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A DERP's DERs and interconnection installation must meet all applicable international, national, provincial and local construction and safety codes.

Any DERP may operate 60 Hz, three-phase or single-phase generating equipment, in parallel with EDTI's distribution system and in accordance with the EDTI Interconnection Operating and Maintenance Agreement, provided that the equipment and DERP meet or exceed the requirements of this guideline.

Sections 1.1, 1.2 and 1.3, respectively, define the following technical requirements:

  • The distribution system's technical requirements (the DERP's equipment must be able to operate within the ranges specified in this section).
  • Technical requirements to be met by the DERP.
  • Technical requirements to be met by the facilities interconnecting the producing facility with the distribution system.

These requirements promote safe operation and minimize the impact on electrical equipment within the EDTI distribution system, including other customers. These requirements do not address the protection for the DERP's DER equipment. It is the responsibility of the DERP to provide such protection. The DERP is responsible for protecting its DER equipment in such a manner that utility system outages, short-circuits or other disturbances, including excessive zero-sequence currents and ferroresonant overvoltages, do not damage the DERP's DER equipment. As required in this guideline, the DERP's protective equipment must also prevent excessive or unnecessary tripping that would affect EDTI's reliability and the quality of power provided to other customers.

The DERP is required to install, operate and maintain, in good order and repair and in conformity with good electrical practice, the facilities required by this guideline for safe parallel operation with EDTI's distribution system.

1.1 EDTI's Distribution System

1.1.1 General Characteristics

Each distribution circuit on EDTI's distribution system is normally radial, supplied from a single substation. EDTI's distribution circuits operate at 4.16 kV, 13.86 kV or 24.94 kV nominal line-to-line voltages.

Some areas of the downtown core of Edmonton are operated as a secondary network system, with multiple paths for power to flow from an EDTI substation to the customers. Reverse power flow from the customer back through the secondary network is not permitted. Refer to the requirement for reverse power protection (see section 1.3.18).

Three-phase primary voltage service is available on 13.86 kV and 24.94 kV circuits.

1.1.2 System Frequency

The Alberta Interconnected Electric System (AIES) operates nominally at 60 Hz alternating current (AC). Frequency deviations are typically 59.7 Hz to 60.2 Hz for small contingencies that cause modest disturbances, when the AIES remains intact and connected to the Western system.

For large contingencies, much larger frequency deviation can occur. These variations can be experienced when a portion of the AIES becomes islanded.

1.1.3 ​Voltage Regulation

The CSA Standard CAN3-C235-83 Preferred Voltage Levels for AC Systems 0 to 50,000 V provides general guidance for appropriate performance as shown in Table 1.

Table 1: CSA-recommended voltage variation limits for circuits up to 1,000 V, at service entrance2

​Nominal system voltage ​Extreme operating conditions ​ ​ ​
​Normal operating conditions
















2CAN3-C235-83, Table 3

1.1.4 Power Quality

All interconnected equipment must comply with EDTI's standards for power quality. The following industry standards provide guidance for appropriate performance.

  • Voltage flicker: IEEE Std 519-2014 IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power System.
  • Harmonics: IEEE Std 519-2014 IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power System.

According to IEEE 519-2014, the recommended practice for utilities is to limit the maximum individual frequency voltage harmonics to 3% of the fundamental frequency, and the voltage total harmonic distortion to 5% on the utility side of the point of common coupling (1 kV < V ≤ 69 kV). These harmonic voltage limits can be used as system design values for the "worst case" under normal operation.

1.1.5 Voltage Unbalance

Distribution facilities are typically three-phase systems incorporating single-phase distribution taps. Under normal operating conditions, the voltage unbalance on EDTI's distribution system may reach 3%, due to unbalanced loading and single-phase regulation. Voltage unbalance will be calculated using the following formula derived from CSA-61000-4-30-04:


V2 is the negative-sequence voltage (fundamental frequency component).

 V1 is the positive-sequence voltage (fundamental frequency component).

1.1.6 Fault Levels

Fault levels, including maximum allowable fault levels, vary significantly throughout a distribution system. These must be considered in the design of the interconnection. Fault levels and X/R ratios must be evaluated for the equipment selected.

The DERP's facility must not increase fault levels on EDTI's distribution or transmission system above each system's design levels for maximum faults (see section 1.3.11).

1.1.7 System Grounding

Distribution facilities are typically operated as effectively (solidly) grounded and wye-connected at the source substation bus.

Distribution facility grounding must conform to the Alberta Electrical and Communication Utility Code (2013).

1.1.8 Fault and Line Clearing
​To maintain the reliability of the distribution system, EDTI typically uses automatic reclosing. The DERP needs to take into consideration line reclosing when designing DER protection schemes. This is to ensure that the DER is disconnected from EDTI's distribution system prior to the automatic reclosing of breakers. The DERP may reconnect when EDTI's distribution system voltage and frequency return to the normal range and are stabilized.
1.1.9 Limit for DER Connection
No international or national standard specifies the maximum level of DER connection on a distribution circuit or a substation bus. Whenever the hosting capacity for a given distribution circuit is not specified, EDTI will limit the aggregated DER connected on distribution circuits at the substation bus; to the bus’s minimum load EDTI defines hosting capacity for a given distribution circuit as the level of DER connection that maintains a safe, reliable and acceptable operation of the distribution circuit.

1.2 DER facility

1.2.1 Smart Inverters
In general, there are three types of generators - synchronous generators, induction generators and inverters. EDTI requires that smart inverters, as defined in IEEE Std-1547 be used in an inverter-based DER facility.
1.2.2 Mitigation of Adverse Effects

Adding a DER facility to EDTI's distribution system can adversely affect the electric service to existing or future electric customers. The DERP shall work with EDTI to mitigate any unfavourable conditions.

If the DER facility is adversely affecting customers, EDTI reserves the right to disconnect the DER until the concern is mitigated. The DERP will be responsible for any costs incurred as a result of these actions.

1.2.3 Synchronism

Any DER facility that can create a voltage while separated from EDTI's distribution system must have synchronization facilities to allow its connection to EDTI's distribution system.

Synchronization facilities are not required for induction generators that act as motors during start-up, drawing power from the electric system before they themselves generate power.

EDTI cannot synchronize to a DER facility. The DER facility is responsible for synchronizing and maintaining synchronization with EDTI's distribution system. A proposed synchronizing scheme must be submitted and outlined in the Interconnection Operating and Maintenance Agreement.

Synchronization equipment must prevent connection to EDTI's distribution system when the DERP's synchronous generator and/or EDTI's distribution system is operating outside the limits showing in Table 2.3

Table 2: Synchronization parameter limits for synchronous interconnection to EDTI's distribution system

​Aggregate rating of DER units (kVA) ​Frequency difference (Δf, Hz ​Voltage difference (ΔV, %) ​Phase angle difference (ΔΦ, °)
​0–500 ​0.3 ​10 ​20
​501–1,500 ​0.2 ​5 ​15
​> 1,500 ​0.1 ​3 ​10

Distribution facilities typically allow for automatic reclosing of electrical circuits after a variable time delay. The DERP is responsible for protecting their facility from the effects of such reclosing.

DERs can automatically restart following automatic reclosing of distribution facility electrical equipment. DERs that automatically restart must have a time delay on restart with an adjustable range of 1–5 minutes.4 EDTI will coordinate the settings of DER restart time delays so that DERs on any circuit can restart in a staggered order to prevent unfavorable conditions from arising during restart.

3IEEE 1547-2003, Table 5

4IEEE 1547-2003, Clause 4.2.6

1.2.4 Voltage Regulation and Power Factor

The DERP is responsible for ensuring that the voltage levels at the point of common coupling (PCC) are maintained within the guidelines prescribed by EDTI. Voltage levels must be at least equal to the voltage levels at all circuit load conditions, prior to the interconnection. DERs shall not actively regulate the voltage at the PPC, except in the following cases:

  • Synchronous generators must be equipped with excitation controllers capable of controlling voltage. The generator bus voltage set point shall be stable at, as well as adjustable to, any value between 95% and 105% so that EDTI can maintain CSA voltage limits on its system.
  • Inverter-type generators must be capable of adjusting the power factor in the range of ±0.9. The DERP may operate outside that range only by prior agreement with EDTI.
  • EDTI will define voltage and reactive power control requirements on a project-by-project basis. The DERP and EDTI will identify the exact transformer ratio to allow the best voltage regulation on the system and determine whether an on-load tap changer is needed.
  • In order to coordinate with its existing voltage-control devices, EDTI may require that the DER operate in a power factor control mode. This means operating within a constant power factor set point range. The voltage/power factor regulator must be capable of controlling the power factor of the DER between +0.9 and -0.9. EDTI shall determine the actual set point between these limits.
  • In power factor control mode, the DER must have a voltage override that causes it to reduce excitation if the voltage at the PCC exceeds an upper limit to be specified by EDTI. The normal upper limit is 105% of nominal voltage; however, the DER shall have a provision to adjust this upper limit to between 100% and 110% of nominal voltage. The DER must also have a provision for a time delay between sensing an excursion of the upper voltage and initiating control action. The power factor control equipment in the DER must allow for the adjustment of this time delay between 0 and 180 seconds. EDTI will specify the required time delay.
1.2.5 Frequency Control

When EDTI's distribution system frequency is in an abnormal range, as specified in Table 3, the DERs shall cease to energize EDTI's distribution facilities within the clearing time indicated. Clearing time is the time between the start of the abnormal condition and the DERs ceasing to energize EDTI's distribution facilities.

Adjustable under-frequency trip settings shall be coordinated with EDTI's distribution operations.

Islanded operations are not allowed for DERs connected to EDTI's distribution system (see section 1.3.16). DERs with stand-alone capability, that serve isolated systems, must be capable of controlling the frequency of the system to between 59.7 Hz and 60.2 Hz for normal operation.

Table 3: Interconnection system response to abnormal frequencies5

​DER size ​Frequency range (Hz) ​Clearing time (s)
​≤ 30 kW ​> 60.5 ​0.16
​< 59.3 ​0.16
​> 30 kW ​> 60.5 ​0.16
​< {59.8 – 57.0} (adjustable set point) ​Adjustable 0.16 to 300
​< 57.0 ​0.16

5IEEE Std 1547-2003, Table 2

1.2.6 Voltage Unbalance

The phase-to-phase voltage unbalance must not exceed 1%6 for any three-phase DER facility, as measured both with no load and with balanced three-phase loading. Voltage unbalance will be calculated using the same formula as in section 1.1.5. Single-phase DERs must not adversely unbalance the three-phase system. When they are connected in multiple units, an equal amount of DER capacity must be applied to each phase of a three-phase circuit. The group of DERs must also maintain balance when one unit trips or begins generating before or after the others. A single-phase DER may be connected alone, only if it does not cause voltage unbalance on EDTI's distribution system in excess of 2%.7

6IEEE Std 141-1993 (Red Book), section 3.8.3

7AUC Guide for Generator Interconnection, section 3.2.5

1.2.7 Grounding
​A ground grid of sufficient size to handle the maximum available ground fault current shall be designed and installed in order to limit step and touch potentials to safe levels as set forth in ANSI/IEEE Std 80 IEEE Guide for Safety in AC Substation Grounding. All electrical equipment must be grounded in accordance with the Alberta Electrical and Communication Utility Code (AECUC) and the Canadian Electrical Code's electrical and safety regulations. The ground grid must be approved by EDTI.
1.2.8 Resonance and Self-Excitation of Induction Generators

Resonance should be considered in the design of the DERP's facility, as certain resonance can cause damage to existing electrical equipment, including the electrical equipment of the DERP. Engineering analysis by the DERP should be a part of the design process to evaluate the existence of and to eliminate the harmful effects of:

  • Ferroresonance in the transformer (see Appendix VII, Note 1).
  • Sub-synchronous resonance due to the presence of series capacitor banks (see Appendix VII, Note 2).
  • Resonance with other customers' equipment due to the addition of capacitor banks to the distribution system (see Appendix VII, Note 3).

For DERPs connecting induction generators, the adverse effects of self-excitation of the induction generator during island conditions should be assessed and mitigated. The intent is to detect and eliminate any self-excited condition (see Appendix VII, Note 4).

The engineering analysis of resonance and the assessment of the effect of self-excitation of induction generators should be submitted to EDTI for approval or further evaluation.

1.2.9 Single-Phase DER Facilities

The aggregate generation on a single phase shared secondary should not exceed 10 kW.8 For dedicated distribution transformer services, the limit of a single-phase generating facility shall be the transformer nameplate rating.

8AUC Micro-Generator Application Guideline, section B

1.3 Interconnection


​An interconnection review study will be conducted when a DERP applies for DER connection to EDTI's distribution system. The study will evaluate the impact that the DER will have if it is connected to EDTI's distribution system (the scope of the study is outlined in Appendix I).

1.3.1 Safety

Abnormal conditions can arise on the EDTI's distribution system that require a response from the connected DER. This response contributes to the safety of utility mai.ntenance personnel and the general public, as well as the avoidance of damage to connected equipment, including the DER as per IEEE 1547​9

The connection, installation and operation of a DER facility shall not create a safety hazard to EDTI's personnel, customers, general public and personnel working in the DER facility.

Safety is of primary concern and shall be the main consideration when designing a DER facility. The primary objective of this technical guideline is to provide interconnection specifications to ensure that safety will be maintained.

9section 4.2, IEEE 1547-2003


1.3.2 Point of Common Coupling

Point of common coupling (PCC) is the point where EDTI's electrical facilities or conductors are connected to the DERP's facilities or conductors, and where any transfer of electric power between the DERP and EDTI takes place. The PCC will be identified in the design and on the single-line diagram. EDTI will coordinate design, construction, maintenance and operation of the facilities on the distribution side of the PCC. The DERP is responsible for the design, construction, maintenance and operation of the facilities on the DER side of the PCC.

In specific cases, either EDTI or the DERP may own equipment located on the other's side of the PCC. For example, EDTI may own and operate communications, supervisory, or metering equipment, which is located on the DERP's side of the PCC.

The DERP must provide a site with the necessary space for EDTI to install current transformers, potential transformers, switching equipment, meters, and any other controls or communications equipment required to interconnect with the DER facility. The site is to be approved by EDTI and a 120 V alternating-current power service is to be available for the use of portable tools.

All voltage, frequency and harmonic parameters, as specified in the following sections, shall be met at the PCC unless otherwise stated.

1.3.3 Point of Disconnection

To provide a means of electrically isolating EDTI's distribution system from the DER, a manual and visible disconnect switch must be installed at the point of common coupling (PCC). Where the DER facilities are located far from the PCC, the DERP may be allowed to install a local point of isolation next to the DER. EDTI and the DERP will mutually agree on the exact location of the switch.

If the switch is to be located on EDTI's side of the PCC, it will be installed by EDTI at the DERP's expense. If the switch is to be located on the DERP's side of the PCC, it must be supplied and installed by the DERP.

When the interconnection involves three-phase DERs, the disconnect switch must be gang operated to simultaneously isolate all three-phases.

All disconnect switches must:

  • Be within 5 m (horizontal) of the PCC, or an EDTI-approved location.
  • Be capable of being opened at rated load.
  • Viewing windows, through which the status (open or closed) of the disconnect switch can be verified readily.
  • Be readily accessible to EDTI operating personnel on a 24-hour basis.
  • Have provision for being locked in the "open" position.
  • Disconnect all ungrounded conductors of the circuit simultaneously.
  • Be externally operable without exposing the operator to contact with live parts.
  • Be capable of being closed onto a fault with complete safety for the operator.
  • Be capable of being energized from both sides.
  • Plainly indicate whether it is in the "open" or "closed" position.
  • Provide safe isolation for EDTI personnel from the generators and all other possible customer sources of power.
  • Be labelled with an EDTI switch number.
  • Meet applicable Canadian Electrical Code (CEC) Part I and Part II standards.
  • Be installed to meet all applicable codes.
  • Be inspected and maintained annually.

The disconnect switch on the DER side of the interconnection transformer will be owned and maintained by the DERP. Refer to Appendices II, III and IV for a sample configuration.

On a site that interconnects multiple generators, one disconnect switch must be capable of isolating all the generators simultaneously.

A withdrawable circuit breaker is an acceptable disconnect device.

The DERP shall follow EDTI's switching, clearance and tagging procedures. EDTI shall instruct the DERP on these procedures.

There may be other means of meeting the requirements of this section. EDTI must approve any other means.

1.3.4 Phasing
Since phasing is not standardized across distribution facilities, the phase sequence and the direction of rotation must be coordinated between the DERP and EDTI's system.
1.3.5 Voltage Flicker

The DERP must not cause excessive voltage flicker on EDTI's facilities. Any voltage flicker at the point of common coupling that is caused by the generating facility should not exceed the limits defined by the maximum borderline of irritation curve identified in Figure10.3 of IEEE Std 519-2014 IEEE Recommended Practices and Requirements for Harmonic Control in Electric Power Systems. The customer will also comply with the short- and long-term flicker limits as specified in Tables 2 and 6 of CAN/CSA-C61000-3-7 Electromagnetic Compatibility (EMC) - Part 3-7. This is necessary to minimize any adverse voltage effects that could be experienced by other customers on EDTI's distribution system.

Induction generators may be connected and brought up to synchronous speed (as an induction motor) if these flicker limits are not exceeded. The DERP must submit to EDTI the expected number of starts per specific time period and the maximum starting current draw data in order to verify that the voltage dip due to starting is within the IEEE limits. At no time should the voltage drop exceed 5% as measured on EDTI's side of the point of common coupling (PCC). Otherwise, the DERP will be required to install corrective step-switched capacitors or apply other techniques to bring voltage fluctuations to acceptable levels. These corrective measures could, in turn, cause ferroresonance; therefore, EDTI must review any measures undertaken on the DERP's side of the PCC.

1.3.6 Harmonics

The DERP's operation of its DER facilities must not cause an unacceptable level of harmonics. Maximum harmonic current distortion limits for a DER facility, measured at the point of common coupling (PCC), are as specified in Table 4.

The objective of the current distortion limits in CSA C22.3 No. 9-08 is to limit the harmonic injection from individual customers. This is to ensure that they do not cause unacceptable levels of voltage distortion to normal system characteristics. Ideally, the voltage distortion would then be limited to 3% of the fundamental frequency for individual harmonic frequencies and 5% voltage total harmonic distortion on EDTI's side of the PCC.

DER facilities must not inject DC current greater than 0.5% of the full rated output current into EDTI's distribution system under normal or abnormal operating conditions.

Table 4: Maximum harmonic current distortion in percent of current (I)a 10

​Individual harmonic order h (odd harmonics)b ​h < 11 ​11 < h < 17 ​17 < h < 25 ​25 < h < 35 ​35 ≤ h ​Total demand distortion (TDD)
​​Percent (%) ​4.0 2.0​ 1.5 ​0.6 0.3​ 5.0​


a. I = the greater of the distribution system maximum load current integrated demand (15 or 30 min) without the DER unit; or the DER unit rated current capacity, transformed at the PCC when a transformer exists between the DER unit and the PCC.

b. The maximum distortion values specified in this table are for odd harmonics. To obtain maximum distortion values for even harmonics, multiply the value in the corresponding h-range by 25%.

10Table 3, IEEE 1547-2003 and Table 1, CSA C22.3 9-08

1.3.7 Inadvertent Energization of EDTI’s Facilities

​When EDTI's facilities are de-energized for any reason, the DERP's DER must not energize EDTI's facilities for any of the following reasons.

  1. The DER can cause power power quality problems to EDTI's customer.
  2. The DER can cause out-of-phase reclosing between EDTI's distribution system and the DER facility.
  3. The DER can create safety hazards to EDTI's personnel, customers and general public.
1.3.8 Network System Interconnection

DER facilities that export power onto EDTI's distribution system will not be allowed to connect to the downtown secondary network system. This restriction is required because EDTI uses network protectors for reverse power protection throughout the downtown's networked underground distribution system. Network protector manufacturers and IEEE C37.108 specifically recommend against using network protectors in a DER application.

For non-exporting DER facilities, EDTI may allow parallel operation on the network system if:

  • The DERP installs reverse power protection for the facility (see section 1.3.18).
  • Reverse power protection settings prevent any cycling operation of network protectors due to the output of the DER.
  • The network equipment loading and fault-interrupting capacities are not exceeded by the addition of the DER.
1.3.9 Dedicated Transformer

EDTI reserves the right to require a DERP's facility to connect to the distribution system through a dedicated transformer. A dedicated transformer is a transformer with a secondary winding that serves only one customer. The transformer may be necessary to:

  • Ensure conformance with EDTI safe work practices.
  • Enhance service restoration operations.
  • Prevent detrimental effects on other EDTI customers.

The dedicated transformer that is part of the normal electrical service connection of a DERP's facility may meet this requirement if no other customers are supplied from it. A dedicated transformer may not be required if the installation is designed and coordinated with EDTI to protect the EDTI system and its customers adequately from potential problems caused by the operation of the DER.

1.3.10 Interconnection Grounding

Grounding configurations shall be designed to provide:

  • Solidly grounded distribution facilities.
  • Suitable fault detection to isolate all sources of fault contribution, including the DER, from a faulted line or distribution element.
  • A circuit to block the transmission of harmonic currents and voltages.
  • Protection of the low-voltage side from high-fault-current damage.

For three-phase DERs, the EDTI-supplied distribution transformer will normally be a grounded-wye configuration on both the low- and high-voltage side.

The preferred configuration for a DERP-owned interconnection transformer is delta connection on the DER side of the transformer and a grounded-wye configuration on EDTI's side of the transformer. If this configuration is not possible, the configuration chosen must still address the above concerns. The winding configuration for DER interconnection transformers must be reviewed and approved by EDTI.

If an interconnection transformer with delta connection on EDTI's side is used, a special interconnection review study will be conducted to avoid temporary over-voltage issues if a line-to-ground fault on a distribution circuit were to occur.

1.3.11 Interrupting Device Ratings and Fault Levels

The design of the DER facility must consider the fault contributions from both the distribution facility and the generating facility to ensure that all circuit fault interrupters are adequately sized. EDTI will inform the DERP of the present and anticipated future fault contributions from the interconnected electric system, including fault-level design limits.

For generators that have time-variant fault-contribution characteristics, the characteristic producing the highest fundamental frequency fault current shall be used for maximum fault current calculations. For synchronous and induction generators, the subtransient reactance shall be used.

Inverter-type systems are different from rotating machines in that fault currents are typically only marginally greater than full-load current.

EDTI will perform fault-level calculations based upon the DERP's supplied data for proposed generator and transformer impedances. EDTI will advise the DERP if the proposed facility exceeds EDTI's maximum design fault levels. If the calculated fault contribution from the proposed facility increases the fault levels on either EDTI's distribution or transmission system above the maximum design levels, then the DERP will be required to redesign its facility to reduce fault level contributions. This may involve one or more of the following mitigation techniques:

  • Select a generator with a larger subtransient impedance.
  • Select a higher-impedance generator transformer.
  • Install current-limiting reactors or other fault-current-limiting devices.

If the fault-level contributions from the facility cannot be reduced to an acceptable level, then EDTI will not allow the interconnection of the DERP's facility.

1.3.12 Phase and Ground Fault Protection

The DERP must install protective devices to detect and promptly isolate the DER facility for faults occurring in the DER facility or on EDTI's distribution system.

The DER facility's protective devices must fully coordinate with protective relays on the electric system. The DERP must calculate the protective-device settings and submit the relay characteristics and settings to EDTI for review and approval.

The DER facility must be able to detect the following situations and isolate itself from the distribution facility:

  • A short-circuit between any phase(s) and ground.
  • A short-circuit between phase(s).
  • Loss of any phase(s).
1.3.13 Overvoltage and Undervoltage Protection

The DERP will operate its generating equipment in such a manner that the voltage levels on EDTI's distribution system are in the same range as if the generating equipment were not connected to the distribution system.

The DERP must install necessary relays to trip the DER circuit breaker when the voltage, measured phase-to-ground, is outside predetermined limits.

The DERP's interconnection device shall cause the generator to cease to energize EDTI's distribution system within the clearing time as indicated in Table 5. The clearing time is the time between the start of the abnormal condition and the interconnection device ceasing to energize EDTI's distribution system.

Table 5: Interconnection system response to abnormal voltages11

​Voltage range (% of base voltage)a ​Clearing time (s)
​V < 50 ​0.16
​50 ≤ V < 80 ​2.00
​110 < V < 120 ​1.00
​V ≥ 120 ​0.16

a. Base voltages are the normal system voltages stated in CSA Standard CAN3-C235-83.

The DERP may reconnect when EDTI's system voltage and frequency return to normal range and are stabilized for a time period up to 5 minutes.12

1110IEEE 1547-2003, Table 1

12IEEE Std 1547-2003, Clause 4.2.6

1.3.14 Overfrequency and Underfrequency Protection

The DERP must install frequency-selective relays to separate the DER(s) from EDTI's distribution system in cases of extreme variations in frequency.

Underfrequency and overfrequency relaying that automatically disconnects DERs from the distribution system must be time delayed in accordance with IEEE Std 1547-2003 as noted in section 1.2.5. The DERP may reconnect when EDTI's distribution system voltage and frequency return to a normal range and are stabilized for a time period up to 5 minutes.

1.3.15 Unbalanced Phase Protection
​The DERP should be aware that single-phase protection devices exist on EDTI's distribution system. Unbalanced current conditions caused by open conductors on the distribution system can subject the generator to a high level of negative-sequence current. The DERP is responsible for protecting its generating equipment from the effects of excessive negative-sequence currents in the event of single phasing. Negative-sequence current relaying is recommended.
1.3.16 Anti-Islanding Protection

In most cases, the DER facility will routinely operate as a part of the interconnected system. A problem on EDTI's distribution system could lead to the DER becoming islanded and inadvertently acting as the sole power resource for one or more of EDTI's customers. This could result in damages to those customers and liability to the DERP because of irregularities in power quality. The DERP's generator must be equipped with anti-islanding protection designed to prevent the DER from being connected to a de-energized EDTI circuit. The anti-islanding protection should meet the following requirements:

  • Upon loss of voltage in one or more phases of EDTI's distribution system, the DER facility shall automatically disconnect from EDTI's distribution system within 0.6 s.
  • All DER facilities must have passive anti-islanding protection, including:
    • Underfrequency/overfrequency protection (section 1.2.5).
    • Undervoltage/overvoltage protection (section 1.3.13).
    • Reverse power protection.
  • All DER facilities shall have active anti-islanding protection, which can be:
    • Sandia Frequency Shift.
    • Sliding Mode Frequency Shift.
    • Active Frequency Drift.
    • Other method approved by CSA or ANSI/IEEE or UL.
  • The DERP shall demonstrate to EDTI that it shall not sustain an island for longer than 0.6 s.

Transfer trip for anti-islanding protection may be required as stipulated in section 1.3.17.

Damages that are caused by a failure to separate safely during an islanding event will be the responsibility of the DERP.

1.3.17 Requirements for Transfer Trip

No international or national standard specifies the minimum DER rating at which transfer trip (TT) is required. However, EDTI should set up the threshold for TT installation on a DER site to avoid DERs running in islanding mode. In general, the minimum load on a 15 kV or 25 kV circuit is about 2,000 kVA. If a DER with 1,000 kW rating is connected to a circuit without TT, it is possible for the DER to run in islanding mode to feed the customers on the circuit when the circuit breaker is open. Therefore, all synchronous generators and inverter-based generators that are rated 1,000 kW or larger with the ability to export power onto EDTI's distribution system must be equipped with transfer trip protection or an EDTI-approved anti-islanding relay that performs the equivalent function of transfer trip. This is to ensure that these generators do not island in the event of a substation breaker or intermediate automatic circuit recloser opening.

General requirements are as follows:

  1. A DER end-open signal must be sent to EDTI's circuit breaker relay to make sure the breaker is safe to reclose after tripping on a fault.
  2. Generator lockout or lockout of the main breaker (for DER facilities that want to operate in isolation) must occur at the point of common coupling location within 0.6 s13 of the EDTI substation circuit breaker or the automatic circuit recloser opening.
  3. Fail-safe lockout must occur within 6 s of communication loss.
  4. The DERP is responsible for detecting and tripping in the event of communication loss.

If transfer trip protection is installed for a DER, the DER must operate on the specified circuit. When the DER is transferred to another circuit from the specified circuit, the DER must be turned off.

Synchronous generators and inverters of less than 1,000 kW may also require this protection, depending upon the characteristics of the particular distribution circuit to which they are connected. EDTI will inform the DERP of the requirements in these cases.

DERs of less than 1,000 kW should have provision for the capability to receive EDTI trip signals and cease generation; i.e., they should have provision for the installation of transfer trip. The actual implementation is not required when the DER is commissioned but may be requested by EDTI at a later date to be implemented at the DERP's cost.

Unless the DERP can demonstrate that there is no potential for self-excitation, transfer-tripping requirements also apply to induction generators.

13A circuit breaker or a recloser can be reclosed within 0.6 s.

1.3.18 Reverse Power Relay Protection

Reverse power protection must be installed on non-exporting or export-limited generating facilities that are connected in parallel to the EDTI system.

Until the DERP has received all the necessary licences and permits to operate in a parallel manner, all generating facilities require reverse power protection while connected in parallel to the EDTI system. An option for the DERP is to install an interlocking device that will prevent any electrical connection between the generator and the EDTI distribution system.

The setting for the reverse power protection (IEEE device 32) shall be the export or non-export limit as agreed to by EDTI, with a maximum 2 s time delay.14

14San Diego Gas & Electric Company Rule 21, sheet 155

1.3.19 Visibility and Controllability

Where a DER could adversely affect the power system, the DERP must have systems in place to inform EDTI what protective operations occurred or failed to occur. An example of an adverse effect would be the DERP's generator providing inflow into a fault.

Each DER unit of 250 kW or more or aggregate of 250 kW or more at a single point of common coupling (PCC) shall have the provision15 for EDTI to monitor remotely the DER's connection status, total real power output, total reactive power output, per-phase voltage and per-phase current at the point of DER connection.

Each DER unit of 250 kW or more or aggregate of 250 kW or more at a single PCC shall be controllable (turned off) by EDTI's operators. This controllability must be implemented through 100% utility infrastructure. The DERs will be turned off only in extreme conditions—e.g., if EDTI's distribution system must be stabilized after a blackout or if a threat to public safety exists.

15IEEE Std 1547-2003, Clause 4.1.6, provision means that the data required is available, necessary devices for communication from the DER control to EDTI's communication devices are in place when the DER is commissioned.

1.3.20 Protection from Electromagnetic Interference

The influence of electromagnetic interference must not change the state or operation of the interconnection between EDTI's and the DERP's systems.

The DER facility interconnection must have the capability to withstand electromagnetic interference environments in accordance with either of the following:

  1. ANSI/IEEE Std C37.90.2-2004 IEEE Standard for Withstand Capability of Relay Systems to Radiated Electromagnetic Interference from Transceivers
  2. CAN/CSA-CEI/IEC 61000-4-3-07 (R2015), using Level X, 35 V/m, in accordance with IEEE C37.90.2

The DERP shall provide documentation to show compliance with a. or b. above.

1.3.21 Surge Withstand Performance
​The interconnection system must have the capability to withstand voltage and current surges in accordance with the environments described in IEEE/ANSI C62.41-2002 or C37.90.1-2012.
1.3.22 Special Interconnection Protection

In some cases, it will be necessary to provide for special generator-specific protection and controls, such as for loss of synchronism between the DERP and EDTI.

Unbalance conditions can occur in the distribution system, especially under system-fault conditions, and the design of the interconnection facilities should consider this.

For wye-delta interconnection transformers, the unbalance fault current could damage the generator interconnection transformer. The damage can occur because of the circulating current that occurs in the delta winding of the interconnection transformer in its attempt to balance the fault current. Therefore, the design may require protection for the transformer to address this potential problem.

In cases where the DERP wants to separate automatically from EDTI's system and begin isolated operation, additional devices may have to be installed to effect separation.

1.4 Typical Interconnection Protective Requirements


Typical interconnection requirements for safely operating the DERP's generating equipment in parallel with EDTI's distribution system are specified below. Specific interconnection locations and conditions may require more restrictive protective settings or hardware, especially when exporting power to the EDTI system. EDTI shall notify the DERP of special circumstances as soon as possible. An example of a restrictive area for DER interconnection is EDTI's downtown network system. The DERP will need to work closely with EDTI to determine whether interconnection and operation within a specific network system is possible.

Protective relays shall provide DER status and analog values specified in section 1.3.19. Alarms generated by the DER facility are to be monitored by the DERP, and appropriate action should be taken by the DERP.

Only utility-grade protection devices are approved for interconnection protection.

Protective relays, electric conversion devices or other devices can comply with this guideline by demonstrating that they can accomplish the required protective function as specified in Table 6, Table 7 and Table 8.

Table 6: Protection for single-phase generators interconnection control, protection and safety equipmenta

Single-phase connected to secondary system

Protection requirement for DER ≤ 50 kWb
 Interconnect disconnect device ✓​
 Generator disconnect device ✓​
 Undervoltage/overvoltage trip ✓​
 Overfrequency/underfrequency trip ✓​
 Overcurrent ✓​
 Synchronizing control (manual or automatic)c manual or automatic​
 Anti-islanding (inverter type) ✓​
 Synch-checkc (at point of common coupling) ​✓



a. Exporting to EDTI's system may require additional operational/protection devices and will require coordination of operations with EDTI.

b. For single-phase generators larger than 50 kW, consult with EDTI on the required interconnection control, protection and safety equipment.

c. For synchronous and other types of generators with stand-alone capability.

Table 7 – Protection for three-phase generators Interconnection control, protection and safety equipmenta

Three-phase connected to primary or secondary system (all devices are three-phase unless otherwise specified)

Device # Protection requirement 150 ≤ DER ≤ 1,000 kW DER > 1,000 kW
  Interconnect disconnect device
  Generator disconnect device
  Synchronizing controlb manual (M) or automatic (A) M or A A
25 Synch-checkb (at point of common coupling)
27 Undervoltage
59 Overvoltage
59N Neutral overvoltagec
50/51 Instantaneous/timed overcurrent d d
50/51N Instantaneous/timed neutral overcurrent
81 Overfrequency and Underfrequency
32 Directional power e


TT Transfer trip or equivalent relay f
  Telemetry data communication g
  Anti-islanding for inverters (IEEE Std 929, UL 1741, CSA C22.2 No. 107.1)



a. Exporting to EDTI's system may require additional operational/protection devices and will require coordination of operations with EDTI.

b. For synchronous and other types of generators with stand-alone capability.

c. Only required for generators that have their interconnection transformer's primary winding ungrounded. Used in conjunction with three PTs in broken delta configuration rated for line-to-line voltage. For detecting ground faults on the distribution system.

d. A timed overcurrent relay with voltage restraint (51 V) may also be required to prevent nuisance trips.

e. Only required for non-exporting or export-limited generators.

f. Transfer trip or equivalent protective relay function required for all synchronous generators rated 1,000 kW and larger with export capability. May also be required for exporting synchronous generators under 1,000 kW, depending upon characteristics of the distribution circuit. EDTI will advise.

g. Telemetry required for DERs greater than 250 kW.

Table 8 – Protection for closed transition switching Interconnection control, protection and safety equipment generators connected to primary or secondary system

For six cycles or less (closed transition switching)16

Interconnect disconnect device
Generator disconnect device
Undervoltage trip
Overvoltage trip
Overfrequency and underfrequency trip
Neutral overvoltage tripa or neutral overcurrent tripa
Directional power trip
Synchronizing controlb Manual or automatic
Synch-checkb (at point of common coupling)



a. Selection depends upon grounding connection of interconnection transform


b. For synchronous and other types of generators with stand-alone capability.

16 CSA C22.3 No.9-08, clause 7.4.13

17IEEE 1547, section 1.3

1.4.1 Single-Phase Generators
​Refer to Table 6 for the protective functions required to meet this guideline. Inverter-type generators must meet the criteria in IEEE 929 Recommended Practice for Utility Interface of Photovoltaic (PV) Systems and be certified to UL 1741 and/or CSA C22.2 No.107.1.
1.4.2 Three-Phase Synchronous Generators

Refer to Table 7 for the protective functions required to meet this guideline.

The DERP's generator circuit breakers must be three-phase devices with electronic or electromechanical control.

The DERP is responsible for:

  • Properly synchronizing its generator with EDTI's distribution system.
  • Ensuring that the interconnection protection device settings coordinate with EDTI's protective device settings.
1.4.3 Three-Phase Induction Generators and Three-Phase Inverter Systems
​Refer to Table 7 for the protective functions by DER size required to meet this guideline. Inverter-type generators must meet the applicable criteria in IEEE 929 and be certified to UL 1741 and/or CSA C22.2 No.107.1.
1.4.4 Generators Paralleling for Six Cycles or Less (Closed Transition Switching)

DERs that are parallel with EDTI's distribution system for six cycles or less shall be provided with the following protection functions:

  1. Undervoltage protection, which shall ensure that the DER is not capable of energizing EDTI's distribution system if the distribution system is de-energized.
  2. A six-cycle timer to ensure that the DER facility will not be parallel with EDTI's distribution system for more than six cycles.
  3. A manual or automatic synchronization check.

DERPs whose generators meet these criteria can submit an application and sign an Interconnection Operating and Maintenance Agreement, once they have met the other requirements of this guideline.

1.4.5 Mitigation of Protection System Failure

Relays with self-diagnostic check features provide information on the integrity of the protection system and should be used whenever possible. The protection system should be designed by a qualified professional engineer, or a competent technical person, working with EDTI engineers to ensure that this self-checking feature is integrated into the overall protection system for the safe and reliable operation of the power system.

Depending upon the system and its design, where relays with this self-diagnostic feature do not trip the appropriate breaker(s), sufficient redundant or backup protection should be provided for EDTI's distribution system. The malfunctioning relay should also send a signal to notify operating personnel to initiate an investigation of the malfunction.

Older electromechanical relays do not generally come with self-diagnostic features. In these instances, design of protection and control systems should have fail-safes to maintain the integrity of the protection system in the case of malfunctions.

The DER facility requires a reliable power supply. Batteries and ancillaries, where provided, shall have adequate capacity and rating to ensure the operation of all protection functions when the principal source of power fails. Protection functions shall remain operational for the period of time specified in Table 3 and Table 5 following EDTI's distribution system disturbances or loss of supply from EDTI's distribution system. This may be achieved using batteries and a charger connected to the main supply or by using an uninterruptable power supply with sufficient capacity. Capacitors (i.e., capacitor trip) shall not be used for this purpose.

1.5 Interconnection Protection Approval


The DERP must provide complete documentation on the proposed interconnection protection for review against the requirements of this guideline and for potential impacts on EDTI's distribution system.

The documentation should include:

  • A completed application form.
  • An overall description of how the protection will function.
  • A detailed single-line diagram (substation name, circuit name, voltage levels, transformer ratings, point of common coupling, etc.). For examples see Appendix II, III, and IV.
  • Protection component details (manufacturer and model).
  • Protection component settings (trigger levels and time values).
  • Disconnect switch details (manufacturer, model and associated certification).
  • A site plan (substation name, circuit name, voltage levels, primary circuit route, secondary circuit route, transformer location, DER location, electrical room, etc.).

The DERP shall revise and resubmit the protection information for any proposed modifications.