Capacitor bank unbalance protection (ANSI 60)
- Capacitor bank: equipment, parts and configurations
- Capacitor bank trips
- Resonance in capacitor banks
- Automatic control of capacitor banks
- Harmonic filters and IEEE 1531
- Special-purpose switch for banks and motors
References
- IEEE Std C37.99-2000, IEEE Guide for the Protection of Shunt Capacitor Banks: cl. 1 (scope and purpose), 4 to 12 and annexes A to D; Table 1 (bank and system protection).
- IEEE Std 18-1992, IEEE Standard for Shunt Power Capacitors, and IEEE Std 1036-1992, IEEE Guide for Application of Shunt Power Capacitors, cited by the guide.
Frequently asked questions
Does IEEE C37.99 apply to pole-mounted capacitor banks on distribution lines?
No. The guide excludes pole-mounted banks on distribution circuits and capacitors connected to rotating machines. It applies to power and filter banks in substations, including extra-high-voltage banks.
Is it a mandatory standard?
It is a guide: it gives application recommendations, not test requirements. It uses «should» for what is recommended and «shall» for what the guide itself considers necessary. Contractual or regulatory requirements are set by each project.
What protects a capacitor bank first?
Unbalance protection. It detects that elements or units have been lost, alarms and trips before the healthy units exceed their allowable continuous voltage. System overcurrent and overvoltage protection back it up but do not replace it.